North London Strategic Flood Risk Assessment

North London Strategic Flood Risk Assessment
North London
Strategic Flood Risk
Assessment
August 2008
Produced for
North London Waste Plan
Prepared by
Ian Bakewell
Knights House
2 Parade
Sutton Coldfield
West Midlands
B72 1PH
T 0121 355 8949
F 0121 355 8901
E [email protected]
Strategic Flood Risk Assessment
North London
Final - August 2008
ii
Document Control Sheet
Report Title
Strategic Flood Risk Assessment
Revision
1
Status
Final
Control Date
August 2008
Record of Issue
Issue Status Authors
01
Draft
Ian Bakewell
Sarah Luff
Matthew Griffin
02
Draft
03
Final
Ian Bakewell
Sarah Luff
Matthew Griffin
Ian Bakewell
Sarah Luff
Date
Check
Nov
2007
Apr
2008
August
2008
Faruk Pekbeken
Faruk Pekbeken
Faruk Pekbeken
Date
Nov
2007
Apr
2008
August
2008
Authorised
Faruk Pekbeken
Faruk Pekbeken
Faruk Pekbeken
Date
Nov
2007
Apr
2008
August
2008
Distribution
Organisation
Contact
North London Waste Plan
Archie Onslow
1
London Borough of Barnet
Nick Lynch
1
London Borough of Camden
Brian O’Donnell
1
London Borough of Enfield
Tom Rumble
1
London Borough of Islington
Sakiba Gurda
1
London Borough of Hackney
Bob Dolata
1
London Borough of Haringey
Jill Warren
1
Copies
London Borough of Waltham Forest Brian Whiteley
1
Environment Agency
1
Strategic Flood Risk Assessment
North London
Final - August 2008
Vicky Boorman
iii
Strategic Flood Risk Assessment
North London
Final - August 2008
iv
Contents
Document Control Sheet ...........................................................................................iii
Contents ......................................................................................................................v
Table of figures .........................................................................................................xv
Tables........................................................................................................................xvi
Glossary..................................................................................................................xviii
Executive Summary ....................................................................................................1
1
Introduction ........................................................................................................4
1.1
Background..........................................................................................................4
1.2
Project Objectives ................................................................................................5
1.3
SFRA Scope ........................................................................................................6
1.4
Methodology ........................................................................................................6
1.5
Setting the Scene.................................................................................................7
2
1.5.1 London Borough of Barnet
8
1.5.2 London Borough of Camden
9
1.5.3 London Borough of Enfield
9
1.5.4 London Borough of Hackney
10
1.5.5 London Borough of Haringey
10
1.5.6 London Borough of Islington
11
1.5.7 London Borough of Waltham Forest
11
Planning Policy and Flood Risk ......................................................................14
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2.1
National Planning Policy and Guidance..............................................................14
2.1.1 Planning and Compulsory Purchase Act 2004
14
2.1.2 Planning Policy Statement 1
15
2.1.3 Planning Policy Statement: Planning and Climate Change: Supplement
to PPS1
15
2.1.4 Planning Policy Statement 25
2.2
16
The Sequential Test and Exception Test ............................................................16
2.2.1 The Sequential Test
16
2.2.2 The Exception Test
20
2.3
Planning Responsibilities ...................................................................................20
2.4
Other Planning Statements and Policies ............................................................22
2.5
The Thames Catchment Flood Management Plan (CFMP) ................................22
2.6
Making Space for Water.....................................................................................23
2.7
The Pitt Review – Final Report...........................................................................24
2.8
Flood Risk Management Key Stakeholders........................................................25
2.9
2.8.1 Environment Agency
25
2.8.2 Thames Water and Three Valleys Water
26
2.8.3 British Waterways
26
Flood Risk Responsibility ...................................................................................27
2.9.1 Main Rivers
27
2.9.2 Ordinary Watercourse
27
2.9.3 Riparian Owners
28
3
Review of Development Framework ...............................................................30
3.1
Regional Planning Policy – The London Plan.....................................................30
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3.2
Lower Lee Valley Opportunity Area Planning Framework...................................33
3.3
Water Matters, Draft Water Strategy ..................................................................34
3.4
Local Planning Policy – The UDP and LDF Core Strategies...............................34
4
Data Collection and Validation........................................................................38
4.1
Mapping and Topographic Data .........................................................................38
4.2
Flood Defences ..................................................................................................39
4.3
Thames Water Data ...........................................................................................39
4.4
Clean Water Supply Data...................................................................................39
4.5
River Lee and Brent Data...................................................................................39
4.5.1 Historic Flood Records
39
4.5.2 Flood Modelling Results
40
4.6
Regional Flood Risk Appraisal ...........................................................................40
4.7
Lower Lee Flood Risk Management Strategy.....................................................43
4.8
Other Drainage Studies......................................................................................43
4.9
4.8.1 The Drain London Project
43
4.8.2 Central London Forward
44
4.8.3 North London Strategic Alliance
44
Strategic Flood Risk Assessments .....................................................................45
4.9.1 East London SFRA
45
4.9.2 Lower Lee SFRA
45
4.9.3 Enfield SFRA
46
4.9.4 Islington SFRA
47
4.9.5 Olympic FRA
47
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4.10 Community Risk Registers .................................................................................48
4.11 Groundwater Data..............................................................................................48
4.12 Flood Records....................................................................................................48
4.12.1
Individual Boroughs
48
4.12.2
Transport for London
48
4.12.3
London Underground
49
4.12.4
The London Fire Brigade
49
4.13 Public Reservoirs ...............................................................................................49
4.14 British Waterways ..............................................................................................49
4.15 Low Confidence Data.........................................................................................50
4.15.1
EA Asset Data
50
4.15.2
Barnet Flooding Records
50
4.16 Data Gaps..........................................................................................................50
4.16.1
Thames Water
50
4.16.2
London Underground Flood Records
51
5
Sources of Flooding.........................................................................................52
5.1
Introduction ........................................................................................................52
5.2
Key Watercourses..............................................................................................52
5.3
5.2.1 Lower River Lee (Lee)
54
5.2.2 River Lee Tributaries
55
5.2.3 River Brent and Tributaries Upstream of the Brent Reservoir
56
5.2.4 Subterranean and Culverted Rivers
57
Fluvial Flooding ..................................................................................................57
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5.3.1 Historic Fluvial Flood Events
58
5.3.1.1
March 1947 floods
58
5.3.1.2
December 2000 Floods
58
5.3.1.3
Silk Stream Floods
58
5.3.1.4
Other Localised Fluvial Flooding
58
5.3.1.5
Haringey Council Flood Management Strategy Report Review
59
5.3.2 Tidal Flooding
59
5.3.3 Flood Risk Modelling and Mapping
60
5.3.3.1
Flood Mapping Overview
60
5.3.3.2
Review of Modelling Techniques
60
5.3.3.3
River Lee Modelling and Mapping
61
5.3.3.4
River Brent Modelling and Mapping
61
5.3.4 Flood Zone 3b - Functional Floodplain
61
5.3.4.1
Lower Lee Valley Functional Floodplain
61
5.3.4.2
Upper Brent Valley Functional Floodplain
62
5.3.5 Flood Defences and Inline Structures
62
5.3.5.1
Silk Stream Flood Alleviation Scheme
66
5.3.5.2
Salmons Brook Flood Alleviation Scheme
66
5.4
5.3.6 Overtopping
66
5.3.7 Breach
67
Groundwater Flooding........................................................................................67
5.4.1 Geology
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ix
5.5
5.4.2 Groundwater Flooding
69
5.4.3 Borehole Data and Groundwater Contour Data
69
5.4.4 GARDIT
70
5.4.5 Groundwater trends
71
Sewer and Surface Water Flooding....................................................................72
5.5.1 Sewer Flooding
72
5.5.2 Thames Water Flood Database
73
5.5.3 Impermeable Surfaces and Urban Creep
75
5.5.4 Basement Flooding
75
5.5.5 Other surface water flooding records
75
5.5.5.1
Barnet Flood Hot Spots
75
5.5.5.2
Review Transport For London flood records
75
5.5.5.3
Floods in Camden – Report of the Floods Scrutiny Panel
76
5.5.5.4
London Fire Brigade Flood Calls
77
5.6
5.7
Artificial Drainage Bodies ...................................................................................77
5.6.1 Reservoirs
77
5.6.2 Canals
79
5.6.3 New River
80
Infrastructure Failure ..........................................................................................80
5.7.1 Water Infrastructure
80
5.7.2 Flood Defence Infrastructure
81
5.7.3 Bridge and Culvert Blockage
81
5.7.4 Overview
81
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5.8
Source Pathway Receptor Model .......................................................................81
5.8.1 Scale of Consequences
83
5.9
Community Risk Registers .................................................................................83
6
Strategic Flood Risk Assessment...................................................................86
6.1
Sustainability and Climate Change.....................................................................86
6.2
Fluvial Flooding ..................................................................................................88
6.2.4.1
6.2.1 Lower Lee Flood Risk Zone
88
6.2.2 Silk Stream and Dollis Brook Flood Zones
89
6.2.3 Actual Flood Risk
89
6.2.4 Recommendations for Further Work
89
River Lee Catchment
89
6.3
Groundwater Flooding........................................................................................90
6.4
Assessment of Surface Water and Sewer Flooding............................................91
6.5
6.4.1 Pluvial Flood Records
91
6.4.2 Surface Water Flood Risk
92
6.4.3 Basement Flooding
94
6.4.4 Recommendations for Further Work
94
6.4.5 Reservoirs
95
6.4.6 Recommendations for Further Work
95
Emergency Planning Measures for Flooding ......................................................95
6.5.1 London Boroughs
95
6.5.2 Environment Agency
96
6.5.3 Civil Contingencies Act
96
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6.5.4 The Pitt Review
97
7
Application of Sequential Flood Risk Test .....................................................98
7.1
Flood Risk Zone Categories ...............................................................................98
7.2
Application of Sequential Test ............................................................................99
7.3
Assessment of Risk within Zones 2 and 3 ..........................................................99
7.3.1 Application of the Exception Test
99
7.4
Brownfield Development ..................................................................................100
8
Local Policy Guidance ...................................................................................102
8.1
Introduction ......................................................................................................102
8.2
Policy Recommendations.................................................................................102
8.2.1 Policy
102
8.2.2 Development Control
104
8.2.3 Technical
109
9
Guidance for Developers ...............................................................................112
9.1
Development and the management of residual flood risk .................................112
9.2
Drainage Assessments ....................................................................................114
9.3
Sustainable Urban Drainage Systems..............................................................116
9.3.1 The SUDS management train
116
9.3.2 SUDS Selection
117
9.3.3 Pervious Surfaces
121
9.3.4 Green Roofs
122
9.3.5 Soakaways
124
9.3.6 Infiltration trenches
125
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10
9.3.7 Infiltration Basins
125
9.3.8 Filter strips
125
9.3.9 Swales
126
9.3.10
Bio-retention areas
126
9.3.11
Detention Basins
127
9.3.12
Ponds
127
9.3.13
On/Off-line storage
128
9.3.14
Adoption and Maintenance of SUDS
128
9.3.15
Land Drainage Consent
128
Conclusions ...................................................................................................130
10.1 Summary for Barnet .........................................................................................130
10.2 Conclusions for Barnet .....................................................................................130
10.3 Summary for Camden ......................................................................................131
10.4 Conclusions for Camden ..................................................................................131
10.5 Summary for Enfield.........................................................................................132
10.6 Conclusions for Enfield.....................................................................................132
10.7 Summary for Hackney......................................................................................132
10.8 Conclusions for Hackney..................................................................................133
10.9 Summary for Haringey .....................................................................................133
10.10 Conclusions for Haringey .................................................................................134
10.11 Summary for Islington ......................................................................................134
10.12 Conclusions for Islington ..................................................................................134
10.13 Summary for Waltham Forest...........................................................................135
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10.14 Conclusions for Waltham Forest ......................................................................135
11
Recommendations .........................................................................................136
11.1 Recommendations for Further Work.................................................................136
11.2 Recommendations for Boroughs ......................................................................136
11.3 Recommendations for Developers ...................................................................137
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Table of figures
Figure 1 - North London Study Area.......................................................................... 8
Figure 2 - Planning Hierarchy and Flood Risk ......................................................... 21
Figure 3 - Leith House Borehole Level .................................................................... 71
Figure 4 - Illustration of a Pervious Pavement using Block Paving ........................ 122
Figure 5 - Illustration of an extensive green roof.................................................... 123
Figure 6 - Illustration of an Intensive Green Roof .................................................. 123
Figure 7 - Illustration of a Simple Intensive Green Roof......................................... 124
Figure 8 - Illustration of a traditional soakaway design .......................................... 124
Figure 9 - Cross section through an infiltration basin ............................................. 125
Figure 10 - Illustration of a swale........................................................................... 126
Figure 11 - Illustration of an engineered bio-retention area.................................... 126
Figure 12 - Illustration of a detention basin following a period of heavy rain .......... 127
Figure 13 - Illustration of a pond in a high density housing development in
Bicester, Oxfordshire ........................................................................... 127
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Tables
Table 1 - Definition of Flood Zones.......................................................................... 18
Table 2 - Flood Risk Vulnerability Classification ...................................................... 19
Table 3 - Flood Risk Vulnerability and Flood Zone Compatibility ............................. 20
Table 4 - London Plan Opportunity Areas that adjoin the Blue Ribbon Network ...... 33
Table 5 - Summary of UDPs and Policies................................................................ 35
Table 6 - Summary of Boroughs Development Plan Documents Timescales taken
from the Local Development Scheme Reports .......................................... 36
Table 7 - Transportation / Facilities at Risk of Flooding ........................................... 41
Table 8 - Utility Infrastructure within Flood Risk Zones ............................................ 42
Table 9 - Lower Lee Flood Risk Areas..................................................................... 43
Table 10 - Rivers List .............................................................................................. 53
Table 11 - Approximate Number of Properties in Flood Zone 3b in Lee Valley ........ 62
Table 12 - River Lee Tributaries Flood Defence Grades 4 and 5............................. 64
Table 13 - River Brent Tributaries Flood Defence Grades 4 and 5 .......................... 65
Table 14 - Thames Water Flooding Records by Borough ........................................ 73
Table 15 - Thames Water Flooding Records, Highlighted Areas ............................. 74
Table 16 - Reservoirs List........................................................................................ 78
Table 17 - Source Pathway Receptor Model ........................................................... 82
Table 18 - Scale of Consequences.......................................................................... 83
Table 19 - Summary of flooding risks as described in Community Risk Register .... 84
Table 20 - Recommended national precautionary sensitivity ranges ....................... 87
Table 21 - Recommended contingency allowances for net sea level rise ................ 87
Table 22 - Locations of Surface Water Flood Risk Zones ........................................ 93
Table 23 - Flood Risk Zones and Development Restrictions ................................... 98
Table 24 - SUDS Score within Haringey, Waltham Forest, Barnet and Enfield ...... 119
Table 25 - SUDS Score Technique within Camden, Islington and Hackney .......... 120
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Appendices
Appendix A - Maps 1 to 24
140
Appendix B - Data Summary
142
Appendix C - EA Guidance
144
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Glossary
The following are the main abbreviations and terms used throughout this report.
Other terms and abbreviations have a local meaning and are defined where they
occur.
AOD
Above Ordnance Datum
CIRIA
Construction Industry Research and Information Association
DEFRA
Department of the Environment, Food and Rural Affairs
DEM
Digital Elevation Model
EA
Environment Agency
FEH
Flood Estimation Handbook
Fluvial
Pertaining to a river
FRA
Flood Risk Assessment
GARDIT
General Aquifer Research, Development and Investigation
Team
GIS
Geographical Information System
ISIS
Hydrodynamic software produced by Wallingford Software
JFLOW
Two-dimensional dynamic flood model, produced by JBA
Consulting
JW DPD
Joint Waste Development Plan Document
LDF
Local Development Framework
LFB
London Fire Brigade
LPA
Local Planning Authority
NFCDD
National Flood and Coastal Defence Database
NLWP
North London Waste Plan
OFWAT
The economic regulator for the water and sewerage industry in
England and Wales
OLF
Olympic and Legacy Facilities
Pluvial
Pertaining to precipitation
PPS1
Planning Policy Statement 1: Delivering Sustainable
Development
PPS3
Planning Policy Statement 3: Housing Objectives
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PPS6
Planning Policy Statement 6: Planning for Town Centres and
Retail
PPS 25
Planning Policy Statement 25: Development and Flood Risk
PPG 25
Planning Policy Guidance note 25: Development and Flood
Risk
Response Times
The time a catchment takes to respond to rainfall and for the
flow to enter a point of discharge. Factors such as catchment
slope and soil characteristics will affect this time.
RFRA
Regional Flood Risk Appraisal
RSS
Regional Spatial Strategy
SFRA
Strategic Flood Risk Assessment
SPG
Supplementary Planning Guidance
SUDS
Sustainable Urban Drainage Systems
TFL
Transport for London
Tidal
Pertaining to the movement of coastal waters
TUFLOW
A finite-difference 2D and 1D flood and tide simulation by
WBM Pty Ltd and The University of Queensland
UKCIP
UK Climate Impacts Program
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Executive Summary
The North London Boroughs of Barnet, Camden, Enfield, Hackney, Haringey,
Islington and Waltham Forest are in the process of compiling their Local
Development Framework (LDF) to guide future development needs of the Boroughs.
The seven Boroughs have a history of co-operating on waste matters, having
combined to prepare a Joint Waste Development Plan Document (JWDPD) also
known as the North London Waste Plan (NLWP). Due to an already active
collaboration between the seven Boroughs the NLWP was identified as the most
appropriate means for the Strategic Flood Risk Assessment (SFRA) to be procured.
Mouchel was commissioned in July 2007 to undertake a SFRA in order to ensure
that flood risk is considered as part of the spatial planning process.
The objectives of the SFRA were predominantly informed by the requirements of
Planning Policy Statement 25, which requires decision makers involved in the
planning process to consider regional and local flood risk issues when planning
development.
The Primary aims of the SFRA were:
•
Identify the areas within North London that are at risk of flooding for all Flood
Zones identified in table D1 in PPS 25, and within Flood Zone 3, the
variations in the actual flood risk including the effect of any formal or informal
flood defences.
•
Identify the risk of flooding due to surface water either in the form of flash
flooding due to surface water run-off, rising groundwater, inadequate
drain/sewer capacity or inadequate drain/sewer maintenance
•
Identify the likely effects of climate change on flood risk
•
Identify catchment areas and the potential for development to affect flood risk
in areas beyond the individual Borough boundaries
•
Provide the basis for allocating sites in the Local Development Framework
(LDF) including, if necessary, applying the sequential test approach to site
allocation within the indicative flood plain.
•
Provide a clear rationale for assessing the merits of potential development
allocations based on a sequential flood risk assessment, taking into account
the flood risk vulnerability of proposed uses (table D2, PPS25)
•
Recommend policy options for dealing with the range of flood risks and
provide guidance for developers
Strategic Flood Risk Assessment
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•
Recommend appropriate monitoring and review methods
All forms of flooding were investigated, primarily by compiling and reviewing relevant
information provided by a wide variety of sources, but primarily the Environment
Agency, the North London Boroughs, London Fire Brigade and Thames Water.
In general only limited data pertaining to sewer flooding has been obtained from
Thames Water. While Thames Water has provided extracts from their flooding
database, the sensitivity of the data restricts them from identifying individual flooding
problems. Other data requested from Thames Water, such as GIS extracts of the
main sewer lines and modelling data or results of Drainage Area studies, has not
been made available for the study. The data obtained so far is insufficient to enable
a thorough investigation of sewer flooding within the study area and the time
constraints of the project. Without this data no verification or quantification of surface
water flood risks could be undertaken.
A source pathway receptor model was used to assess those flood sources which
had the greatest consequences for each of the borough as shown in Table 18. The
primary source of flood risk was determined to be the posed by the watercourses
both fluvial and tidal in each of the concerned boroughs. The risk of flooding from
secondary sources was in general found to be low, also the information required to
make detailed assessments of the secondary sources was less available such as the
sewer and canal information.
The findings of the study were used to assess the flood risk across the study area
and recommendations for further work were provided. Guidance on applying the
sequential test to developments in the North London Boroughs has also been
provided.
The findings of the SFRA were used to advise on local planning policy issues and
provide guidance to developers on the management of residual flood risk and
surface water drainage through the use of Sustainable Urban Drainage Systems.
These findings included recommendations for potential work which may be required
for further stages of the SFRA and the maintenance of the ‘live’ SFRA document.
Strategic Flood Risk Assessment
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1
Introduction
1.1
Background
Planning Policy Statement 25: Development and Flood Risk (PPS25) issued in
December 2006 by the Department for Communities and Local Government; sets
out the national policy for land use planning and flood risk management in England.
The policy highlights the requirement for each Local Planning Authority (LPA) to
consider flood risk and flood risk management within local development documents
and ensure that informed decisions on the flood risk attributed to new developments
are made by those involved in the planning process. Key to this risk based approach
is the production of a Strategic Flood Risk Assessment (SFRA) report to consider
catchment wide flood risk issues. SFRAs are required to be produced by individual
or groups of LPAs in conjunction with their Local Development Framework (LDF).
Mouchel were commissioned in July 2007 to undertake a SFRA for the seven North
London Boroughs, Barnet, Camden, Enfield, Hackney, Haringey, Islington and
Waltham Forest. The seven Boroughs have a history of co-operating on waste
matters, having combined to prepare a Joint Waste Development Plan Document
(JWDPD). The proposed JWDPD is also known as the North London Waste Plan
(NLWP) and is the preferred approach to implementing the principles of sustainable
waste management for all controlled waste streams. As an already active
collaboration between the seven Boroughs the NLWP was identified as the most
appropriate vehicle through which the SFRA could be procured. The report is
intended to provide each individual Borough with the evidence base required to
develop their LDFs. As such, the focus of the report is not entirely on the activities of
the North London Waste Plan, but on the overall development within each Borough
in the context of flood risk.
At the time of Mouchel’s appointment Enfield and Islington had already produced
their own separate SFRAs, and Waltham Forest had commissioned a SFRA
covering a small area within their Borough. It is intended that this report comprises
one over arching SFRA covering all seven of the North London Boroughs and in
order to achieve this aim it was agreed that the Enfield and Islington SFRAs would
be reviewed as part of this study and where appropriate data will be incorporated
into this report. Where necessary the data in the Enfield and Islington SFRAs will be
updated to include more recent information and any deficiencies in the assessment
will be improved to ensure a consistent level of detail across all seven Boroughs.
While this approach may lead to some duplication of data from the Islington and
Enfield SFRAs, it is important for this to take place to ensure that those using the
North London SFRA for cross Borough planning purposes, such as the North
London Waste Plan, have a single document containing all base line data related to
Strategic Flood Risk Assessment
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Final - August 2008
4
flooding. The status of the Waltham Forest SFRA is currently unknown and no
review of this document has taken place.
1.2
Project Objectives
The objectives of this SFRA are based on the brief agreed with the North London
Waste Plan which has predominantly been informed by the requirements of PPS25.
The primary objective of any SFRA is to provide decision makers involved in the
planning process with a better understanding of local flood risk issues thus enabling
flood risk to be considered at the earliest stages of the planning process. The SFRA
is aimed at identifying areas most suitable for sustainable development through the
application of the sequential test as set out in PPS25. A summary of the objectives of
this study are summarised below:
•
Identify the areas within North London that are at risk of flooding for all Flood
Zones identified in table D1 in PPS 25, and within Flood Zone 3, the
variations in the actual flood risk including the effect of any formal or informal
flood defences.
•
Identify the risk of flooding due to surface water either in the form of flash
flooding due to surface water run-off, rising groundwater, inadequate
drain/sewer capacity or inadequate drain/sewer maintenance
•
Identify the likely effects of climate change on flood risk
•
Identify catchment areas and the potential for development to affect flood risk
in areas beyond the individual Borough boundaries
•
Provide the basis for allocating sites in the Local Development Framework
(LDF) including, if necessary, applying the sequential test approach to site
allocation within the indicative flood plain.
•
Provide a clear rationale for assessing the merits of potential development
allocations based on a sequential flood risk assessment, taking into account
the flood risk vulnerability of proposed uses (table D2, PPS25)
•
Recommend policy options for dealing with the range of flood risks and
provide guidance for developers
•
Recommend appropriate monitoring and review methods
In addition to informing the LDF the aims of the SFRA are to provide advice on flood
risk management policies, inform the local sustainability appraisal and provide
advice on the requirements for local development flood risk assessments. Although
the objectives of an SFRA are defined in PPS25, where appropriate the scope of this
assessment has been tailored to suit the unique requirements of each Borough,
Strategic Flood Risk Assessment
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Final - August 2008
5
ensuring that all sources of flood risk are addressed at a level consistent with the
scale of consequences and the risk which they pose to development and
infrastructure within the North London sub-region.
The majority of the North London Borough area is located outside of the fluvial
floodplain, and it was determined early on in the project that a high level assessment
of flood risk would be appropriate for the majority of the study area. However, where
fluvial flood zones exist the scope of the assessment has been widened to provide
sufficient level of detail to enable the application of the exception test within flood risk
zones by determining the residual risk within Flood Zones 2 and 3.
1.3
SFRA Scope
This is a Level 1 SFRA. Any further work will be conducted at a later date. The scope
and objectives of a SFRA are often described through a three tiered process (levels
1, 2 and 3). Level 1 assessments should cover an entire Borough and provide an
LPA with the flood risk information required to undertake the spatial planning process
implemented through their LDF. Where significant development is identified at the
Regional Spatial Strategy stage, such as the Olympics or Thames Gateway, further
assessment constituting a level 2 and 3 assessment may be undertaken, with
coverage of the planned development areas only.
A level 1 SFRA is a coarse assessment using information already available to
identify areas of flood risk. From this initial review of existing data it can be
determined whether a level 2 assessment is required, generally a level 2
assessment is required where development allocation will require the application of
the exception test or where there is either high uncertainty about flood risk data.
Where flood risk data with high uncertainty, or missing data is identified, further work
such as hydraulic modelling may be required. The need to infill or improve data will
partly depend on the spatial relationship between the areas of potential flooding and
planned development zones. Any further work will be identified but not undertaken
as part of this study. A level 3 assessment is when a detailed study is to be
undertaken if the Level 2 SFRA concludes that further quantitative analysis is
required to assess flood risk issues.
1.4
Methodology
For simplicity the study has been broken down into three stages, the first is a data
collection and validation exercise which assesses the suitability of all data and
identifies any missing or incomplete data. The second stage involves a review of all
flood risk sources identified through a thorough review of data collected during stage
one. In order to ensure that the study adequately addresses each area of flood risk
to the required level of detail, a source pathway receptor model is created which
identifies the scale of consequence of each flood source. This model is then used to
identify the critical sources of flood risk. Stage three addresses these critical risks in
Strategic Flood Risk Assessment
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6
greater detail and provides further break down of the flood risk such as undertaking
breach or overtopping assessments and identifying areas benefiting from formal or
informal flood defences.
While the report aims to address catchment wide flood risk, in some instances a
flood source may not impact on all Boroughs. In these cases some Boroughs have
been omitted from selected maps to ensure that the report and mapping remains as
concise as possible.
1.5
Setting the Scene
The North London study area is home to approximately 1,676,0001 people from a
range of different social backgrounds. This population maintains local business
activities that are supported by appropriate infrastructure and services. The area has
undergone development and regenerations over the last 20 years that has seen
improvements in social conditions and prosperity. The North London region consists
of 7 Boroughs, Barnet, Camden, Enfield, Hackney, Haringey, Islington and Waltham
Forest. Each borough is predominately urbanised with the density of urbanisation
tending to reduce further from the centre of London. Figure 1 shows the North
London study area and the borough boundaries. More detailed maps can be found in
Appendix A in Maps No.1 to 6).
1
London Fire Brigade (May 2005) Borough Profiles, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
Strategic Flood Risk Assessment
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Figure 1 - North London Study Area
The North London sub-region is drained by a series of watercourses which all form
part of the overall Thames Catchment. The main rivers generally drain in a southerly
direction towards the Thames. However, most of the watercourses in the study area
have been influenced by urbanisation to some degree, with sections of culverted or
canalised watercourses commonplace. Some of the rivers such as the Hackney
Brook have been urbanised to such an extent that they are now incorporated into the
London sewer system. In these built up areas, much of the surface water goes into
the surface water drains before being discharged into watercourses.
1.5.1
London Borough of Barnet
Barnet is an affluent suburb of London with significant Greenbelt and Metropolitan
Open Land areas. Barnet has a total area of 86.7km² and a population of
approximately 330,0002. Barnet is the 4th largest London borough in area and the 2nd
largest in population. Within Barnet Mill Hill East, Colindale and Brent Cross /
Cricklewood are identified as opportunity or intensification areas in the London Plan
and form a significant part of the growth targets for housing and employment in the
sub-region.
The Dollis Brook and Silk Stream are the two dominant watercourses within the
borough, draining the majority of Barnet towards the south where the River Brent
begins at their confluence. Mutton Brook, Deans Brook and Folly Brook are
significant tributaries to these watercourses. The Environment Agency is currently
developing a £1.8million scheme to protect 133 homes and businesses from flooding
attributed to the Edgewarybury Brook in the Silk Stream catchment. East Barnet is
drained by the Victoria Watercourse, Bounds Green Brook and to a lesser extent the
Monkenmead Brook, all of which are tributaries of the Pymmes Brook which is part
of the River Lee catchment.
The south west boundary of Barnet is marked by the River Brent and the Silk Stream
confluence at the Brent Reservoir (Welsh Harp). The 598,000 m² Brent Reservoir is
owned and maintained by British Waterways and is main supply reservoir for the
Grand Union Canal.
2
London Fire Brigade (May 2005) Barnet Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
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1.5.2
London Borough of Camden
Camden is a diverse Borough with a varied mix of land-uses including a number of
entertainment and tourist attractions such as theatres and Camden Market. The
total size of Camden is 21.8km² with a population of approximately 226,000 people3.
It stretches north as far as Hampstead Heath and to the south incorporates the
central London areas of Euston, Kings Cross and Holborn. The Further Alterations to
the London Plan includes Camden within the Central London sub-region and
identifies Kings Cross, Euston and Tottenham Court Road as opportunity areas.
Regents Canal runs from west to east and bisects Camden borough. The River
Fleet, which is formed from two springs on Hampstead Heath is the largest of
London’s subterranean rivers and historically drained the Camden area. The Fleet
has long since been incorporated into the London sewer network although the
traditional route of the Fleet and the large sewer in its place can still be traced in the
south of the Borough as it passes into the City of London. Highgate and Hampstead
Ponds were constructed to increase London’s water supply. They are fed by the
Fleet and are now used by the public for leisure activities. Camden suffered
widespread surface water flooding in August 2002 due to a high intensity rainfall
event.
1.5.3
London Borough of Enfield
Enfield is London’s most northerly borough and one of the largest by land area. The
total area of Enfield is 82.2km² containing an approximate population of 280,0004.
Forty-nine percent of Enfield is designated as green belt or open space. There are
17 industrial estates in Enfield with further areas identified as preferred industrial
locations in the London Plan, all located within the Upper Lee Valley, in the east of
the Borough.
Enfield has the longest length of watercourses among the London boroughs in the
study area. Pymmes Brook, Salmons Brook and Turkey Brook are the principle
tributaries for the River Lee that’s flows south to the River Thames. The River Lee
Diversion Channel forms the Eastern borough boundary. The borough also contains
several large reservoirs that help manage water supply. The New River is an artificial
watercourse that supplies water to the centre of London and flows north to south
through the centre of Enfield.
3
London Fire Brigade (May 2005) Camden Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
4
London Fire Brigade (May 2005) Enfield Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
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1.5.4
London Borough of Hackney
Hackney has one of the most ethically diverse populations in the country and covers
a total area of 19km² with an approximate population of 208,0005. Hackney is a
borough of regeneration, with many developments having taken place over the last
20 years. Nearly one third of the Olympic Park will be located within Hackney and
will be accompanied by associated infrastructure improvements and long term
regeneration.
Regent’s Canal passes across the southern section of Hackney borough between
Shoreditch and Dalston. The man made New River flows through the north west of
the borough to the Stoke Newington Reservoirs. The River Lee travels south towards
the Hackney marshes where it splits into the old River Lee and the Hackney Cut.
The culverted Hackney Brook travels diagonally through the centre of the borough,
although the Brook is now lost and incorporated into the sewer network. Clissold
Park’s eastern lake is one of two which marks the original course of the Hackney
brook.
1.5.5
London Borough of Haringey
Haringey is another particularly diverse borough, with over 160 different languages
spoken. Haringey has a total area of 29.6km², supporting a population of
approximately 225,000 people6. The Borough has a high level of deprivation,
particularly in the east of the borough, where unemployment levels are high. The
London plan identifies the areas of Tottenham Hale and the Upper Lee Valley as
areas of opportunity and the Haringey heartlands as areas suitable for intensification.
The Moselle Brook (River Moselle) flows predominately though Tottenham. It was
originally a natural tributary of the River Lee but has since been mostly culverted and
in now artificially flows into Pymmes Brook. The New River flows south through the
middle of the borough along the eastern boundary of Alexandra Park and into
Finsbury Park. The River Lee forms the eastern Haringey boundary with Waltham
Forest.
5
London Fire Brigade (May 2005) Hackney Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
6
London Fire Brigade (May 2005) Haringey Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
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1.5.6
London Borough of Islington
Islington is situated on higher ground in the central district of London. Islington has
an area of 14.9km² and a total population of approximately 183,0007. It is one of the
smallest London Boroughs by land mass. The dense population with a high
proportion of multiple occupation of most buildings as 60% of households are in flats.
The borough used to supply the city of London with water. Historical Islington
contained sprigs and marshes that were import in the development of London as a
city. However, as the city developed the demand for water increased and the New
River aqueduct was constructed. The New River is no longer in use within Islington,
instead it ends at Stoke Newington and is piped to the Coppermills Water Treatment
Works where it contributes to the north London water supply. Currently, Islington still
has over 35 recorded boreholes.
The Regent’s Canal was constructed in the early 1800’s and flows through Islington,
largely contained within the 886m long Islington tunnel, which runs from Colebrook
Row to the King’s Cross area where the canal emerges.
It is important to note the critical role that the borough plays in London’s transport
network. Due to the high degree of connectivity in the borough, an incident will have
a wider impact than in other boroughs in a short amount of time.
1.5.7
London Borough of Waltham Forest
Waltham Forest is another particularly diverse borough and has the fifth largest
Muslim population in England and the third largest in London (coming after its
neighbouring boroughs, Newham and Tower Hamlets). Waltham Forest is 38.8km²
and has an approximate population of 224,0008. Waltham Forest is one of five host
boroughs in East London for the 2012 London Olympics.
The River Lee and the River Ching are the principal rivers in Waltham Forest. The
Lee in Waltham Forest is a largely artificial route and is tidal as far as Low Hall
Playing Fields with the Lee Bridge Road marking the mean high water limit. The Lee
Flood Relief Channel was completed in 1976 to prevent a repeat of the flooding that
took place in 1947 and is a wide concrete culvert that rejoins the River Lee at
Hackney Marshes. The River Lee forms the eastern borough boundary with Hackney
at Hackney Marshes.
7
London Fire Brigade (May 2005) Islington Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
8
London Fire Brigade (May 2005) Waltham Forest Borough Profile, http://www.londonfire.gov.uk/about_us/borough_profiles0506.asp
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The Lee Valley also contains the Chingford and Walthamstow Reservoirs which
together represent one of the largest expanses of open water in London. Many other
smaller ponds are also found within the Epping Forest.
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2
Planning Policy and Flood Risk
The enactment of the Planning and Compulsory Purchase Act 2004 instigated major
alterations to national, regional and local planning policies. Regional Spatial
Strategies (RSSs) replaced Regional Planning Guidance, with the London Plan
being published in February 2004. Following on from the RSSs, Local Planning
Authorities are required to replace existing Unitary Development Plans with a suite of
documents known as Local Development Frameworks (LDFs), which provide
guidance on the use and development of land. LDFs must conform to the
overarching RSS.
The North London SFRA is a freestanding document that will be part of each
Boroughs overall LDF. The assessment is undertaken in the context of Planning
Policy Statement 25: Development and Flood Risk (PPS 25) published in December
2006. This statement replaces Planning Policy Guidance note 25: Development and
Flood Risk (PPG 25), published in 2001, and is part of the Government’s ongoing
strategy of replacing Planning Policy Guidance with Planning Policy Statements that
provide “statements of government policy on nationally important land use and other
planning matters, supported where appropriate by locational framework”9
2.1
2.1.1
National Planning Policy and Guidance
Planning and Compulsory Purchase Act 2004
The Planning and Compulsory Purchase Act 2004 received Royal Assent in May
2004. The Act influences the planning process at both regional and local levels, with
the requirement for development of RSSs and LDFs. The Act is designed to
accelerate the planning process, make planning decisions more predictable and
provide a more sustainable approach to planning.
Regional planning bodies and LPAs will now have a statutory duty to ensure that
development documents promote sustainable development and Sustainability
Appraisals must be carried out as part of the LDF process. Annual reports on the
achievement of locally set policies and targets are required from LPAs, ensuring they
promote and implement good quality development rather than just facilitating the
planning process.
9
Department of Communities and Local Government Planning Policy Guidance 1: General
Policy and Principles, 1997
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2.1.2
Planning Policy Statement 1
The overarching planning policies and guidance from which the other PPSs will
follow is set out in Planning Policy Statement 1: Delivering Sustainable
Development, published in 2005. PPS1 clearly identifies the requirement for future
development to take account of sustainability and flood risk. The following extracts
taken from PPS1 identify the need for regional and local plans to take account of
flood risk and flood management.
“Regional planning bodies and local planning authorities should ensure that
development plans contribute to global sustainability by addressing the causes and
potential impacts of climate change.”
“Development plan policies should take account of environmental issues such as:
the potential impact of the environment on proposed developments by avoiding new
development in areas at risk of flooding and sea-level rise, and as far as possible, by
accommodating natural hazards and the impact of climate change.”
“Key objectives should include ensuring that developments are sustainable, durable
and adaptable (including taking account of natural hazards such as flooding)…”
The introduction of PPS1 and subsequently PPS25 (see chapter 2.1.4) is a clear
Government led policy change towards the management of flood risk through the
planning process. Those involved in the planning process must be provided with
clear guidance in order to effectively action this policy change.
2.1.3
Planning Policy Statement: Planning and Climate Change: Supplement to PPS1
In December 2007 the Government published a supplement to PPS1 entitled
“Planning and Climate Change”. This policy set key planning objectives relating to
the delivery of spatial strategies which aim to “secure new development and shape
places that minimise vulnerability, and provide resilience, to climate change.”
The supplementary policy sets out key principles which planning authorities should
apply during decision making about spatial strategies;
“new development should be planned to minimise future vulnerability in a changing
climate;” and “climate change considerations should be integrated into all spatial
planning concerns;”
The supplementary policy also highlights the importance of Sustainability Appraisals
and Strategic Environmental Assessment when developing planning strategies.
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The policy does not deal directly with flooding issues except for the requirement for
considering flood risk when selecting land for development. Planning Policy
Statement 25 remains the key planning policy development and flood risk.
2.1.4
Planning Policy Statement 25
The introduction of PPS 25 in December 2006 outlined the responsibility of Regional
Planning Bodies and Local Planning Authorities to prepare and implement planning
strategies which help to deliver sustainable development by ensuring that flood risk
is understood and managed effectively as an integral part of planning process. This
is primarily achieved through preparation of Strategic Flood Risk Assessments
(SRFAs) or Regional Flood Risk Appraisals (RFRAs) as appropriate. These look at
catchment wide flooding issues with the aim to appraise, manage and reduce risk by:
•
“Identifying land at risk and the degree of risk of flooding from river, sea and
other sources in their areas”
•
“Only permit development in areas of flood risk when there are no reasonably
available sites in areas of lower flood risk and benefits of the development
outweigh the risks from flooding”
•
“Reduce flood risk to and from new development through location, layout and
design, incorporating sustainable drainage systems (SUDS)”
SFRAs must be prepared by Local Planning Authorities in consultation with the
Environment Agency, emergency response teams and the local drainage authority.
The SFRA should build upon existing flood maps by taking into account other
sources of flooding in order that it can provide a basis from which to apply the
Sequential Test and Exception Test in development allocation.
2.2
2.2.1
The Sequential Test and Exception Test
The Sequential Test
To determine the suitability of land for development in areas at risk of flooding, a
sequential risk based approach should be applied at the outset of the planning
process. The aim of the Sequential Test is to guide new developments to areas with
the lowest probability of flooding.
Flood zones are the basis of the sequential approach. Zones 2 and 3 are shown on
Environment Agency Indicative Floodplain Maps with Flood Zone 1 being all land
falling outside Zones 2 and 3.
In areas at risk of sea or river flooding, preference should be given to locating
development in Flood Zone 1. If there is no reasonably available site in Flood Zone
1, the flood vulnerability of the proposed development should be taken into account
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in locating the development in Flood Zone 2 and then Flood Zone 3. The vulnerability
classification of proposed development is provided in Table 2 which is extracted from
PPS25.
New development should be located at sites with the lowest probability of flooding
from all sources within each Flood Zone, as indicated by the SFRA.
Table 1 below defines each Flood Zone referring to the probability of sea and river
flooding only, ignoring the presence of all existing flood defences; Table 3 defines
the vulnerability classification of proposed developments against flood zone
locations.
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Table 1 - Definition of Flood Zones
Flood Zone
Zone 1
Low Probability
Zone 2
Probability
Zone 3a
Probability
Medium
High
Definition
Appropriate Uses
< 1 in 1000 annual probability
of river or sea flooding in any
year (<0.1%)
All uses of land are appropriate in this
zone.
1 in 100 to 1 in 1000 annual
probability of river flooding
(1% - 0.1%) or between a 1
in 200 and 1 in 1000 annual
probability of sea flooding
(0.5% - 0.1%) in any year.
“Water-compatible”, “less vulnerable” and
“more vulnerable” uses of land and
essential infrastructure are appropriate in
this zone.
1 in 100 or greater annual
probability of river flooding
(>1% ) or a 1 in 200 or
greater annual probability of
flooding from the sea (>0.5%)
in any year.
“Water-compatible” and “less vulnerable”
uses of land are appropriate in this zone.
Subject to the Sequential Test being
applied, the “highly vulnerable” uses of
land are only appropriate in this zone if the
Exception Test is passed.
“Highly vulnerable” development should
not be permitted in this zone.
“More vulnerable” uses of land and
“essential infrastructure” should only be
permitted in this zone if the Exception Test
is passed.
Essential infrastructure permitted in this
zone should be designed and constructed
to remain operational and safe for users in
times of flood.
Zone 3b
The
Functional Floodplain
Land where water has to flow
or be stored in times of flood.
SFRAs should identify this
Flood Zone (land which
would flood either an annual
probability of 1 in 20 (5%) or
greater in any year or is
designed to flood in an
extreme (0.1%) flood, or at
another probability to be
agreed between the LPA and
the Environment Agency,
including water conveyance
routes).
Only the water-compatible uses and the
essential infrastructure that has to be there
should be permitted in this zone. It should
be designed and constructed to:
•
remain operational and safe for
users in times of flood;
•
result in no net loss of floodplain
storage;
•
not impede water flows: and
•
not increase flood risk elsewhere
Essential infrastructure in this zone should
pass the Exception Test.
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Explanation of Flood Risk Probability
Flood events and flood risk zones are commonly referred to in terms of the probability that
a particular flood event will occur. A flood event with a probability of 1% is defined as an
event that has a 1 in 100-year or greater chance of occurring in any one year.
Table 2 - Flood Risk Vulnerability Classification
Essential
Infrastructure
•
Essential transport infrastructure (including mass evacuation
routes) which has to cross the area at risk, strategic utility
infrastructure, including electricity generating power stations
and grid and primary substations.
Highly Vulnerable
•
Police stations, Ambulance stations and fire stations and
Command Centres and telecommunications installations
required to be operational during flooding.
Emergency dispersal points
Basement dwellings
Caravans, mobile homes and park homes intended for
permanent residential use.
Installations requiring hazardous substances consent
•
•
•
•
More Vulnerable
•
•
•
•
•
•
Less Vulnerable
•
•
•
•
•
•
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Hospitals
Residential institutions such as residential care homes,
children’s homes, social services homes, prisons and hostels
Buildings used for : dwellings houses, student halls of
residence, drinking establishments, nightclubs and hotels
Non-residential uses for health services, nurseries and
educational establishments
Landfill and sites used for waste management facilities for
hazardous waste
Sites used for holiday or short-let caravans and camping,
subject to a specific warning and evacuation plan.
Buildings used for: shops, financial, professional and other
service, restaurants and cafes; hot food takeaways, offices,
general industry, storage and distribution, non-residential
institutions not included in ‘more vulnerable’ and assembly
and leisure.
Land and buildings used for agriculture and forestry.
Waste treatment (except landfill and hazardous waste
facilities).
Minerals working and processing (except for sand and gravel
working).
Water Treatment Plants
Sewage Treatment Plants (if adequate pollution control
measures are in place).
19
2.2.2
The Exception Test
The Exception Test should be applied by decision-makers once the Sequential Test
has been applied and unable to deliver acceptable sites located in zones of lower
probability of flooding. The Exception Test can be applied, as shown in PPS25 Table
D.1, as a method of managing flood risk while still allowing necessary development
to occur. However, the exception test should not be used to justify highly vulnerable
development in the high risk flood zones.
The Exception Test should be applied when “more vulnerable” development and
“essential infrastructure” cannot be located in Zones 1 or 2 and “highly vulnerable”
development cannot be located in Zone 1. The circumstances where the exception
test may be applied are shown in Table 3.
Table 3 - Flood Risk Vulnerability and Flood Zone Compatibility
Flood Risk
Vulnerability
classification
Essential
Infrastructure
Water
Compatible
Highly
Vulnerable
More
Vulnerable
Less
Vulnerable
Zone 1
Exception
Test required
Zone 2
Zone 3a
Exception
Test required
Zone 3b
Exception
Test required
Exception
Test
required
Source Annex D PPS 25
Further guidance on the exception test is provided in section 7.3.1.
2.3
Planning Responsibilities
DEFRA has overall policy responsibility for flood risk in the England and the
Environment Agency has statutory responsibility for flood management and defence.
The Regional Planning body should prepare a Regional Flood Risk Assessment
(RFRA) in consultation with the Environment Agency to inform their Regional Spatial
Strategy.
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The Local Planning Authority is responsible for the planning system which aims to
direct development away from areas at highest flood risk. They are required under
PPS 25 to produce a SFRA in consultation with the Environment Agency, to inform
their sustainability appraisals, land allocations and development control policies and
the to undertake the application of the sequential test for new development.
At a site level developers should consult the Environment Agency, sewerage
undertakers, highways authorities and any other relevant bodies to supply
information for a Flood Risk Assessment of the site. This is to provide information to
the Local Planning Authority from which they can reach a decision on the
development application. PPS25 requires that planning applications for
development proposals of 1 hectare or greater in Flood Zone 1 and all proposals for
new development located in Flood Zones 2 and 3 should be accompanied by a FRA.
Figure 2 - Planning Hierarchy and Flood Risk
NATIONAL
Legislation, Planning Policy Statements
REGIONAL
REGIONAL
Regional Flood Risk Assessments (RFRA)
Regional Spatial Strategies (RSS)
Catchment Flood Management Plans (CMFP)
Thames Estuary 2100 (TE2100)
LOCAL
LOCAL
Local Development Framework (LDF)
Strategic Flood Risk Assessments (SFRA)
SITE
SITE
Site Specific Plans
Site Flood Risk Assessments
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2.4
Other Planning Statements and Policies
PPS3 and PPS6 sets out the Government’s national policy framework for delivering
Housing Objectives and national policy on Planning for Town Centres and Retail
Developments. Although not directly related to the SFRA they should be read
together with other relevant statements of national planning policy including PPS 25.
PPS3 states that physical restrictions such as flood risk should be taken into account
when identifying locations for development but emphasis is placed upon the priority
for “development to be located on previously developed land, in particular vacant
and derelict sites and buildings.” The national annual target is that at least 60
percent of new housing should be provided on previously developed land.
PPS6 requires that a sequential approach be applied to retail developments in
identifying suitable locations with growth largely encouraged in existing centres.
While this has obvious benefits, it may also direct future development towards areas
of high residual flood risk.
These two planning documents highlight that in exceptional circumstances, with
sufficient justification, development within areas of higher residual flood risk may be
permitted.
The North London River Restoration Strategy produced by the Environment Agency
highlights the benefits of river restoration in North London and promotes the use of
river restoration in the management of flood risk.
2.5
The Thames Catchment Flood Management Plan (CFMP)
The Thames CFMP was published for consultation in December 2007 by the EA.
The plan covers the fluvial Thames catchment and its tributaries and reviews the
present and future flood risk. The Thames CFMP identifies the most sustainable
direction for managing fluvial flood risk within the Thames catchment for the next 50
to 100 years. There are specific messages, objectives and actions for each of the
rivers within this SFRA (Brent, Lower Lee and all its tributaries). These have been
recommended after consideration of the opportunities and constraints in the
catchment and should be considered when forming flood risk policy.
There are four main messages that apply to the whole of the Thames CFMP area:
1.
2.
3.
4.
Flood defences cannot be built to protect everything
Climate change will be the major cause of increased flood risk in the future
The floodplain is our most important asset in managing flood risk
Development and urban regeneration provide a crucial opportunity to
manage the risk
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In addition, the plan emphasises the importance of land use planning, the role of the
natural floodplain, use of sustainable approaches in urban areas, flood defences and
increasing the resilience of the properties while reducing the consequences of
flooding. These policies are inline with the development approach defined in PPS25.
2.6
Making Space for Water
Recent flooding incidents such as those experienced in 1998 and 2000 highlighted
the need for the Government to develop an integrated strategy for managing future
flood risks in England. In response to this, DEFRA carried out a consultation
exercise known as “Making Space for Water” with stakeholders in 2004 to debate the
future direction of UK flooding strategy.
Policies outlined in Making Space for Water follow the governments’ ideal of
sustainable development with an overall strategy aim of managing risks from
flooding whilst working to deliver the greatest environmental, social and economic
benefit.
The Making Space for Water strategy highlighted the importance of a holistic
approach when dealing with all forms of flooding. This is especially significant in
urban areas where complex interaction of drainage systems can exist leading to
difficulty in identifying responsibility for dealing with floods. The government propose
that different authorities responsible for different parts of the drainage system work
together to manage flood risk and take a long term strategic approach.
The Governments commitment to the use of Sustainable Urban Drainage Systems
(SUDS) is outlined in the strategy through the support of the National SUDS Working
Group working to address key issues such as ensuring adoption, ownership and
operation responsibility is clearly defined.
It is highlighted that approximately 10 per cent of England is located within mapped
areas of flood risk with a significant proportion of previously developed land sighted
for redevelopment also being within areas of higher flood risk. The report advises
planning authorities to ensure that the minimum standard of protection be provided
for the lifetime of the development in line with PPS25 when considering
developments located within flood plains. Measures to reduce flood risk through land
use planning are dealt with in Section 7 of this report.
Making Space for Water also refers to the need for Regional Spatial Strategies and
Local Development Frameworks to take full account of current and future flood risks
and incorporate the sequential approach in PPS25 and integrated approach with
catchment flood management plans.
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2.7
The Pitt Review – Final Report
The Pitt Review was undertaken in the 10 months after the 2007 Summer Floods
that lead to 13 deaths, 55,000 properties being flooded and the largest loss of
essential services since World War 2.
The report reviewed many different aspects of flood risk management, regulations,
the water industry, roles and responsibilities in association with flood risk, the
technical methods of risk assessment and further issues connected with them. The
Pitt Review made 92 recommendations that are summarised in the Foreword to the
Sectaries of State (see extract below). Some of these recommendations will be
highlighted in the report.
Extract from the Foreword of the Pitt Review Final Report (Page vii)
•
We believe that there must be a step change in the quality of flood
warnings. This can be achieved through closer cooperation between the
Environment Agency and Met Office and improved modelling of all forms of
flooding. The public and emergency responders must be able to rely on this
information with greater certainty than last year.
•
We recommend a wider brief for the Environment Agency and ask councils
to strengthen their technical capability in order to take the lead on local flood
risk management. More can be done to protect communities through robust
building and planning controls.
•
During the emergency itself, there were excellent examples of emergency
services and other organisations working well together, saving lives and
protecting property. However, this was not always the case; some decision
making was hampered by insufficient preparation and a lack of information.
Better planning and higher levels of protection for critical infrastructure are
needed to avoid the loss of essential services such as water and power.
There must be greater involvement of private sector companies in planning
to keep people safe in the event of a dam or reservoir failure. Generally, we
must be more open about risk.
•
We can learn from good experience abroad. People would benefit from
better advice on how to protect their families and homes. We believe that
levels of awareness should be raised through education and publicity
programmes. We make recommendations on how people can stay healthy
and on speeding up the whole process of recovery, giving people the
earliest possible chance to get their lives back to normal.
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2.8
2.8.1
Flood Risk Management Key Stakeholders
Environment Agency
The Town and County Planning Order was amended in 2005 to make the
Environment Agency a statutory consultee for development where flood risk is an
issue under any of the following criteria:
•
Development within 20m of the bank top of a Main River
•
Any culverting operation or development which controls the flow of any river or
stream
•
Development other than minor development in Flood Zones 2 & 3
•
Development in Flood Zone 1 where there are critical drainage problems
•
Any development exceeding one hectare in extent
Major development is defined as residential development with 10 or more dwellings
or site area of 0.5 hectare or more, or a non-residential development of 1000 square
metres or more.
The EA are a statutory consultee and have to be consulted in the town and country
planning process. The EA’s representations to local planning authorities relate to the
environmental matters that the EA are responsible for reviewing, including flooding.
The EA only comment on planning policies or applications – the EA do not decide
them. The EA aims to protect flood plains from inappropriate development. The EA
local offices can provide advice on development issues. Further details of
government policy on development and flood risk, are found in Planning Policy
Statement 25, published by the Department for Communities and Local Government
in England.
Where a LPA is minded to approve a major development against the objection of the
EA, the LPA must notify the Secretary of State10 who will review the application
against the requirements of PPS25 and call in the application for determination if
necessary. If this is the case and the development proceeds against EA advice the
Association of British Insurers have made it clear that it is highly unlikely that
insurance against flood risk will be available for such developments.
10
Circular 04/06 (Communities and Local Government): The Town and Country Planning
(Flooding) (England) Direction 2007
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Under the Water Resources Act 1991, the EA have powers to maintain and improve
main rivers, in order to ensure the efficient passage of flood flow and to manage
water levels. These powers allow the EA to do work but they do not oblige the EA to
carry out either maintenance or construction of new works on main rivers. Further
explanation of the EA’s responsibilities are available are outlined in the ‘Living on the
Edge’ publication. For further information, please contact the EA at their Area Office
in Hatfield at the following address.
Environment Agency
North East Area Office,
Apollo Court,
2 Bishops Square Business Park,
St Albans Road West,
Hatfield,
Hertfordshire,
AL10 9EX
2.8.2
Thames Water and Three Valleys Water
Thames Water as the Sewerage Undertaker in North London are responsible for
surface and foul drainage discharge from developments where disposal is to the
adopted sewer network. SFRAs are required to take account of any specific capacity
problems associated with these artificial drainage networks. Developers are
responsible for consulting sewerage undertakers directly for surface and foul water
disposal issues.
Thames Water is responsible for the day to day maintenance of the network and
looking after its interest in any associated planning issues.
Thames Water is also the main supplier of potable water within the North London
area and is responsible for the day to day maintenance and operation of the clean
water distribution network.
The North of Barnet is supplied by Three Valleys Water. Three Valleys Water are
responsible for the water supply, but not the surface and foul water discharge. Areas
supplied include Arkley, Barnet, Colindale, Edgware, Finchley, Hadley Wood, New
Barnet, North Finchley, Mill Hill and Totteridge.
2.8.3
British Waterways
British Waterways is a public corporation that manages more than 2,200 miles
(3,540 km) of canals and rivers in England, Scotland and Wales. It is the
responsibility of British Waterways to ensure no flooding occurs from the canal
networks.
Within the London Borough of Barnet, British Waterways manages the Brent
Reservoir. The Brent Reservoir, also known as the Welsh Harp, supplies water to the
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Grand Union Canal. The Regents canal is the primary canal for the study area,
connecting the Grand Union Canal with the Thames. Further information can be
found in section 5.6.2.
British Waterways are under no obligation to receive discharge from the surface
water networks into the canals but this may prove to be a suitable option during
extreme rainfall events.
2.9
Flood Risk Responsibility
Organisations that are responsible for providing flood defence are known as
operating authorities. They have different powers and responsibilities as follows:
•
The EA is responsible for works on all main rivers.
•
Local Authorities look after ordinary watercourses (different tiers of Local
Authorities; Counties, Metropolitan, Unitary and Districts have differing flood
defence responsibilities).
The powers given to the operating authorities to carry out works are all permissive,
which means they can choose either to carry out works, or not at their discretion.
2.9.1
Main Rivers
Main rivers are usually large streams and rivers, but also include smaller
watercourses of strategic drainage importance. The designation of every
watercourse is held on an official document known as the main river map, held by
the EA and can include any structure or appliance for controlling or regulating the
flow of water in, into or out of a main river. The decision as to what is designated
‘main river’ is made by DEFRA. The EA have powers to maintain and improve main
rivers as well as carrying out flood defence works. However, responsibility for
maintenance of main rivers remains with riparian owners as discussed in section
2.9.3. The EA has a duty to exercise general supervision over flood defence works.
2.9.2
Ordinary Watercourse
All other watercourses are defined as ‘ordinary watercourses’ and the local Borough
Council is responsible for making sure that the districts land drainage system
performs satisfactorily. An ordinary watercourse is every river, stream, ditch, drain,
dyke, cut, sewer (other than public sewer) and passage through which water flows
and which does not form part of a main river11. The Land Drainage Act 1991 gives
11
EA Publication, Living on the Edge: A Guide to the Rights and Responsibilities of Riverside
Occupation, available on the EA website.
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local authorities powers to deal with obstructions in ordinary watercourses. If the
obstruction impedes the flow, the council may serve notice on the riparian owner to
remove the obstruction.
2.9.3
Riparian Owners
The owner of land or property adjacent to a watercourse is known in legal terms as
the "riparian owner" of the watercourse. Riparian owners have certain rights and
responsibilities which are primarily established through Common Law. Where the
watercourse forms the boundary between two properties, owners usually posses the
land up to the centre of the watercourse unless the property deeds indicate
otherwise.
Riparian owners are required to maintain a watercourse in such a condition that the
free flow of water is not impeded, including maintaining banks and clearing natural
and man-made debris regardless of its origin. The riparian owner has the
responsibility to accept the flood flows through their land, even if caused by
inadequate capacity downstream. Riparian owners have no duty to improve the
drainage capacity of a watercourse. Where structures such as trash screens or
culverts are present, riparian owners have a duty to clear debris. Failure to uphold
responsibilities can result in legal action in some circumstances.
Riparian owners can refer to the EA’s “Living on the Edge” booklet for further details
on their rights and responsibilities.
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3
Review of Development Framework
3.1
Regional Planning Policy – The London Plan
The London Plan is the Regional Spatial Strategy for London produced by the
Mayor. The plan covers a wide range of issues from employment and resources to
housing and transport.
Chapter 2 of The London Plan outlines the key components of the spatial
development strategy. Policy 2A.5 highlights “Opportunity Areas” where as part of
producing the Sub-Regional Development Frameworks a sustainable development
programme will be created for each Opportunity Area. The London Plan highlights
Cricklewood Brent Cross, Colindale, King’s Cross, Stratford, Tottenham Hale and the
Lee Valley as opportunity areas, capable of accommodating substantial new jobs or
homes. These areas generally include major brownfield sites with capacity for new
development and increases in density.
Policy 2A.6 highlights area for “intensification” where development should exploit
public transport and accessibility and potential for increases in residential,
employment and other uses through higher densities and more mixed and intensive
use. Arsenal/Holloway, Farringdon/Smithfield, Haringey Heartlands/Wood Green and
Mill Hill East have been highlighted as intensification areas.
Chapter 4C of The London Plan concentrates on the river and water network and the
inter-relationships of all of London’s waterways, referred to as the “Blue Ribbon
Network”. It highlights the importance that development and use of water and
waterside land should respect natural forces in order to ensure that future
development and uses are sustainable and safe.
The London Plan outlines 34 policies referring to the Blue Ribbon Network, a number
of which refer directly to flooding and flood plains.
Policy 4A.12 Flooding states that “in reviewing their DPDs, boroughs should carry
out strategic flood risk assessments to identify locations suitable for development
and those required for flood risk management. Within areas at risk from flooding
(flood zones) the assessment of flood risk for development proposals should be
carried out in line with PPS25”.
Policy 4A.13 Flood risk management “Where development in areas at risk from
flooding is permitted, (taking into account the provisions of PPS25), the Mayor will,
and boroughs and other agencies should, manage the existing risk of flooding, and
the future increased risk and consequences of flooding as a result of climate change,
by:
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•
protecting the integrity of existing flood defences
•
setting permanent built development back from existing flood defences to
allow for the management, maintenance and upgrading of those defences to
be undertaken in a sustainable and cost effective way
•
incorporating flood resilient design
•
establishing flood warning and emergency procedures
Opportunities should also be taken to identify and utilise the areas for flood risk
management, including the creation of new floodplain or the restoration of all or part
of the natural floodplain to its original function, as well as using open space in the
flood plain for attenuation of flood water.
The Mayor will, and boroughs and other agencies should, take fully into account the
emerging findings of the Thames Estuary 2100 Study, the Regional Flood Risk
Appraisal and the Thames Catchment Flood Management Plan.”
Policy 4A.14 Sustainable drainage “The Mayor will, and boroughs should, seek to
ensure that surface water run-off is managed as close to its source as possible in
line with the following drainage hierarchy:
•
store rainwater for later use
•
use infiltration techniques, such as porous surfaces in non-clay areas
•
attenuate rainwater in ponds or open water features for gradual release to a
watercourse
•
attenuate rainwater by storing in tanks or sealed water features for gradual
release to a watercourse
•
discharge rainwater direct to a watercourse
•
discharge rainwater to a surface water drain
•
discharge rainwater to the combined sewer
The use of sustainable urban drainage systems should be promoted for
developments unless there are practical reasons for not doing so. Such reasons may
include the local ground conditions or density of development. In such cases the
developer should seek to manage as much runoff as possible on site and explore
sustainable methods of managing the remainder as close as possible to the site.
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The Mayor will encourage multi agency collaboration (GLA Group, Environment
Agency, Thames Water) to identify sustainable solutions to strategic surface water
and combined sewer drainage flooding/overflows.
Developers should aim to achieve greenfield run off from their site through
incorporating rainwater harvesting and sustainable drainage. Boroughs should
encourage the retention of soft landscaping in front gardens and other means of
reducing, or at least not increasing the amount of hard standing associated with
existing homes.”
Policy 4A.15 Rising groundwater In considering major planning applications in
areas where rising groundwater is a existing or potential problem, the Mayor will and
boroughs should, expect reasonable steps to be taken to abstract and use that
groundwater. The water may be used for cooling or watering purposes or may be
suitable for use within the developments or by a water supply company”.
The London Plan promotes the use of SUDS and highlights the importance that
developers and local planning authorities to work together with water supply and
sewerage companies to enable the inspection, repair or replacement of water supply
and sewerage infrastructure.
Supplementary Planning Guidance (SPG) has been produced to provide additional
information to support the implementation of the London Plan.12
The SPG outlines the Mayors “essential” and “preferred standards” to include the
importance of the use of SUDS wherever practical and the need to “achieve 50%
attenuation of the undeveloped site’s surface water runoff at peak times” as an
essential standard and “achieve 100% attenuation of the undeveloped sites surface
water runoff as peak times” as a preferred standard.
12
Supplementary planning guidance, Sustainable design and construction Mayor of London,
May 2006
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Section 4.9 identifies Opportunity Areas or Areas for Intensification, some of which
includes or adjoins parts of London’s Blue Ribbon Network. The areas included in, or
influencing this study are listed in
Table 4.
Table 4 - London Plan Opportunity Areas that adjoin the Blue Ribbon Network
3.2
Opportunity Areas
Relevant water spaces
Cricklewood/Brent Cross
River Brent
Colindale
River Brent
King’s Cross
Regents Canal
Lower Lee Valley
Bow Back Rivers, River Lee
Stratford
Bow Back Rivers, River Lee
Tottenham Hale
River Lee, Pymmes Brook
Haringey Heartlands/Wood Green
Moselle Brook
Lower Lee Valley Opportunity Area Planning Framework
The Lower Lee Valley Opportunity Area Planning Framework sets out in more detail
the Mayor of London’s plans for the area extending through the Lee Valley from the
Thames in the south, to Leyton in the north. The regeneration area extends into the
south of both the Boroughs of Hackney and Waltham Forest and any policies and
proposals outlined therein must also be taken forward by those Boroughs.
The Framework rightly identifies flood risk as a key issue and a potential threat to
creating sustainable development in the Lee Valley regeneration area. The salient
points of interest are as follows.
A significant proportion of the regeneration area is included within flood zone 3,
although protected by defences to a varying standard of protection. The areas of
Leyton and Hackney Wick are considered to be at “actual flood risk”, defined as
having a greater than 1% probability of flooding.
The framework sets out a number of policies relating to flood risk, the aim of which is
to ensure that future developments do not “put people and property at unacceptable
risk” and to ensure that the flood risk to 3rd parties is not increased by any future
development.
The framework also identifies that those areas that are at actual risk of flooding will
require site specific or strategically planned defences of some form or another in
order that development classed as “more vulnerable” in PPS25 might be accepted.
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Such development must also first pass the sequential and exception tests as set out
in PPS25.
3.3
Water Matters, Draft Water Strategy
The Mayor of London’s draft water strategy was issued for consultation in March
2007 and sets out to provide a London wide strategy for managing water resources
and flooding, particularly aiming to prepare London for the stresses that population
growth and climate change will have on the existing water management
infrastructure. The strategy is designed to compliment and develop further the
policies set out in the London Plan. To ensure a consistent policy approach any
policies set out by individual Boroughs in their LDDs should be developed to
compliment those of both the London Plan and draft water strategy. As such, those
preparing planning policies relating to flood risk should ensure that they are fully
conversant with the final water strategy document when it is released by the Greater
London Authority.
3.4
Local Planning Policy – The UDP and LDF Core Strategies
The Unitary Development Plan (UDP) is a ”land use” plan produced by each London
Borough and forms the development plans used for the purpose of Section 54A of
the Town and Country Planning Act 1990. The UDP’s set out each Boroughs
policies regarding planning and transportation.
The Local Development Framework (LDF) is the spatial planning strategy introduced
in England and Wales by the Planning and Compulsory Purchase Act 2004. The
LDF is to be made up of a series of planning documents that together will set out the
overall planning vision, looking forward 10 to 15 years, in general conformity with the
London Plan. The LDP will replace the existing UDP in each borough. All boroughs
have published a Local Development Strategy (LDS) which sets out the councils 3
year plan explaining how the LDF will be produced. In the interim period the UDPs
still steer planning policy in the Boroughs and in order to identify potential
development locations and existing flood risk management policies, each Boroughs’
UDP has been reviewed. Table 5 offers a brief summary of the policy coverage from
each UPD. In general the policies set out in the Boroughs UDPs are outdated and
will require modification to bring them into line with PPS25.
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Table 5 - Summary of UDPs and Policies
UPD
Authority
/ Policy
Flood Risk &
Development
Brownfield
Development
SUDS
Environmental
Conservation
Groundwater
Watercourse
Enhancement
Opportunity
/Development
Areas
Comments
Cricklewood, Brent
Barnet
ENV9
ENV 10
O12/O13
Cross, West Hendon,
Colindale, Mill Hill East
Camden
Enfield
Hackney
GD12
EQ9
GD13
HO20/E18
N6
RC1
ENV1
O6/O7/O8/O9
ST13/ST42
ST12/EQ26
Kings Cross Railway
Lands
Promotes Flood
Mitigation
Tottenham Hale,
Haringey
ENV1R
Haringey Heartlands,
ENV 2
Wood Green
Islington
D35
Waltham
Forest
WPM 18
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ENV25
ENV8/ENV21/ENV23
WPM
19
35
WPM 15
WPM 15
WPM 15
Leyton, Blackhorse
Road
The delivery schedule of the Local Development Framework Development Plan
Documents for each of the boroughs is indicated in the Borough’s Local
Development Scheme reports. Local Development Schemes set out the timetable for
delivery of LDF documents and are therefoe subject to change. Borough websites
will provide the most recent version of the Local Development Scheme. In addition a
few of the Development Plan Documents have already been produced and adopted
by the Borough Councils. A summary of the Borough’s delivery dates for each of the
documents is given in Table 6. The dates for the Area Action Plans show the start
date of the first one and the adoption date of the final Area Action Plan for each
borough. Some of the Area Action Plan may have already been completed ahead of
the final adoption date.
Table 6 - Summary of Boroughs Development Plan Documents Timescales taken from the Local
Development Scheme Reports
Document
Title
Core
Strategy
Core /
Development
Policies
Barnet
Start: ?
Adopt: Aug
10
Start: Sep
07
Examination:
Oct 10
Joint Waste
Management
Plan
Start: Jun 07
Area Action
Plans
Start: ?
Camden
Enfield
Hackney
Haringey
Islington
Waltham
Forest
Start:
Mar 07
Adopt:
Sep 10
Start:
Jan 07
Adopt:
Jul 09
Start: Apr
04
Adopt:
Oct 07
Start: ?
Start: Jul
07
Adopt:
Dec 10
Start: Jan 06
Start: Oct
07
Adopt:
May 11
Start: Apr 06
Start: Jun
07
Adopt:
Dec 10
Start: Jun
07
Adopt:
Dec 10
Start: Jun 07
Start: Jun 06
Adopted:
Feb 08
Start: Apr
05
Adopt:
May 11
Adopted:
Dec 07
Adopted
Jul 06
Adopted Jun
07
Start:
Mar 07
Adopt:
Sep 10
Adopt:
Dec 09
Start:
May 04
Adopt:
Dec 10
Adopt: Jan
08
Examination:
Oct 08
Start:
Jun 07
Adopt:
Dec 10
Start:
Jun 07
Adopt:
Dec 10
Start: Jun
07
Adopt:
Dec 10
Adopt: Mar
10
Adopted
Jun 06
Start:
Jan 07
Adopt:
Jun 10
Start:
May 05
Adopt:
Dec 10
Statement of
Community
Involvement
Start: ?
Adopted
Jun 06
Adopted
Jun 06
Start: Jun
04
Adopt:
Sept 06
Site
Allocations
/Schedule
Adopted Aug
05
Start:
Sep 08
Adopt:
Feb 11
Start:
Nov 07
Adopt:
Jul 10
Start:
May 04
Adopt:
Dec 10
Start: Oct
07 Adopt:
May 11
Start: Apr 06
Examination:
Oct 08
Continuous
Production
Adopted
Jun 06
Start:
Jan 07
Adopt:
Start: Apr
04
Adopt:
Adopted
Dec 07
Start: Apr 06
Examination:
Oct 08
Proposals
Map
Adopt: Dec
10
Adopt: Jun
07
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Adopt: Dec
10
Examination:
Dec 08
Jul 09
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37
4
Data Collection and Validation
A large amount of data has been made available from a variety of sources for this
SFRA study. All incoming data has been scrutinised to determine its authenticity and
validated where appropriate to determine its accuracy and suitability for use. Not all
data was deemed suitable for inclusion in the study. Appendix B - Data Summary,
presented at the end of this chapter lists all data and reports acquired for the study
including the source, its relevance to the scheme and any individual licensing
restrictions.
The majority of the data for this study has been provided by the Environment Agency
and participating London Boroughs. A brief explanation and summary of the key data
sources is summarised in the following chapters.
4.1
Mapping and Topographic Data
Mapping data for this study has been provided by each of the London Boroughs and
is licensed for use in this study only. The preferred mapping source for this study is
Ordnance Survey’s MasterMap topography layer, which can be viewed at up to
1:1250 scale. Ordnance Survey Landline mapping is also used to represent the
wider catchment area as MasterMap data is only provided to cover the study
boundary. However, to date, not all Boroughs have been able to provide MasterMap
data and alternative sources have been used.
A Digital Terrain Model (DTM) has been provided by the EA for use in the study. The
DTM is supplied in an Arcview compatible format and consists of LiDAR data
provided in 500m x 500m files at a 0.5m grid cell. The DTM is a bare earth terrain
model with building and vegetation removed by filtering the original Digital Surface
Model using the supervised classification technique. The data has been briefly
reviewed to ensure that buildings or vegetation have been correctly identified and
filtered. LiDAR data coverage is unavailable for significant areas of the North London
sub-region. Supplementary topographic data to fill missing gaps is not available
from the EA but has been requested from the individual Boroughs. The Nextmap
DTM data has been be made available to supplement the missing areas. The areas
with no LiDAR coverage are generally the regions where no main rivers are present,
which predominantly covers Camden, Islington, Hackney and Haringey.
The EA has also provided GIS layers to identify the location of watercourses, canals,
flood zones, green belt and public open space areas within the seven boroughs. The
location of green belt and metropolitan land has also been obtained for Barnet
separately.
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4.2
Flood Defences
The Environment Agency has extracted information about flood defences in the
study area. The location, standard of protection and condition grade of the flood
defences and structures have been provided from the National Flood and Coastal
Defence Database (NFCDD) in GIS format.
4.3
Thames Water Data
Thames Water maintains records of flooding arising from surcharging of adopted
drains and sewers within its responsibility. Thames Water has provided an extract
from this register, known as the DG5 register. The data is provided with a reference
by truncated post code only so the exact location or source of flooding is not
identified. It does however enable some comparison with other sources of flooding
data such as that received from the London fire Brigade.
In general only limited data pertaining to sewer flooding has been obtained by the
study team. The data obtained so far is insufficient to enable a thorough investigation
of sewer flooding within the study area and the time constraints of the project. No
GIS extracts of the main sewer lines, modelling data or results of Drainage Area
studies have been obtained. Some additional information about the main sewer lines
in Haringey were made available in paper format.
Other data requested from Thames Water has not been made available for the study
and without this data no verification or quantification of surface water flood risks
could be undertaken. Further discussion on the missing data and the subsequent
limitations of the study are included later in the report.
4.4
Clean Water Supply Data
Neither Thames Water nor Three Valleys Water were contacted for details of
potential clean water supply flood risks due to the unpredictable nature of such risks
and the sensitivity of reproducing the information in a publicly available document.
The flood risks associated with clean water supply pipework predominantly occurs
from bursts, which compared with other sources of flood risk are generally small in
nature and extremely difficult if not impossible to predict. As such this information is
unlikely to be beneficial to the planning process and has not been pursued further as
part of this study.
4.5
4.5.1
River Lee and Brent Data
Historic Flood Records
The Environment Agency maintains records of past flood events which are used in
conjunction with river modelling in the preparation of the flood risk maps made
available to the public via the internet. The EA have some historic flood data for
North London which has been made available in GIS format. Haringey and Enfield
councils were also able to provide additional historic flood records.
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4.5.2
Flood Modelling Results
The EA have commissioned numerous flood studies for the main rivers in the study
area. Most of the designated main rivers have been mapped and modelled under
section 105 of the Water Resources Act 1991.
The River Lee catchment has been well studied over the years with the most recent
being the ‘Hydrology and Mapping Study’ (2007) undertaken by Halcrow Group
Limited using ISIS hydraulic modelling software. The Holyhill Brook and the
upstream end of Turkey Brook, Salmons Brook above the Hounsden Gutter
confluence are also modelled in JFLOW.
The finial version of the River Brent Mapping study report was completed by Jacobs
in 2007. Flood levels have been determined using an ISIS hydraulic model divided
into two models for manageability. The Upper Brent hydraulic model includes the
Dollis Brook, Mutton Brook, Folly Brook and the River Brent to the Brent Reservoir
inlet. The Silk Stream was modelled separately a couple of years earlier due to the
pressing need to develop a flood alleviation scheme. The flood zones produced have
been incorporated into the mapping.
4.6
Regional Flood Risk Appraisal
The draft Regional Flood Risk Assessment was issued for consultation in June 2007,
with a view to finalising the document in late 2007. The report has been produced by
the Greater London Authority and gives a “broad consideration of flood risk” for the
whole of London, identifying the types and the spatial implications of flood risk for the
region. The report is considered to be a live document and will continue to be
updated to reflect changes in legislation, policy and developments into climate
change investigations.
The RFRA identifies that 40% of the areas of opportunity and 30% of the areas
identified for intensification are located within flood risk zones, much of it in the Lee
Valley. Applying the sequential test at a regional level would appear to show that
alternative sites are not available without encroaching onto green belt or other
protected areas. This further highlights the importance of addressing flood risk within
the Lee Valley such that development of these sites can be carefully planned to
ensure that the sequential and exception tests can be applied at a local level. It also
identifies the requirement for strategic review of flood defence protection in the
Lower Lee Valley.
For the North London sub-region, fluvial flood risk is identified as the prime flood risk
source especially with regard to the Lee Valley, where the River Lee Flood Relief
Channel is identified as having a standard of protection below the current required
standard. The additional development in the upstream catchment is believed to have
reduced the level of protection that Flood Relief Channel provides.
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The River Brent is identified as being sufficiently protected from flood risk, although
the upstream tributaries in Barnet have suffered localised flooding in the past.
The RFRA surmises that Canals poses a low flood risk but the River Lee Navigation
may convey flood waters from other watercourse and so needs to be considered
within SFRAs and FRAs. The RFRA does not address reservoir flood risk as this is
dealt with under different legislation.
The impacts of flood risk on the area are considered with particular interest on the
critical infrastructure. Several tube lines, utilities and important facilities are within the
flood plain, these are detailed in Table 7 and Table 8. This information is important
for use in emergency planning and is included in the RFRA to help stimulate cooperation between emergency and spatial planners. Most of the locations have been
plotted on Map 24.
Table 7 - Transportation / Facilities at Risk of Flooding
Transportation / Facilities at Risk of
Flooding
Watercourse Presenting Risk
Colindale Hospital
Partially within Silk Stream Floodplain
Edgware Hospital
Wholly within Silk Stream Floodplain
Burnt Oak Tube Station
– Northern Line
Silk Stream floodplain
Tottenham Hale Tube Station
- Victoria Line
River Lee floodplain
Source: Regional Flood Risk Appraisal
Strategic Flood Risk Assessment
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Final - April 2008
41
Table 8 - Utility Infrastructure within Flood Risk Zones
Borough
Location Within Borough
Infrastructure in Floodplain
Enfield
Leeside Road N18
Gas
Enfield
Albany Road N18
Gas
Enfield
Bolton Road N18
Unknown
Enfield
Balham Road N9
Unknown
Enfield
Lee Park Way N18
Waste North London Waste to Energy
Enfield
Picketts Lock Lane N9
Deephams Sewage Treatment Works
Enfield
Woodhall Road EN3
Gas
Enfield
Edison Road EN3
Electricity
Enfield
Brancroft Way EN3
Electricity Brimsdown Power Station
Enfield
Lee Valley Reservoirs EN3
Water Reservoirs including pumping
stations
Enfield
Brancroft Way EN3
Electricity Sub Station
Enfield
Hadley Road EN2
Drainage Pumping Station
Enfield
Station Road N11
Gas
Enfield
Dendridge Close EN3
Unknown
Hackney
Millfields Road E5
Electricity
Hackney
Millfields Road E5
Waste
Haringey
Leeside Road N17
Electricity
Haringey
Marsh Lane N17
Drainage Pumping Station
Haringey
Reform Row N17
Unknown
Waltham Forest
Westdown Road E15
Unknown
Waltham Forest
Osier Way E10
Sewage
Waltham Forest
Clementina Road E10
Gas
Waltham Forest
South Access Road E17
Council Depot
Waltham Forest
Coppermill Lane E17
Water Coppermills Waterworks
Waltham Forest
South of Lockwood Reservoir
E17
Water Pumping Station
Waltham Forest
west of Chingford Road E4
Electricity
Waltham Forest
May Road E4
Unknown
Waltham Forest
Harbert Road E4
Water Pumping Station
Waltham Forest
Lee Valley Reservoirs E4
Water Reservoirs
Source: Regional Flood Risk Appraisal
Strategic Flood Risk Assessment
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4.7
Lower Lee Flood Risk Management Strategy
The Lower Lee Flood Risk Management Strategy is an Environment Agency
produced document which aims to identify a strategy for managing flood risk over the
next 100 years.
Downstream from Ware it is identified that the flood defences (including the flood
relief channel) provide protection against storms with a greater than 5% probability of
occurrence (or 1 in 20 year return period flood). Further upstream the level of
protection is reduced even further to a 20% probability (or 1 in 5 year return period
flood). Development in the upstream catchment and climate change is likely to
further erode this level of protection in future. The standard of protection provided by
the Lower Lee Valley defences is a significant threat to the opportunity areas
identified in the London Plan.
The principle fluvial flood risk areas are identified in the Lower Lee Flood Risk
Management Strategy are detailed in Table 9.
Table 9 - Lower Lee Flood Risk Areas
Flood Risk Area
Watercourse
Enfield Lock to Stewardstone
Lower Lee
Waltham Forest
Lower Lee
Lee Valley Trading Estate
Lower Lee
Walthamstow Area
Lower Lee
Grange Park
Salmons Brook
Chingford
Ching
Upper Edmonton
Pymmes
4.8
Other Drainage Studies
In recent years, the need for integrated has increased and so the number of multi
authority organisations has grown. With flooding and drainage has become more of
a cross boundary issue various wide area studies, forums and working groups that
focus on different issues have been established. Below are these known to cover the
London area that includes the North London Boroughs in this study.
4.8.1
The Drain London Project
The Drain London Project is to look into issue of surface water management in the
London area. The project aims to:
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43
Map the layout, ownership and capacity of surface water drains
Assess the location, frequency, severity and cause of surface water
flooding in London, and the impact of surface water flows on the tributary
river network
Assess the capacity of the surface water drainage network and urban river
networks to manage future increases in rainfall
Identify current and future flood hot spots and their causes
Identify and prioritise solutions and determine responsibility to deliver
actions
4.8.2
Central London Forward
Central London Forward (CLF) is a local authority-led partnership for Central London
set up in 2007 that comprises of six boroughs in the Central Activities Zone (as
defined in the London Plan). The boroughs are: City of London, City of Westminster,
Kensington & Chelsea, Camden, Islington, and Southwark. Further more the private
sector and third sector feed into CLF through Advisory Panels and Local Strategic
Partnerships.
Alongside East London, Central London will be amongst the main growth areas in
London. So in light of the huge growth predicted in this part of London over the next
10 years, Central London Forward’s purposes are:
4.8.3
•
To influence policy on major issues affecting Central London, including
making the case for additional resources.
•
To promote the strategic importance and needs of Central London with a
focus on sustainable economic development and the improvement of the
quality of life of workers, residents and visitors.
•
To identify and co-operate on areas of mutual interest to partners, including
tourism, the Olympics and its legacy
North London Strategic Alliance
The North London Strategic Alliance (NLSA) was established in 1999 as the subregional strategic partnership for North London, bring together public, private and
voluntary organisations. The NLSA currently works across Barnet, Enfield, Haringey
and Waltham Forest. However this is a transitional period and the new sub regional
arrangements being progressed by the Greater London Authority means NLSA will
Strategic Flood Risk Assessment
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44
also be looking to develop relationship across Camden, Hackney, Islington and
Westminster.
The NLSA provides a forum for different interest groups within the Lee Valley to
discuss and plan development activities across borough boundaries. The NLSA
website can be found at www.nlsa.org.uk
4.9
Strategic Flood Risk Assessments
A number of Strategic Flood Risk Assessments have been produced or are in
production which may influence the North London SFRA. These also include the
Enfield and Islington SFRAs which are to be reviewed as part of this report and the
relevant flood risk data and mapping incorporated where appropriate.
The following sections offer a brief review of each SFRA including identifying the key
interfaces with the North London SFRA.
4.9.1
East London SFRA
The East London SFRA was commissioned by the Thames Gateway London
Partnership to cover the boroughs of Barking and Dagenham, Bexley, Greenwich,
Havering, Lewisham, Newham, Redbridge and Tower Hamlets. The project was
completed in 2005 and the scope of the study was designed to meet with the
requirements for PPG25. Since then PPS25 has been published and contains a
stronger emphasis on flood risk management through the planning process. The
East London SFRA does not contain all of the information required by Boroughs to
spatially plan development within the requirements of PPS25.
The East London SFRA has one key point of interaction with the North London
SFRA which is the River Lee valley. The East London SFRA assesses the
downstream end of the River Lee in Newham and Tower Hamlets before it joins the
River Thames. The SFRA reviews the delineation of flood risk zones around the
Lower Lee Valley and covers the southern portions of Waltham Forest, Hackney and
to a lesser extent, Haringey.
The hydraulic models derived for this study could be used for any further
assessment required for the 3 Boroughs listed above, although further definition of
the flood plains may be required to improve accuracy in areas which were actually
outside of the East London SFRA study area.
4.9.2
Lower Lee SFRA
The London Development Agency has commissioned a SFRA covering the Lower
Lee Valley. The Lower Lee SFRA is intended to support the regeneration of the
Lower Lee Valley and in particular development associated with the 2012 Olympic
Games and its associated infrastructure. There are likely to be significant overlaps
Strategic Flood Risk Assessment
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between the Lower Lee and North London SFRAs. To facilitate the SFRA the LDA
has commissioned a model to look at fluvial flood risk and breach hazards. This
model could provide the majority of data on residual flood risk for the Waltham
Forest and Hackney Boroughs. The requirement for this data and the
recommendations for further work, including flood risk is discussed in section 11.1.
The Thames Estuary 2100 is an EA initiative that aims to determine the appropriate
level of flood protection needed for the Thames Estuary for the next 100 years.
Thames Estuary 2100 is the first step of the process and will help shape the way in
which future flood defence schemes are designed and managed. Taking action now
will allow time for research, design and the physical construction of the defences.
4.9.3
Enfield SFRA
The London Borough of Enfield has completed a comprehensive level 1 SFRA in line
with PPS25. The assessment provided a background to the main sources of flooding
and includes sufficient detailing of the flood risk zones to enable application of the
sequential test at a Borough level. The report recommends further assessment of the
flood zones around four Area Action Plans, such that the exception test may be
applied during the spatial planning of these future developments.
Where appropriate the flood risk information produced in the Enfield SFRA will be
incorporated into the overall North London SFRA maps to provide consistency. In
some instances, such as assessment of surface water and sewer flooding, additional
information has been obtained for this study which has not been included in the
Enfield SFRA. This information is discussed further in section 4.12. Again for
consistency this data will be utilised in the same way for all Boroughs. This may
develop the assessment of certain flood sources to a greater extent than the Enfield
SFRA, however, it is not anticipated that the results will contradict the Enfield SFRA
findings.
In Enfield, the bench mark for fluvial flooding was the 1947 flood on the River Lee
and its tributaries. Since then many alleviation schemes have been undertaken to
reduce fluvial flooding including the River Lee Flood Relief Channel. Even now more
sustainable flood risk management schemes are being developed. The assessment
of groundwater and surface water flooding would appear to suggest that Enfield does
not have a significant problem associated with these flood risks, other than a small
number of localised problems that could be expected in any urban environment.
The London Borough of Enfield are intending to extend the scope of their SFRA to
include a level 2 assessment, covering only allocated sites.
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4.9.4
Islington SFRA
The Islington SFRA has been completed; however, it is not comprehensive enough
to fulfil the brief of an SFRA as described in PPS25. The SFRA does consider some
aspects of flood risk in the borough of Islington but there are some sources of
flooding that are either not addressed in sufficient detail, or not reviewed at all. The
SFRA identifies different sources of flooding yet only fluvial flooding has been
investigated in any detail.
The map of Islington and its watercourses only shows open channels. The Canal
that flows through the borough has not been discussed. The sewage and main water
systems as sources of flooding have only been mentioned in regards to who is
responsible for their maintenance. In addition, surface water and ground water
flooding has not been considered or investigated other than a brief statement from
the EA.
A review of various policies that effect or manage water and flood water were
included. However, no developer or planner guidance is available on the application
and use of PPS25 in regards to Flood Risk Assessments and sustainable
development. In addition, no SUDS guidance is provided in the main assessment.
Appendix 5 does discuss SUDS but there is no discussion on what SUDS are and
which SUDS techniques might be appropriate for use in the London Borough of
Islington (LBI).
The short comings of this document will be addressed and further investigation into
other sources of flooding undertaken as part of this study, such that the LBI have a
comprehensive assessment of flood risk consistent with their neighbouring North
London Boroughs.
4.9.5
Olympic FRA
The Olympic FRA was published by the Olympic Delivery Authority in May 2007 and
the study boundary includes part of the Lower Lee Valley in the Boroughs of
Hackney and Waltham Forest. The FRA supports the planning application of the
Olympic and Legacy facilities. The FRA uses the Lower Lee Valley Regeneration
Strategy computational model (licensed from the London Development Agency) and
updates it to create the Olympic and Legacy Facilities (OLF) baseline model. The
model is a linked 1D - 2D fully hydrodynamic TUFLOW model with input hydrographs
provided by a routing model. An allowance for climate change is included in the flow
and tidal components in line with PPS25. The tidal influence on the River Lee is
taken into account by assuming a 1 in 20 annual probability tidal level with a level
hydrograph extracted from the River Thames ISIS model. The modelling work
undertaken for this study could form the basis of any level 2 assessment required for
the Boroughs of Waltham Forest and Hackney if such an assessment is identified as
being necessary.
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4.10
Community Risk Registers
The Civil Contingencies Act 2004 places a legal duty on Category 1 responders to
produce a Community Risk Register. The Resilience forums that cover several
authorities in an area produce a Community Risk Register. The study area contains
two Resilience Forums each of which have their own Community Risk Register; the
North Central London Resilience Forum and the North East London Resilience
Forum. The North Central London Community Risk Register covers Barnet, Camden,
Enfield, Hackney, Haringey and Islington while the North East London Community
Risk Register includes Waltham Forest. Both the registers identify flooding from two
main sources: surface water, coastal and fluvial as hazards.
4.11
Groundwater Data
Borehole data has been received from the Environment Agency stating details of the
boreholes and groundwater levels dating back to January 2004. The information
about the boreholes is not filtered and shows ALL the boreholes both active and
disused known to the EA in the study area. Boreholes are predominately located
towards the south of the region towards the Thames, and within the Lee catchment
area. The boreholes are sparsely located within the Brent catchment area. Borehole
records dating back further than 2004 have also been requested. It should be noted
that 217 of the 509 borehole are of an unknown status or type, 71 of the 509 are
disused and 105 of the 509 are for observational information. The EA were also able
to provide the depth Groundwater contours. It is hoped that these records will allow
assessment of the long term trend in groundwater levels.
4.12
Flood Records
Flood records have been collected and collated from a range of sources to produce
the most complete flood history of North London as possible. Records have been
obtained from the borough councils, Transport for London, London Fire Brigade and
the EA. Other organisations were asked for information but were unable to share
their records.
4.12.1
Individual Boroughs
All the boroughs were asked for their flood records. Camden and Enfield were able
to provided GIS mapping of recent flood events that occurred within their borough.
Haringey were able to provide an Inception Report for Haringey Flood Management
Strategy. The Enfield SFRA also contains locations of flood records. Barnet provided
a range of flood records, most of which related to isolated highway flooding
incidents.
4.12.2
Transport for London
Transport for London is responsible for the management of the 580km of main road
within the M25. They maintain a list of flood events related to these road assets. A
Strategic Flood Risk Assessment
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48
list of flooding incidents has been obtained, dating only from May 2006 to August
2007.
4.12.3
London Underground
Several requests have been made for information on flooding incidents at
underground stations within the seven London Boroughs but no response has been
received.
4.12.4
The London Fire Brigade
The London Fire Brigade (LFB) have provided an extract of calls received about
flooding in the seven London boroughs from April 1999 to August 2007. The 15,500
calls handled by the LFB were concerned with different types of flooding that include
fluvial, surface water and burst pipes. The data has been filtered down to exclude
calls where no location has been recorded and calls that appear unrelated to
flooding. The records were then reduced further by using data from grouped flood
events that were defined by receiving 12 or more calls in 48 hours. These were then
cross referenced with rain gauge data from the EA to try and determine whether
events could be attributed to pluvial flooding or whether events may be attributed to
other factors.
4.13
Public Reservoirs
A public list of reservoirs as defined by the 1975 Reservoirs Act in the North London
SFRA study area has been received from the Environment Agency as part of their
GIS mapping layers. This data list specifies the locations, capacities, age, type and
the operators of each reservoir. A request has been made for further details
regarding the condition and maintenance of the reservoirs but no information has
been received.
The EA are responsible for the enforcement of the Reservoirs Act which defines
reservoirs as having 25,000m2 of storage above the natural ground level. The EA are
responsible for the management of EA owned reservoirs. The list provided by the EA
provides details of any reservoirs within the study boundary which come under this
category. There are other reservoirs which are smaller or not raised above ground
which are not included as part of this assessment. Reservoirs set below the natural
ground level have a much lower flood risk and smaller reservoirs, particularly those
associated with clean water supply have reduced flooding consequences due to their
size.
4.14
British Waterways
A request has been made for information on the canals in the North London region.
However, no information has been received from British Waterways.
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4.15
4.15.1
Low Confidence Data
EA Asset Data
The standard of protection data extracted from the EA’s National Flood and Coastal
Defence Database (NFCDD) has been reviewed. For nearly all of the flood defences
and structures in the study area a standard of protection of up to a 1 in 5 years is
listed. In areas where known flood alleviation schemes have been constructed such
as the Silk Stream where a large amount of work is complete there is no
acknowledgement of these formal defences on the database extract. These
defences also provide a higher standard of protection. The EA have commented that
5 is the default value they use for unknown standards of protection. However, with
this many unknowns, there is low confidence in the NFCDD standard of protection
data set.
4.15.2
Barnet Flooding Records
The data provided by Barnet on their flooding hotspots covered a two year period.
However no dates were associated to the flood events data making it difficult to
analyse flood patterns by comparing records with rainfall data.
4.16
Data Gaps
Data required to enable assessment of the primary source of flooding has been
made available for the study, predominantly by the EA. However, the information
required to undertake a thorough assessment of secondary sources of flood risk
such as sewer flooding and infrastructure failure is not widely available. Without the
requisite background information on these flood risk sources a comprehensive
assessment has not been possible.
4.16.1
Thames Water
Information pertaining to sewer flooding is particularly limited, which has resulted in a
low confidence on the potential risks and impacts of such flooding included in the
study. While Thames Water have provided extracts from their flooding database, the
sensitivity of the data restricts them from identifying individual flooding problems. The
flood records are identified by truncated postcode only. No hydraulic models or
results of hydraulic studies of the sewer network were made available, which
prevented a thorough assessment of the potential limitations of the network.
Thames Water’s response to these requests is quoted as follows:
“Due to the complexities of the sewage and surface water networks and the infinite
number of development opportunities at this point of the planning process, it is not
possible to accurately assess areas which will be affected by flooding as a result of
future development. To ensure all future development is sustainable detailed
computer modelling of development sites needs to be carried out. To do this the
exact location and scale of development needs to be known. The LPA will work
Strategic Flood Risk Assessment
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closely with the water company to ensure that development will not be allowed to
precede the delivery of essential infrastructure by rejecting un-sustainable
developments or attaching 'Grampian' style planning conditions on sites where
essential infrastructure is required.”
4.16.2
London Underground Flood Records
Several attempts were made to get information and data regarding historic flooding
of the tube stations but no response or information was provided.
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5
Sources of Flooding
5.1
Introduction
The following section provides details of the specific flood sources which pose a risk
to the North London Boroughs. The sources have been identified through the review
of the data supplied in the initial phase of the study. This initial assessment is used
to inform the source pathway receptor model presented at the end of this chapter,
which in turn focuses the needs of this study into further areas of investigation.
Assessment of flood risk is generally approached by considering the probability of
occurrence of a particular flood event along with the consequences. The probability
of flooding is usually determined through computational modelling and reviewing
historic flood events. In extreme cases the consequences may be loss of life;
however, less severe consequences would normally include economic loss,
inconvenience and disruption to public services.
Flood Risk in the North London study area has been assessed principally through
the review of existing data sources. The assessment focuses on fluvial flooding as
the primary source of flood risk and on secondary sources, including reservoirs,
artificial drainage, groundwater, surface water, sewer flooding and the tidal influence
on the Lee. A brief assessment of infrastructure failure is also included. The
accuracy and thoroughness of any assessment into secondary sources is
unfortunately limited by the data available. In most cases the opportunity to fill in
such data gaps is outside of the scope of this study.
The main sources of information used for the assessment have been provided by the
EA and the participating North London Borough Councils through the provision of
historic flood records.
5.2
Key Watercourses
The key watercourses affecting the study area are listed in Table 10 along with
significant details regarding each. The key water features in the study area are
represented on map 9 and the individual catchment areas are shown on map 11.
The following sections provide a brief description of the key watercourses including
details of any hydraulic studies which may inform this study.
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Table 10 - Rivers List
Watercourse Name
River
Status
Boroughs
Affected
Barnet Ditches
Mimmshall Brook
River Brent
Broomsfields Ditch
Deans Brook
Deers Hill Road
Dollis Brook
Edwarebury Brook
Edgeware Brook
Hendon Cemetery Drain
Mutton Brook
Oakhampton Road Drain
Springwood Crescent Drain
Silk Stream
Tramway Ditch Colindale
Watling Ditch
Folly Brook
Victoria Water Course
Cuffley Brook
Enfield Ditch
Boundary Ditch
Glenbrook South Drain
Holyhill Brook
Hounsden Gutter
Leeging Beech Brook
Small Lee
Merryl Hills Brook
Monken Mead Brook
Salmons Brook
Sadlers Mill Stream
Turkey Brook
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Main River
Bounds Green Brook
Main River
Pymmes Brook
Main River
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Barnet
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield
Enfield,
Barnet
Enfield,
Barnet
Hackney,
Waltham
Forest,
Enfield,
Haringey
Hackney,
Waltham
Forest
Haringey
Haringey
Haringey
River Lee
Main River
Lee Navigation (Lower)
Lee New Cut
Moselle Brook
Stonebridge Brook
Main River
Main River
Main River
Main River
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Length
(km)
3.1
12.5
25.1
0.79
5
0.3
13.7
1.8
9
1.4
5
0.2
1.0
8
0.5
1.8
1.1
0.4
9.2
1.7
0.8
1.6
4.0
1.2
9.0
2.7
4.0
14.4
6.6
13.7
Catchment
Area (ha)
2,918
10,150
4,149
4,158
3.7
Hydraulic
Studies
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
JFLOW
ISIS
TUFLOW
ISIS
JFLOW
JFLOW
JFLOW
ISIS
JFLOW
ISIS
ISIS
TUFLOW
ISIS
ISIS
15.5
4,427
TUFLOW
33
141,234
ISIS
22.9
ISIS
9.5
8.5
1.7
ISIS
TUFLOW
ISIS
726
5.2.1
Intercepting Drain
Main River
The Ching
Main River
Dagenham Brook
Main River
Eastern Flood Channel
Main River
Haringey,
Enfield
Waltham
Forest
Waltham
Forest
Waltham
Forest
5.4
6.8
ISIS
1,747
ISIS
5.3
ISIS
9.1
ISIS
Lower River Lee (Lee)
The River Lee is one of the largest Thames tributaries which drains a large rural
catchment to the north of London. The total catchment is about 1415 km2 and
extends as far north as Luton, encompassing a large part of Hertfordshire and parts
of west Essex.
The Lee catchment within the study area can be characterised as developed flood
plain with built flood defences, with tributaries which rise in the green belt with
undeveloped natural flood plains. The Lee Valley forms the eastern borders of
Enfield, Haringey and Hackney and borders Waltham Forest to the West. The River
Lee flows into the River Thames through the Borough of Newham. The downstream
side of Lee Bridge Road marks the tidal extent on the River Lee which is within the
Boroughs of Waltham Forest and Hackney.
The River Lee experienced severe flooding in 1947 and since then the Lee has been
heavily altered and defended. The development of several man-made channels has
provided flood relief in the area by increasing conveyance capacity through the
catchment. The River Lee Flood Relief Channel was designed to protect against the
1947 event which was believed to be approximately a 1 in 70 year flood. This is
below the level of protection that might be considered acceptable for new
development. Furthermore the level of protection is known to have been reduced
further by the extensive development in the upper catchment.
The EA provides a flood warning service to properties in flood zones 2 and 3 along
the River Lee. The EA will aim to provide a lead time of 2 hours wherever possible.
Many of the tributaries of the Lee have fast response times due to their small urban
catchments and impermeable London Clay associated with North London.
In recent years, the River Lee has been studied quite intensively due to its
importance in the London 2012 Olympics. The most recent was the ‘Lee Flood Risk
Mapping Study’ (March 2007) undertaken by Halcrow for the EA which used
TUFLOW and ISIS to model the watercourses. The May 2007 Olympic FRA and the
Lower Lee Valley SFRA models are more detailed for the area downstream of Lee
Bridge Sluices than the Lee Hydrology and Mapping Study. Further information
about the Olympics can be found in section 4.9.5.
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The EA is concerned that there is insufficient space available for much needed flood
alleviation schemes. They are pursuing a long term approach that could consider
ideas like land swaps to remove more vulnerable development from the floodplain
and release land for flood storage. The Thames CFMP provides further more
localised guidance on the approaches most applicable to the sub-catchments as
further discussed in the following sections.
The hydrology of North London is influenced primarily by the high urbanisation and
impermeable geology of the London Clay which is typical of the area. The Upper Lee
catchment is founded on the chalk uplands and is predominantly rural. The soils are
slowly permeable in nature which gives the Upper Lee an increased response time.
The Lee and its tributaries in North London have little supply from groundwater
sources and rise quickly during rainfall. Rainfall in clay areas cannot penetrate into
the ground and instead runs off quickly into the rivers. The rivers flow over the
London Clay which overlies the chalk aquifer and generally prevents interaction of
surface water with the groundwater.
5.2.2
River Lee Tributaries
The River Lee has many tributaries that enter the watercourse mostly from the west.
These watercourses form a dense drainage network across the western side of the
Lower Lee catchment.
The catchment includes several large tributaries which are prone to localised
flooding; these include Salmons Brook, Turkey Brook and Pymmes Brook. Many of
the Lee tributaries have extensively developed catchments which experience a rapid
response to rainfall. The EA provides a flood warning service to properties in flood
zones 2 and 3 along the Lee Tributaries.
The Ching Brook, located in the northern part of the Waltham Forest, arises at
Connaught Water and flows south then west to enter the River Lee to the north of
Banbury Reservoir. The total catchment area for the Ching Brook is 1747 hectares.
Turkey Brook drains a more rural catchment with an urban extent of just 4%, the total
area is 4,158 hectares. The source of Turkey Brook lies just outside the northern
boundary of Enfield. Its main tributary is the Cuffley Brook which rises 5 kilometres to
the north of Enfield and flows through predominately agricultural land to meet with
Turkey Brook. Turkey Brook then flows eastwards to the confluence with the Small
River Lee, before entering the River Lee to the north of the King George V reservoir.
The other main tributary of the Turkey Brook is the Holyhill Brook.
The Salmons Brook catchment covers a total area of 4149 hectares. The rivers
tributaries include Merryl Hills Brook, Leeging Beech Brook, Gelnbrook South Drain,
Saddlers Mill Stream and Houndsden Gutter. The Salmons Brook rises in the east of
Barnet and runs through fairly flat agricultural land for approximately 6km, picking up
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a number of small tributaries from the south. The underlying geology of the
catchment is predominantly London Clay with sporadic gravel beds. As it enters the
urban areas of Enfield the watercourse is culverted in a number of locations. The
Salmons Brook joins the Pymmes Brook to the south of the North Circular Road.
The Pymmes Brook catchment covers a total area of 4,427 hectares with an urban
extent of 44%. The heavily engineered watercourse passes through over 4
kilometres of concrete lined channels and almost 2 kilometres of culvert. The river
rises in Barnet and flows through Hadley Wood, flowing eastward through Enfield
towards the River Lee, picking up Bounds Green Brook along the way.
The Thames CFMP suggests that whilst these urban watercourses experience rapid
runoff, floodplain encroachment and modified channels, they still have sufficient river
corridors that to support more sustainable approaches. The implementation of
sustainable approaches would be most appropriate during the development or
redevelopment of sites along with maintaining existing open space.
5.2.3
River Brent and Tributaries Upstream of the Brent Reservoir
A small reach of the River Brent connects Dollis Brook to the Brent reservoir. For
completeness this reach will be included within the Brent tributaries. The River Brent
tributaries within the study area are Folly Brook, Dollis Brook, Springwood Crescent
Drain, Watling Ditch, Silk Stream, Hendon Cemetery Drain, Tramway Ditch
Colindale, Oakhampton Road Drain, Edgeware Brook, Mutton Brook flow into the
Brent Reservoir.
Some of the tributaries have been recently taken on as main rivers by the EA due
the level of fluvial flood risk. These are the Broomfields Ditch, Deer Hill Road Brook,
Edwarebury Brook, Deans Brook, Hendon Cemetery Drain, Watling Ditch, Tramway
Ditch Colindale, Springwood Crescent Drain and Oakhampton Road Drain.
The Dollis Brook becomes the Brent once it confluences with Mutton Brook in the
South of Barnet. The Dollis Brooks tributaries include Folly Brook, Hendon
Cemetery Drain, Deers Hill Road and Oakhampton. The Brent confluences with the
Silk Stream at the Brent Reservoir.
The Silk Streams main tributary is Deans Brook which flows south, meeting
Springwood Crescent Drain, Broomfields Ditch and Edwarebury Brook before
forming the Silk Stream at the confluence with Edgware Brook. Watling Ditch and
Tramway Ditch both enter the Silk Stream before it reaches the Brent Reservoir.
The CFMP suggested that flood risk along these watercourses should be accepted
in the short term but the long term flood risk should be prevented from rising beyond
the existing level. It is suggested that the as opportunities arise through redevelopment the urban layout can be changed to ensure greater flood resilience and
sustainability.
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All these watercourses are defended in places and a low standard of protection is
offered to properties in the floodplain. In recent years, a flood alleviation scheme has
raised the standard of protection to properties along the silk stream. The EA provides
a flood warning service to properties in flood zones 2 and 3 along the Brent
Tributaries. The Silk Stream is noted for being flashier than the neighbouring
watercourses.
5.2.4
Subterranean and Culverted Rivers
Historically a number of watercourses in London have been integrated into the urban
environment through canalisation and culverting. In some cases these watercourses
have been entirely incorporated into the sewer network and are often referred to as
London’s “Lost Rivers”.
The River Fleet is one such subterranean river. The River Fleet historically originates
from springs on Hampstead Heath and drains to the Thames approximately via
Kentish Town, Camden Town and Holborn. Through Camden and the City of London
the Fleet is entirely incorporated within the sewer network, owned and maintained by
Thames Water. For the purposes of this study it is considered as a sewer. The
catchment of what was the River Fleet is shown on map 11, the catchment is
extracted from Flood Estimation Handbook (FEH) and has not been verified for
accuracy. The Fleet would have been the main drainage body for the Camden area
and any future development activities in Camden could have significant impacts on
flood risk within the City of London if they are not adequately managed.
The Hackney Brook is also a subterranean river located in the Borough of Hackney,
which is also now incorporated into the sewer network. Before 1860 the Hackney
Brook had been heavily culverted and in was increasing being used as a sewer. In
1860, the Hackney Brook was incorporated into the Northern High Level Sewer. Map
No. 11 in Appendix A, shows the catchment as extracted from the FEH. Again the
catchment has not been verified for accuracy and is intended as a guide only.
The Moselle Brook in the Borough of Haringey has been mostly culverted with a
small reach of the river above ground in Tottenham Cemetery. Culverting of key
sections of the Moselle started in 1836 and further culverting of the watercourse took
place in 1906. The original culverting is believed to have taken place in order to
reduce flood risk. However, this often has the opposite effect.
5.3
Fluvial Flooding
Fluvial flooding occurs when the capacity of a watercourse is exceeded and flood
plains become inundated. In urban environments, man made structures within the
watercourse or flood plain can also cause inundation of areas outside of the natural
floodplains. Fluvial flooding also occurs when flood defences are overtopped or
breached. Overtopping usually results in a gradual inundation of the defended areas
and is usually easier to predict through flood warnings. Breaching can be much more
difficult to predict and can result in rapid inundation with little warning.
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5.3.1
Historic Fluvial Flood Events
Historic flood records have been supplied from a variety of sources. The key flood
events are summarised below and referenced on map 10 included in Appendix A.
5.3.1.1
March 1947 floods
The floods of 1947 were the largest since records began over 100 years previous to
this event. A large rainfall event and thawing of snow in the Thames catchment
combined with a storm surge in the North Sea, causing flooding in the River Thames
and its associated tributaries.
The 1947 floods also affected some of the Lee tributaries including the Cuffley
Brook, Turkey Brook, Salmons Brook (predominately between Leeging Beech Brook
and Hounsden Gutter, and the Ching.
The areas immediately around the Warwick Reservoirs were flooded which include
the Walthamstow Marshes and Nature Reserve downstream, Leyton Industrial
Village, the Cromwell Estate, Riding School, Roxwell Trading Park, Forest Business
Park, Fairways Business Park, filter beds south of the Lee Bridge at Hackney Marsh.
Since 1947 areas that were inundated by flood waters have been developed. This is
most noticeable between the Stonebridge Brook junction with the Lee Navigation
(Lower) and where the Pymmes Brook meets the Salmons Brook.
The flooding in 1947 led to the construction of the River Lee Flood Relief Channel,
which became operational in 1976. There has been no major flooding in this region
since then, although the flood relief channel almost reached full capacity in 1987,
1993 and 2000.
5.3.1.2
December 2000 Floods
In October 2000, flooding occurred at the confluence between Salmons Brook and
Sadlers Mill Stream, and along a reach of the Eastern Flood Channel at Douglas
Eyre playing fields and in Walthamstow Marshes nature reserve. The most severe
flood event occurred when the Montagu Road culvert became overloaded. All of the
flood locations are within flood risk zone 3.
5.3.1.3
Silk Stream Floods
The Silk Stream flooded twice in the 1990’s, once in 1992 and once in 1999. The
trigger to these flood events is not known, however, the lead time of the
watercourses would indicate that they are susceptible to flooding caused by short,
intense rainfall events typical of summer storms.
5.3.1.4
Other Localised Fluvial Flooding
Other smaller more localised flood events have occurred since 1947. Most of these
flood events took place in the summer as a result of intense rainfall events. All of the
historic flooding is located within flood risk zones and in some instances (where no
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hydraulic study has been undertaken) it is evident that the flood risk zones have
been derived from the historic flood events.
5.3.1.5
Haringey Council Flood Management Strategy Report Review
The Borough of Haringey commissioned a report to consider flood risk from a
spectrum of sources in their borough and produce appropriate strategies for the
management of flood risk. The report identifies the council as the riparian owner on
much of the Moselle Brook meaning that they have a greater responsibility than
realised prior to this report.
A number of flooding incidents were reported, the most significant of which was
flooding occurring in July 1965. The area around White Hart Lane appeared to be
most affected including the White Hart Lane Station and properties on The
Roundway. Montagu Road flooded during this event and is also reported to have
flooded in 1956. White Hart Lane is also reported to have flooded in 1993 with the
Wedges Yard Trash Screen a particular flood hazard.
An outline analysis of the affects of flooding on transportation suggested that rail and
tube lines are unlikely to be flooded but may be affected if the embankments are
eroded or washed away.
It was concluded that Haringey was at risk from fluvial, sewer and pluvial flooding but
not groundwater or tidal flooding. Several strategies were put forward that included
the improvement of flood risk mapping, emergency planning and development
planning.
The Haringey flood locations are represented on maps 10, 13 and 20 in appendix A.
5.3.2
Tidal Flooding
Tidal flooding can also occur as a result of overtopping or breaching of flood
defences, wave action or where tide-locking causes ponding of fluvial or surface
water flows.
Parts of the south of Waltham Forest and Hackney are located within the tidal reach
of the River Thames, which extends up the Lower Lee Valley to the Lee Bridge
sluices. The flood levels in the River Lee can therefore be affected by tidal
processes as very extreme tidal events in the Thames could result in flooding of
these areas. The areas are defended against such flooding with local flood defences
and crucially, the Thames Barrier, which currently provides tidal flooding protection in
excess of 0.1% annual probability flood event. The Thames Barrier combined with
local flood defences means that any extreme tide level would have to be
accompanied by a breach in flood defence to result in severe flooding. Localised
flooding could still occur where surface water outfalls become tide-locked, causing
short term ponding behind defences.
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5.3.3
Flood Risk Modelling and Mapping
5.3.3.1
Flood Mapping Overview
Section 105 of the Water Resources Act, 1991 required the Environment Agency to
produce a series of maps showing the extent of areas at risk of flooding from tidal or
fluvial sources. These maps have subsequently been modified and released under
various guises. The latest incarnation of these maps is the flood zone mapping which
have been provided to Local Planning Authorities in electronic format for use as a
spatial planning tool.
The zones defined by the flood risk maps are those areas considered to be at risk
from annual fluvial or tidal flooding with a probability of 0.1% or more. The flood risk
maps generally provide a broad high level assessment of flooding. The level of
accuracy does vary from catchment to catchment. The Brent and Lee outlines are of
good quality from detailed studies. It is important to note that there are a small
number of models in the flood maps do not account for structures within the flood
plains such as bridges and culverts. In addition, most of the models do not take
climate change into account but often a separate climate change outline is available.
Flood Zones show the undefended flood extents but the EA do have defended
outlines available. Both the River Brent and River Lee studies have mapped the
Areas Benefiting from Defences. The flood risk maps only provide an indication of
flood risk from fluvial or tidal sources. The zones are consistent with the approach to
flood risk assessment set out in PPS25. These maps form the basis of the initial
flood risk assessment, however for the purposes of applying the sequential test and
exception test additional assessment may be required in some catchments. This is
discussed further in Section 0.
5.3.3.2
Review of Modelling Techniques
The flood zones in North London are generally produced using either ISIS or JFLOW
hydraulic models. The ISIS models were constructed to update the existing simplified
steady state models. The models were improved using a 1D hydrodynamic approach
that is supplemented by 2D TUFLOW modelling. The JFLOW modelling technique
uses a 2D wave equation to determine flood levels over short discreet sections of
watercourse, using automated hydrology calculations. The model is based on a
national Digital Elevation Model (DEM) with a 0.5m – 1m accuracy. JFLOW
modelling provides a fairly quick an invaluable method of determining flood maps
where more accurate hydraulic modelling is not available. While the JFLOW
modelling may predict flood plains and allow some degree of spatial planning to take
place the accuracy is insufficient for assessing site specific flood risk. The accuracy
of flood risk mapping from the JFLOW modelling can be significantly lower than that
produced from ISIS modelling and a review of the flood risk zones produced using
this method is discussed in section 0.
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5.3.3.3
River Lee Modelling and Mapping
The River Lee Modelling and Mapping Study was undertaken to improve the
accuracy of flood zone 2 and 3 outlines for the EA’s flood map while identifying the
areas that benefit from existing flood defences. The modelled area extends north
and south out of the study area along the Lower River Lee providing a complete
picture of flood risk in the Lower Lee Valley.
The Lower River Lee and tributaries flood outlines were modelled using ISIS and
TUFLOW by Halcrow for the 1 in 5, 20, 100, 100 + 20% and 1000 year return
periods. The return periods of 1 in 2, 10, 25, 50 and 200 year events were modelled
but the outlines were not mapped.
The recent ‘Lee Flood Risk Mapping Study’ by Halcrow remodelled the Cobbins
Brook, Nazeing Brook and Boundary Ditch in ISIS and TUFLOW produced ‘notable
improvements’ to the flood risk mapping.
The ISIS modelling of the River Lee catchment predominantly covers the urban
areas. However there are two reaches of watercourses in the urban area that have
not been modelled in ISIS which are the upper reaches of the Hounsden Gutter and
the upper reach of Pymmes Brook. The flood risk zones for these stretches of
watercourse are defined using JFLOW modelling.
5.3.3.4
River Brent Modelling and Mapping
The River Brent and its associated watercourses have recently been modelled and
mapped using a new ISIS study. This revised mapping has been incorporated into
this SFRA. This study covers Dollis Brook, Folly Brook, Mutton Brook and Deans
Brook. The Silk Stream was modelled and mapped in a separate study.
5.3.4
Flood Zone 3b - Functional Floodplain
The Flood Zone 3b shows the 1 in 20 year flood extent known as the Functional
Floodplain. This area as stated by PPS25 is suitable for water-compatible uses and
essential infrastructure as described in table 2. The Flood Zone 3b has been
obtained from the Environment Agency as a GIS layer from the more recent
modelling studies in the River Lee and River Brent Valleys as shown on map 8.
5.3.4.1
Lower Lee Valley Functional Floodplain
In the Lower Lee Valley, the 1 in 20 year extent is present mostly in open spaces.
However there are some developed areas within flood zone 3b (see Table 11
below). The River Lee Flood Relief Channel was built to provide a standard of
protection up to a 1 in 70 year flood event, which means that most of the man made
channels contain the 1 in 20 year event. There are two areas; one area to the north
of the River Ching confluence with the River Lee and one area to the west of Enfield
Island, that are indicated to be within the 1 in 20 year floodplain.
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Table 11 - Approximate Number of Properties in Flood Zone 3b in Lee Valley
Watercourse
Number of Properties
Salmons Brook
4
Pymmes Brook
74
River Ching
130
River Lee
78
Bow Back River
84
A large number of existing properties and their associated land appear within the 1 in
20 year flood plain on the tributaries of the River Ching, Pymmes Brook and Bow
Back River. The remaining area of flood zone 3b along these watercourses is mostly
in the undeveloped floodplain of the river corridor.
5.3.4.2
Upper Brent Valley Functional Floodplain
The Silk Stream mapping for Flood Zone 3b shows the flood outline post scheme for
a 1 in 25 year event from the 2006 Halcrow study. On the upper reaches, flood
alleviation work has taken place to protect properties to a minimum standard of 1 in
20 years.
On the Dollis Brook, Mutton Brook and the River Brent a more substantial 1 in 20
year flood envelop is present mostly in the narrow undeveloped river corridor.
Approximately 40 properties are already present within flood zone 3b.
The functional floodplain also includes areas which are designed to flood. The Silk
Stream FAS are located within the Upper Brent catchment. This is discussed further
in section 5.3.5.1.
5.3.5
Flood Defences and Inline Structures
The majority of the flood defences in North London are located along the Lower Lee
and are characterised by heavily engineered channels and hard engineered
defences. This section describes the significant flood defences and culverts within
the catchment including a discussion on the standard of protection they offer and
their general condition. Flood defences along the main rivers in the study area are
inspected on an annual basis by the EA. The condition of these defences is graded
between Poor (5) and excellent (1). Maps 14, 15, 16, 17, 18 and 19 show the
location and condition grade of the defences across the study area along with the
type of defence.
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The River Lee has been altered within the Lower Lee Valley in order to manage flood
risk and prevent a reoccurrence of the 1947 floods, whilst encouraging development
in the area. The Lee New Cut and Eastern Flood Channel are both artificial channels
designed to take flood waters away from the main river. The main river itself forms
the Lee Navigation that since the River Lee Act 1766 has been operated like a canal
that runs from Hertford to the Thames. The Act authorised improvement works, the
construction of locks and new channel sections which include the Limehouse Cut, a
connecting canal at the southern end of the watercourse.
River Lee flood relief channel and associated structures were designed to have a
standard of protection of approximately 1 in 70 return period. However, urbanisation
of the upstream catchment has significantly reduced this standard of protection in
some locations. Overall the standard of protection of the defences is generally above
2% (1 in 50) annual probability, but is as low as 5% (1 in 20) in some areas. In some
locations the standard of protection is greater than 1% (1 in 100) annual probability.
It is intended to produce a map highlighting the areas with defences which have a
low standard of protection, however, identifying the exact standard of protection of
each defence, and therefore the areas of highest risk has not been entirely possible
due to the limited data in the NFCDD. Further information on flood defence levels will
be required to enable these areas to be mapped with any degree of accuracy. Where
possible the standard of protection by locality is described below. Although there is a
lack of standard of protection information for the catchment, the Lee and Brent
mapping studies produced defended outlines which would give an idea of risk.
The flood defences on the fluvial reach of the Lower Lee through Hackney affords a
high standard of protection (greater than 1 in 100 years (1%), with the exception of
properties along Mandeville, Oswald and Pedro Streets. Other open spaces in this
area also have a lower standard of protection.
Properties along the Lower Lee through Waltham Forest appear to be protected to a
standard lower than 1 in 100 years (1%), in particular properties near the confluence
with the Ching Brook, from Uplands Business Park to Douglas Eyre Playing Fields
including Eward Road, the area between Coppermill Lane and Salop Road and from
St James Park down to Roxwell Trading Centre, including Cromwell Estates and
residential properties near Lee Bridge Road. Other areas with low standards of
protection include Walthamstow Marshes and the railway sidings opposite Hackney
marsh.
The defences along the River Lee are estimated to have a residual life of 50 to 60
years. While most of these assets are assessed as being in good to fair condition,
several reaches of the Lee tributaries are in a wider variety of conditions with some
sections being in poor condition. These are listed in Table 12 below.
The Housden Gutter, Pymmes Brook, Moselle Brook, Interceptor Drain, Saddlers
Stream, The Ching and Bounds Green Brook all have associated flood defences.
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These defences are a mixture of culverts, embankments, piling and channel
reinforcement. The standard of protection data provided by the EA from NFCDD
shows that 1.7% of recorded flood defences have a known level of protection.
In the Olympics area of the River Lee further work is being conducted to improve the
watercourses. Some of the proposed watercourse structures are having additional
work undertaken on them. This includes the setting back of flood defences on
Waterworks River, the in-filling of Pudding Mill River, the culverting and extending of
Hennikers Ditch and inlet improvements.
Currently British Waterways (BW) is building structures on the Prescott Channel and
Three Mills Channel that will prevent tidal ingress up these channels (currently the
River Lee is tidal up to Lea Bridge Sluices at Lea Bridge Road). The intention will be
to prevent tidal ingress and maintain a roughly constant water level (approx
2.3mAOD) upstream of the structures which will enable 24hr navigation of the
Prescott, Three Mills and Waterwork rivers, and allow for boat movements between
these channel and the rest of the navigation channels (Old River Lea, City Mill River,
Lee Navigation, Limehouse Cut). River flows coming down the Lee catchment will
discharge through the structures when tide levels permit (by lowering of gates at
Prescott Channel).
When tide levels are high there will be no fluvial discharge through the structures
and water levels will build up until the tide falls. The design and operation of the
structures will be such that there should be no increase in flood risk as a result of the
structures, either through the impounded section or on tributaries that discharge into
the impounded section. all structures should be complete and fully automated by
October 2008.
Table 12 – River Lee Tributaries Flood Defence Grades 4 and 5
Asset Reference
Grade
Watercourse
0625353BO0101R02
4
Bounds Green Brook
0625353CH0101R03
5
River Ching
0625353CH0101R04
4
River Ching
0625353CH0101R10
5
River Ching
0625353CH0102R03
5
River Ching
0625353DA0101R02
5
Dagenham Brook
0625353DA0103R05
4
Dagenham Brook
0625353MB0103R02
4
Moselle Brook
0625353PY0103R02
5
Pymmes Brook
0625353PY0202L03
4
Pymmes Brook
0625353PY0202R03
5
Pymmes Brook
0625353PY0304R05
4
Pymmes Brook
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0625353SA0103R02
5
Salmons Brook
0625353TU0202R02
4
Turkey Brook
0625353BO0102L02
5
Bounds Green Brook
0625353CH0103R06
4
River Ching
0625353TU0101L05
4
Turkey Brook
The River Brent tributaries are less heavily defended. The defences in the Upper
River Brent catchment are estimated to have a residual life of 11 to 20 years. The
Silk Stream has a greater number of defences that vary between fair and poor
condition while the defences on the Dollis Brook range from good to poor. Table 13
shows the defences in poor condition. The proposed Silk Stream flood alleviation
scheme should improve flood defences in this region to between 1 in 20 and 1 in 25
standard of protection.
Table 13 - River Brent Tributaries Flood Defence Grades 4 and 5
Asset Reference
Grade
Watercourse
0623838MU0102R02
4
Mutton Brook
0623838WD0101R03
4
Watling Ditch
0623838DE0104R02
4
Deans Brook
0623838SC0101R02
4
Springwood Crescent Drain
0623838HD0101R05
4
Hendon Cemetery Drain
0623838HD0101R03
4
Hendon Cemetery Drain
0623838BF0101R03
4
Broomfield Ditch
0623838BF0101R05
4
Broomfield Ditch
0623838BF0101R07
4
Broomfield Ditch
0623838HD0101R07
4
Hendon Cemetery Drain
0623838EB0101R02
4
Edwarebury Brook
0623838EB0101R04
4
Edwarebury Brook
0623838EB0101R06
4
Edwarebury Brook
0623838ED0101R03
4
Edgeware Brook
0623838ED0101L03
4
Edgeware Brook
Informal flood defences are structures or man made objects which unintentionally act
as flood defences. Examples of these would include railway embankments across a
flood plain, buildings that block the flow path of the water and changes in ground
profiles which may provide flood storage.
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5.3.5.1
Silk Stream Flood Alleviation Scheme
Along the Silk Stream in Barnet the EA is currently constructing a £1.8 Million Flood
alleviation Scheme to protect 746 properties.
The project began in 1992 after 293 properties were flooded in Edgware. Once the
scheme is finished, it will comprise of a series of 6 storage reservoirs that will provide
a standard of protection of between 1 in 20 to 1 in 25 years. A higher standard of
protection has not been provided because a higher protection scheme did not meet
the required DEFRA cost benefit criteria. The primary barrier to an improved scheme
is the difficulty in finding larger sites for flood storage.
The four flood storage areas that are constructed are at Prince Edward Playing
Fields, RAF Stanmore, Summerhouse Lakes and Bentley Heath. While the
Edwarebury Park and Bury Farm storage areas are currently in construction.
5.3.5.2
Salmons Brook Flood Alleviation Scheme
In October 2000, 192 properties were flooded when the complex culvert
arrangement at the confluence of Sadlers Mill Stream and Salmons Brook reached
full capacity due to heavy rainfall. The arrangement at the Montagu Road culvert
prevented Sadlers Mill Stream from flowing into the Salmons Brook. The 2000
floods lead to the development of the proposed Salmons Brook Flood Alleviation
Scheme.
The scheme is designed to protect 1700 properties against a minimum 1 in 75 year
flood event. Amended designs will go to public consultation in late 2007 with
construction planned for 2009-10, subject to funding and land agreements.
The scheme consists of improvement works at four locations in Enfield. These are
the creation of new food storage areas at Hog Hill and Enfield Golf Course upstream
of the previously affected properties. The works at Montagu Road Area consist of a
bypass channel from Salmons Brook to Pymmes Brook, the raising of banks, culvert
improvement works and environmental enhancement. While in the Eleys Industrial
Estate in Edmonton the river banks will be raised.
5.3.6
Overtopping
Overtopping of flood defences occurs when water levels exceed the protection level
of raised flood defences. The worst case occurs when the fluvial or tidal levels
exceed the defence level as this can lead to prolonged flooding. Less severe
overtopping can occur when flood levels are below defence levels, but wave action
causes cyclic overtopping, with intermittent discharge over the crest level of the
defence. Flood defences are commonly designed with a freeboard to provide
protection against overtopping from waves.
The risk from overtopping due to exceedance of the flood defence level is much
more significant than the risk posed by wave overtopping. Exceedance of the flood
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defence level can lead to prolonged and rapid flooding with properties immediately
behind the defences at highest risk.
5.3.7
Breach
Breaching of flood defences can cause rapid inundation of areas behind flood
defences as flow in the river channel discharges through the breach. A breach can
occur with little or no warning, although they are much more likely to concur with
extreme river levels or tides when the stresses on flood defences are highest. Flood
water flowing through a breach will normally discharge at a high velocity, rapidly
filling up the areas behind the defences, resulting in significant damage to buildings
and a high risk of loss of life.
Breaches are most likely to occur in soft defences such as earth embankments
although poorly maintained hard defences can also be a potential source of breach.
The condition grading of the North London flood defences has been obtained from
the National Fluvial and Coastal Defence Database (NFCDD) which is maintained by
the EA.
5.4
Groundwater Flooding
It is estimated that groundwater flooding affects a few hundred thousand properties
in the UK13. Groundwater flooding most commonly occurs in low lying areas which
are underlain by permeable rocks or aquifers. Flooding occurs when the
groundwater table rises up from the permeable rocks to the ground surface, flooding
low lying areas or occurring as intermittent springs. Flooding is most likely to occur
after prolonged periods of rainfall when a greater volume of rain will percolate into
the ground, causing the groundwater table to rise above its usual level. Low lying
areas are generally more prone to groundwater flooding because the water table is
usually at a much shallower depth and groundwater flow paths tend to travel in a
direction from high to low ground. Areas prone to groundwater flooding also often
experience surface water flooding problems.
Groundwater flooding occurs much more slowly than other forms of flooding and the
risks to people are generally low, however, the slow onset of groundwater flooding is
mirrored by the time that flood water can take to dissipate back into the ground
unless there is an alternative flow path for the flood water.
Localised groundwater flooding can also occur around specific geological features,
such as areas of permeable soils overlying impermeable strata. Such features where
they occur in North London are discussed in section 5.4.1.
13
UK Groundwater Forum, http://groundwateruk.org/html/faq3.htm
Strategic Flood Risk Assessment
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Final - August 2008
67
5.4.1
Geology
North London is almost entirely underlain by the London Clay formation which
overlays a significant chalk aquifer. The London Clay layer varies in thickness from
less than 10m near the Lee Valley to over 100m in the areas of higher ground in
Camden and Barnet. The clay layer is almost entirely impermeable which has a
considerable impact on lead times of fluvial flows in many of the watercourses,
especially when combined with intense urban development. The upstream
catchment in the River Lee comprises a predominantly chalk soil, which results in
increased permeability and slower response times in the watercourse.
In places the London Clay layer is overlain by deposits of gravels and silts. This is
most prominent in the Lee Valley and East of Hackney where alluvium deposits from
the River Lee are in evidence. There are also notable outcrops of gravels and silts
further to the west in Enfield, Stanmore gravels in Barnet and gravel outcrops on
Hampstead Heath. These gravel and silt deposits are much more permeable than
the underlying clay layer and flooding can occur at the edges of these deposits and
outcrops when the groundwater percolating through the permeable layer meets the
impermeable clay layer, causing the water to flow out at surface level, appearing as
small springs. The locations of the most prominent of these geological features are
discussed below.
The Lee Valley consists of a layer of gravels and silts deposited by the river within its
natural flood plain that covers a layer of London Clay. Silts and gravels are also
found in smaller quantities along the flood plains of other main rivers in the area,
which include the River Brent, Dollis Brook, Silk Stream, Salmons Brook, Pymmes
Brook and the River Ching. The Lee Valley is the lowest lying area within North
London and is therefore susceptible to groundwater flooding. Groundwater levels in
shallow deposits in the Lee Valley are hydraulically linked to the watercourses
through the alluvium deposits and may be responsive to rainfall events and the
corresponding increases in fluvial flows. However for the major aquifer, the chalk,
this is not always true. During prolonged rainfall events the groundwater table may
experience a relatively short term rise which could cause localised flooding incidents.
Further to the west of the Lee Valley, within Enfield there are a number of drift
deposits of silts and sands. A small number of groundwater flooding incidents are
known to have occurred in the vicinity of these deposits. Groundwater flooding
history is discussed further in the following section.
Hampstead Heath lies on a silty sand layer on top of the London clay. During rainfall
events water drains through the sands before reaching the impermeable layer
beneath, causing the formation of springs which feed the Highgate Ponds and form
the source of the River Fleet.
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68
In the Finchley and Hendon area to the north of Hampstead, a Till, chalky sandy clay
and gravel outcrop lies on the surface that may lead to a localised groundwater
flooding, although there is no history of groundwater flooding in this location.
5.4.2
Groundwater Flooding
Very few groundwater flooding records have been provided by the EA and all of
those that are recorded lie within the London Borough of Enfield. The locations of the
flooding incidents are shown on map 20.
Almost all of the recorded incidents occur within the upland areas of the Borough in
the vicinity of the drift deposits which overlay the impermeable London Clay. A small
cluster of flood events also occur further west within the Borough. These three
events are located on the down slope from an outcrop of Stanmore Gravels, which
may account for these localised flooding incidents.
A small number of flooding incidents are located in the Lee Valley to the north of the
Borough. These flooding incidents are most likely to be attributed to the low lying
areas where the groundwater table is relatively close to the surface.
Identifying the cause of each of these flood incidents is outside the scope of this
report, however, it does identify a number of flooding trends which should be
considered at a site specific stage.
5.4.3
Borehole Data and Groundwater Contour Data
The EA has supplied a series of borehole records which provide a snapshot of the
groundwater levels across the study area on recorded date of February 2007. The
borehole data matches the groundwater levels provided, validating in places the
contours provided. The boreholes are sparsely located across the study area,
especially in the larger boroughs of Barnet and Enfield. The majority of the borehole
data is situated at the lower end of the Lee catchment in Waltham Forest, Hackney
and further south of the study area. Boreholes in the Brent catchment are very
limited.
The groundwater contours representing depth below ground are shown on map 12.
These groundwater levels are for the chalk aquifer underlying the confining clay,
which are not necessarily the level of the groundwater beneath any particular
location and relate to the peizometric pressure of the groundwater at that point. This
means, the water level in the borehole would rise up to the stated value due to the
pressure under the confining clay, it does not mean that the water in the ground is at
that level. Therefore, flooding from the chalk aquifer is not that likely in areas where
the chalk is confined by the clay even though groundwater levels may read that
groundwater is only a few meters below ground level.
The groundwater profile through London shows relatively little change in elevation,
however, the topography of the North London sub-region shows significant variation,
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69
with a general fall in an easterly direction from the higher ground in Barnet to the Lee
Valley, where much of the areas is only a few metres above sea level. As expected,
groundwater levels are closest to the surface around watercourses, particularly in the
low lying Lee Valley. The groundwater levels in the Lee catchment are significantly
closer by approximately 30m to the surface than within the Brent catchment. A
region of higher ground between river catchments stretches from central Camden
and up through central Barnet to the north west of the overall assessment region.
Throughout this region the groundwater levels are at a depth of 80-90m below the
surface.
The borehole data provided records the depth of the groundwater table in the chalk
aquifer. It does not record the intermittent groundwater levels which occur where the
gravel and silt deposits overlay the London Clay layer.
5.4.4
GARDIT
During the 19th and 20th centuries significant volumes of groundwater were
abstracted from the deep aquifer below London to support the industries prevalent at
the time. This intensive abstraction of groundwater had a dramatic impact on the
groundwater levels. As these industries began to decline the volume of water
extracted correspondingly reduced and groundwater levels began to rise back to
their natural levels. The decline in abstraction began in the 1940’s and from around
the 1970’s onwards groundwater levels began to rise dramatically, at more than 1m /
year14 in many areas. Building foundations and infrastructure which had previously
been designed with little regard for groundwater began to be at risk as the water
table began returning to its natural level. It is estimated that during the period of
heavy abstraction groundwater levels may have lowered by as much as 90m15, and a
corresponding rise could cause a substantial flood risk to critical infrastructure and
basements, while also causing instability to building foundations.
In 1992 GARDIT (General Aquifer Research, Development and Investigation Team),
an umbrella organisation consisting of Thames Water, London Underground and the
EA, was formed to address the issue of rising groundwater. The GARDIT group
established a five stage solution to maintain groundwater levels at an acceptably
agreed level on an area by area basis. The solution involved reusing existing
boreholes and creating a series of new boreholes which would extract a total of
70Ml/day across London. The project was due to be fully implemented by 2005 and
as such groundwater levels are expected to stabilise to a manageable level.
14
CIRIA Special Publication 69 (1989) The Engineering Implications of rising groundwater
levels in the deep aquifer beneath London
15
Thames Water Central London Rising Groundwater
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70
The GARDIT report states that “as a result of the GARDIT strategy and the controls
imposed on central London, the whole of the London Basin Chalk aquifer is
becoming a highly managed entity”. Also included is that the ground water contours
“have barely changed compared to last year, so the groundwater level can now be
considered static.”
The monitored area consists of observation boreholes stretching from Hatfield in the
north to Epsom in the south, with Dagenham and Dartford on the eastern boundary
and Staines and Uxbridge to the west.
The continued increase in use of abstraction licences is demonstrated within the
report. It is intended that “substantial further increase in abstraction will control
groundwater levels at significantly lower levels than previously seen.”
Groundwater trends
The EA’s groundwater monitoring team have provided historic levels of a borehole
located within the City of London Boundary (other more relevant borehole data has
also been requested but not received) at Leith House, Gresham Street. A graph of
the boreholes recorded levels from 1990 to 2006 are presented in Figure 3. A plan
showing the location of the borehole is included in Appendix A, (Map No. 12).
Figure 3 - Leith House Borehole Level
Leith House Borehole level (mAOD)
-30
-35
Level (mAOD)
5.4.5
-40
-45
-50
-55
11/08/1987
07/05/1990
31/01/1993
28/10/1995
24/07/1998
19/04/2001
14/01/2004
10/10/2006
06/07/2009
Date
The borehole record shows the trend of rising in groundwater during the 1990’s with
an approximate rise of 2m/year occurring from 1990 to 1998. Since then levels have
stabilised below -35m AOD (Above Ordnance Datum) as a result of abstraction of
groundwater across London.
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North London
Final - August 2008
71
The actions of the GARDIT team will significantly reduce the future risk of
groundwater flooding in London providing the current abstraction rates are
maintained indefinitely. However, a residual risk of groundwater flooding could still
remain to some new developments or below ground infrastructure.
The impacts of the groundwater levels within the chalk aquifer on proposed
development and the planning process are not considered further in this report as
the overall risk of groundwater flooding is considered to be low given the current
stability in groundwater levels. However, groundwater flooding is still a potential
issue within some of the permeable soils which overlay the London Clay including
the River Lee Valley. Further discussion on this groundwater flooding problem is
discussed in Section 0.
5.5
5.5.1
Sewer and Surface Water Flooding
Sewer Flooding
Sewer and surface water flooding generally results in localised short term flooding
caused by intense rainfall events which overload the capacity of sewers or run off
adjacent land as sheet flow. Flooding can also occur as a result of blockage, poor
maintenance or structural failure. Sewer systems in London are often very old,
particularly within the Boroughs of Camden, Islington and Hackney. These older
sewers were sometimes designed to convey storms of relatively low return periods
such as a 1 in 10 year rainfall event. Even new surface water systems are designed
to a minimum standard of 1 in 30 years, much less than the 1 in 100 year standard of
protection expected from fluvial flooding. As a result sewer flooding events where
they occur can often be frequent, although the scale of consequence is generally
smaller than those associated with fluvial flooding. Some of the London sewer
network is a combined system with storm and foul drainage served by a single
sewer. This makes flash flood events particularly inconvenient and unpleasant as
floodwaters will often be contaminated with sewage.
The annual rainfall for the North London area is 640mm, somewhat less than the
national annual average of 897mm. Rainfall in London also experiences less
seasonal variation than other areas of the country, with the winter months
experiencing only marginally higher rainfall than summer months, however, the
rainfall in summer months will often occur in a smaller number of rainfall events
leading to intense rainfall peaks which can lead to flash flooding and overloading of
sewer systems. Climate change predictions indicate that these intense summer
storms will become more frequent. Over time the standard of protection of existing
sewers will reduce leading to an increase in localised flooding incidents.
Sewer flooding does not always respect the topography of a catchment and flooding
can just as easily occur at the head of a network as it can near to the outfall.
However, flood events occurring at the downstream end of a drainage system are
likely to be more severe due to the sheer volumes of flow involved. London’s sewer
network is generally protected from such large scale flooding by storm overflows
Strategic Flood Risk Assessment
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Final - August 2008
72
which discharge high storm flows from the sewer system into watercourses, thus
preventing flooding from the sewer network. In the event that an extreme fluvial
event coincided with heavy rainfall within the catchment, storm water would be
unable to discharge via storm overflows and would therefore surcharge within the
sewer network. This could result in significant sewer flooding problems. Without
access to hydraulic sewer models it is not possible to determine the extent of this
risk or the areas most likely to be affected. Naturally low lying areas such as the river
valleys and the areas immediately behind the locally raised flood defences could be
potential receptors of such a flood event.
The Pitt Review looked at sewer flooding and considered the current regulations that
allow the automatic right to connect surface water to sewers. The report
recommends that this right should be removed (Recommendation 10) to encourage
greater consideration of SUDS in the management of storm water runoff.
It is essential that any new development takes account of known sewer flooding
problems to ensure that the development is not put at risk and that the development
does not worsen an existing problem. Future development if not adequately planned
can increase the flood risk from sewer flooding and in some cases cause new flood
problems to occur. Potential increases in surface water or sewage discharge from
new development must be adequately managed and mitigation measures
introduced.
5.5.2
Thames Water Flood Database
Thames Water have provided an extract of their flood register for use in this study,
however, the data is referenced by truncated postcode only and therefore cannot
specifically identify a particular flooding problem. This is not a unique situation as
Water Authorities have in general been reluctant to release flooding data for use in
other SFRAs due to the sensitive nature of the information. The results of the
Thames Water data is referenced on map 13 in appendix A. Table 14 below shows
the number of flood records by Borough.
Table 14 - Thames Water Flooding Records by Borough
Borough
No. of Properties on Thames
Water Flooding Database
Barnet
Camden
Enfield
Hackney
Haringey
Islington
Waltham Forest
42
90
5
47
12
1
55
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Water Authorities are only required to maintain records of flood events which
occurred more frequently than once in ten years. This is primarily down to targets set
by OFWAT, the regulating body of the water industry. This inconsistent approach in
recording sewer flooding compared with the EA’s recording of fluvial flooding can
make it difficult to assess the risks and consequences of sewer flooding. It is
possible that a less frequent but substantial flood risk from sewers exists in the North
London, but without the requisite information these potential risks cannot always be
addressed by a pro-active approach, such as mitigation at the planning stage of a
new development. Instead such risks often have to be addressed through a
reactionary approach once a flood event had occurred.
It should be noted that the flood records provided by Thames Water may not be a
complete and accurate record of flood events in the boroughs over the last 10 years.
Some minor flooding incidents may go unreported, particularly if no property is
affected by such flooding.
The Thames Water records shows a reasonable correlation with other historic flood
records, including those of the Camden floods discussed later in this chapter. A
substantial number of the postcode zones include a small number of isolated
flooding incidents with no correlation to the adjacent postcode zones, however, there
are five locations which should be highlighted, either because they contain a higher
number of flooding incidents or there is a cluster of postcodes all containing flood
events. These locations are listed in Table 15 and account for approximately half of
the events on the Thames list.
Table 15 - Thames Water Flooding Records, Highlighted Areas
North London
Borough(s)
Affected
Location
Edgware and Cannon
Hill
Other London
Borough(s)
Affected
Comments
Flooding occurs around the
Barnet
Harrow
upper tributaries of the Silk
Stream
West Hampstead,
Cricklewood, South
Hampstead and west to
Barnet / Camden
Brent
Church End
Stamford Hill
Hackney
Leyton, Leytonstone
Waltham Forest
South Tottenham
Haringey
Strategic Flood Risk Assessment
North London
Final - August 2008
Flooding incidents located
south of Seven Sisters Road.
Flooding occurs around the
Stonebridge Brook Area
74
The scale and exact cause of the individual flooding problems is unknown and
further investigation of each incident is clearly outside of the both the timescale and
economic constraints of this study. However, the data has been combined with other
flood history data discussed below and used to identify those areas which may be
susceptible to surface water, this approach discussed in more detail in chapter 6.
5.5.3
Impermeable Surfaces and Urban Creep
Over the past couple of decades, there has been a growing trend of paving front
gardens in urban areas to provide additional car parking. This practise has lead to
urban creep that increases the impermeable area and the associated run off.
The Communities and Local Government website provides an Impact Assessment of
Permeable Surfaces (January 2008) which considers the problem of paving front
gardens further. This document provides evidence about the problem and reviews
the permitted development rights for permeable surfacing.
The Pitt Review also considers the growth of paving front gardens and made a
recommendation (Recommendation 9) that householders should no longer be able
to lay impermeable surfaces as a right on front gardens and that the government
should consult on whether to extend this to back gardens and business premises.
5.5.4
Basement Flooding
Flooding of basements of buildings is flooding of space below ground level. In the
mildest case this may involve seepage of small volumes through walls, temporary
loss of services. In more extreme cases larger volumes may lead to the catastrophic
loss of stored items and failure of structural integrity.
5.5.5
Other surface water flooding records
5.5.5.1
Barnet Flood Hot Spots
The London borough of Barnet has provided a list of flood locations within Barnet
over the last 2 years. The flood locations also include the associated reasons for
flooding. A substantial number of the flood events were attributed to maintenance of
gullies and this data has been removed from the data set.
The remaining recorded flood events are generally in close proximity to a major
watercourse. The large majority are in the west of the borough along stretches of
the Silk Stream and Edgware Brook, predominately along the western banks of the
watercourses. These incidents generally correlate with the Thames Water flood
records. The Edgware area appears particularly prone to surface water flooding
problems. Other significant flooding problems are identified in the Colindale area
and around the Silk Bridge in West Hendon.
5.5.5.2
Review Transport For London flood records
Transport for London is the authority that manage and maintain public transportation
within the M25. On request they have made their flooding records available for use in
Strategic Flood Risk Assessment
North London
Final - August 2008
75
this study. The records comprise an extract of 288 records covering the Greater
London area in regard to flood incidents from July 2006 to August 2007. These were
reduced to relevant 16 records that fall within the study area. Of these, 6 incidents
occurred in the summer and 3 are attributed to the same flood event.
The TFL data is a list of reported flood incidents on transport routes within the M25.
There are numerous events in south Barnet and south Hackney. Islington and
Camden have very few or no recorded TFL flood events.
Hackneys flood events are concentrated around Wick Road which appears to be in a
depression and accounts for 6 of the incidents. Those in Enfield are located on the
watercourses of Pymmes Brook, Bounds Green Ditch and Salmons Brook, either
within or very close to the flood plain. These events are likely to be attributed to the
interaction between the watercourse and highway or surface water drainage.
Waltham Forest has flood events along the A406 North Circular with the majority of
these being a reasonable distance from the River Lee and the Ching. They are
unrelated to any other flood records reviewed as part of this study. It may be
attributed to inadequacies with the local highway drainage.
The TFL flooding in Barnet was linked to watercourses in the east of the borough,
the Dollis Brook and the Mutton Brook.
5.5.5.3
Floods in Camden – Report of the Floods Scrutiny Panel
The report investigates the serious floods that occurred in some parts of the Borough
on the 7th August 2002. The flooding inflicted considerable damage on some
Camden residents and their homes, public services and facilities, and private
businesses.
High rainfalls levels and flood events are a recurring feature in Camden due to the
nature of summer thunderstorms and the topography of Hampstead. The report
suggests that the similarities between floods in 1975 and 2002 conclude that these
flood events have not been recently created by changes in the global climate.
The flood event on the 7th August 2002 was caused by excessive rainfall causing the
main sewer system to become completely inundated. The surcharge pressure forced
the water to back onto the streets through manholes and gully gratings and into
residents’ homes at basement and ground floor level. It was stated that “Any blocked
or otherwise deficient Camden Council highway gullies could not have caused
flooding on this scale” as the flood water could not drain to the trunk sewer.
This was supported by Thames Water’s evidence confirmed that the flooding was
caused by its sewer system reaching maximum capacity very quickly so that surface
water could not be drained at the rate as the rain fell.
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76
Following the flood event Thames Water is to make further funding cases to OFWAT
to relieve more properties from flooding and they indicated that flooding issues in
Camden will be picked up as part of their prioritisation programme.
The report suggests numerous recommendations to ensure the councils and key
agencies responses are improved including task groups, regular maintenance of
gullies and pipe networks and emergency response systems.
Map 22 shows the roads affected by flooding during the August 2002 floods. The
map shows a reasonable correlation between the Thames Water records although
the Camden floods would appear to be more widespread than is identified on
Thames records. This can be attributed to the fact that some of the properties
flooded in 2002 will not be included on the Thames Water database unless they
flood twice in ten years.
5.5.5.4
London Fire Brigade Flood Calls
The London Fire Brigade record all incoming calls including those about flooding.
Across the seven boroughs since April 1999 to August 2007, the London Fire
Brigade received 15,515 calls regarding flooding, all of which have been made
available for this study. Of those calls 247 calls were immediately discounted as they
had no locational information. The remaining calls were divided into two categories,
‘Water provision or removal, including flooding’ and ‘Flood calls’. The differentiation
between the two categories is related to how the Fire Brigade mange their service.
When a series of calls are received in a short period of time an immediate response
can not be sent to each incident. ‘Batch Mobilisation’ takes place when the
appliances deal with one call and go straight to the next call. This is likely to happen
if an intensive down pour occurred and localised flooding took place. The batch
mobilisation calls are denoted as ‘flood call’.
Due to the large number of incidents it was necessary to look at groups of incidents
rather than individual calls. The calls were sorted into date order and groups of 12 or
more events occurring within a 48 hour period were selected. 52 of these “flood
groups” were noted and the distribution of the calls across the seven boroughs was
examined. Rainfall data from the nearest rain gauge has then been reviewed in order
to determine whether the flooding incidents can be related to sewer flooding or
surface run-off. This assessment is discussed further in section 6.4.
5.6
5.6.1
Artificial Drainage Bodies
Reservoirs
Reservoirs as defined by the Reservoirs Act 1975 are bodies of water above natural
ground level that hold at least 25,000 cubic metres of water. Within the study area
there are 17 reservoirs that managed and maintained in accordance with the
Reservoir Act 1975. These are listed in Table 16 and shown on map 9. The
enforcement of this legislation is conducted by the Environment Agency, who
maintain the Public Reservoirs List.
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77
The Pitt Review considered Reservoirs and Dams in a separate chapter that
reviewed the existing situation and legislation. Recommendation 57 highlights the
need for inundation mapping and emergency management plans to be created for
both large and small reservoirs. Currently, the Reservoir Act would not cover all the
reservoirs present in the North London boroughs, yet these smaller reservoirs in a
densely populated area would still present a risk.
The largest of the reservoirs are in the Lower Lee Valley and most of these
reservoirs are owned and operated by Thames Water. The King George V and the
William Girling reservoirs have a combined capacity of nearly 30,000,000 m³. Both
these reservoirs are within the 1:1000 year floodplain. They are both nonimpounding reservoirs with raised embankments around them. The inflows to these
two reservoirs are pumped from the Eastern Flood Channel.
There are also two Flood Storage Reservoirs under construction by the EA, one on
Salmons Brook and the other at Stoney Wood in Barnet. These are also included in
the above list.
The New River (discussed further in section 5.6.3), whilst not included on the
reservoir list may also be considered in the same context as reservoir flooding.
Sections of the New River are elevated above ground level and a failure of the
embankment at these locations would result in a significant discharge of flow.
Table 16 - Reservoirs List
Reservoir
Location
Undertaker (Owner)
Category
Capacity
3
(m )
Banbury
Near Chingford
Thames Water Ltd
Non-impounding
2846000
Brent
Near Willesden
British Waterways
Impounding
1632330
East Warwick
Near Walthamstow
Thames Water Ltd
Non-impounding
905000
Grovelands Park Lake
Near Southgate
London Borough of Enfield
Impounding
40000
Highgate Pond No.2
Near Camden
Corporation of London
Impounding
36000
Highgate Pond No.3
Near Camden
Corporation of London
Impounding
46000
High Maynard
Near Walthamstow
Thames Water Ltd
Non-impounding
462000
Hornsey
Near Hornsey
Thames Water Ltd
Non-impounding
102000
King George V
Near Chingford
Thames Water Ltd
Non-impounding
13970000
Lockwood
Near Walthamstow
Thames Water Ltd
Non-impounding
1787000
Salmons Brook FAS
Near Edmonton
Environment Agency
Impounding
Stoke Newington (East)
Near Stoke Newington
Thames Water Ltd
Non-impounding
182000
Stoke Newington (West)
Near Stoke Newington
London Borough of Hackney
Non-impounding
227000
Stoney Wood FSA
Barnet
Environment Agency
Impounding
56000
Trent Park Lake
Near Enfield
London Borough of Enfield
Impounding
25000
Walthamstow No.4
Near Walthamstow
Thames Water Ltd
Non-impounding
591000
Not Listed
Walthamstow No.5
Near Walthamstow
Thames Water Ltd
Non-impounding
713000
West Warwick
Near Walthamstow
Thames Water Ltd
Non-impounding
805000
William Girling
Near Chingford
Thames Water Ltd
Non-impounding
16500000
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Final - August 2008
78
There are two types of reservoirs, impounding and non-impounding. Impounding
reservoirs are defined as those constructed by damming a watercourse to intercept
flows. Impounding reservoirs often pose the greatest flood risk as inflows to the
reservoir are generally uncontrolled and dams are usually elevated above the
downstream valley. An uncontrolled release of water from an impounding reservoir
can result in catastrophic flooding leading to loss of life. The most serious cause of
flooding would be attributed to a dam breach.
The responsibility for managing flood risk from reservoirs lies with the owners of
each reservoir, while the EA have a duty to ensure that this responsibility is
enforced. Through this enforced process of regular inspection and maintenance the
potential risk of catastrophic reservoir failure is managed such that the probability of
occurrence is low.
The Water Act 2003 requires that Reservoir Flood Plans be produced for certain
reservoirs. Defra is in the process of producing a ‘Guide to Emergency Planning for
UK Reservoirs’, which will go out to consultation in Summer 2008. They are
expected to become a legal requirement in Spring 2009 when the Secretary of State
in England will direct undertakers to produce flood plans for reservoirs where failure
could have a major impact. It is not yet clear what criteria will be used to determine
which reservoirs require Flood Plans but this information would be provided in future
versions of this SFRA.
Other reservoirs, not covered by the Reservoirs Act are also present within the North
London study area. Many of these will be clean water supply reservoirs, which are
covered. This study does not address the risk associated with these reservoirs,
predominantly due to the relatively small size, combined with the low risk associated
with the type of failure that would be required to cause a flooding incident. The
location of these reservoirs is also considered sensitive and it would not be
appropriate to map the locations of such assets.
5.6.2
Canals
Regents Canal was constructed in 1820 to form the London arm of the Grand Union
Canal. The Grand Union Canal came into being in January 1929 from an
amalgamation of several different canals and was later extended in 1932. It was
formed from the amalgamation of 11 different canals connecting London to
Birmingham, stretching 135 miles and has 160 locks.
The Regents Canal stretches from Limehouse basin in Docklands to Paddington
passing through Mile End, Hackney, Islington, King’s Cross, Camden, Regents Park
and Little Venice. Within this reach of the Regents Canal there are three tunnels;
Islington Tunnel (886 meters) and Maida Hill Tunnel (251 meters) and the smaller
Eyre'
s tunnel (48 meters).
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79
Currently, the Camden lock system apparently holds a head of water of some 25
miles. The risks associated with the canal were sufficient that in the WWII war
barriers were closed across the canal during air raids. These barriers are now
unserviceable. In the past, British Waterway have commented that damage to canal
walls from developer is quite common.
5.6.3
New River
The New River is a water supply aqueduct, constructed in 1613 to supply drinking
water to London from Ware in Hertfordshire. The New River is the owned and
operated by Thames Water. It is located to the west of the Lee Valley, and runs north
to south, parallel to the path of the River Lee. The original route and size of the
aqueduct has been altered to increase carrying capacity as demand increased. The
New River’s carrying capacity is now up to 220 megalitres (48 million gallons) per
day. The length however has been shortened to 38km (24 miles), ending at storage
reservoirs in Stoke Newington, northwest Hackney.
The New River water level is regulated by sluice gates, designed to allow flow to
meet the requirements of the pumping stations and reservoirs. The Artificial
Recharge Scheme consists of a series of boreholes which enable surplus treated
water to be stored in the chalk aquifer and then pumped into the New River as
required.
The New River contains many sections which are raised above the existing ground
level. These include the crossing over the M25 close to junction 25 and several
sections within residential areas (see map 9 for its route). Should any of these raised
sections fail they could pose a significant flood risk to adjacent properties. The
inspection and maintenance of these raised sections of the New River is therefore
critical.
5.7
Infrastructure Failure
Infrastructure Failure can be generally be associated with the following sources,
Reservoir, Canals, Burst Water Mains, Blocked sewers or failed pumping stations. In
extreme cases infrastructure failure has the potential to release large volumes of
water at high velocities with little warning time. This makes flooding from
infrastructure very different from other types of flooding. Review of Infrastructure
failure at a strategic level is generally hampered by limited or sensitive information,
which cannot be made available.
Infrastructure failure causing flooding can be linked to the following three categories
of infrastructure.
5.7.1
Water Infrastructure
Water artificially contained in large water bodies such as reservoirs and canals
contain a significant risk. CIRIA C542 Risk Management for UK Reservoirs provides
guidance on the application of risk assessment and risk management procedures to
Strategic Flood Risk Assessment
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Final - August 2008
80
UK reservoir practice. The main reservoirs and water bodies are primarily located in
the Lower Lee Valley as shown in Map 9.
Canals are at risk of overtopping and breach in a similar manner to watercourses.
Canals can breach at any point making risk assessments a difficult proposition. The
Grand Union Canal and New River are the two main artificial watercourses within the
study area. Others include the Intercepting Drain and Eastern Flood Channel, all
located within the Lower Lee Valley.
5.7.2
Flood Defence Infrastructure
Flood defence infrastructure includes flood defences, flap valves, pumping stations,
culverts and any other structure which may help to alleviate flooding. To be able to
perform a risk assessment it is necessary to understand the reasons for the defence,
its conditions and any potential failure mechanisms, as well as the consequences of
any infrastructure failure.
5.7.3
Bridge and Culvert Blockage
Blockages of this nature can cause, be caused by and magnify flooding.
Unprotected culverts may become blocked whilst debris and security screens
obscured with debris restrict flow. This is often made amplified during floods due to
the increased amount of debris carried in the flow. The large quantity of culverts in
such an urbanised area increases the likelihood of blockages but it cannot be
predicted.
5.7.4
Overview
Risks of this infrastructure failure need to be assessed in a site specific flood risk
assessment and assessments into the condition of the infrastructure may be
required.
5.8
Source Pathway Receptor Model
The seven North London boroughs form a large area with each borough affected by
a different range of flood sources. The strategic level at which this report is targeted
permits only a broad level of assessment of flood risk and the timescales required by
the North London Boroughs do not allow a detailed assessment of all flood risks. In
order to focus the SFRA it is necessary to identify sources of flooding with the largest
potential consequences. To this end a source pathway receptor model has been
created in Table 17. The model identifies sources of flooding while approximately
assessing the potential scale of consequences of such flooding. The consequences
of each flood source are described on a Borough by Borough basis.
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81
Table 17 - Source Pathway Receptor Model
Source
Fluvial Flooding from
the River Lee and
Tributaries
Fluvial Flooding from
the River Brent
Tributaries
Tidal Flooding from
the River Lee
Surface Water /
Combined Sewer
Flooding / Overland
Flow
Groundwater
Flooding
Pathway
Inundation of
floodplains /
overtopping of flood
defences / breaching of
flood defences
Inundation of
floodplains /
overtopping of flood
defences / breaching of
flood defences
Overtopping of flood
defences / breaching of
flood defences / wave
Action / ponding of
surface water behind
defences
Exceedance of sewer
capacity
Blockage of Pipework
Potential Scale of Consequence
Receptor
Properties and
infrastructure within
the defined flood
plain and in locality of
Pathway
Properties and
infrastructure within
the defined flood
plain and in locality of
Pathway
Properties and
infrastructure within
the defined flood
plain and in locality of
Pathway
Properties and
infrastructure in the
locality of Pathway
Assessment in SFRA
Barnet
Camden
Enfield
Hackney
Haringey
Islington
Waltham Forest
Medium
N/A
Large
Medium
Large
N/A
Large
Assessment of fluvial flood levels, flood
risk zoning and defence conditions.
Potentially large
consequence and risk of loss
of life
Large
N/A
N/A
N/A
N/A
N/A
N/A
Assessment of fluvial flood levels, flood
risk zoning and defence conditions.
Potentially large
consequence and risk of loss
of life
Medium
Low risk due to presence of local defences
and Thames Barrier. Review of condition
of flood defences to be undertaken.
Potentially large
consequences and risk of
loss of life. Low probability of
occurrence due to flood
defences including the
Thames Barrier.
Small
Moderate flooding possible some further
assessment of flood risk required. Study is
limited by the information provided
Thames water have provided
limited information on flood
history. London Fire Brigade
and Borough Council
information on flood history
Very Small
Groundwater levels obtained from EA.
Comparison with ground levels and
research into future trends in groundwater
levels
Limited information on which
to base assessment.
Potential for flood risk to
infrastructure.
Large
Limited information available but low
likelihood for flooding to occur due to
enforced management of risks. Reservoir
flood inundation plans are in the process
of being produced as required by changes
in legislation.
Limited information available
on which to base assessment
N/A
Small
N/A
Medium
N/A
Very Small
Medium
Small
N/A
Very Small
N/A
Small
Tidelocking of outfalls
Rising Groundwater
Development in low
lying areas or
developments with
basements
Comment
Very Small
Very Small
Very Small
Very Small
Very Small
Very Small
Infrastructure, Tube
Stations
Infrastructure Failure
from Reservoirs /
Canals (including
the New River) /
blocked Sewers /
burst water mains
Overtopping of reservoir
embankments
Breach of reservoir or
canal embankments
Outlet failure
Strategic Flood Risk Assessment
North London
Final - April 2008
Watercourse,
property and
infrastructure
downstream of
infrastructure
82
Medium
Medium
Large
Large
Large
Medium
5.8.1
Scale of Consequences
The scale of consequences used in the source pathway receptor model are
presented in Table 15. The assessment of the consequences are approximate only
and intended to guide the study towards the pertinent flooding issues and allow
further assessment of the critical flood sources affecting planning decisions. They
make no consideration of the likelihood of flooding.
Table 18 - Scale of Consequences
Scale
Consequence
Very Large
> 2000 Buidlings
Large
500 – 2000 Buildings
Medium
90 – 500 Buildings
Small
40 – 90 Buildings
Very Small
0 – 40 Buildings
The consequences outlined in the above table provide a very simplified approach
based solely on the number of properties affected, however, within the Source
Pathway Receptor model other factors are considered. For instance the number of
properties affected by infrastructure failure is largely unknown at this stage of the
study, however, it is clear that such failure could have sever consequences based on
other factors such as the velocity and rate of onset of such flooding.
5.9
Community Risk Registers
The Civil Contingencies Act 2004 places a legal duty on Category 1 responders to
produce a Community Risk Register. The Resilience forums that cover several
authorities in an area produce a Community Risk Register. The study area contains
two Resilience Forums each of which have their own Community Risk Register; the
North Central London Resilience Forum and the North East London Resilience
Forum. The North Central London Community Risk Register covers Barnet, Camden,
Enfield, Hackney, Haringey and Islington while the North East London Community
Risk Register includes Waltham Forest.
Both the registers identify flooding from two main sources: surface water, coastal
and fluvial as hazards. The level of risk varies between areas. It should be noted that
the hazard sub-category titles are not particular clear and are difficult to align with
existing system of defining flooding throughout England and Wales. In future reviews
of the Community Risk Register, it would be ease communications between planners
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83
and emergency planners, if the hazard sub-categories titles were brought into
alignment with the existing system.
Table 19 Summary of flooding risks as described in Community Risk Register (May 2008)
Area
Hazard
Coastal
Flooding
North
Central
Fluvial
Flooding
Structural
Coastal
Flooding
North
East
Fluvial
Flooding
Structural
Strategic Flood Risk Assessment
North London
Final – August 2008
Magnitude
(Risk Ref)
Major
(H19)
Major
Local
(HL16)
Localised
(HL17)
Major
(H20)
Major
(H21)
Major
Local
(HL18)
Significant
Local
(HL19)
Localised
(HL20)
Reservoir
Failure
(H44)
Major
(H19)
Major
Local
(HL16)
Localised
(HL17)
Major
(H20)
Major
(H21)
Major
Local
(HL18)
Significant
Local
(HL19)
Localised
(HL20)
Reservoir
Failure
(H44)
Likelihood
Impact
Risk
Unlikely
Significant
Very High
Rare
Significant
High
Possible
Moderate
High
Rare
Catastrophic
High
Unlikely
Catastrophic
Very High
Unlikely
Catastrophic
Very High
Possible
Significant
Very High
Possible
Moderate
High
Negligible
Catastrophic
Medium
Rare
Catastrophic
High
Rare
Moderate
Medium
Rare
Moderate
Medium
Rare
Catastrophic
High
Probable
Moderate
High
Unlikely
Significant
Very High
Possible
Moderate
High
Unlikely
Significant
High
Negligible
Catastrophic
Medium
84
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85
6
Strategic Flood Risk Assessment
The following chapter aims to further assess the flood risk from with the key flood
sources identified in the source pathway receptor model. Where available data
permits the assessment is intended to provide planners who will use this document
with the appropriate information required to make informed and sustainable planning
decisions and also provide developers with baseline data from which site specific
flood risk assessments can be developed. The degree of assessment attributed to
sources has been dictated by the approximate risk posed by each source partly
assessed through the source pathway receptor model approach, the accuracy of
flood risk data already available and also the limitations on data that would be
required for further assessment.
6.1
Sustainability and Climate Change
There is now a common consensus that climate change will continue to have a
detrimental impact on flood risk on a global scale. Around the United Kingdom, sea
levels are predicted to rise and changes in weather patterns are likely to lead to
longer winter storms and increasingly intensified summer storms. Such changes in
sea levels and weather patterns will lead to an increase in the frequency and scale of
flooding, with predicted rises in fluvial flows, rising groundwater and increases to
peak rainfall run-off and volumes all contributing to an increase in flood risk. Future
development must take climate change into account when considering sustainable
development and PPS 25 explicitly states that future development must take into
account climate change when considering flood risk.
A number of climate change studies exist which use global climate predictions to
model potential impacts on sea level rise and rainfall. Despite these various studies
there is still uncertainty about the extent of global warming and therefore the
potential impact on flood risk. The UK Climate Impacts Program (UKCIP)16 has
published a series of scenarios based on varying global emission levels which form
the basis for the London’s Warming Technical Report17. The scenarios presented in
UKCIP02 use 4 different scales of emissions to model the potential impact on rainfall
and sea levels. The output of the models provides a range of results which could be
applied to this strategic flood risk assessment through sensitivity testing of the
computational hydraulic models used to determine flood risk zones. However, the EA
studies which provide predicted flood zones and levels for this report generally use
16
UK Climate Impact Partnership (2002) Climate Change Scenarios for the United Kingdom:
The UKCIP02 Scientific Report
17
London Climate Change Partnership (2002) London’s Warming: A Climate Change
Impacts in London Evaluation Study
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86
the precautionary allowances stated in PPS25 as the basis for climate change
consideration. PPS25 recommends an increase in fluvial flows of up to 20% and an
increase in peak rainfall intensity of up to 30% by 2115. The main climate change
tables from PPS25 (B.1 and B.2) are reproduced below in Table 20 and Table 21.
To review all of the hydraulic studies in relation to the UKCIP02 scenarios is
considered too onerous and unnecessary for a study of this nature. The
precautionary allowances in PPS25 are considered appropriate for both the SFRA
and site specific FRA process since a more complex approach will not guarantee
greater certainty.
The EA flood zones which form the basis of the zoning for this report do not include
an allowance for climate change, however, the Lower Lee Valley study applied an
adjustment of 20% to fluvial flows to determine the increase in zoning. Climate
change forecasts are likely to be periodically revised in years to come. UKCIP are
expected to release further climate change models in 2008. Site based flood risk
assessments for future development would be expected to make use of the most up
to date guidance on climate change allowances where appropriate.
Table 20 Recommended national precautionary sensitivity ranges
1990 to
2025
2025 to 2055
2055 to 2085
2085 to
2115
Peak rainfall intensity
+5%
+10%
+20%
+30%
Peak river flow
+10%
Parameter
+20%
Offshore wind speed
+5%
+10%
Extreme wave height
+5%
+10%
Table 21 Recommended contingency allowances for net sea level rise
Net Sea Level Rise (mm/yr) relative to 1990
Administrative Area
1990 to
2025
2025 to
2055
2055 to
2085
2085 to
2115
4.0
8.5
12.0
15.0
3.5
8.0
11.5
14.5
2.5
7.0
10.0
13.0
East of England, East
Midlands, London, South
East England (south of
Flamborough Head)
South West
NW England, NE England
(north of Flamb’g Head)
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87
6.2
Fluvial Flooding
Unsurprisingly, the source pathway receptor model identifies fluvial flooding as the
most significant flood risk to North London. The number of properties at risk from
fluvial flooding combined with the low standard of protection of some flood defences
give rise to significant flood risk issues. The Lee Valley poses a considerable flood
risk, affecting 4 of the 7 Boroughs, Enfield, Hackney, Haringey and Waltham Forest.
The Lower Lee Valley is defined as an opportunity area in the London Plan despite
having considerable deficiencies in the standard of flood protection within much of
the area. Any development which passes the sequential test within the Lower Lee
Valley will still require careful consideration of the residual flood risks in order to
ensure that a safe and sustainable development can be achieved.
The Silk Stream, Dollis Brook and their tributaries also pose inherent flood risk, albeit
to a lesser scale than the Lee Valley. The Silk Stream Flood Alleviation Scheme will
go some way to mitigating the flood risks in this area although the standard of
protection offered by the scheme is only approximately 1 in 25 years.
As discussed in section 5.3.3, the flood risk zones provided by the EA have been
derived using two separate methods. Map 21 shows the usage of these two
methods, showing the differing extents of JFLOW and alternative modelling studies.
6.2.1
Lower Lee Flood Risk Zone
The Lee Valley has been extensively modelled in recent years and the flood extents
are entirely derived from these studies. The majority of the Lee Valley tributaries
have also been included in these studies. Only the upper reaches of Holyhill Brook,
Salmons Brook, Hounsden Gutter, Monken Mead Brook and Bounds Green Brook
are modelled using the JFLOW modelling process. The upper reaches of these
watercourses generally extend into Green Belt or Metropolitan Open Land and
therefore the low certainty regarding the flood risk zones will not affect the
application of the sequential test during the allocation of development sites. As such
no refinement of these flood risk zones through a level 2 SFRA is deemed
necessary.
The exception to this is the Hounsden Gutter that passes through a highly urban
area but is using an older and less detailed JFLOW model and to a lesser extent
Bounds Green Brook, where the JFLOW modelling extends into both urban and rural
areas, see Map 9 and 21. Some consideration must be given to whether these
watercourses lie within proposed development areas as the JFLOW derived flood
risk zones contain a high degree of uncertainty. Where development is proposed
near to these flood risk zones a buffer strip of 20m could be added to flood risk
zones to allow for any such inaccuracies. In Annex E of PPS25, (Para. E6) state that
“where decision-makers have been unable to allocate all proposed development and
infrastructure in accordance with the sequential test, taken account of the flood
vulnerability category of the intended use, it will be necessary to increase the scope
Strategic Flood Risk Assessment
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Final – August 2008
88
of the SFRA to provide the information necessary for application of the exception
test.”
6.2.2
Silk Stream and Dollis Brook Flood Zones
The Silk Stream and Dollis Brook Flood Zones included in this SFRA have recently
been remodelled by Jacobs for the EA using ISIS. The new flood risk zones have
been included in the flood zone mapping. These are considered suitable for use in
sequential testing for allocation of development sites.
6.2.3
Actual Flood Risk
The flood risk zones shown on map 8 and discussed above do not take into account
flood defences, either formal or informal. As such the flood risk zones do not always
represent the actual risk of flood inundation. Where areas of high flood risk are
identified, such as the Lee Valley it is appropriate to assess the actual flood risk,
both to defended and undefended areas, taking into account overtopping and breach
risks to determine the flood hazards. Such an assessment is essential where
development is planned in areas behind defences, particularly in the Lee Valley
(identified as an opportunity area) where the standard of protection is often less than
the 1% annual probability. Such an assessment falls within the remit of a level 2
SFRA and is therefore outside of the scope of this report. Further discussion on the
requirement, extent and recommendations for a level 2 SFRA is included in section
6.2.4.
6.2.4
Recommendations for Further Work
The improved level of assessment which comes from a Level 2 SFRA is generally
required where high uncertainty exists over the existing data or where areas of high
risks are identified. A level 2 SFRA is not required to cover the whole LPA area in the
way that a level 1 SFRA must.
The principal purpose of a Level 2 SFRA is to facilitate application of the Sequential
and Exception Tests. These studies should focus only on proposed development
areas where there is data with high uncertainty or high flood risk. Within the North
London Boroughs there does not appear to be a requirement for a level 2 SFRA.
6.2.4.1
River Lee Catchment
The Lower Lee Valley and its tributaries are extensively modelled and the flood risk
zones provided by the EA have a high confidence level associated with them. As
such no further delineation of these flood zones are required, although allocation of
sites around the upper reaches of the Hounsden Gutter and Bounds Green Brook
should allow for a 20m buffer strip to account for inaccuracies in the flood risk zones.
The information contained in the flood risk zones is sufficient for allocation of sites
using the sequential test.
The Lower Lee Valley has extensive flood plains and a number of interacting flood
defences, including the Lee Flood Relief Channel. The raised flood defences provide
Strategic Flood Risk Assessment
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Final – August 2008
89
varying standards of protection and actual flood risk will vary considerably from the
undefended flood risk zone map. There is limited scope for large scale development
elsewhere in many of the Boroughs and the sequential test is likely to place a
number of development sites within flood zone 3 in the Lower Lee Valley. The
sequential test requires that where more than one potential site exists within the
same flood zone, development is prioritised into the areas of lowest probability of
flooding. In order to demonstrate this, Boroughs will need to understand the actual
flood risk while also having information on flood hazards including those from
overtopping and breach analysis. Data on depth of flooding, flood velocities and the
rate of onset of flooding will be required to delineate sites within flood zone 3, such
that the sequential test can be fully demonstrated.
Waltham Forest and Hackney may already be sufficiently covered in the London
Development Agency commissioned Lower Lee Valley SFRA and it is recommended
that this be reviewed before they undertake any further work. If the LDA are not able
to make the Lower Lee SFRA available for use by the Boroughs it may be possible
to obtain the Lower Lee model under license from the LDA and undertake their own
study. This approach would inevitably involve some duplication of work already
undertaken in the Lower Lee SFRA and is not the preferred option.
6.3
Groundwater Flooding
Groundwater flooding records have been combined with geological and topographic
plans (where available) to identify a series of groundwater flood risk locations. The
risk map (map 23) identifies areas close to the outcrops and deposits of gravels and
silts and also the low lying areas within the Lee Valley where the groundwater table
is close to the ground surface and in hydraulic connectivity with the watercourses.
The Silk Stream and Dollis Brook valleys are also identified due to the presence of
the alluvium deposits overlying the impermeable clay layer.
The groundwater flood risk zones within the river valleys clearly overlap with the
fluvial flood risk zones which will take precedent when undertaking the sequential
test. The groundwater flood risk zones in the upper catchment slopes should not be
taken into account when allocating the sites using the sequential test as the
groundwater flood risk is relatively low compared with other sources of flooding and
can be adequately mitigated at a site flood risk assessment level. Other factors away
from flooding are likely to have a greater bearing on the allocation of site.
The information should be used by developers when considering flood risk at a site
specific level. As well as assessing flood risk to their own development, developers
should also consider the impact that their site could have on groundwater flows
elsewhere in the catchment. Large basements or strip foundations can impede
groundwater flow causing springs to arise on adjacent sites. Care should be taken in
the groundwater flood risk zones to ensure that this is adequately addressed. The
provision of basements or conversion of basements into flats in areas of
Strategic Flood Risk Assessment
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Final – August 2008
90
groundwater flood risk must be carefully considered by both developers and
planners.
6.4
Assessment of Surface Water and Sewer Flooding
A number of flood records relating to surface water and sewer flooding have been
obtained as part of the study and were reviewed in detail in section 5.5. This section
attempts to combine these flood records and use them in conjunction with
topographic maps to identify potential overland flow paths. It would be beneficial to
review the maps against sewer record plans to identify areas at risk. However, in the
absence of such data this exercise could not be completed for this version of the
SFRA.
6.4.1
Pluvial Flood Records
The largest data set relating to flooding was received from the London Fire Brigade.
Over 15000 individual incidents were provided and these records have been
grouped together into flood groups and correlated with high rainfall events taken
from gauge data received from the EA. The correlation attempts to try and determine
whether events could be attributed to pluvial flooding or whether events may be
attributed to other factors such as infrastructure failure.
In total 55 grouped flood events were identified and of these 23 events were
successfully cross referenced with the EA rain gauge data. Rainfall data for the
other events was not available from the EA and it has not been possible to determine
if these events were as a result of pluvial flooding.
The 23 events are displayed on map 20 through the marking of points where a flood
incident was recorded. The points have been divided by the sources of information
as not all the events were associated with dates. The points show an even split of
events across the summer and winter seasons. Most of the calls were recorded in
the southern part of the study area within the heavily urbanised areas.
The densest cluster of flood calls occurs in central and southern Islington and
Hackney. These flood events do not appear to correlate with Thames Water flood
records or other sources of flood risk. The incidents are generally spread across the
Boroughs and as such they do not highlight any significant areas of flood risk. Once
topographic information is available for these Boroughs, a further review of the
flooding incidents will be considered against the topography to determine if there are
any particularly vulnerable locations within these Boroughs.
A small number of flood events were recorded in the Edgware area of Barnet, which
correlates with both the Barnet and Thames Water records.
A small number of events were also recorded in Camden and the locations of these
events also correlate with the flood records of the 2002 floods in Camden. It is
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91
noticeable that the Camden floods of 2002 do not significantly appear in the flood
data sets derived. This may be attributed to a lack of available rainfall data.
A number of flood events are recorded on the Hackney / Haringey border in the
Stamford Hill area. These flood events correlate with the Thames Water flood
records where a significant flood risk area is identified.
The number of events recorded in Waltham Forest and Enfield are relatively low and
do highlight any significant surface water or sewer flood risk areas other than those
previously identified from inspection of the other sources of flood records.
6.4.2
Surface Water Flood Risk
For the purpose of the SFRA, there is a need to better understand the risk from
surface water flooding and the performance of the urban drainage system during
flood events.
Unlike fluvial flood mapping which is well recorded and has plenty of information
available for future developments. This is not the case for overland flooding, which
may be attributable mainly to large impermeable areas and incapacity in the surface
water drainage systems. Therefore consideration must be made to upstream and
downstream conditions, soil conditions, age of drainage systems, topography and
any local evidence of surface water during the planning process.
Map 23 shows the likely drainage paths of the surface water run off based on the
topography and the drainage catchments of the study area. However, the urban
nature of the study area will have a large influence the surface water flow paths and
pooling locations.
Five surface water flood risk areas are listed below and are marked on Map 23.
These have been identified from reviewing all available flood records relating to
surface water and sewer flooding. Further assessment of these flood zones has not
been possible without information on the local sewer networks. Thames Water was
unable to make this information available to inform the study.
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Table 22 – Locations of Surface Water Flood Risk Zones
Location
North London
Borough(s)
Affected
Suggested Planning
Considerations
Site specific flood risk
Edgware and Cannon
Hill
assessments or surface water
Barnet
drainage strategy to be
encouraged for all developments
if not required under PPS25
Comments
Flooding occurs around the
upper tributaries of the Silk
Stream. Culverted sections may
be responsible for such flooding
Site specific flood risk
assessments or surface water
West Hampstead,
Cricklewood, South
Hampstead and west to
drainage strategy to be
Barnet / Camden
encouraged for all developments
if not required under PPS25.
Church End
Development of basement flats to
be restricted
Site specific flood risk
Stamford Hill
Hackney /
Haringey
assessments or surface water
drainage strategy to be
encouraged for all developments
Flooding incidents located south
of Seven Sisters Road.
if not required under PPS25
Site specific flood risk
assessments or surface water
Leyton, Leytonstone
Waltham Forest
drainage strategy to be
encouraged for all developments
if not required under PPS25
Site specific flood risk
assessments or surface water
South Tottenham
Haringey
drainage strategy to be
encouraged for all developments
Flooding occurs around the
Stonebridge Brook Area
if not required under PPS25
*Other Flood Risk Zones may be added to this list once topography plans are obtained for
Camden, Islington and Hackney.
The Edgware and Cannon Hill areas of Barnet (and Harrow) are located at the head
of the Silk Stream and Edgeware Brook where a number of smaller tributaries
combine. A number of the watercourses are culverted and the capacity of these
culverts may be responsible for the flooding problem which exists in this area. Future
development should take account of this potential flooding problem. Aiming to restrict
the run-off from future development and considering the potential for on-site flooding.
Strategic Flood Risk Assessment
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Final – August 2008
93
The area may benefit from a combined flood analysis, reviewing both the fluvial and
sewer flooding in one combined assessment.
The areas of West Hampstead, Cricklewood and South Hampstead would appear to
be the areas at most risk from pluvial flooding within the North London areas. This
flood risk extends to a lesser extent to Church End in the Barnet and also into the
east of Camden, which experienced flooding during the 2002 Camden Floods. The
extent of the 2002 Camden floods is shown on Map 22. The cause of these floods
was attributed to surcharged sewers which could not cope with the volume of run-off.
Without access to sewer records this cannot be verified. As such the flood risk zones
are identified to highlight the potential for flood risk in these areas.
A number of surface water and sewer flood risk areas have been identified and
development within these areas should consider measures to reduce flood risk and
ensure that new development considers the residual risk. Some types of
developments such as basement flat conversions may be considered inappropriate
within these areas without a thorough flood risk assessment. However, in the
majority of the North London area, surface water flood risk is limited and is
considered a minor influence to the sequential test when allocating development
sites. In addition, the Pitt Review supported the introduction of Surface Water
Management Plans in Recommendation 18.
6.4.3
Basement Flooding
In the Central London boroughs of Camden and Islington, there are a large number
of basements that are used for commercial and residential purposes. This number
reduces slightly as you move north to the suburban boroughs of Barnet, Hackney,
Haringey, Enfield and Waltham Forest.
Islington report 12,800 basement properties in their borough. While there is low
fluvial flood risk to the borough the greater likelihood of surface water flooding poses
a threat to these properties and other like them in Camden. In other boroughs,
basement properties face flooding from a variety of different sources of flooding such
as fluvial, tidal, surface water, sewer or groundwater depending upon their location.
Basement flooding is a new area of consideration for planner. At present the best
example of Basement Flooding Guidance for development is a note prepared by the
EA to assist planners in Hammersmith and Fulham. This can be found in Appendix
C.
6.4.4
Recommendations for Further Work
Further assessment of the surface water risk is recommended to better define the
surface water flood risk zones. However, this cannot take place without the cooperation or collaboration of Thames Water who have set out their stance regarding
the SFRA process. Engagement of Thames Water is recommended to determine
whether any flood relief schemes are in place to alleviate any of the flooding within
these zones.
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As further information is made available about the location of basements and the
results of surface water modelling for the boroughs would allow identification of
basement properties with the highest risk of flooding. Boroughs should use the EA
Guidance Note on the flood risk for basement development issued to Planners
Hammersmith and Fulham to assist them in planning decisions while policy is being
formed.
6.4.5
Reservoirs
In the North London boroughs there are a large number of reservoirs (large one on
the reservoir register and smaller ones that are not). These reservoirs are either for
commercial or recreation purposes with the most noticeable reservoirs present in the
Lee Valley. While there is low risk of the reservoirs failing in the North London
boroughs, the consequences of such an event are severe to down stream areas.
Reservoirs that are too small to be on the register, pose more risk of failing due to
the lack of regulation governing their maintenance and management. In Spring 2009,
all reservoirs under the 1975 Reservoir Act, will require the development of a Flood
Plan and associated inundation mapping.
6.4.6
Recommendations for Further Work
Due to the densely populated nature of North London, it is recommended that each
of the boroughs takes the time to locate the smaller reservoirs and assimilate
information about them on a par with the reservoirs on the register. It is also
suggested that the emergency planners liaise with the reservoir operators and work
with them on the development of the reservoirs flood plans and inundation mapping.
The reservoir operators are encouraged to work with and share information with the
Borough Councils so to reduce the risk to development in the future from the
reservoirs.
6.5
6.5.1
Emergency Planning Measures for Flooding
London Boroughs
All seven boroughs have Emergency Planning Units that handle incidents of
flooding. The boroughs all participate in the North East Local Resilience Forum for
Waltham Forest and the North Central Local Resilience Forum for the other six
boroughs. The Community Risk Registers for both these forums are available on the
London Fire Brigade Website.
The individual boroughs have different types of flooding to deal with and plan for
accordingly. At present the London Boroughs have not specified whether they
currently have flood incident plan. The only details of the Local Authorities response
are outlined in their Emergency Plan. The London Borough of Enfield has stated that
they are currently looking at writing a specific plan in order to enhance their response
to a flood event.
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6.5.2
Environment Agency
Under the Civil Contingencies Act 2004, the Environment Agency is a Category 1 or
Core Responder to incidents including the management of fluvial and coastal flood
events. The Environment Agency operates a flood incident management system that
includes flood warning, incident management, river and river structure management
and awareness raising on flood preparation, safety, mitigation and cleaning up after
a flood. The Environment Agency fully participates in the North East Local Resilience
Forum for Waltham Forest and the North Central Local Resilience Forum for the
other six boroughs.
The Environment Agency has developed a multimedia Flood Warning System. There
are four levels of warnings which are Flood Watch, Flood Warning, Severe Flood
Warning and All Clear. The warnings are aimed at communities in flood zones two
and three and delivers messages direct to the public, businesses, emergency
services and the media via phone, fax, sms (text messaging), email and pager.
People are able to register for the service if they call the Floodline service on 0845
988 1188. This warning system will be further developed to allow online registration
for the service.
The Environment Agency has one Flood Warden who has provided a service to
properties within Hemming Road and Bransgrove Road that are at risk of fluvial
flooding from Edgeware Brook and close to Silk Stream and Deans Brook for over
two years. There three further Flood Wardens in the North London area. The EA is
continuously trying to establish further Flood Warden Schemes but frequently find
that people do not consider volunteering as a flood warden while they do not feel the
threat of flooding. Local Authorities are encouraged to promote new and existing
flood warden schemes.
To support all the activities, the EA in association with other drainage authorities are
undertaking a targeted flood awareness programme throughout the population with
interests in the flood zones 2 and 3. This programme includes the use of local
advertising campaigns, open meetings, awareness raising road shows and mail
shots.
6.5.3
Civil Contingencies Act
The Civil Contingencies Act states that Category 1 presents those organisations at
the core of the response to most emergencies (e.g. emergency services, local
authorities, NHS bodies). Category 2 responders are less likely to be involved in the
heart of the planning work but will be heavily involved in incidents that affect their
sector. Examples include Health and Safety Executive, transport and utility
companies). Category 2 responders have a lesser set of duties - co-operating and
sharing relevant information with other Category 1 and 2 responders. The Resilience
Forums that cover several authorities in an area produce a Community Risk
Register.
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6.5.4
The Pitt Review
The final provided many recommendations relating to emergency planning and
business continuity management. This section highlights a few of the key issues,
however, it is suggested that planners and emergency planners read the Pitt Review
as this report will have repercussions on improving the UK’s resilience to flooding in
the future.
Recommendation 13 should be noted by Local Authorities that there will be more
requirements in the future to promote business continuity and property flood
resistance and resilience by businesses as part of there already agreed duties under
the Civil Contingencies Act 2004.
Other recommendations promoted in the Pitt Review suggest Local Authorities take
more of a lead on local flood risk management (Recommendation 14 and 15), while
improving their in-house technical capabilities in Flood Risk Management
(Recommendation 19). In addition, the final report recommends that the Local
Authority takes on responsibility to collate and map main food risk management and
drainage assets including the ownership and the condition of these assets
(Recommendation 16).
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7
Application of Sequential Flood Risk Test
PPS25 encourages LPAs to undertake the sequential flood risk test to help gain an
understanding of the potential flood risk associated with future development. The key
aspect of the sequential flood risk test is the assessment of the flood risk zones in
accordance with PPS25. This is done by delineation of the flood risk zones using
topographic data and flood modelling results. Following the application of the
sequential test, further separation of development sites can be assessed using the
exception test. This requires consideration of individual development risks such as
the risk to vulnerable users and also further appraisal of area in Flood Zone 3 by
determining the risk from overtopping and breach.
The sequential and exception tests are risk based assessment tools intended to
ensure that preference is given to development of sites of little or no risk of flooding
over areas of higher risk. The approach uses flood risk zones as defined in section
2.2.
7.1
Flood Risk Zone Categories
The following Flood Zone risk categories are highlighted on the flood zone map,
which should be employed when applying the sequential test to development
proposals.
Table 23 - Flood Risk Zones and Development Restrictions
Flood Zone
Description and Development Restrictions
Zone 1
All Development types acceptable
Zone 2
Exception Test Required for Highly Vulnerable Development
Zone 3a
More Vulnerable development and essential infrastructure requires Exception Test,
Highly Vulnerable Development not permitted.
Land where water has to flow or be stored in times of flood. Only water-compatible
Zone 3b
uses and essential infrastructure that has to be there should be permitted. Exception
test required for essential infrastructure.
For vulnerability of development classes refer to section 2.2.
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7.2
Application of Sequential Test
The objective of the Sequential Test is to steer development to areas at the lowest
probability of flooding from various sources. The test logically guides Local Planning
Authorities through site selection by identifying whether a suitable site is available in
the low probability zone (Flood Zone 1). If no site is available in Flood Zone 1 then
Flood Zone 2 can be considered depending on the vulnerability of the development,
as outlined in Table D.2 in PPS25. This vulnerability classification will then allow
further consideration of appropriate locations of the proposed development. Table
D.1 in PPS25 defines the suitability of different flood zones for the proposed
development’s vulnerability classification. Site selection must review the sites with
the lowest risk of flooding first before considering sites in higher risk areas.
7.3
7.3.1
Assessment of Risk within Zones 2 and 3
Application of the Exception Test
The Exception Test is only applied after the Sequential Test and is intended to
ensure that vulnerable type of property are not developed in areas of high flood risk.
Three conditions must be met in order to pass the exception test:
Criteria for Passing the Exception Test
a) The Development must provide wider sustainability benefits to the community that
outweigh the flood risk. Where appropriate the benefits should contribute to the Core
Strategy’s Sustainability Appraisal
b) The development site must be on developable previously developed land or if it is not
on previously developed land there should be no reasonable alternative sites on
developable previously developed land
c) A site specific flood risk assessment must demonstrate the development will be safe
without increasing flood risk elsewhere and also demonstrate the reduction of flood risk
The exception test is applied only after the sequential test has been applied as is
only applicable when “more vulnerable” development and “essential infrastructure”
cannot be located in flood zones 1 or 2 and “highly vulnerable” development cannot
be located in flood zone 1. It should not be used to justify “highly vulnerable”
development in flood zones 3a, or “less vulnerable”, “more vulnerable” and “highly
vulnerable” development in flood zone 3b. LPAs are required to undertake parts A
and B of the test with developers usually undertaking parts C.
The Environment Agency provide clear guidance on LPA responsibilities for
undertaking the sequential and exception tests when allocating development sites.
The following website provides step by step guidance on assessing development
proposals under the sequential test.
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http://www.pipernetworking.com/floodrisk/sequential.html
However it should be noted that flood risk standing advice will soon no longer be
available from piper networking it will soon be available from the EA website.
7.4
Brownfield Development
PPS3 encourages LPAs to give preference to previously developed site and disused
properties prior to allocating greenfield sites for development. These brownfield sites
characterise a substantial amount of the potential development land within the North
London study area.
Brownfield Development Advice
If there are two sites with the same probability of flooding, the brownfield site should be
developed in preference to the greenfield site.
If the brownfield site has a higher probability of flooding than the greenfield site, then a
trade-off must be made between benefits and disadvantages of the two sites. Should the
LPA consider that developing on the brownfield site is beneficial then appropriate
mitigation measures need to be included in the design and guidance provided in PPS25
regarding elderly and vulnerable occupants still applies.
A site located within a flood risk zone that has been previously developed does not
automatically warrant planning approval for a replacement development. The flood
risk assessment and management of the new application should be dealt with in the
same manner as a greenfield site.
In practice some of the PPS25 flood zonings are difficult to apply to London since
development already lies within Zones 2 and 3 which would not now pass the
sequential test. However, when considering re-development of these areas the
sequential test must be considered in full and development should not be allocated
within flood zone 3b.
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8
Local Policy Guidance
8.1
Introduction
The evolution from Unitary Development Plans to Local Development Frameworks
provides an opportunity for the North London Boroughs to introduce planning policies
which are consistence with both national and regional policy.
The findings of the North London SFRA have been used to produce a series of draft
policy recommendations outlined in this section. The policy recommendations are
split into the following three themes: Policy, Development Control and Technical. The
Boroughs may choose to adapt and adopt some of these policies where applicable
at a local level. These policy recommendations are written to reflect the national and
regional policies outlined in the London Plan. The wording and approach of these
recommended policies would need to be reviewed in the context of other local
policies. It should be noted that these do not in anyway reflect actual policies of the
boroughs and they are provided only as a starting point from which the Boroughs
can develop their own appropriate flood risk policies in consultation with the
Environment Agency.
For the Boroughs to assess the current and future policy recommendations,
consultation of and information from the latest versions of the Planning Policy
Statement on Development and Flood Risk, Sewers for Adoption and Flood
Estimation Handbook would be suggested. Sewers for Adoption provides sewer
network design guidance that is used by the water companies. The document has
specific sections on flood capacity design. While the Flood Estimation Handbook
provides information and guidance on both flood estimation and flow design.
8.2
8.2.1
Policy Recommendations
Policy
01: Incorporation of SFRA findings into the Core Strategies and other
Development Framework Documents
In order to ensure a robust approach to flood risk appraisal, management and
reduction, the findings of the SFRA need to be incorporated into each of the
Boroughs’ Development Framework Documents and support documents. This will
ensure a holistic and robust approach to flood risk management, ensuring that the
matter is taken into account at all stages of the planning process. The findings of the
SFRA demonstrate the level of flood risk within the boroughs. Key issues identified
by the SFRA will be a priority for future spatial planning in the seven North London.
Future policy should seek to address how defences will be maintained and address
how development can be accommodated. These issues are considered in greater
detail in subsequent policy recommendations. The Boroughs should endeavour to
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ensure that the findings of the SFRA feed into policy preparation and is incorporated
within planning policy.
02: The Core Strategy should include a clear policy statement on flood risk in
urban areas
In accordance with PPS12, the Core Strategy should set out the key spatial
elements of the planning framework for the area and include core policies. All other
development plan documents must be in conformity with the Core Strategy. In
setting out a spatial vision for the boroughs, a clear borough-wide policy statement
on flood risk should be included to set a clear policy hierarchy on planning for flood
risk. This should reflect the approach of pro-active management of flood risk through
the reduction of causes of flooding to existing and future development. It is
recommended that a specific policy on flood risk is included within the Core Strategy.
This should focus on ensuring that, where possible:
• Development is located in the lowest risk area
• New development is flood-proofed to a satisfactory degree and does not increase
flood risk elsewhere
• Surface water is managed effectively on site
• The Borough applies the sequential approach when determining planning
applications
• Flood storage and SUDS used where practicable
Site specific and other detail should then be included in subsequent development
plan documents as they are prepared.
03: Linkages with other Flood Risk Management Strategies
The Boroughs should ensure that in developing and taking forward the findings of
the SFRA they have regard to other developing strategies that consider flood risk
management in the area to ensure that wider concerns are reflected where
appropriate. The Thames Catchment Flood Management Plan (CFMP) has been
produced and is considered within the SFRA. The CFMP will also be updated like
the SFRA. Other SFRA reports such as the Lower Lee, the Upper Lee, the Brent and
the Olympics development and the Blue Ribbon Strategy are included and updates
will be required as changes are applied. Also the outputs form these studies
especially the CFMP should be considered for taking forward in policy
recommendations.
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04: Flood Risk Guidance should be included in the Area Action Plans
Action plans should comply with Core Strategies, and recommendations for Action
plans should be included for Core Strategies. The Area Action Plans are currently
being produced and should include a planned approach to flood risk within the
document that is based on the recommendations of the SFRA. This will include
reviewing any associated documents to ensure compliance with the SFRA
recommendations. The precise scope of the flood risk policy to be included is not
within the scope of this report. However, the findings have identified the following
recommendations for inclusion:
• Highly vulnerable development should be avoided in the high and medium flood
zones
• More vulnerable development should not be located on the ground floor
• Flood proofing and flood resilience should be incorporated in the overall design of
any development
• Access/egress points and specified refuge points that meets EA Guidance layout in
Appendix C.
• Any proposals will need to demonstrate that emergency planning measures have
been taken into account and, where appropriate, secured in perpetuity through legal
agreements
• Any particular requirements relating to flood risk and specific designations
It is recognised that the Strategic Development Areas (SDAs) within the Area Action
Plans (AAPs) have specific design and development issues that are subject to
Supplementary Planning Documents (SPDs) providing detailed design guidance.
Further design policy in relation to flood risk, as relevant and sympathetic to each
SDA should be included within the SPDs. However, the Boroughs should be
cautious of the approach of adopting the SPDs in order that they may be used in
advance of the formal adoption of the Area Action Plans.
8.2.2
Development Control
05: Development adjacent to flood defences, reservoirs and canals
Development adjacent to flood defences should be set back from defences,
reservoirs and canals to allow for the replacement/repair of the structures. Any future
raising of the standards of protection provided by these structures should be done in
a sustainable and cost effective way. Boroughs should ensure that development
does not breach or undermine flood defence, reservoir or canal structures in any
way. All new development should be set back 16 metres from tidal rivers, eight
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metres from main rivers and five metres from canals. For reservoirs, it is
recommended that discussions with the Operator and their Inspecting Engineer of
the Reservoir in question.
06: Appropriate development of urban centres
The SFRA identifies some areas of North London being at a high risk of flooding.
Particular care should be given to policy development for the urban centres, which
act as a social, economic and regeneration hub. In line with other recommendations
and the approach adopted in the guidance, the Boroughs should develop policies
which comment on the appropriateness of vulnerable uses in the urban centres.
Particular issues for consideration include:
• Guidance on the suitability of land uses on the ground floor of more vulnerable
development
• The incorporation flood proofing and flood resilience measures in the overall
development design
• The use of Sustainable Urban Drainage Systems (SUDS) (See Chapter 9 for more
information)
• Demonstration that emergency planning measures have been designed into the
proposed development
• The location and appropriateness of uses
07: Flood risk and housing market renewal
Areas where housing market renewal are located in mixed Flood Zones ranging from
Zone 1 to Zone 3 would involve the simple application of the Sequential Approach.
However due to the specific role of housing market renewal in terms of regenerating
housing demand in various areas care should be given to their development.
The boroughs should ensure that flood risk is considered as part of the spatial
planning and regeneration of the housing market renewal. These areas should take
account of the flood risk and adopt any measures necessary to ensure minimal risk
to future communities in the immediate area and wider areas. Regeneration should
aim to prioritise brownfield sites over greenfield sites in accordance with wider
sustainability policies and development should seek to reduce run-off rates.
These developments will seek to incorporate flood proofing and resilience through
design, take advantage of greenspace water storage where possible and any other
measures possible. The emerging Area Action Plans should include a policy on flood
risk reflecting the recommendations of the SFRA to ensure that a planned approach
to flood risk is embedded within the document.
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The Pitt Review Recommendation 11 suggests that building regulations should be
revised to ensure that all new or refurbished buildings in high flood risk areas are
flood resistance or resilient. While in Recommendation 12 the reviewers believe that
the Local Authority should include flood resistance and resilience activities should be
included under the home improvement grants for properties at high flood risk.
08: The Functional Floodplain should be protected from development
PPS25 defines the Functional Floodplain as Flood Zone 3b where the water will flow
or will store water during times of flood. The guidance states that only Water
Compatible Development and/or Essential Infrastructure should be permitted within
this zone. The Functional Floodplain has an essential role to play in the effective
long term flood risk management of North London. The Functional Floodplain should
be reviewed each time the SFRA is updated to ensure that latest information is
available and in use. This is support by Recommendation 7 in the Pitt Review.
09: Develop Flood Risk and Design Policy
Flood risk management in the highest risk zones is likely to affect the design of new
development. This could use non-invasive techniques such as site layout or
landscaped swales to address flood risk management issues. Alternatively, this
could involve invasive design solutions to ensure that developments are flood
resilient through the use of particular materials or flood exclusion measures such as
barriers to doorways.
The Boroughs should develop policy to address flood risk management and design
from the non-invasive measures to the specific design issues relating to design
sensitive areas like Listed Buildings and disabled access in relation to raised floor
levels. On small sites there are difficulties in providing the required length of ramp to
meet Disability Discrimination Act requirements.
10: Develop policy for basement dwellings in flood zones 3 and 2
PPS25 states that while flooding can not be prevented, its impacts can be avoided
and reduced through good planning and management. The risk to people residing in
basement dwellings could be a real time threat in a flood event. The Boroughs
should seek to prevent self-contained basement dwellings within Flood Zone 3 and 2
to minimise the flood risk to occupants. Any basement development should ensure
that no sleeping accommodation is provided at basement level and internal access
to higher floors should be maintained. All development in Flood Zones 3 and 2
should adopt flood proofing measures to minimise risk to people and property in a
flood event.
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11: Consideration of Sustainable Urban Drainage Systems (SUDS) for surface
water runoff
A key mitigation measure the Boroughs should consider is the potential increase of
use of surface water storage. Development should seek to ensure that surface water
run-off is managed as close to its source as possible through the use of surface
water storage options in public open spaces, car parks and green-roofs. The use of
SUDS should be promoted for developments unless there are practical reasons for
not doing so. The developer should restrict peak run-off from the site to the
undeveloped greenfield run-off rate for both developments on greenfield and
brownfield sites. SUDS can then be used to attenuate the surface flood water and
relieving pressure on the drainage network. Measures which could be adopted
include:
• Compensatory storage and floodplain compensation
• Sustainable Urban Drainage Systems (more information available in Chapter 9)
• No increase in the development footprint
• Surface water run-off must be restricted to existing levels with a reduction if
possible
However, where surface water management options are proposed consideration
needs to be given to their adoption and maintenance. However, it is realised
changes at the national level are required to ensure the adoption issue do not hinder
the uptake of sustainable urban drainage systems. Recommendation 20 of the Pitt
Review Final Report urges the government to resolve the ownership issue of SUDS.
12: Flood proofing for all new development
The Boroughs should develop a flood proofing policy for their area in consultation
with key partners such as the Environment Agency, the insurance industry and
developers. Flood proofing measures incorporated would reflect on the level of risk
and the vulnerability of the development use, as defined on the Flood Risk Maps and
PPS25. In developing the flood proofing policy, the borough should consider all
sources of flood risk and should therefore address the surface water drainage
management of sites, sustainable drainage systems and specific design measures
including:
• Electrical circuits lowered from the ceiling, raised sockets
• Flood gates to doors
• Air brick covers
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• Horizontal plaster boards
• Damp proof membranes
Any such guidance or standards should be the subject of continual review and
updating to ensure it reflects the latest techniques and guidance. Further information
is contained in ‘Improving the Flood Performance of New Buildings – Flood Resilient
Construction’, Department of Communities and Local Government, May 2007 and
www.ciria.org. This policy suggestion is supported by Recommendation 11 of the Pitt
Review.
13: Windfall sites
Windfall sites are sites which are not the subject of an allocation in an adopted
development plan. These sites should also be considered against flood risk
management policy, as outlined above. Such sites, dependant on their location
within the identified flood zone maps and the use, may need to be subject to the
Sequential and Exception Tests, as relevant, and are likely to require site-specific
flood risk assessments.
14: Guidance on the application of the Sequential and Exception Tests
The SFRA has identified areas that have a relatively high risk of flooding. In
accordance with PPS25, the Sequential Test and Exception Test shall be applied in
allocating sites and reviewing applications for permission. Evidence will need to be
provided to demonstrate that there are no reasonably available alternatives sites in
areas with a lower probability of flooding that would be appropriate for the type of
development.
Chapter 7 in the SFRA provides guidance on the process of applying the Sequential
and Exception Tests, based on the approach identified in PPS25. It is recommended
that a policy statement on the application of the Sequential and Exception Tests is
included in the each of the Boroughs’ Development Framework and explicitly
requires developers to approach the Borough and other key organisations to discuss
flooding issues.
15: Location of Development Types
The SFRA shows that some urban areas are within Flood Zone 3 and 2. The historic
development pattern North London has led to concentrations of particular types of
land use for example, the River Lee corridor frontage historically host industrial and
commercial land uses. However, several of these locations are adjacent to rivers and
are at the high to medium risk of flooding. Applications for development in Zones 2
and 3 must be accompanied by a local flood risk assessment. Developers should
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ensure that flood risk is managed through a process of sequential design, mitigation
and implementation of flood resilience measures where appropriate.
Boroughs should ensure that development is located in areas of lowest probability of
flooding when taking into account all relevant flood sources, while ensuring that
development achieves an overall reduction in flood risk. Given the likely competition
for land between more/highly vulnerable developments within North London, less
vulnerable uses should be directed to higher risk sites or should at least be required
at ground floor level in the high risk locations. This approach would lower risk to the
vulnerable uses and would help to safeguard land at lower risks of flooding for more
vulnerable development such as housing, hospitals and residential institutions.
However, flood risk should not stifle regeneration and the loss of land that currently
hosts less vulnerable uses to development of a more vulnerable classification, such
as ground floor residential should be avoided where possible. The Boroughs should
work with its development partners to address the implications of flood risk.
16: Emergency Planning and Evacuation Routes
The London Resilience Forum is currently undertaking a strategic flood plan which
will address emergency planning issues in North London. The Boroughs should
incorporate the findings of the SFRA within their Emergency Plans, in consultation
with its key stakeholders. This should specifically identify strategic evacuation routes
(‘red routes’) to enable emergency services to continue work during a flood event.
The flood risk of key command centres and emergency facilities, and the adequacy
of the level of protection which they are afforded, should be assessed using this
SFRA.
The Emergency Plan should identify key strategic locations for protection in flooding
emergencies and the locations of refuge areas which are capable of remaining
operational during flood events. Based on the findings of this SFRA, there may be
some works required to enable the implementation of the Emergency Plan. Legal
agreements should be sought where necessary to ensure that any maintenance
requirements are carried forward in perpetuity. See Appendix C for EA Guidance on
‘Safe Access’
8.2.3
Technical
17: Development of Surface Water Management Plan (SWMP)
The SFRA incorporates flood risk from surface water flooding. This should be
developed to more accurately identify areas that are at risk from surface water
flooding, and be reflected in the policy approach for those areas. A SWMP (or
similar) should be developed to work towards solutions to urban flooding, developed
in partnership with other North London Boroughs and other key partners, including
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Thames Water and the Environment Agency to ensure integrated working. This is
supported by Recommendation 18 of the Pitt Review.
18: Working in Partnership
The SFRA process has involved the Boroughs working in partnership with the
Environment Agency. While Thames Water and British Waterways were consulted
they participation in the SFRA Process was limited. In future revisions of this SFRA
this partnership and working involvement needs to improve. The sharing of
knowledge and information on a continual basis also needs to improve, to ensure the
SFRA and any subsequent flood risk management policy is based on the latest and
best information available, with mutual agreement with partners in its
implementation.
The London Resilience Team requested the SFRA to consider the implications of
flood risk on existing utilities. To date the utilities information required to undertake
this exercise has not been made available by the utility companies. It is understood
that the data has been supplied to the London Resilience Team and many of the
utilities companies were unwilling to supply the data for a second time. This
demonstrates a wider reluctance of many organisations other than the Environment
Agency to engage in the SFRA process. This is supported by Recommendation 17
of the Pitt Review.
19: SFRA Review
The SFRA is a living document and should be subject to rolling review, to ensure
that new guidance and data is incorporated within the study. Issues that could trigger
a review include the availability of new modelling data, occurrence of a major flood
event occurs, revised DCLG advice issued, change to the preferred River Lee, River
Brent and River Thames Flood Risk Management Strategy and any significant
planning issues arising. Data and mapping should be reviewed regularly to identify
any updates or additional data available. A review should be undertaken at least
every one to two years or to tie in with the development and review of the Core
Strategy, or as required by key stakeholders responsible for flood risk management.
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9
Guidance for Developers
9.1
Development and the management of residual flood risk
The following section is provided as a guide to developers on the management of
flood risk and provision of flood risk assessments for future development. The
guidance is based on recommendations arising from the findings of the SFRA while
also following the requirements of PPS25.
At a site level, developers should consult the Environment Agency, sewerage
undertakers, highways authorities and any other relevant bodies to supply
information for a Flood Risk Assessment (FRA) of the site. This will provide
information to the Local Planning Authority from which they can reach a decision on
the development application.
Developers are recommended to consult with the LPA over all developments in
Flood Zones 2 and 3 at the earliest opportunity to ensure that the Sequential Test
has been satisfied. This should be done before a FRA is undertaken as it may be
the case that the proposal is refused on Sequential Test grounds.
Planning applications for development proposals of 1 hectare or greater in Flood
Zone 1 and all proposals for new development located in Flood Zones 2 and 3
should be accompanied by a local FRA. For developments in Flood Zones 2 and 3,
the FRA should identify opportunities to reduce the probability and consequence of
flooding.
With reference to PPS25, the objectives of an FRA are to establish:
•
Whether a proposed development is likely to be affected by current or future
flooding from any source.
•
Whether it will increase flood risk elsewhere
•
Whether the measures proposed to deal with these effects and risks are
appropriate
•
Whether the developer will be able to demonstrate that the site will be safe,
without increasing flood risk elsewhere and where possible will reduce flood
risk overall (part c of the exception test).
The scope of each FRA should be agreed with the LPA, EA and any other relevant
consultees but it may include some or all of the following outputs:
•
Development description and location
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•
Definition and assessment of all potential sources of flooding including:
fluvial, coastal and tidal, estuarial, groundwater, artificial drainage systems,
infrastructure failure.
•
Assessment of the probability of flooding (existing and post development)
•
Assessment of the sequence of flooding across the site, rate of rise of water
level, flow velocities, depths and the duration of flood (existing and post
development and including consideration of climate change)
•
Detailed development proposals including estimation of the volume of runoff
likely to be generated by the development and assessment of the hydraulic
performance of the artificial drainage system for both storm and foul whether
existing or proposed.
•
Flood risk management measures and assessment of long term sustainability
•
Assessment of the change in flood risk to the surrounding area caused by the
development site both upstream and downstream including volumes of
displaced water and flood levels
•
Residual risks to both the development site and the surrounding area after
inclusion of any mitigation measures. Where new or modified structural
measures are provided, an assessment of their behaviour in extreme events
should be provided.
Developments within medium or high risk flood zones should be further classified to
ensure that flood resilience measures are considered to manage the residual risk of
flooding. These measures include:
•
The location of most vulnerable elements of a development within the areas
of lowest risk.
•
Maintenance of access/egress routes
•
Location of critical infrastructure.
•
The use of flood resilient and securely fastened infrastructure, seats etc
•
Planning of escape routes. In areas of high and medium risk it may not be
possible to ensure dry egress routes in time of flood. In these cases
consideration should be given to the likely occupancy during flooding, the
availability of safe refuge, potential for provision of key services and the
expected duration of inundation.
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Reference should be made to the following sources:
9.2
•
C624 Development and flood risk - guidance for the construction industry
CIRIA 2004” for details of best practice.
•
National Flood Forum at www.floodforum.org.uk
•
Damage Limitation Guide and Flood Product Advice Guides by the EA
•
Preparing for Floods by DCLG.
•
Flood Resistance and Resilience solutions: an R& D scoping Study Joint
DEFRA and EA.
•
Further information and guidance about the production of flood risk
assessments can also be found at www.pipenetworking.com/floodrisk
•
EA developer guidance can also be found at
www.environment-agency.gov.uk/developers
Drainage Assessments
All developments which increase the overall impermeable area of a site or result in
an increase in foul water discharge should require a surface and foul water
management strategy, which must comply with the policies outlined in the LDF.
All drainage design should comply with the latest national and regional design
guidance. At present these are Sewers for Adoption 6th Edition and, Document H –
Drainage and Waste Disposal of the Building Regulations (BR part H). The integrity
of the existing drainage should be maintained in compliance with the Building
Regulations Part C.
Responsibility for drainage is fragmented which makes management more complex.
Sewerage undertakers are responsible for the public sewerage system that serves
most urban areas. Within the North London study area, Thames Water is the
responsible sewerage undertaker. Thames Water is responsible for the drainage of
flow arising from the boundaries of developments. Property owners are responsible
for drainage within the boundary of their development.
Where new developments are proposed the planners role is to ensure that the
development can be effectively drained from above and below ground without any
detrimental effects downstream.
Guidance from Sewers for Adoption 6th Edition and PPS25 is as follows:
•
PPS25 states that “In general terms, sewers should be designed to ensure
that no flooding occurs above ground level for events with a return period in
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the range of 30 to 50 years, depending on the development type”. Sewers
for Adoption 6th edition states that “all systems should be designed not to
flood any part of the site in a 1 in 30 year return period design storm”
•
“any underground storage to be constructed to attenuate the 1 in 30 year
event should be sited within the system being offered for adoption. Storage
over and above the 1 in 30 year event should not be sited within the
adoptable system” (Sewers for Adoption 6th Edition)
•
Sewers for Adoption 6th Edition states that “During extremely wet weather,
the capacity of the surface water sewers may be inadequate. Under such
conditions surface water may escape from the system. Checks should be
made to ensure that an adequate level of protection against flooding of
properties is achieved”. Following PPS25 guidance, for return periods
greater than 1 in 30 years, surface flooding of open spaces such as car parks
or landscaping is acceptable for short periods of time.
•
“In designing the site sewage and layout, developers should demonstrate
flow paths and potential effects of flooding resulting from storm events
exceeding the design criteria.” Design of the drainage system should ensure
that water is routed away from any vulnerable property and no flooding of
property should occur as a result of a 1 in 100 year event including allowance
for climate change.
•
PPS25 states that “The development rate of runoff into a watercourse or
other receiving water body should be no greater than the existing rate of
runoff for the same event.” This should also be applied to discharges to
sewer system as per the majority of the North London.
Developers should be encouraged to reduce runoff rates from previously developed
sites as far as possible and supplementary planning guidance of the London Plan
outlines the Mayors “essential” and “preferred standards” to include the importance
of the use of SUDS wherever practical and the need to achieve:
•
“at least 50% attenuation of the undeveloped site’s surface water runoff at
peak times” (essential standard)
•
“100% attenuation of the undeveloped sites surface water runoff as peak
times” (preferred standard).
Areas located within the critical drainage areas should apply the preferred standard.
All drainage design should be undertaken in close liaison with Thames Water and
where the scale of development permits, in liaison with the Environment Agency.
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Further guidance is provided in Designing for exceedance in urban drainage, good
practice (CIRIA C 635) 2006.
9.3
Sustainable Urban Drainage Systems
Surface water drainage systems developed in line with the concept of sustainable
development are referred to as Sustainable Urban Drainage Systems (SUDS).
SUDS are made up of one or more structures used to manage environmental risks
arising from urban runoff and to provide environmental enhancement wherever
possible.
Both PPS1 and PPS25 require that Regional Planning Bodies and LPAs should
promote SUDS. To comply with PPS25 Regional Spatial Strategies should include
specific policies to encourage the use of SUDS. In response to this The London Plan
highlights the importance of using SUDS in new developments wherever possible:
Policy 4C.8
“Boroughs should seek to ensure that surface water run-off is managed as close to
its source as possible. The use of SUDS should be promoted for developments
unless there are practical reasons for not doing so. Such reasons may include the
local ground conditions or density of development. In such cases the developer
should seek to manage as much runoff as possible on site and explore sustainable
methods of managing the remainder as close as possible to the site.”
SUDS should be designed to mimic natural catchment processes as closely as
possible. In order to do this, surface water drainage needs to be considered
throughout the design process.
9.3.1
The SUDS management train
The SUDS Manual describes how design can be split into four stages often referred
to as a “management train”. These four stages are outlined below:
1. Prevention – the use of good site design and maintenance to prevent runoff
and pollution e.g. rainwater reuse/harvesting;
2. Source control – control of runoff at or near to its source e.g. soakaways,
green roofs, permeable pavements;
3. Site control – management of water in a defined area e.g. routing water from
roofs to a detention basin;
4. Regional control – management of runoff from a number of sites, typically in a
large balancing pond or wetland.
The SUDS management train highlights prevention as the first step. This may just
involve good site design and maintenance or features such as rainwater harvesting.
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Figure 4 – Illustration of an office building with a rainwater harvesting pond at base
(Source: Design-build network website)
Rainwater from roofs and hard surfaces such as car parks can be stored and used in
and around properties. The collected water can be used for a range of purposes
such as flushing toilets and irrigation. While the system is in place for rainwater
harvesting, where attenuation is required extra storage volume can be provided
within the storage tanks to attenuate storm water flows.
The second stage of the management train is source control. New development or
re-development should ensure that surface water run-off is managed as close to its
source as possible.
There are many different SUDS techniques that can be used within a development
site, however, not all techniques will be suitable for all locations. Therefore, it is
important that site constraints are identified in order that appropriate SUDS
technique can be used.
Despite the relatively high density of development in the North London Boroughs, a
number of suitable SUDS techniques exist which can be incorporated into new
developments to help manage flood risk.
9.3.2
SUDS Selection
The method for assessing the suitability of SUDS techniques based on site
characteristics has been adapted from (CIRIA Sustainable Drainage Systems,
Hydraulic, structural and water quality advice 2004” (C609)).
The seven boroughs have been split into two groups that share similar
characteristics such as local ground conditions or density of development.
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The selection of SUDS techniques is divided into five sections based on the following
criteria:
•
Hydrological performance
•
Land use characteristics
•
Physical and site features
•
Community and environment
•
Economic and maintenance
A weighting score is given to each criteria, scored from 1 to 3, with 1 having least
weighting. The SUDS are scored out of 5 with 5 being good. The score is then
multiplied by the weighting to give the score for that element of the SUDS technique.
For the purpose of this SFRA, the suitability of SUDS has been based purely on
hydrological performance, land use characteristics and physical site features.
The community and environment, economic and maintenance criteria should be
considered by developers in choosing appropriate SUDS in accordance with specific
site objectives. The matrix provides a guide as to which SUDS features might be
most useful, however, developers are ultimately responsible for assessing the most
suitable techniques as part of their overall drainage strategy.
The total scores have been shaded as follows to highlight the most appropriate
SUDS techniques:
Red = <60 Unlikely to be suitable for use
Orange = 60-70 May be possible to design into schemes
Green = >70 A good SUDS option
The results are shown in Table 15 and Table 16 below.
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sources, pollutant removal
priority?
may be required.
Is water
Attenuation required to
quantity control
prevent downstream
a priority?
flooding.
Is flow rate
If flow can be reduced it
control a
would have a positive
priority?
impact on flood risk.
Is groundwater
No, high Groundwater
recharge
levels in north London.
On-/off-line storage
Storm water wetlands
Extended detention ponds
Grassed filter strips
15
12
6
12
9
9
12
9
12
15
3
3
15
12
9
6
6
9
10
9
15
12
15
9
15
3
12
12
6
9
6
12
12
6
12
12
12
12
12
1
3
3
3
1
3
3
1
1
1
2
2
2
3
2
10
10
10
8
4
8
6
6
8
6
8
8
8
1
5
5
1
4
1
1
3
3
3
3
3
3
4
1
5
5
5
5
5
5
5
5
5
5
5
5
5
1
5
5
3
5
2
2
5
3
1
1
1
1
5
1
5
5
5
5
5
4
3
3
1
5
4
3
5
1
5
5
5
5
5
5
4
5
5
5
5
4
5
80
77
62
60
43
61
60
50
63
60
67
62
65
Green roofs
Wet ponds
15
Infiltration basin
15
Swales
Filter drains
Mixture of roof runoff
removal a
Filtration techniques
Infiltration devices
Hydrological
Is pollutant
Bio-retention
3
Assessment
Weighting
Criteria
Pervious Pavements
Table 24 - SUDS Score within Haringey, Waltham Forest, Barnet and Enfield
Land use
required?
Suitability to
Development will be
type of
varied.
development
Catchment
It has been assumed that
Area
all catchments will be 2 –
Physical site features
8ha
Site slope
A site slope of 0 to 10%
has been assumed.
Space required
Space is less of a
constraint.
Soil infiltration
Geology is impermeable
rate
in the west being clays
moving to permeable
gravels in the east.
(assumed >10-6m/s,<10-3m/s)
Water table
Greater than 1m depth to
depth
water table across area
Total Score
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Filter drains
Infiltration basin
Extended detention ponds
Wet ponds
10
10
8
4
8
6
6
6
6
8
10
4
removal a
be from roof runoff.
priority?
Pollutant removal is not a
3
15
12
9
6
6
9
9
9
15
12
15
9
15
3
12
12
6
6
6
12
12
6
12
9
9
9
12
1
3
2
3
1
3
3
2
1
1
2
2
2
1
1
5
5
5
3
2
4
3
3
3
3
4
5
3
1
5
5
5
5
5
5
5
5
5
1
1
1
5
1
5
5
5
5
5
5
5
5
5
5
5
5
5
3
15
15
9
15
6
6
6
15
3
3
3
3
15
1
5
5
5
5
4
4
2
1
1
4
4
4
5
1
5
5
5
5
5
5
5
5
5
5
5
4
5
80
76
62
59
46
61
56
56
56
50
56
52
70
Green roofs
On-/off-line storage
Infiltration devices
10
Storm water wetlands
Swales
Most instances runoff will
Bio-retention
Grassed filter strips
Is pollutant
Pervious Pavements
2
Assessment
Weighting
Criteria
Filtration techniques
Table 25 - SUDS Score Technique within Camden, Islington and Hackney
Land
use
Hydrological
priority.
Is water
Attenuation required to
quantity control
prevent downstream
a priority?
flooding.
Is flow rate
If flow can be reduced it
control a
would have a positive
priority?
impact on flood risk.
Is groundwater
No, high groundwater
recharge
levels experienced in
required?
these boroughs.
Suitability to
Assumed that all sites will
type of
be in dense urban areas.
development
Catchment
Assumed that due to
Area
nature of development, all
catchments will be <2ha
Physical site features
Site slope
A site slope of 0 to 10%
has been assumed.
Space required
Limited space for SUDs
across these boroughs
Soil infiltration
Geology is impermeable
rate
in the west being clays
moving to permeable
gravels in the east.
(assumed >10-6m/s,<10-3m/s)
Water table
Assumed greater than 1m
depth
depth to water table
across boroughs
Total Score
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In the boroughs of Haringey, Waltham Forest, Barnet and Enfield, pervious
pavements, green roofs and storage facilities such as wet ponds were cited as being
the most applicable SUDS features. However these are not the only SUDS options
that are available; the other SUDS techniques are dependent on the local site
conditions and the proposed development. These options include various infiltration
techniques like swales, bio-retention and other attenuation methods such as on/off
line storage. These have gained a lower score as they are not as effective at flow or
water quality control however they could be considered should a suitable site or
development be available. Filter strips and drains were considered unlikely to be
appropriate for use due to the consistent low scores as a result of their overall
unsuitability within the development areas.
For the more densely urban areas including Camden, Islington and Hackney, the use
of permeable pavements, green roofs and on/off line storage were identified as the
most appropriate techniques. Bio-retention and swales are considered to be limited
in their use in these areas. Their use is dependent on the local site conditions and
the proposed development. The remaining SUDS techniques are unlikely to be used
due to the space required, their suitability for the type of development, their limited
flow control and limited pollution control.
It should be noted that the all SUDS features should be considered and selection
should be undertaken by developers on a site by site basis.
A brief summary of potential SUDS techniques is included in the following sections.
For more detailed information including design methods, refer to CIRIA C697, The
SUDS Manual.
9.3.3
Pervious Surfaces
Pervious pavements allow rainwater to infiltrate through the surface into underlying
construction layers where water is stored prior to infiltration to the ground, reused or
released to a surface water drainage system or watercourse at an attenuated rate.
While pervious pavements are a good choice of SUDS for use within urban areas,
consideration of the proximity of basements and foundations must be made. Where
pervious pavements are located within 5m of foundations or basements, an
impermeable membrane liner is required to prevent infiltration.
Pervious pavements can either be made from porous materials which allow
infiltration across their entire surface e.g. gravels, grass and porous concretes, or
permeable surfaces which are made from impermeable materials with voids to allow
infiltration e.g. brick paving.
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Figure 4 - Illustration of a Pervious Pavement using Block Paving
(Source: CIRIA website)
Pervious pavements can be used for both infiltration and attenuation collecting water
from paved areas and roof catchments. They have been shown to reduce both the
peak flow rate and total runoff volume from developments. They can be designed to
temporarily store runoff from storm events with a return period of 1 in 100 years and
are suitable for incorporation into rainwater utilisation projects.
Pervious surfaces can be incorporated into soft landscaping and oil interceptors can
be added to improve pollutant retention and removal. In urban areas where there is
a high percentage of hard cover the use of pervious surfaces for car parks and hard
areas is a valuable technique that should be used wherever possible.
9.3.4
Green Roofs
“Green roofs comprise a multi layered system that covers the roof of a building with
vegetation cover/landscaping/permeable car parking, over a drainage layer. They
are designed to intercept and retain precipitation, reducing the volume of runoff and
attenuating peak flows.” (CIRIA Sustainable Drainage Systems, Hydraulic, structural
and water quality advice 2004”)
Rooftops form a major part of the cityscape, but have been vastly under utilised.
The use of green roofs can reduce the size of downstream SUDS and drainage
infrastructure that is required.
There are two main types of green roof as described below:
Extensive green roof
This covers the whole roof area with low growing, low maintenance plants. They
usually comprise of 25 – 125mm thick soil layer in which a variety of hardy, drought
tolerant, low level plants are grown. Extensive green roofs are designed to be self
sustaining and cost effective and can be used in a wide variety of locations.
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Figure 5 - Illustration of an extensive green roof
(Source: The Greater Vancouver Regional District (GVRD) Web site)
Intensive green roof
Intensive green roofs are landscaped areas which include planters or trees and are
usually publicly accessible. They may include irrigation and storage for rainwater.
They often require more maintenance and impose a greater load on the roof
structure than extensive green roofs.
Figure 6 - Illustration of an Intensive Green Roof
(Source: Trelleborg website information green roofs)
Intensive roofs can be adapted to be simple intensive green roofs. These are
vegetated with lawns and still require maintenance, however, they impose a reduced
load on the roof structure and are less expensive.
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Figure 7 - Illustration of a Simple Intensive Green Roof
(Source: Rehbein Environmental Solutions website)
While green roofs are designed to absorb most of the rainfall from an ordinary rainfall
event (up to a 2 year return period), during larger storms there will still be a need to
discharge excess water to the buildings drainage system.
9.3.5
Soakaways
Figure 8 - Illustration of a traditional soakaway design
(Source: SUDS website)
Soakaways are drainage structures with high available storage. Surface water runoff
is directed to the soakaway where the storage volume provides attenuation of flows
and gradual infiltration to the surrounding soil. Soakaways can be designed to store
rapid runoff from a single house, several buildings or highway areas. Long, thin
soakaways are called infiltration trenches.
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9.3.6
Infiltration trenches
Infiltration trenches are shallow trenches filled with rubble or stone that creates
temporary storm water storage which is then filtered through the stone material and
conveyed downstream or infiltrated into the soil. Infiltration trenches can significantly
reduce runoff rates and volumes. They can easily be incorporated into site
landscaping and fit well beside roads.
9.3.7
Infiltration Basins
Infiltration basins are vegetated depressions designed to store runoff and infiltrate it
gradually into the ground.
Figure 9 – Cross section through an infiltration basin
(Source: SUDS website)
9.3.8
Filter strips
Filter strips are vegetated strips of gently sloping land designed to accept runoff as
overland flow. They are often used as a pre treatment technique before other SUDS
techniques such as swales. They have low construction costs and can easily be
integrated into landscaping to provide aesthetic benefits. They however offer limited
attenuation during extreme rainfall events. They are best suited to treating runoff
from relatively small drainage areas such as roads, small car parks and pervious
surfaces. They are not suitable for use on contaminated sites or in areas of
groundwater vulnerability.
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9.3.9
Swales
Figure 10 – Illustration of a swale
(Source: CIRIA website, Site is in Reddich Worcestershire)
Swales are linear vegetated drainage features in which surface water can be stored
or conveyed. When used alongside roads, swales can replace conventional gullies
and drainage pipes. They are easy to incorporate into landscaping, offer good
reductions in both runoff rates and pollutant removal. They are ideal for use as
drainage systems on industrial sites because any pollution that occurs will be visible
and can be dealt with before it causes damage to a receiving watercourse.
9.3.10
Bio-retention areas
Figure 11 – Illustration of an engineered bio-retention area
(Source: North Carolina division of water quality website)
Bio-retention areas are shallow landscaped areas. They are typically under drained
and rely on engineered soils and enhanced vegetation and filtration to remove
pollution and reduce runoff downstream. They are designed to treat surface water
runoff from frequent rainfall events. During larger rainfall events, excess water is
passed forward to other drainage facilities. Bio-retention areas are very effective in
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removing urban pollution and are well suited to use in highly impervious areas such
as car parks, along side highways and roads or as landscaped islands draining
impervious areas.
9.3.11
Detention Basins
Figure 12 – Illustration of a detention basin following a period of heavy rain
(Source: Sustainable Urban Drainage Systems Network website)
These are vegetated surface storage basins that provide flow control through
attenuation of storm water runoff and controlled release. Detention basins are
normally dry except during and immediately after a storm event. In some instances
the land may also function as a recreational facility e.g. playground or sports field.
9.3.12
Ponds
Figure 13 – Illustration of a pond in a high density housing development in Bicester, Oxfordshire
(Source: Environment Agency website)
Ponds can provide both storm water attenuation and treatment. Runoff from each
rain event is detained and treated in the pond through sedimentation and biological
uptake. Ponds can provide valuable aesthetic and wildlife value to a development
site.
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9.3.13
On/Off-line storage
On-line or off-line storage refers to tanks or other underground storage structures.
Tanked storage collects and stores runoff to be released at the required rate into the
receiving watercourse or sewer. On/Off-line storage can take the form of oversized
pipes, concrete storage and cellular storage systems. They can be applied where
space is a constraint and above ground SUDS features such as ponds are not
suitable. Tank storage systems do not provide any pollutant removal potential and
should, where possible be used in conjunction with other SUDS features. The
primary benefit of On/Off-line storage is their ability to attenuate run-off to the
greenfield run-off rate.
9.3.14
Adoption and Maintenance of SUDS
There is currently no specific provision for the adoption of SUDS techniques by
Water Authorities. However, within urban areas much of the development drainage
will fall within private development ownership e.g. green roofs, meaning that the
adoption scenario should not be a constraint to the use of SUDS.
Owners of developments using SUDS should be provided with an owners’ manual to
include details of the location of SUDS, a maintenance plan, brief summary of how
they work and identification of areas where activities are prohibited e.g. stockpiling
materials on pervious surfaces.
Maintenance inspections can generally be carried out monthly to include activities
such as grass cutting, plant control and debris removal. If SUDS systems are
properly monitored and maintained, any deterioration in performance can be
managed out.
9.3.15
Land Drainage Consent
The Water Resources Act 1991 and associated byelaws require those wishing to
undertake works in, over, under or adjacent to main rivers to obtain a formal consent
for the work from the EA. This is to ensure that such activities do not cause or make
worse an existing flooding problem, interfere with the existing watercourse, and do
not adversely affect the local environment, fisheries, wildlife, and flood defences.
These consents are referred to as ‘flood defence consents’; in the past they were
sometimes called ‘land drainage consents’, after the old legislation that applied.
Under the Land Drainage Act 1991, you also need the EA’s consent to construct a
culvert or flow control structure (such as a weir) on any ordinary watercourse. It
should be noted that the EA generally oppose culverts and in-channel structures
because of their environmental impacts.
Once an application of the full details of the proposed work is submitted, it will take at
least two months to process. It is recommended to all developers to talk to the EA as
early as possible to avoid delays and wasted effort. Works will not approved if they
are believed harm the environment or increase flood risk, even if the works appear to
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be sound from an engineering or structural point of view. The EA promote and
encourage the use of ‘soft engineering’ methods to control erosion. Such as the use
of natural materials like woven willow spilling or natural planting to limit erosion
where practical, rather than steel sheet piles, unless conditions at the location
require that piles are used.
Work must not be conducted without consent. If found without a consent then the
consequences could be expensive. The EA can reclaim the cost of whatever action
is decide as necessary to remove or alter your work. Or, they can require that the
work be removed and the site returned to its original state. Carrying out works
without prior consent or failing to rectify problems may be a criminal offence.
The EA consent only covers the impact of the structure on flood risk and the
environment. It does not:
confirm that a proposed structure is of sound design.
check whether your plan complies with other legislation.
allow you to carry out works on land or rivers that you do not own.
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10
Conclusions
The main objective of this level 1 SFRA was to enable the 7 North London Boroughs
of Barnet, Camden, Enfield, Hackney, Haringey, Islington and Waltham Forest to
apply the sequential flood risk test when allocating development sites, thus ensuring
compliance with PPS25, but more importantly, providing sustainable development
throughout the Borough.
A review of all flood sources was undertaken culminating in a source pathway
receptor model which assessed the risks and consequences posed by the different
sources of flooding. However, the review and assessment of risks from sewer and
surface water flooding was undertaken due to the limited information provided by
Thames Water.
10.1
Summary for Barnet
The LB of Barnet is split between the catchments of the River Brent and the River
Lee. In recent years Barnet as seen many flood risk management activities, which
include hydraulic modelling, improvements to the flood extent mapping and the
construction of flood alleviation schemes.
The Silk Stream, Dollis Brook and tributaries have recently been modelled and the
flood extent mapping results have been included in the SFRA. The flood risk zones
included in this report are based on the less accurate JFLOW modelling results.
Pymmes Brook has been modelled as part of the Lower Lee Tributaries and the
most up to date results are shown on map 8.
Barnet also contains the Brent Reservoir that lies on the southern boarder with LB
Brent. Flood management plans and supporting inundation mapping is will be a legal
requirement from spring 2009.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
10.2
Conclusions for Barnet
The primary source of flood risk to the LB Barnet was found to be from fluvial
flooding from Dollis Brook, Silk Stream, Pymmes Brook and their associated
tributaries.
The Brent Reservoir is considered to present a low risk to Barnet. It is anticipated
that the Flood Management Plans and associated inundation mapping will provide a
more accurate appraisal and assessment of flood risk presented by the reservoir.
Surface water flooding in Barnet presents a low to moderate risk to the borough
while sewer flooding is also noted for being low risk. Areas with historical sewer
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flooding are low but this assessment is based on limited information from Thames
Water.
Groundwater flooding was found to be a relatively low risk.
10.3
Summary for Camden
Camden has no fluvial watercourses within its borough boundaries. The Regents
Canal does flow through the borough but the locations of raised canal banks that
could pose a flood risk are yet to be identified as attempts to obtain information from
British Waterways have been unsuccessful. The Canal could also be considered as
a reservoir as in places embankments have been constructed to create the
watercourse.
Surface water flooding has a well document and recent history. The 2002 Camden
floods highlighted the vulnerability of particular areas (shown in map 22).
The two small reservoirs in Hampstead Heath are part of a series of ponds owned by
the City of London Corporation. These reservoirs lie within the River Fleet
catchment. The flood management plans and supporting inundation mapping is
anticipated to be a legal requirement from spring 2009.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
10.4
Conclusions for Camden
The LB of Camden has a particularly high risk of flooding from sewer and surface
water flooding, while fluvial flood risk remains low due to the lack of watercourses. At
present the Canal presents and unknown risk to the borough. A more detailed
assessment of the flood risk posed by the Canal to the surrounding properties is
required in close partnership with British Waterways.
Surface water flooding zones are in need of further investigation within Camden due
to the high level of risk and historic precedent. A more detailed assessment of sewer
flooding would also be desirable but this would require the cooperation of Thames
Water in releasing the necessary data for a review and analysis to be undertaken.
Where sewer and surface water flooding may occur the consequences are unlikely
to restrict development providing that mitigation for surface water flooding is applied
using the precautionary approach.
Groundwater flooding was found to be a relatively low risk.
The two small reservoirs on Hampstead Heath are considered to present a low risk
to Camden. It is anticipated that the Flood Management Plans and associated
inundation mapping will provide a more accurate appraisal and assessment of flood
risk presented by the reservoir.
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10.5
Summary for Enfield
Enfield is completed located in the River Lee catchment and has a number of
tributaries that form sub-catchments in the Borough. The flood risk zones for the
River Lee and its tributaries are well defined from the extensive studies already
undertaken. The flood risk zones are defined on map 8.
In addition, two large water supply reservoirs, King George V and William Girling
reservoirs are located within Enfield and owned by Thames Water. It is anticipated
that flood management plans and supporting inundation mapping is will be a legal
requirement from spring 2009. The New River water supply aqueduct passes
through Haringey and holds the water in many places above ground level.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
However, a small number of groundwater flooding problems were identified in Enfield
which are attributed to drift deposits of gravels and silts overlying the impermeable
London Clay.
10.6
Conclusions for Enfield
The primary source of flood risk to Enfield Borough was found to be from fluvial
flooding, with the Lower Lee, Pymmes Brook, Salmons Brook and tributaries
providing the highest flood risk.
The King George V and William Girling reservoirs do pose a risk to the downstream
properties. It is anticipated that the Flood Management Plans and associated
inundation mapping will provide a more accurate appraisal and assessment of flood
risk presented by the reservoir. In addition, the New River is consider to pose a
limited flood risk as it is flow is controlled by pumping stations demand.
An assessment of risks from sewer and surface water flooding was undertaken
which indicated that the risk of sewers flooding is generally low across the Borough
with a small number sewer flood risk zones.
Groundwater flooding was found to be a relatively low risk. However the local
geology does increase the risk of groundwater flooding and these risk zones were
highlighted.
10.7
Summary for Hackney
Hackney is located in the River Lee catchment and has a number of tributaries that
form sub-catchments in the Borough. The flood risk zones for the River Lee are well
defined from the extensive studies already undertaken. Some of the Olympic
development works are being undertaken in the south of the borough and include
watercourse improvements. However there is a tidal influence on the Lower Lee in
this borough that could affect the fluvial flooding.
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Two large water supply reservoirs, Stoke Newington East and West reservoirs are
owned by Thames Water and the LB Hackney are located within the borough to
receive water from the New River. The flood management plans and supporting
inundation mapping for reservoirs will be a legal requirement from spring 2009. The
Regents Canal does flow through the borough but the locations of raised canal
banks that could pose a flood risk are yet to be identified as this information was not
obtained from British Waterways.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
10.8
Conclusions for Hackney
The primary source of flood risk to Hackney Borough was found to be from fluvial
flooding, with the Lower Lee provides the highest flood risk.
The Stoke Newington East and West reservoirs do pose a limited risk to the
downstream properties. It is anticipated that the Flood Management Plans and
associated inundation mapping will provide a more accurate appraisal and
assessment of flood risk presented by the reservoir. In addition, the New River is
consider to pose a limited flood risk as it is flow is controlled by pumping stations
demand.
An assessment of risks from sewer and surface water flooding was undertaken that
indicated that the risk of flooding from sewers is generally low except in the north of
the borough. However, this assessment is based on limited information from Thames
Water.
Where sewer and surface water flooding may occur the consequences are unlikely
to restrict development if appropriate mitigation is included in the design. However, a
more detailed assessment of surface water flooding in partnership with Thames
Water would be of significant benefit to the borough.
Groundwater flooding was found to be a relatively low risk.
10.9
Summary for Haringey
Haringey is located in the River Lee catchment and has a number of tributaries that
form sub-catchments in the Borough. The flood risk zones for the River Lee are well
defined from the extensive studies already undertaken. The New River water supply
aqueduct passes through Haringey and has some storage capacity in the Hornsey
Reservoir.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
Additionally, a small number of groundwater flooding problems were identified that
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are attributed to drift deposits of gravels and silts overlying the impermeable London
Clay.
10.10
Conclusions for Haringey
The primary source of flood risk to Haringey is fluvial flooding, with the Lower Lee,
Moselle Brook and Stonebridge Brook providing the highest flood risk. While the
New River poses a limited flood risk as it is flow is controlled by pumping stations
demand.
An assessment of risks from sewer and surface water flooding was undertaken
which indicated that the risk of sewers flooding is generally low across the Borough
with a small number sewer flood risk zones. However, a more detailed assessment
of surface water flooding in partnership with Thames Water would be of significant
benefit to the borough.
Groundwater flooding was found to be a relatively low risk. However the local
geology does increase the risk of groundwater flooding and these risk zones were
highlighted.
10.11
Summary for Islington
Islington has no fluvial watercourses within its borough boundaries. The Regents
Canal does flow through the borough but the locations of raised canal banks that
could pose a flood risk are yet be identified as attempts to obtain information from
British Waterways have been unsuccessful.
Surface water flooding has a recent history in Islington with the 2002 floods and
other more localised flood events. Unlike Camden the detailed information of the
floods were not recorded.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
10.12
Conclusions for Islington
The LB of Islington has a particularly high risk of flooding from sewer and surface
water flooding, while fluvial flood risk remains low due to the lack of watercourses. At
present the Canal presents and unknown risk to the borough. A more detailed
assessment of the flood risk posed by the Canal to the surrounding properties is
required in close partnership with British Waterways.
The borough has a moderate risk of flooding from sewer and surface water flooding
that has been identified from a variety of sources. Further assessment of sewer
flooding would also be desirable with the cooperation of Thames Water. Where
sewer and surface water flooding may occur the consequences are unlikely to
restrict development if appropriate mitigation is included in the design. In addition,
recording of flood events would benefit future flood studies.
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Groundwater flooding was found to be a relatively low risk.
10.13
Summary for Waltham Forest
Haringey is located in the River Lee catchment and has a number of tributaries that
form sub-catchments in the Borough. The flood risk zones for the River Lee are well
defined from the extensive studies already undertaken.
Waltham Forest has 11 medium sized reservoirs within the borough that are owned
by Thames Water. The flood management plans and supporting inundation mapping
for reservoirs will be a legal requirement from spring 2009.
GARDIT operate an on going abstraction scheme across London to maintain the
level of the groundwater table which is assisted by the impermeable geology.
10.14
Conclusions for Waltham Forest
The primary source of flood risk to Waltham Forest Borough was found to be from
fluvial flooding, with the Lower Lee, Pymmes Brook, Salmons Brook and Silk Stream
providing the highest flood risk.
Surface water and sewer flooding poses a moderate flood risk to the borough. A
more detailed assessment of sewer flooding would also be desired but this would
require the cooperation of Thames Water in releasing the necessary data for a
review and analysis. Where sewer and surface water flooding may occur the
consequences are unlikely to restrict sustainable development.
The reservoirs do pose a risk to the downstream properties. It is anticipated that the
Flood Management Plans and associated inundation mapping will provide a more
accurate appraisal and assessment of flood risk presented by the reservoir.
Groundwater flooding was assessed to be a low risk.
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11
Recommendations
11.1
Recommendations for Further Work
It should be noted that it is difficult to produce meaningful recommendations if bodies
do not provide the relevant needed information on flood risk. The following areas of
further work are recommended to be undertaken either by individual Boroughs or
through a continued partnership arrangement.
11.2
Recommendations for Boroughs
1. Update the information on flood defence levels to enable accurate
assessment of standards of protection and assessment of areas benefiting
from flood defences.
2. Assess areas at actual risk of flooding in the Lower Lee Valley and tributaries
adjacent to the Lee, Turkey Brook, Pymmes Brook and Salmons Brook.
Determine high residual flood risk through breach and overtopping,
determining rate of onset of flooding, flood depths and velocities.
3. Boroughs to determine own area action plans and review in conjunction with
allocated sites to determine which areas need coverage.
4. Strategic review of flood defences to ensure that future development of the
Lower Lee Valley can occur.
5. Undertake further assessment of the surface water risks to better define the
surface water flood risk zones. This cannot take place without the cooperation or collaboration of Thames Water who have set out their stance
regarding the SFRA process. Engagement of Thames Water is
recommended to determine whether any flood relief schemes are in place to
alleviate any of the flooding within these zones.
6. Engage with British Waterways to identify raised canal embankments. A
detailed study as part of the Level 2 SFRA to consider the level of risk posed
by the head of water held at the Camden Lock. The boroughs concerned
would benefit from undertaking a joint study on this subject.
7. Review the document after 12 months and then every 2 – 3 years afterwards.
Aspects to be reviewed would include:
•
Flood Zone outlines
•
Planning and Flood Risk Policy at national, regional and Local
Authority levels
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•
Flood risk schemes and drainage initiatives
•
Update and edit information about various watercourses
•
Update of progress on conducting recommendations
8. Review Emergency Flood Plans and update in accordance with the SFRA
updates.
9. Review all reservoirs and ponds that are not on the reservoir register to
consider the risk they pose to the surrounding area.
10. Review the DEFRA “Guide to Emergency Planning for UK Reservoirs”
consultation document when issued in summer 2008 and ensure that
Emergency Planners review flood plans when they become available.
11. Flood data collection in the boroughs by the boroughs. By collecting their own
flood information in a common format with other boroughs would allow the
boroughs to share the information and improve upstream and downstream
planning with particular regard to surface water flooding.
12. The boroughs in partnership with key organisations such as Thames Water
and the Environment Agency should seek to develop a Surface Water
Management Plan.
13. This SFRA has been developed through an interactive process involving
different bodies. In future revisions, this interaction needs to develop into a
partnership that improves the sharing of knowledge and information on a
continual basis to ensure the SFRA and any subsequent flood risk
management policy is based on the latest and best information available.
14. Each borough is considering their own requirements for a level 2 SFRA
where necessary.
15. Where opportunities exist, the Environment Agency recommends for
developers to consider the opening up of culverts and the setting back of
defences to allow hydrological and environmental improvements.
11.3
Recommendations for Developers
1. Review Area Action Plans once available.
2. Review new policies and strategies on a continual basis and when
appropriate provide constructive feedback on policy or strategy in question.
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3. Review proposed development site with consideration of flood risk posed by
reservoirs (either registered or non-registered under the Reservoirs Act 1975)
and canals or other artificial water bodies.
4. Provide adequate clearance of development activities from the footings /
foundations / toe of canals flood defences, reservoirs and other artificial
watercourse structures.
5. Develop closer working relationships with the Local Planning Authority, EA,
Thames Water and other organisations to reduce the risk of flood risk to the
occupants, neighbours and users of the proposed site through all stages of
its development and use.
6. In preparing a site specific FRA or a Drainage Strategy developers much
consider consequences of potential flooding from upstream areas. Similarly
the developer must consider the effect of overland flows to downstream areas
in the event of design exceedence from their site specific drainage system.
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References
1) Department of Communities and Local Government Planning Policy
Guidance 1: General Policy and Principles, 1997
2) Supplementary Planning Guidance, Sustainable design and construction.
Mayor of London, May 2006
3) London County Council, Main Drainage of London Descriptive Account, 1930
4) CIRIA Special Publication 69 The Engineering Implications of Rising
Groundwater levels in the deep aquifer beneath London, 1989
5) Thames Water Central London Rising Groundwater
6) UK Climate Impact Partnership, Climate Change Scenarios for the United
Kingdom: The UKCIP02 Scientific Report, 2002
7) London Climate Change Partnership, London’s Warming: A Climate Change
Impacts in London Evaluation Study, 2002
8) http://te2100.dialoguebydesign.net/dbyd.asp
9) EA / DEFRA, Flood Risk Assessment Guidance for New Development Phase
2, 2005
10) EA Publication, Living on the Edge: A Guide to the Rights and
Responsibilities of Riverside Occupation, 2007 available on the EA website.
11) Department of Communities and Local Government, Planning Policy
Statement 25: Development and Flood Risk, 2005
12) Sustainable Urban Communities, Building for the Future, ODPM, 2003
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ABI, July 2004
14) Preparing for Floods, ODPM October 2003
15) Design Guidance on Flood Damage to Buildings, BRE, 1996
16) Hother, J., Applying Reliability Risk Analysis to Flood Risk Management, 39th
DEFRA Flood and Coastal Management Conference, 2004
17) Steven Wade, Mohammed Hassan & Valerie Bain, Flood risks to people in
defended areas, (HR Wallingford Ltd) presented at the London CIWEM
conference, January 2005.
18) Flood Resilient Homes, Association of British Insurers, April 2004
19) ABI, 2005; Statement of principles on the provision of Flood Insurance –
Updated Version, Association of British Insurers, November 2005
20) DEFRA, 2004; Making Space for Water-Developing a New Government
Strategy for Flood and Coastal Erosion Risk Management in England.
21) Environment Agency 2003; Your Environment – 1953 Floods, Timetable of
Events
22) Lancaster, J.W., Preene, M. and Marshall, C.T. 2004; Development and
Flood Risk – guidance for the construction industry (C624). CIRIA
Publications. ISBN 0-86017-624-X
23) Development and Flood Risk: A Practical Guide Companion to PPS25 ‘Living
Draft’ a consultation paper.
24) Environment Agency 2007. Managing Flood Risk, Thames Region
Catchment Flood Management Plan, Summary Document, Consultation,
January 2007.
25) LCCP Report – “Climate Change and London Transport Systems”
26) London Catchment Abstraction Management Plan
27) Groundwater levels in the Chalk-Basal Sands Aquifer of the London Basin
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