Table of Contents Maryland Stormwater Design Manual VOLUME ONE

Table of Contents Maryland Stormwater Design Manual VOLUME ONE
Table of Contents
Maryland Stormwater Design Manual
VOLUME ONE
Page
List of Tables ................................................................................................................................... i
List of Figures ................................................................................................................................. ii
Preface.............................................................................................................................................iv
Chapter 1:
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Purpose of Manual ............................................................................................................1.1
Why Stormwater Matters: Impact of Runoff on Maryland Watersheds...........................1.3
1.1.1 Declining Water Quality .......................................................................................1.5
1.1.2 Diminishing Groundwater Recharge and Quality ................................................1.7
1.1.3 Degradation of Stream Channels ..........................................................................1.8
1.1.4 Increased Overbank Flooding .............................................................................1.10
1.1.5 Floodplain Expansion .........................................................................................1.11
General Performance Standards for Stormwater Management in Maryland..................1.13
How to Use the Manual ..................................................................................................1.16
1.3.1 Volume One ........................................................................................................1.16
1.3.2 Volume Two (Appendices) ................................................................................1.17
Revising the Manual .......................................................................................................1.19
What’s New ....................................................................................................................1.19
Symbols and Acronyms ..................................................................................................1.21
Chapter 2:
2.0
2.1
2.2
2.3
2.4
2.5
2.6
Introduction to the Manual
Unified Stormwater Sizing Criteria
Unified Stormwater Sizing Criteria ..................................................................................2.1
Water Quality Volume (WQv) ..........................................................................................2.2
Recharge Volume Requirements (Rev).............................................................................2.5
Channel Protection Storage Volume Requirements (Cpv)................................................2.8
Overbank Flood Protection Volume Requirements (Qp2 or Qp10) .................................2.12
Extreme Flood Volume (Qf ) ..........................................................................................2.13
Design Examples: Computing Stormwater Storage Requirements ................................2.14
NOTE: The Maryland Stormwater Design Manual has been revised. Changes are identified
as Supplements (e.g., Supp. 1) and occur throughout the design manual. When there are
conflicts between supplemental and original requirements, the newest shall supersede.
TOC-1
Supp. 1
2.7
2.8
Acceptable Urban BMP Options ....................................................................................2.37
2.7.1 Urban BMP Groups ............................................................................................2.37
2.7.2 Structural BMPs that do not fully meet the WQv Requirement .........................2.39
Designation of Stormwater Hotspots .............................................................................2.41
Chapter 3: Performance Criteria for Urban BMP Design
3.0
3.1
3.2
3.3
3.4
3.5
Performance Criteria for Urban BMP Design ..................................................................3.1
Stormwater Ponds .............................................................................................................3.2
3.1.1 Pond Feasibility Criteria .......................................................................................3.8
3.1.2 Pond Conveyance Criteria ....................................................................................3.9
3.1.3 Pond Pretreatment Criteria..................................................................................3.10
3.1.4 Pond Treatment Criteria......................................................................................3.10
3.1.5 Pond Landscaping Criteria..................................................................................3.11
3.1.6 Pond Maintenance Criteria .................................................................................3.12
Stormwater Wetlands......................................................................................................3.16
3.2.1 Wetland Feasibility Criteria................................................................................3.21
3.2.2 Wetland Conveyance Criteria .............................................................................3.21
3.2.3 Wetland Pretreatment Criteria ............................................................................3.21
3.2.4 Wetland Treatment Criteria ................................................................................3.21
3.2.5 Wetland Landscaping Criteria ............................................................................3.22
3.2.6 Wetland Maintenance Criteria ............................................................................3.24
Stormwater Infiltration ...................................................................................................3.25
3.3.1 Infiltration Feasibility Criteria ............................................................................3.28
3.3.2 Infiltration Conveyance Criteria .........................................................................3.28
3.3.3 Infiltration Pretreatment Criteria ........................................................................3.29
3.3.4 Infiltration Treatment Criteria ............................................................................3.30
3.3.5 Infiltration Landscaping Criteria ........................................................................3.30
3.3.6 Infiltration Maintenance Criteria ........................................................................3.30
Stormwater Filtering Systems.........................................................................................3.31
3.4.1 Filtering Feasibility Criteria ...............................................................................3.38
3.4.2 Filtering Conveyance Criteria.............................................................................3.38
3.4.3 Filtering Pretreatment Criteria ............................................................................3.38
3.4.4 Filtering Treatment Criteria ................................................................................3.39
3.4.5 Filtering Landscaping Criteria ............................................................................3.40
3.4.6 Filtering Maintenance Criteria............................................................................3.41
Open Channel Systems ...................................................................................................3.42
3.5.1 Open Channel Feasibility Criteria ......................................................................3.45
3.5.2 Open Channel Conveyance Criteria ...................................................................3.45
3.5.3 Open Channel Pretreatment Criteria...................................................................3.45
3.5.4 Open Channel Treatment Criteria.......................................................................3.46
3.5.5 Open Channel Landscaping Criteria...................................................................3.46
Supp. 1
TOC-2
Table of Contents
3.5.6
Maryland Stormwater Design Manual
Open Channel Maintenance Criteria...................................................................3.46
Chapter 4: A Guide to BMP Selection and Location in the State of Maryland
4.0
4.1
4.2
4.3
4.4
4.5
4.6
Selecting the Best BMP at a Site ......................................................................................4.1
Watershed Factors.............................................................................................................4.3
Terrain Factors ..................................................................................................................4.6
Stormwater Treatment Suitability.....................................................................................4.8
Physical Feasibility Factors ............................................................................................4.10
Community and Environmental Factors .........................................................................4.12
Checklist: Location/Permitting Factors ..........................................................................4.14
Chapter 5: Environmental Site Design
5.0
5.1
5.2
5.3
5.4
5.5
5.6
Introduction.......................................................................................................................5.1
Design Process and Planning Techniques ........................................................................5.4
Addressing the Unified Sizing Criteria...........................................................................5.17
Alternative Surfaces........................................................................................................5.41
Treatment Using Nonstructural and Micro-Scale Practices ...........................................5.55
Redevelopment .............................................................................................................5.117
Special Criteria for Sensitive Waters ...........................................................................5.122
Glossary ..................................................................................................................................... G.1
References...................................................................................................................................R.1
TOC-3
Supp. 1
VOLUME TWO: APPENDICES
Appendix A: Landscaping Guidance for Stormwater BMPs
A.1
A.2
A.3
A.4
A.5
General Landscaping Guidance for All Stormwater BMPs.............................................A.2
Specific Landscaping Criteria for BMP Groups..............................................................A.4
A.2.1 Ponds and Wetlands.............................................................................................A.4
A.2.2 Infiltration and Filter Systems ...........................................................................A.12
A.2.3 Bioretention .......................................................................................................A.12
A.2.4 Open Channels ...................................................................................................A.18
A.2.5 Filter Strips and Stream Buffer ..........................................................................A.18
Plant Selection for Stormwater Facilities ......................................................................A.19
A.3.1 Hardiness Zones.................................................................................................A.19
A.3.2 Physiographic Provinces....................................................................................A.21
A.3.3 Hydrologic Zones ..............................................................................................A.25
A.3.3 Other Considerations in Stormwater BMP Landscaping...................................A.26
Stormwater Plant List ....................................................................................................A.27
References......................................................................................................................A.49
Appendix B: BMP Construction Specifications
B.1
B.2
B.3
NRCS-MD Code No 378 Pond Standards/ Specifications ..............................................B.1
B.1.1 Supplemental Pond and Wetland Stormwater Specifications........................B.1.1.1
B.1.2 MDE Dam Safety Small Pond Review Criteria.............................................B.1.2.1
Construction Specifications for Infiltration Practices...................................................B.2.1
Construction Specifications for Bioretention, Sand Filters, and Open Channels.........B.3.1
Appendix C: Step-by-Step Design Examples
C.1
C.2
Shallow Wetland Design Example ...............................................................................C.1.1
Design Example 2 – Water Quality BMPs ...................................................................C.2.1
C.2.1 Design Criteria ..................................................................................................C.2.2
C.2.2 Preliminary Design ...........................................................................................C.2.2
C.2.3 BMP Design Option 1.......................................................................................C.2.8
C-2.3.1
Perimeter Sand Filter (F-3) .............................................................C.2.8
C-2.3.2
Pocket Sand Filter (F-5)................................................................C.2.12
C.2.4 BMP Design Option 2.....................................................................................C.2.16
C.2.4.1 Bioretention System (F-6) ..................................................................C.2.17
C.2.4.2 Infiltration Trench (I-1) ......................................................................C.2.18
C.2.5 BMP Design Option 3.....................................................................................C.2.21
C.2.5.1 Dry Swale (O-1)..................................................................................C.2.21
Supp. 1
TOC-4
Table of Contents
Maryland Stormwater Design Manual
Appendix D: Assorted Design Tools
D.1
D.2
D.3
D.4
D.5
D.6
D.7
D.8
D.9
D.10
D.11
D.12
D.13
D.14
D.15
Testing Requirements for Infiltration, Bioretention, and Sand Filter Subsoils ............D.1.1
Geotechnical Methods for Karst Feasibility Testing ....................................................D.2.1
Short Cut Method for a Wetland Drawdown Assessment ............................................D.3.1
Stormwater Criteria for the Maryland Critical Area IDA Zone ...................................D.4.1
Documentation of BMP Ability to Meet the 80% TSS Removal Requirement ...........D.5.1
Industrial Stormwater NPDES Permit Requirements ...................................................D.6.1
MDE/WMA Overview of the NPDES Stormwater Program .......................................D.7.1
Miscellaneous Details for Compliance with Performance Criteria ..............................D.8.1
MD Stream Use Designations.......................................................................................D.9.1
Method for Computing Peak Discharge for Water Quality Storm .............................D.10.1
Method for Computing the Channel Protection Storage Volume (Cpv) .....................D.11.1
Critical Erosive Velocity for Grass and Soil ..............................................................D.12.1
Method for Designing Infiltration Structures .............................................................D.13.1
Eastern Shore (Delmarva) Dimensionless Hydrograph..............................................D.14.1
Miscellaneous MD SHA Design Charts for Determining Pipe Inlet Control.............D.15.1
Appendix E: Archived Material
E.1
Stormwater Credits for Innovative Site Planning ......................................................... E.1.1
TOC-5
Supp. 1
LIST OF TABLES
No.
Title
Page
1.1
1.2
1.3
Typical Pollutant Concentrations Found in Urban Stormwater........................................1.6
NRCS Estimates of Annual Recharge Rates, Based on Soil Type ...................................1.7
Symbols and Acronyms ..................................................................................................1.21
2.1
2.2
2.3
2.4
2.5
2.6
Summary of the Statewide Stormwater Criteria ...............................................................2.1
Rainfall Depths Associated with the 1,2,10, and 100 Year 24-hour Storm Events........2.11
Summary of General Storage Requirements for Reker Meadows..................................2.19
Summary of General Design Information for Claytor Community Center ....................2.25
Summary of General Storage Requirements for Pensyl Pointe ......................................2.34
Classification of Stormwater Hotspots ...........................................................................2.42
4.1
4.2
4.3
4.4
4.5
4.6
BMP Selection Matrix No. 1: Watershed Factors ............................................................4.5
BMP Selection Matrix No. 2: Terrain Factors..................................................................4.7
BMP Selection Matrix No. 3: Stormwater Treatment Suitability ....................................4.9
BMP Selection Matrix No. 4: Physical Feasibility.........................................................4.11
BMP Selection Matrix No. 5: Community and Environmental Factors.........................4.13
Location and Permitting Factors Checklist.....................................................................4.15
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
Natural Resources & Corresponding Regulatory Authorities ..........................................5.7
Summary of Site Development Strategies ......................................................................5.10
Rainfall Targets/Runoff Curve Number Reductions Used for ESD...............................5.21
Effective RCNs for Extensive Green Roofs ...................................................................5.42
Effective RCNs for Permeable Pavements .....................................................................5.48
ESD Sizing Factors for Rooftop Disconnection .............................................................5.59
ESD Sizing Factors for Non-Rooftop Disconnection.....................................................5.62
Sheetflow to Conservation Area Sizing Factors .............................................................5.67
Solar Reflectance Indices for Typical Paving & Roofing Materials ............................5.123
i
Supp. 1
LIST OF FIGURES
No.
Title
1.1
1.2
1.3
1.4
1.5
1.6
Water Balance at a Developed and Undeveloped Site .....................................................1.3
Relationship between Impervious Cover and the Volumetric Runoff Coefficient...........1.4
Decline in Stream Flow Due to Diminished Groundwater Recharge...............................1.8
Increased Frequency of Flows Greater than the Critical Discharge Rate in a Stream
Channel After Development… .........................................................................................1.9
Change in Hydrograph Following Development............................................................1.11
Change in Floodplain Elevations....................................................................................1.12
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
Location of the Eastern and Western Rainfall Zones in Maryland...................................2.3
Relationship between Impervious Cover and the Water Quality Volume........................2.3
Relationship between Rev and Site Impervious Cover .....................................................2.6
Regions of Maryland Not Subject to the Channel Protection Requirement (Cpv)............2.8
Example of Conventional Stormwater Detention Pond....................................................2.9
Reker Meadows ..............................................................................................................2.15
Reker Meadows: Pre Developed Conditions ..................................................................2.20
Reker Meadows: Developed Conditions ........................................................................2.21
Claytor Community Center.............................................................................................2.22
Claytor Community Center: Pre Developed Conditions ................................................2.26
Claytor Community Center: Developed Conditions.......................................................2.27
Pensyl Pointe...................................................................................................................2.29
Pensyl Pointe: Drainage Area 1-Post Developed Conditions.........................................2.35
Pensyl Pointe: Drainage Area 2–Post Developed Conditions ........................................2.36
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
Example of "Micropool" Extended Detention Pond.........................................................3.3
Example of Wet Pond .......................................................................................................3.4
Example of Wet Extended Detention Pond ......................................................................3.5
Example of Multiple Pond System ...................................................................................3.6
Example of Pocket Pond...................................................................................................3.7
Example of Shallow Wetland .........................................................................................3.17
Example of Extended Detention Shallow Wetland ........................................................3.18
Example of Pond/Wetland System .................................................................................3.19
Example of Pocket Wetland............................................................................................3.20
Example of Infiltration Trench .......................................................................................3.26
Example of Infiltration Basin .........................................................................................3.27
Example of Surface Sand Filter ......................................................................................3.32
Supp. 1
Page
ii
No.
Title
Page
3.13
3.14
3.15
3.16
3.17
3.18
3.19
Example of Underground Sand Filter .............................................................................3.33
Example of Perimeter Sand Filter...................................................................................3.34
Example of Organic Filter ..............................................................................................3.35
Example of Pocket Sand Filter .......................................................................................3.36
Example of Bioretention .................................................................................................3.37
Example of Dry Swale ....................................................................................................3.43
Example of Wet Swale....................................................................................................3.44
4.1
Map of Maryland Showing Key Terrain Factors..............................................................4.6
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.0
5.10
5.11
5.12
5.13
5.14
5.15
5.16
5.17
5.18
5.19
5.20
5.21
Design Process for New Development .............................................................................5.6
Cutaway of a Typical Green Roof ..................................................................................5.44
Examples of Permeable Pavements ................................................................................5.49
Example of Disconnection of Rooftop Runoff ...............................................................5.58
Example of Non-Rooftop Disconnection........................................................................5.64
Example of Non-Rooftop Disconnection........................................................................5.65
Example of Sheetflow to Conservation Area..................................................................5.68
Example of Rain Barrels.................................................................................................5.74
Example of Cistern .........................................................................................................5.75
Example of Submerged Gravel Wetland ........................................................................5.79
Example of Landscape Infiltration .................................................................................5.85
Example of Infiltration Berms ........................................................................................5.89
Example of Dry Wells ....................................................................................................5.93
Example of Micro-Bioretention (Variation 1) ................................................................5.99
Example of Micro-Bioretention (Variation 2) ..............................................................5.100
Example of Micro-Bioretention (Variation 3) ..............................................................5.101
Example of Rain Garden...............................................................................................5.107
Example of a Bio-Swale ...............................................................................................5.111
Example of a Wet Swale...............................................................................................5.112
Example of Enhanced Filters ........................................................................................5.116
Design Process for Redevelopment ..............................................................................5.120
iii
Supp. 1
Chapter 1. Introduction to the Manual..................................................... Introduction and Purpose
1.0
Introduction and Purpose of Manual
Title 4, Subtitle 2 of the Environment Article of Annotated Code of Maryland states that “…the
management of stormwater runoff is necessary to reduce stream channel erosion, pollution, siltation
and sedimentation, and local flooding, all of which have adverse impacts on the water and land
resources of Maryland.” The program designed in the early 1980s to address this finding of the
General Assembly concentrated primarily on controlling runoff increases and mitigating water
quality degradation associated with new development. The counties and municipalities in Maryland
are responsible for administering effective stormwater management programs that “…maintain after
development, as nearly as possible the predevelopment characteristics…” These localities have
performed remarkably in establishing Maryland as a national leader in stormwater management
technology. Over the last 14 years, tens of thousands of best management practices (BMPs) have
been constructed in an attempt to meet program mandates. However, the experience gained since
Maryland’s stormwater statute was enacted has identified necessary improvements and revealed a
need to refocus the approach to fulfill the original intent of this essential water pollution control
program.
Recently, increased emphasis on water quality, resource protection needs, increased BMP
maintenance costs, and identified shortcomings in Maryland’s program have all contributed to basic
philosophical changes regarding stormwater management. The “Maryland Stormwater Design
Manual” is an effort to incorporate the significant experiences gained by the State’s stormwater
community and accommodate much needed improvements for managing urban runoff. It is hoped
that the design standards and environmental incentives provided below will produce better methods
and advance the science of managing stormwater by relying less on single BMPs for all development
projects and more on mimicking existing hydrology through total site design policies. Additionally,
the inherent philosophical change should produce smaller less obtrusive facilities that are more
aesthetic and less burdensome on those responsible for long-term maintenance and performance.
The purpose of this manual is threefold:
to protect the waters of the State from adverse impacts of urban stormwater runoff,
to provide design guidance on the most effective planning techniques, and nonstructural and
structural BMPs for development sites, and
to improve the quality of BMPs that are constructed in the State, specifically with regard to
performance, longevity, safety, ease of maintenance, community acceptance and
environmental benefit.
The BMPs and the required design criteria below represent conventional stormwater
management technology for controlling runoff from new development projects. Based upon
current available research, the Maryland Department of the Environment, Water Management
1.1
Supp. 1
Chapter 1. Introduction to the Manual..................................................... Introduction and Purpose
Administration (MDE/WMA) has evaluated each BMP group and the associated design variants
and has developed standards for each so that all perform similarly. The “General Performance
Standards” outlined in this manual (see Section 1.2, page 1.13) specify those criteria that were
used to create runoff control options that would perform equally. The BMPs contained in this
manual are by no means exclusive. MDE encourages the development of innovative practices
that meet the intent of Maryland’s stormwater management law and can perform according to the
standards in Section 1.2. In the future, should structural or non-structural practices be developed
that meet the standards specified below, MDE will approve their use for controlling new
development runoff.
MDE encourages wise, environmentally sensitive site designs to reduce the generation of runoff
borne pollution. Additionally, Maryland has adopted “Smart Growth” policies that are geared
toward concentrating development where it currently exists thereby reducing “suburban sprawl.”
Therefore, redevelopment is encouraged. A stormwater management policy for redevelopment
is established in the Code of Maryland Regulations (COMAR 26.17.02). Additionally,
redevelopment is defined in both COMAR and this manual.
The policy required in COMAR for redevelopment basically specifies a 50% reduction in
impervious surface area below existing conditions. Because this may be impractical due to site
constraints, environmental site design (ESD) practices are to be used to the maximum extent
practicable (MEP) to meet the equivalent in water quality control of a 50% decrease in
impervious surface area. Various alternative BMPs that do not necessarily meet the performance
criteria established in this manual may be implemented for redevelopment projects provided that
it is demonstrated that impervious area reduction and ESD have been implemented to the MEP.
These alternative BMPs may also be implemented to satisfy the pretreatment volume
requirements established in Chapter 3 below. Individual project designers should contact the
appropriate approval authority for the specific practices allowed for redevelopment and
pretreatment purposes.
The approval of new control technologies, modifications to the practices contained in this
manual, and alternative policies regarding stormwater management for new development is the
responsibility of MDE. Typically, information is submitted to the WMA that describes the
policy or practice. For new BMPs, monitoring data need to be submitted that demonstrate that
the performance criteria in this manual can be met. WMA then reviews this material to
determine if the proposed practice is appropriate for use on new development projects. Because
of local variations in ownership policies, maintenance abilities, cost, design standards,
hydrology, etc., information on practices to be used for redevelopment and pretreatment should
be submitted to the appropriate authority for approval.
NOTE: The Maryland Stormwater Design Manual has been revised. Changes are identified as
Supplements (e.g., Supp. 1) and occur throughout the design manual. When there are conflicts
between supplemental and original requirements, the newest shall supersede.
Supp. 1
1.2
Chapter 1. Introduction to the Manual...........................................General Performance Standards
Section 1.2
General Performance Standards for Stormwater Management in Maryland
To prevent adverse impacts of stormwater runoff, the State of Maryland has developed fourteen
performance standards that must be met at development sites. These standards apply to any
construction activity disturbing 5,000 or more square feet of earth. The following development
activities are exempt from these performance standards in Maryland:
1. Additions or modifications to existing single family structures;
2. Developments that do not disturb more than 5000 square feet of land; or
3. Agricultural land management activities.
The following performance standards shall be addressed at all sites where stormwater management
is required:
Standard No. 1
Site designs shall minimize the generation of stormwater and maximize
pervious areas for stormwater treatment.
Standard No. 2
Stormwater runoff generated from development and discharged directly into
a jurisdictional wetland or waters of the State of Maryland shall be
adequately treated.
Standard No. 3
Annual groundwater recharge rates shall be maintained by promoting
infiltration through the use of structural and non-structural methods. At a
minimum, the annual recharge from post development site conditions shall
mimic the annual recharge from pre development site conditions.
Standard No. 4
Water quality management shall be provided through the use of
ennvironmental site design practices.
Standard No. 5
Structural BMPs used for new development shall be designed to remove 80%
of the average annual post development total suspended solids load (TSS)
and 40% of the average annual post development total phosphorous load
(TP). It is presumed that a BMP complies with this performance standard if it
is:
sized to capture the prescribed water quality volume (WQv),
designed according to the specific performance criteria outlined in
this manual,
constructed properly, and
maintained regularly.
1.13
Supp. 1
Chapter 1. Introduction to the Manual...........................................General Performance Standards
Standard No. 6
Control of the two-year and ten-year frequency storm events is required if
the local authority determines that additional stormwater management is
necessary because historical flooding problems exist and downstream
floodplain development and conveyance system design cannot be controlled.
In addition, safe conveyance of the 100-year storm event through
stormwater management practices shall be provided.
Standard No. 7
To protect stream channels from degradation, the channel protection storage
volume (Cpv) shall be based on the runoff from the one-year frequency storm
event. Environmental site design practices shall be used to the maximum
extent practicable to address Cpv. Any remaining Cpv requirements shall be
addressed using stormwater practices described in Chapter 3.
Standard No. 8
Stormwater discharges to critical areas with sensitive resources [e.g., cold
water fisheries, shellfish beds, swimming beaches, recharge areas, water
supply reservoirs, Chesapeake Bay Critical Area (see Appendix D.4)] may be
subject to additional performance criteria or may need to utilize or restrict
certain BMPs.
Standard No. 9
All BMPs shall have an enforceable operation and maintenance agreement
to ensure the system functions as designed.
Standard No. 10
Every BMP shall have an acceptable form of water quality pretreatment.
Standard No. 11
Redevelopment, defined as any construction, alteration or improvement
exceeding five thousand square feet of land disturbance on sites where
existing land use is commercial, industrial, institutional or multi-family
residential, is governed by special stormwater sizing criteria depending on
the amount of increase or decrease in impervious area created by the
redevelopment.
Standard No. 12
Certain industrial sites are required to prepare and implement a stormwater
pollution prevention plan and file a notice of intent (NOI) under the
provisions of Maryland’s Stormwater Industrial National Pollutant
Discharge Elimination System (NPDES) general permit (a list of industrial
categories subject to the pollution prevention requirement can be found in
Appendix D.6). The requirements for preparing and implementing a
stormwater pollution prevention plan are described in the general discharge
permit available from MDE and guidance can be found in the United States
Environmental Protection Agency’s (EPA) document entitled, “Storm Water
Management for Industrial Activities, Developing Pollution Prevention Plans
and Best Management Practices” (1992). The stormwater pollution
Supp. 1
1.14
Chapter 1. Introduction to the Manual...........................................General Performance Standards
prevention plan requirement applies to both existing and new industrial
sites.
Standard No. 13
.
Standard No. 14
Stormwater discharges from land uses or activities with higher potential for
pollutant loadings, defined as hotspots in Chapter 2, may require the use of
specific structural BMPs and pollution prevention practices. In addition,
stormwater from a hotspot land use may not be infiltrated without proper
pretreatment.
In Maryland, local governments are usually responsible for most stormwater
management review authority. Therefore, prior to design, applicants should
always consult with their local reviewing agency to determine if they are
subject to additional stormwater design requirements. In addition, certain
earth disturbances may require NPDES construction general permit
coverage from MDE (see Appendix D.7).
1.15
Supp. 1
Chapter 1. Introduction to the Manual....................................................... How to Use the Manual
Section 1.3
How to Use the Manual
The Maryland Stormwater Design Manual is provided in two volumes. This first volume provides
designers a general overview on how to size, design, select and locate BMPs at a new development
site to comply with State stormwater performance standards. The second volume contains
appendices with more detailed information on landscaping, BMP construction specifications, stepby-step BMP design examples and other assorted design tools.
Section 1.3.1 VOLUME ONE: STORMWATER MANAGEMENT CRITERIA
The first volume of the manual is organized as follows:
Chapter 1. Introduction to the Manual.
Chapter 2. Unified Stormwater Sizing Criteria. This chapter explains the five new sizing criteria
for water quality, recharge, channel protection, overbank flood control, and extreme flood
management in the State of Maryland. The chapter also outlines the basis for design calculations.
Three step-by-step design examples are provided to familiarize the reader with the new procedures
for computing storage volumes under the five sizing criteria. The chapter also briefly outlines the
six groups of acceptable BMPs that can be used to meet recharge and water quality volume sizing
criteria. Acceptable BMP groups are:
Stormwater Ponds
Stormwater Wetlands
Infiltration Practices
Filtering Systems
Open Channel Practices
Non-structural Practices
Lastly, the chapter presents a list of land uses or site activities that have been designated as
“stormwater hotspots.” If a development site is considered a “hotspot," it may have special
requirements for pollution prevention and groundwater protection.
Chapter 3. Performance Criteria for Urban BMP Design. The third chapter presents specific
performance criteria and guidelines for the design of five groups of structural BMPs. The
performance criteria for each group of BMPs are based on six factors:
General Feasibility
Conveyance
Pretreatment
Treatment Geometry
Landscaping
Maintenance
Supp. 1
1.16
Chapter 1. Introduction to the Manual....................................................... How to Use the Manual
In addition, Chapter 3 presents a series of schematic drawings to illustrate typical BMP designs.
Chapter 4. Guide to BMP Selection and Location in the State of Maryland
This chapter presents guidance on how to select the best BMP or group of practices at a new
development site, as well as environmental and other factors to consider when actually locating each
BMP. The chapter contains six comparative tables that evaluate BMPs from the standpoint of the
following factors:
Watershed Factors
Terrain Factors
Stormwater Treatment Suitability
Physical Feasibility Factors
Community and Environmental Factors
Location and Permitting Factors
Chapter 4 is designed so that the reader can use the tables in a step-wise fashion to identify the most
appropriate BMP or group of practices to use at a site.
Chapter 5. Environmental Site Design
The Stormwater Management Act of 2007 requires establishing a comprehensive process for
stormwater management approval, implementing ESD to the MEP, and ensuring structural practices
(Chapter 3) are used only where absolutely necessary. Implementing ESD not only reduces the
impact of development on the environment, but also reduces the size and cost of stormwater
practices needed at the site. The Chapter includes:
Design Process and Planning Techniques
ESD Sizing Criteria
Alternative Surfaces
Nonstructural and Micro-Scale Practices
Redevelopment Design Process
Special Criteria for Sensitive Waters
The chapter defines ESD and describes planning techniques and design requirements that are used
to implement ESD and treat runoff at the source.
Section 1.3.2 VOLUME TWO: STORMWATER DESIGN APPENDICES
The second volume is provided separately and contains the technical information needed to actually
design, landscape and construct a BMP. Volume Two is divided into four appendices, including:
1.17
Supp. 1
Chapter 1. Introduction to the Manual....................................................... How to Use the Manual
Appendix A. Landscaping Guidance for Stormwater BMPs. Good landscaping can often be an
important factor in the performance and community acceptance of many stormwater BMPs. The
Landscaping Guide provides general background on how to determine the appropriate landscaping
region and hydrologic zone in Maryland. Appendix A also includes tips on how to establish more
functional landscapes within stormwater BMPs and contains an extensive list of trees, shrubs,
ground covers, and wetland plants that can be used to develop an effective and diverse planting plan.
Appendix B. BMP Construction Specifications. Good designs only work if careful attention is
paid to proper construction techniques and materials. Appendix B contains detailed specifications
for constructing infiltration practices, filters, bioretention areas and open channels. In addition,
Appendix B includes a copy of the NRCS Code 378 Standards and Specifications for Ponds.
Appendix C. Step-by-Step Design Examples. A series of design examples are provided in this
appendix to help designers and plan reviewers better understand the new stormwater sizing criteria
and design procedures. Step-by-step design examples are provided for a pond, a sand filter, an
infiltration trench, a dry swale, and a bioretention area.
Appendix D. Assorted Design Tools. This appendix contains an assortment of design tools for
stormwater management, including guidance on geotechnical testing, calculating water balance,
documenting whether a site complies with the Chesapeake Bay Critical Area “10% Rule,” NPDES
stormwater permits, pollution prevention, design details, State Water Use Designations and other
useful design information.
Appendix E. Archived Material and Supplemental Design Guidance. The last appendix
contains material removed from Volume I of the Design Manual for historical purposes. The
appendix also includes guidance material for associated with Design Manual supplements.
Supp. 1
1.18
Chapter 1. Introduction to the Manual............................................................ Revising the Manual
Section 1.4
Revising the Design Manual
The Maryland Stormwater Design Manual establishes minimum performance criteria that should be
met by all techniques and devices used for stormwater management in Maryland. On occasion,
variations or other techniques and devices may be found to function better or be more desirable for
stormwater management by plan approval authorities. As stated above, MDE is responsible for
approving the use of new techniques for controlling runoff from new development. If an approval
authority decides it would like to utilize a revised technique or device on a regular basis, it needs to
prepare a Standard and accompanying Specifications with a cover letter to be submitted to the
MDE/WMA.
A subcommittee consisting of MDE technical personnel will review the revised technique or device
and any supporting data submitted. When the technique or device is approved by the technical
subcommittee, an approval authorization from the Director of WMA and the technical representative
of the local approval authority will be issued. Once the revised or new technique or device has
received approval it can be used on a regular basis within the jurisdiction. If other jurisdictions
desire to utilize the same technique or device then they must seek approval from the technical
subcommittee. A great amount of deviation from the methods within this design manual is not
anticipated, but when better stormwater management can be achieved, revisions will generally be
looked upon favorably.
Section 1.5
What’s New?
This section highlights some of the new stormwater design requirements that are being introduced in
the manual. It is provided to help designers understand how the new manual may affect how they
prepare stormwater plans and practices. At most sites, designers shall now:
Measure the amount of impervious cover created by the development.
Determine if the proposed land use or activity at the site is designated as a “stormwater
hotspot.”
Determine the Use Designation of the receiving water and the condition of the watershed.
Provide a volume that mimics the natural rate of groundwater recharge using structural
and/or nonstructural BMPs (Rev).
Implement ESD to the MEP to mimic predevelopment conditions.
Follow a specific design process to implement a comprehensive site development plan.
Provide water quality and recharge volume storage using approved ESD practices.
Use ESD practices to the MEP to provide Cpv storage. Any remaining Cpv storage
requirements must be addressed using approved BMP options that can meet pollutant
removal targets.
Ensure that the BMP selected meets specific performance criteria with respect to feasibility,
conveyance, pretreatment, treatment, landscaping and maintenance.
Follow new geotechnical testing procedures and provide the contractor with formal
construction specifications.
1.19
Supp. 1
Chapter 1. Introduction to the Manual......................................................................... What’s New
Consider where the BMP is located in relation to natural features and development
infrastructure.
Consider innovative site planning techniques that can reduce both the size and cost of
stormwater practices.
Include operation and maintenance information on approved stormwater management plans.
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1.20
Chapter 1. Introduction to the Manual....................................................... Symbols and Acronyms
Section 1.6
Symbols and Acronyms
As an aid to the reader, the following table outlines the symbols and acronyms that are used
throughout the text. In addition, a glossary is provided at the end of this volume that defines the
terminology used in the text.
Table 1.3 Key Symbols and Acronyms Cited in Manual
A
Af
Asf
Asp
BMP
cfs
Cpv
CMP
CN
df
du
ED
ESD
ESDv
f
fps
hf
HSG
Ia
I
k
MEP
PE
P
Qe
Qf
drainage area
filter bed area
surface area, sedimentation basin
full
surface area, sedimentation basin
partial
best management practice
cubic feet per second
channel protection storage volume
(extended detention of the 1-year
post development runoff)
corrugated metal pipe
curve number
depth of filter bed
dwelling units
24 hour drawdown of the water
quality volume
environmental site design
environmental site design storage
volume
soil infiltration rate
qi
qo
Qp
peak inflow discharge
peak outflow discharge
overbank flood protection volume
qu
unit peak discharge
TR-20
feet per second
TR-55
head above filter bed
hydrologic soil group
initial abstraction
percent impervious cover
coefficient of permeability
Maximum extent practicable
ESD rainfall target
precipitation depth
ESD runoff depth
extreme flood protection volume
TSS
Vf
Vr
Vs
Vt
Vv
WQv
WSE
qp
Rev
Rv
water quality peak discharge
recharge volume
volumetric runoff coefficient
R/W
S
SD
tc
tf
right of way
soil specific recharge factor
separation distance
time of concentration
time to drain filter bed
TP
tt
1.21
total phosphorous
time of travel
Technical Release No. 20 Project
Formulation-Hydrology, computer
program
Technical Release No. 55 Urban Unit
Hydrology for Small Watersheds
total suspended solids
filter bed volume
volume of runoff
volume of storage
total volume
volume of voids
water quality storage volume
water surface elevation
Supp. 1
Chapter 5. Environmental Site Design..........................................................................Introduction
Section 5.0
5.0.1
Introduction
Background
The primary goal of Maryland’s stormwater management program is to maintain after
development, as nearly as possible, the predevelopment runoff characteristics. Traditional
stormwater management strategies treat runoff to mitigate adverse water quality and/or quantity
impacts associated with new development. Designs applying these strategies often combine
centralized structural practices for pollutant removal with channel erosion or flood control
impoundments. These designs are less able to mimic predevelopment conditions because they
focus on managing large volumes of polluted stormwater rather than treating runoff closer to the
source.
A comprehensive design strategy for maintaining predevelopment runoff characteristics and
protecting natural resources is available. This strategy, known as Environmental Site Design or
“ESD,” relies on integrating site design, natural hydrology, and smaller controls to capture and
treat runoff. This chapter provides the foundation to refocus stormwater design from centralized
management to more effective planning and implementation of ESD.
5.0.2
Requirements of the Stormwater Management Act of 2007
The “Stormwater Management Act of 2007” (Act), requires establishing a comprehensive
process for stormwater management approval, implementing ESD to the maximum extent
practicable (MEP), and ensuring that structural practices (see Chapter 3) are used only where
absolutely necessary. The Act also establishes several performance standards for stormwater
management plans. Designers must now ensure that these plans are designed to:
¾
¾
¾
¾
¾
¾
¾
¾
¾
Prevent soil erosion from development projects.
Prevent increases in nonpoint pollution.
Minimize pollutants in stormwater runoff from both new development and redevelopment.
Restore, enhance, and maintain chemical, physical, and biological integrity of receiving
waters to protect public health and enhance domestic, municipal, recreational, industrial and
other uses of water as specified by MDE.
Maintain 100% of the average annual predevelopment groundwater recharge volume.
Capture and treat stormwater runoff to remove pollutants.
Implement a channel protection strategy to protect receiving streams.
Prevent increases in the frequency and magnitude of out-of-bank flooding from large, less
frequent storms.
Protect public safety through the proper design and operation of stormwater management
facilities.
The Act presents a new opportunity to improve Maryland’s stormwater management program.
The original Chapter 5 encouraged ESD through a series of optional credits for the design of
nonstructural practices. Changes in response to the Act not only expand on the ESD practices
first introduced in the Manual but also allow for planning techniques to improve implementation
NOTE: In this chapter, italics indicate mandatory criteria, whereas recommended criteria are
shown in normal typeface.
5.1
Supp.1
Chapter 5. Environmental Site Design..........................................................................Introduction
and overall performance. The remaining sections of this chapter will further define ESD, discuss
planning techniques used in its implementation, and provide design requirements for
nonstructural and micro-scale practices used to treat runoff at the source. For reference
purposes, the original Chapter 5 can be found in Appendix E.1.
5.0.3
Environmental Site Design
Definition
There are many stormwater design strategies that seek to replicate natural hydrology. Sometimes
known as better site design, low impact development, green infrastructure, or sustainable site
design, these strategies all espouse similar techniques. In each, a combination of planning
techniques, alternative cover, and small-scale treatment practices is used to address impacts
associated with development. For consistency, the Act adopts ESD as a more generic
classification for use in Maryland.
Title 4, Subtitle 201.1(B) of the Act defines ESD as “…using small-scale stormwater
management practices, nonstructural techniques, and better site planning to mimic natural
hydrologic runoff characteristics and minimize the impact of land development on water
resources.” Under this definition, ESD includes:
¾ Optimizing conservation of natural features (e.g., drainage patterns, soil, vegetation).
¾ Minimizing impervious surfaces (e.g., pavement, concrete channels, roofs).
¾ Slowing down runoff to maintain discharge timing and to increase infiltration and
evapotranspiration.
¾ Using other nonstructural practices or innovative technologies approved by MDE.
Impacts of Imperviousness
The goal of traditional site design strategies is to maximize development potential by focusing on
the layout of buildings, roads, parking, and other features. Conventional development practices
tend to maximize site imperviousness and contribute to many of the impacts discussed in Chapter
1. These include diminished groundwater recharge, increased flows and runoff volumes,
pollutant accumulation, and elevated water temperatures.
Documentation such as the Impacts of Impervious Cover on Aquatic Systems (Center for
Watershed Protection, 2003) and other studies of Eastern Piedmont and Coastal Plain streams in
Maryland (Morgan and Cushman, 2005) and headwater streams in Montgomery County (Moore
and Palmer, 2005) all indicate that stream biodiversity decreases as impervious cover increases.
There is no simple formula, rule, or threshold for determining how much impervious cover may
be sustained in a given watershed. Generally, stream quality and watershed health diminish
when impervious cover exceeds 10% and become severely degraded beyond 25% (Center for
Watershed Protection, 2003). Results from the Maryland Biological Stream Survey (MBSS)
indicated that in surveyed streams, health was never good when watershed imperviousness
exceeded 15%, (Boward, 1999). These studies establish a fundamental connection between
impervious cover and watershed impairment.
Supp. 1
5.2
Chapter 5. Environmental Site Design..........................................................................Introduction
Integrating the fundamental principles of ESD during the planning process helps minimize the
adverse impacts of imperviousness. The resulting designs reduce the need for costly
infrastructure and maintenance while providing treatment closer to the source. To accomplish
this, the designer must consider the basic concepts found in Section 5.1, Planning Techniques.
5.3
Supp.1
Chapter 5. Environmental Site Design............................................... Design Process and Planning
Section 5.1
5.1.1
Design Process and Planning Techniques
Introduction
The design process described in this section will provide guidance for implementing ESD
planning strategies and practices into a comprehensive site development plan. These techniques
involve protecting natural resources, integrating erosion and sediment controls with stormwater
management practices, minimizing site imperviousness, and using natural conveyance and ESD
practices throughout the site. Applying these techniques early in the design process will ensure
that all available resources have been considered in order to protect streams and waterways from
the impact of land development activities. The design process will require the developer to
adhere to the following procedures to achieve ESD to the MEP:
¾ Following the Design Process for New Development as outlined in the step wise procedures
in Figure 5.1.
¾ Developing a map that identifies natural resource areas and drainage patterns and devising
strategies for protection and enhancement.
¾ Minimizing total site imperviousness by implementing clustered development and other better
site design techniques.
¾ Demonstrating that all practical options for meeting stormwater requirements using ESD
have been explored by using natural areas and landscape features to manage runoff from
impervious surfaces.
¾ Participate in the comprehensive review process for interim plans review and approval at the
conceptual, site development, and final phases of project design.
¾ Integrating strategies for erosion and sediment control and stormwater management into a
comprehensive development plan.
5.1.2
Comprehensive Erosion & Sediment Control and Stormwater Management Review
The Act requires that “a comprehensive process for approving grading and sediment control
plans and stormwater management plans” shall be established. Therefore, county and municipal
stormwater authorities shall establish a coordinated approval process among all appropriate local
agencies. Erosion and sediment control review and approval authorities [e.g., local Soil
Conservation Districts(SCD)] and input from any other local agency deemed appropriate (e.g.,
planning and zoning, public works) shall be included. The process will be tailored to meet local
initiatives and should consider the scope and extent of environmental impacts for individual site
developments. Review agencies involved will provide comments and approval during each of
the following phases of plan development:
1. Concept
2. Site Development
3. Final
At each phase of this review process, the designer will receive feedback provided by the agencies
allowing the developer to incorporate any concerns and recommendations throughout project
planning and design. The concept plan will include site and resource mapping and protection
Supp. 1
5.4
Chapter 5. Environmental Site Design............................................... Design Process and Planning
and conservation strategies. The designer will also provide preliminary stormwater management
ESD calculations. Review of the concept plan will ensure that all important resources have been
mapped, protected, and all opportunities to enhance natural areas have been explored early in the
design process.
The site development plan will establish the footprint of the proposed project and demonstrate
the relationship between proposed impervious surfaces and the existing natural conditions
identified during concept plan design. This will better protect natural resources and buffers and
allow for using ESD practices throughout the site. Included in this step are the preparation of
detailed designs, computations, and grading plans for a second comprehensive review and
approval. This ensures that all options for implementing ESD have been exhausted. After
approval from the review agencies, the applicant will then proceed with final plan preparation
including the design of any structural practices needed to address remaining channel protection
requirements. Final plans will go to both the stormwater and erosion and sediment control
review agencies for approval.
The design process and planning techniques described in this section will provide guidelines for
protecting natural areas, minimizing imperviousness, using available landscaping for ESD
practices, and integrating stormwater and erosion and sediment control strategies. Following this
process will help to achieve the goal of implementing ESD to the MEP. Involving all review
agencies from the beginning of site planning through the more detailed design will foster
feedback and allow for a more efficient review and approval of final plans.
5.1.3
Design Process for New Development
All new development projects shall be subject to the Design Process for New Development as
outlined in the step wise procedures in Figure 5.1.
As described above, the design process will require review and approval during three different
phases of project planning that include the concept, site development, and final stages.
Approving agencies shall use the process outlined in Figure 5.1 as an enforceable mechanism
during review of the plan. Documentation that all steps were followed during project
development and specific rationale to support the proposed design shall be required.
5.1.3.1 Concept Design Phase
The concept design phase is the first step in project development as shown in Figure 5.1. This
step will include the following:
¾
¾
¾
Site and Resource Mapping
Site Fingerprinting and Development Layout
Locating ESD Practices
5.5
Supp.1
Chapter 5. Environmental Site Design............................................... Design Process and Planning
Figure 5.1 Design Process for New Development
Supp. 1
5.6
Chapter 5. Environmental Site Design............................................... Design Process and Planning
Site and Resource Mapping
The resource mapping component will be used as a basis for all subsequent decisions during
project design. During this step, the developer shall identify significant natural resources and
demonstrate that these areas will be protected and preserved. Additionally, options will be
evaluated to enhance important hydrologic functions. Approving authorities may require that
other features be shown depending on site characteristics. This map shall be field verified by the
project designer. Specific areas that should be mapped are organized by government regulatory
authority in Table 5.1 below.
Table 5.1 Natural Resources and the Corresponding Regulatory Authorities:
•
•
•
Federal
Wetlands
Major waterways
Floodplains
•
•
•
•
•
•
•
•
•
State
Tidal and nontidal
wetlands
Wetlands of Special
State Concern
Wetland buffers
Stream buffers
Major waterways
Floodplains
Forests
Forest buffers
Critical Areas
•
•
•
•
•
•
•
•
•
•
•
Local
Steep slopes
Highly erodible soils
Enhanced stream buffers
Topography/slopes
Springs
Seeps
Drainage depressions
Vegetative cover
Soils
Bedrock/geology
Existing drainage areas
The mapping process will identify important natural resources as well as areas that are highly
susceptible to erosion caused by construction activities. Identifying these important resources
and high risk locations and protecting them from disturbance is the first step in the planning
process. When steep slopes and highly erodible soils are found measures need to be taken to
limit disturbance and minimize impacts. This may be done by using information developed by
the local SCDs. These offices maintain lists that identify highly erodible soil map units for each
county in Maryland. Additionally, steep slopes are defined as those with gradients of 20 percent
or more and moderately steep slopes fall within the range of 10 to 30 percent (USDA NRCS,
Soil Survey Manual, October, 1993). For the purpose of project planning, steep slopes are
considered to be any mapping unit with a slope class of 15 percent or greater.
While it may not be practicable to eliminate earth disturbing activities exclusively on the basis of
soil erodibility or slope alone, constraints are warranted when both steep slopes and highly
erodible soils occupy the same area within the development footprint. Areas with highly
erodible soils and slopes equal to or greater than 25 percent should be incorporated into adjacent
buffers, remain undisturbed, protected during the construction process, and/or preserved as open
space.
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Chapter 5. Environmental Site Design............................................... Design Process and Planning
Strategies to protect steep slopes and highly erodible soils include:
¾
¾
Identify and map all highly erodible soils and steep slopes; and
Protect areas with highly erodible soils on slopes equal to or greater than 25 percent from
earth disturbing activities.
In addition to preserving sensitive areas during disturbances, the environmental benefits of other
existing natural resources should be maximized by incorporating protection strategies into the
overall goals of the project. Protecting these resources up front in the planning process will
allow their many functions to be utilized for infiltration, flow attenuation, groundwater recharge,
flood storage, runoff reduction, nutrient cycling, air and water pollution reduction, habitat
diversity, and thermal impact reduction. When ESD practices are located later in the planning
process, these protected areas may be further enhanced by using them to meet stormwater
requirements.
Natural resource protection and enhancement strategies include:
¾
¾
¾
¾
Protecting large tracts of contiguous open space, forested areas, and other important
resources through conservation easements.
Identifying afforestation opportunities in open space areas and setting aside land for
natural regeneration.
Identifying important resource areas that may be expanded such as stream buffers and
floodplains.
Minimize disturbance to highly permeable soils.
Site Fingerprinting and Development Layout
After conserving and protecting sensitive resources has been addressed, the next step in the
planning process involves locating the approximate location of buildings, roadways, parking lots,
and other impervious areas. These site improvements should be placed at a sufficient distance to
protect the conservation areas. Protecting these resources will involve enhancing or expanding
forested and stream buffers of adequate widths based on site characteristics.
Minimum buffer widths may be expanded based on receiving stream characteristics, stream
order, adjacent land slopes, 100-year floodplain, wetlands, mature forests, vegetative cover,
depth of the groundwater table, and the presence of spring seeps and other sensitive areas.
Several studies have suggested that minimum buffer widths could be based on site specific
functions (Palone and Todd, 1998) including: bank stability and water temperature moderation
(50 feet), nitrogen removal (100 feet), sediment removal (150 feet), or flood mitigation (200
feet). The approving agency may enhance existing buffer requirements depending upon resource
protection goals identified at the local level.
After the development footprint has been established, consideration should be given to natural
drainage areas and how runoff will travel over and through the site. Sheetflow and existing
drainage patterns should be maintained and discharges from the site should occur at the natural
location wherever possible. New drainage patterns result in concentrated flow leaving the site at
Supp. 1
5.8
Chapter 5. Environmental Site Design............................................... Design Process and Planning
an inappropriate or unstable location, as well as creating erosion, sediment transport, and stream
channel stability problems. The use of storm drains and engineered conveyance systems should
be minimized by using vegetated swales and other natural systems so that forests, buffers and
overland flow characteristics remain intact. Planning for on-site and off-site drainage patterns
must be done early in the design process to establish a stable outfall for downstream discharges.
Some of the strategies listed below can be used to establish nonstructural practices such as
sheetflow to natural areas. These protection and enhancement tools, can then double as
important strategies for meeting on-site stormwater requirements.
Strategies for site layout and connecting landscape features include:
¾
¾
¾
¾
¾
¾
Plan the building footprint and layout to protect conservation areas.
Evaluate opportunities to enhance/expand forested, wetland, and stream buffers.
Grade the site so that runoff will flow from impervious areas directly to pervious areas or
other natural conveyance systems.
Maintain natural flow paths between the site and upstream and downstream systems.
Maintain sheetflow and natural overland flow processes wherever feasible.
Provide stable conveyance of runoff off-site.
In addition to the site fingerprinting techniques described above, other strategies may be used to
protect important natural resources. One type of practice that encompasses many of these design
techniques in residential developments is clustering. This practice allows for concentrating
development in one area, thereby reducing the distance between individual lots, the length of
subdivision roadways, and overall impervious areas. It will also allow for protecting open space
and buffer areas and reduce clearing and grading in natural areas.
Commercial and industrial developments offer other opportunities to reduce impervious cover.
Because parking lots are the dominant land cover for most commercial and industrial projects,
designers can minimize the surface area dedicated to parking and use ESD practices in
landscaped areas for stormwater treatment. Table 5.2 below provides a list of better site design
techniques that may be used to reduce site imperviousness, protect environmentally sensitive
areas, and provide more open space. More details and information may be found in, Better Site
Design: A Handbook for Changing Development Rules in Your Community (Center for
Watershed Protection, 1998 and Schueler, 2000b).
Locating ESD Practices
Reducing the impervious area in residential, commercial, and industrial development enhances
the space available for landscaped features (e.g., parking lot islands, medians, plazas). Many of
the micro-scale practices discussed in this chapter are tailored to fit in these smaller landscaped
areas. When strategies for reducing imperviousness and protecting natural resources are
combined with design options that distribute ESD practices throughout a site, the resulting plans
will provide an effective means to address stormwater requirements at the source. After the site
footprint has been established, preliminary calculations for determining stormwater requirements
using ESD can be provided and potential management areas can be identified. The concept plan
shall include a drawing or sketch identifying the preliminary location of ESD practices.
5.9
Supp.1
Chapter 5. Environmental Site Design............................................... Design Process and Planning
Table 5.2 Summary of Site Development Strategies
Better Site Design
Recommendations
Technique
Using narrower,
Streets may be as narrow as 22 ft. in neighborhoods serving low traffic
shorter streets,
volumes; open space designs and clustering will reduce street lengths;
rights-of-way, and
rights-of-way can be reduced by minimizing sidewalk width, providing
sidewalks
sidewalks on one side of the road, and reducing the border width
between the street and sidewalks.
Cul-de-sacs
Allow smaller radii for turn arounds as low as 33 ft.; use a landscaped
island in the center of the cul-de-sac and design these areas to treat
stormwater runoff.
Open vegetated
Allow grass channels or biofilters for residential street drainage and
channels
stormwater treatment.
Parking ratios,
Parking ratios should be interpreted as maximum number of spaces; use
parking codes,
shared parking arrangements; minimum parking stall width should be
parking lots, and
less than 9 ft. and stall length less than 18 ft.; parking garages are
structured parking
encouraged rather than surface lots.
Parking lot runoff
Parking lots are required to be landscaped and setbacks are relaxed to
allow for bioretention islands or other stormwater practices in
landscaped areas.
Open space
Flexible design criteria should be provided to developers who wish to
use clustered development and open space designs.
Setbacks and
Relax setbacks and allow narrower frontages to reduce total road length;
frontages
eliminate long driveways.
Driveways
Allow for shared driveways and alternative impervious surfaces.
Rooftop runoff
Direct to pervious surfaces.
Buffer systems
Designate a minimum buffer width and provide mechanisms for longterm protection.
Clearing and grading Clearing, grading, and earth disturbance should be limited to that
required to develop the lot.
Tree conservation
Provide long-term protection of large tracts of contiguous forested
areas; promote the use of native plantings.
Conservation
Provide incentives for conserving natural areas through density
incentives
compensation, property tax reduction, and flexibility in the design
process.
(Adapted from Center for Watershed Protection, 1998)
Review of Concept Plans
Concept plans should be submitted to the appropriate review agencies and include the
information discussed above along with a narrative to support the design. The narrative should
describe how important natural areas will be preserved and protected, and show how ESD may
be achieved for meeting on-site stormwater requirements. Local review authorities may require
additional information at this phase, however, at a minimum a concept plan should include the
following elements:
Supp. 1
5.10
Chapter 5. Environmental Site Design............................................... Design Process and Planning
•
•
•
•
•
•
•
•
•
A map of all site resources shown in Table 5.1.
Field verification from the project engineer of the natural resource map.
Proposed limits of clearing and grading.
Location of proposed impervious areas (buildings, roadways, parking, and sidewalks).
Location of existing and proposed utilities.
Preliminary estimates of stormwater requirements.
Preliminary location of ESD practices.
Stable conveyance of stormwater at potential outfall locations.
A narrative that supports the concept and describes how the design will achieve.
o Natural resource protection and enhancement.
o Maintenance of natural flow patterns.
o Reduction of impervious areas through better site design, alternative surfaces, and
nonstructural practices.
o Integration of erosion and sediment controls into the stormwater strategy.
o Implementation of ESD planning techniques and practices to the MEP.
County and municipal stormwater management agencies are required to have a comprehensive
review process in place so that input is provided for all aspects of development project planning,
design, and construction. The review of concept plans begins this process. Local stormwater
and erosion and sediment control authorities will collaborate to provide coordinated feedback to
the designer before a project proceeds to the more detailed site development phase. This
feedback will accompany the concept plan approval and should be incorporated into future
submissions.
5.1.3.2 Site Development Phase
Preparation of site development plans will include more detailed designs for stormwater
management and erosion and sediment control. During this phase the site footprint will be
finalized with respect to the layout of buildings, roadways, parking, and other structures in order
to develop more detailed design. The following plans will be required for site development
review:
¾
¾
¾
Stormwater Management
Erosion and Sediment Control
An Overlay Showing Stormwater and Erosion and Sediment Control Practices
Stormwater Management Plans
After concept plan approval, the developer should use comments and feedback as a basis for the
next design phase. When the development layout is finalized, the proposed topography may be
determined and final drainage areas established. Natural features and conservation areas can be
utilized to serve stormwater quantity and quality management requirements. Individual ESD
locations will be determined and all alternative surfaces, nonstructural, and micro-scale practices
will be finalized. When locating and sizing ESD practices, the primary objective is to manage
runoff as close its source as possible by using vegetated buffers, natural flow paths, sheetflow to
natural areas, and landscape features. ESD practices are then designed according to sizing
5.11
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Chapter 5. Environmental Site Design............................................... Design Process and Planning
requirements specified later in this chapter and discharge computations and storage volumes
provided. Calculations and details will be submitted to the review agencies to verify the design
approach. Section 5.2 provides more information on sizing requirements and design
specifications for all ESD practices. A narrative will also be required to justify that the design
will achieve ESD to the MEP.
Erosion and Sediment Control Plans
After concept plan approval, the final grading and proposed drainage areas during construction
will also be established. This is critical to developing erosion and sediment control plans.
Erosion and sediment control plans prepared at this phase will include measures for:
¾
¾
¾
¾
Preservation
Phasing and construction sequencing during each stage of development
Design of sediment controls
Stabilization strategies
Preservation
Comments received during concept plan review should be used as a basis for preparing erosion
and sediment control plans. Strategies to preserve sensitive resources, ensure soil stability, and
prevent erosion begin with protecting those areas during project construction. Erosion and
sediment control plans should identify areas to be protected by marking the limit of disturbance,
sensitive areas, buffers, and forested areas that are to be preserved or protected. In addition,
infiltration and recharge areas that need to be protected from fine sediments and compaction
should be identified. Plans should also note that all protected areas be marked in the field prior
to any land disturbing activity.
Phasing and Sequences of Construction During Each Stage of Development
The site development plan will provide sequences of construction for each stage of development.
These include initial clearing and grubbing, rough grading, site development, and final grading.
Because initial and final flow patterns will not apply to all intermediate phases, these sequences
should consider flow pattern changes, drainage areas, and discharge points at transitional phases
of the construction process. Phased plans need to ensure that erosion and sediment controls
adequately address the changing runoff patterns.
Erosion and sediment control strategies for minimizing erosion during interim grading include:
¾
¾
¾
Interim plans to address grade changes and flow patterns during clearing and grading,
rough grading, site development, and final grading.
Slope length and steepness reductions.
Clean water diversions around or through a site that discharges to a stable outlet.
Supp. 1
5.12
Chapter 5. Environmental Site Design............................................... Design Process and Planning
Design of Sediment Controls
Water handling practices need to provide erosion protection during site grading operations. This
may be done by diverting runoff away from highly erodible soils, steep slopes, and disturbed
areas by using dikes, swales, or reverse benches. Similarly, runoff can be safely conveyed from
the top of slopes to a stable outfall using pipe slope drains or channels. Check dams may be
needed to reduce velocities and prevent erosion. Runoff from all discharge points shall provide a
stable outlet.
Stabilization Strategies
When vegetation is removed and soil disturbance occurs, the extent and duration of exposure
should be minimized. All efforts should be made to delay grading operations until it is certain
that final grades can be reached in as little time as possible. Where this cannot be
accommodated, soils shall be stabilized within 14 days of disturbance. The extent and duration
of disturbance should be limited (e.g., 72 hours) and enhanced stabilization techniques such as
soil stabilization matting or turf reinforcement used on areas with highly erodible soils and
slopes greater than 15 percent. Soil exposure should be shortened by the local permitting
authority if warranted by site conditions.
Perimeter controls, perimeter slopes, and extreme grade modifications (e.g., slopes greater than
3:1 or where cuts and fills exceed 15 feet) require stabilization within seven days. Mass clearing
and grading should be avoided with larger projects (e.g., 25 acres) being phased so disturbed
areas remain exposed for the shortest time possible. All other areas should have a good cover of
temporary or permanent vegetation or mulch.
Natural vegetation should be retained in an undisturbed state wherever possible. If it is not
possible to retain natural vegetation, the topsoil should be salvaged, stockpiled on-site, protected
from erosion, and replaced at final grade. Topsoil removal, grading, and filling reduce soil
quality resulting in detrimental impacts on plant growth and increase runoff. Additionally, the
removal of topsoil inhibits biological activity and reduces the supply of organic matter and plant
nutrients. Similarly, unrestricted use of construction equipment can result in soil compaction.
Applicable practices include, but are not limited to, temporary and permanent seeding, sodding,
mulching, plastic covering, erosion control fabrics and matting, the early application of gravel
base on areas to be paved, and dust control. Soil stabilization measures should be appropriate for
the time of year, site conditions, and estimated duration of use. Soil stockpiles must be
stabilized, protected with sediment trapping or filtering measures, and be located away from
storm drain inlets, waterways, and drainage channels. Linear construction activities, including
right-of-way and easement clearing, roadway development, pipelines, and trenching for utilities
shall be phased so that soils are stabilized as quickly as possible.
5.13
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Chapter 5. Environmental Site Design............................................... Design Process and Planning
Strategies to limit the extent and duration that soils are exposed may include:
¾
¾
Minimizing disturbed area.
Phasing earth disturbing activities so that the smallest area is exposed for the shortest
possible time.
Salvaging topsoil for later use.
Stabilizing as work progresses.
¾
¾
Overlay Plan
Many of the stormwater ESD practices deal with alternative surfaces or are nonstructural and
promote hydraulic connection of impervious surfaces with natural landscape features. The
practices for stormwater management and erosion and sediment control may share the same
location while serving different functions. For example, swales used initially to convey
sediment-laden runoff to a trap or basin during the sediment control phase could be used for
water quality treatment and flow attenuation of stormwater runoff at final grade. Similarly,
natural berms and vegetative buffers coupled with traditional sediment filtering controls may be
integrated into the site design and meet both sediment control and stormwater management
requirements.
Once the ESD practices have been located and sized appropriately, consideration to how these
areas will function under proposed conditions is needed. The location of any ESD practice that
requires natural infiltration needs to be identified on the plans and in the field. These areas need
to be protected during construction. An overlay plan should include the location of all ESD
practices to allow for efficient sediment control design and the protection of locations that will
be used to treat stormwater.
An overlay plan should include:
¾
¾
The location of ESD practices on the plan and in the field.
The location of areas that must remain undisturbed, protected, or used for erosion and
sediment control.
Identifiable areas where construction equipment may compact soil and will need
rehabilitation after grading operations.
Removal of sediment from the locations of ESD practices.
Stabilization measures needed to enhance stormwater functions.
¾
¾
¾
Review of Site Development Plans
Site development plans should be submitted to the appropriate review agencies and should
include a stormwater plan, erosion and sediment control plan, an overlay plan, and a narrative to
support the design. Local review authorities may require additional information at this phase,
however, at a minimum a site development plan shall include the following:
•
•
All of the information provided in the concept review.
Comments received by local review agencies during the concept review.
Supp. 1
5.14
Chapter 5. Environmental Site Design............................................... Design Process and Planning
•
•
•
•
•
•
•
•
•
•
•
Determination of final site layout and acreage of total impervious area on site.
Proposed topography.
Proposed drainage areas at all points of discharge from the site.
Proposed stormwater volume requirements for ESD targets and quantity control.
The location and size of ESD practices used to the MEP and all nonstructural, alternative
surfaces, and micro-scale practices used.
Proposed hydrology analysis for runoff rates, storage volumes, and discharge velocities.
Stormwater design details and specifications.
Discharge calculations demonstrating stable conveyance of runoff off site.
Preliminary erosion and sediment control plans showing limits of disturbance, sensitive
areas, buffers, and forests that are to be preserved, proposed phasing, construction
sequencing, proposed practices, and stabilization techniques.
An overlay plan showing the location of stormwater ESD practices and proposed erosion
and sediment controls.
A narrative to support the site development design and demonstrate that ESD will be
achieved to the MEP.
Local stormwater and erosion and sediment control authorities will collaborate to provide
coordinated feedback to the designer before a project proceeds to the more detailed final design
phase. This feedback will accompany the site development approval and should be incorporated
into future submission.
5.1.3.3 Final Plan Design and Review
After site development plan approval, the developer may prepare final designs by incorporating
comments from the appropriate review agencies. After all ESD options have been explored
structural practices may be needed (see Chapter 3) to address additional Cpv requirements. Final
plan approval shall be required for issuing local grading and building permits. Local review
authorities may require additional information at this phase, however, at a minimum final plans
shall include the following information and meet the requirements established in COMAR
26.17.01.05 and 26.17.02 .09:
•
•
•
•
•
•
•
•
•
•
•
All of the information provided in the site development review.
Comments received by local review agencies during the site development review.
Development details and site data including site area, disturbed area, new impervious
area, and total impervious area.
Existing and proposed topography.
Proposed drainage areas.
Representative cross sections and details (existing and proposed structure elevations and
water surface elevations).
The location of existing and proposed structures.
Construction specifications.
Operation and maintenance plans.
As-built design certification block.
Inspection schedule.
5.15
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Chapter 5. Environmental Site Design............................................... Design Process and Planning
•
•
•
•
•
Easements and rights-of-way.
Certification by the owner/developer that all construction will be done according to the
plan.
Performance bonds.
Final erosion and sediment control plans.
Stormwater management report including;
o A narrative to support the final design and demonstrate that ESD will be achieved
to the MEP.
o Table showing the ESD and Unified Sizing Criteria.
o Hydrology and hydraulic analysis of the stormwater management system for all
applicable sizing criteria.
o Final sizing calculations for stormwater controls including drainage area, storage,
and discharge points.
o Final analysis of stable conveyance to downstream discharge points.
o Geotechnical investigations including soil maps, borings, and site-specific
recommendations.
The design process described above is intended to be iterative, as comments from all review
agencies are incorporated during each phase of project design. This will help local jurisdictions
coordinate with other programs requiring environmental review and ensure that development
plans fit priorities for resource protection, enhancement, and restoration. Many counties have
performed restoration assessments on targeted watersheds. The planning process described in
Figure 5.1 and above allows individual site development to be evaluated in the context of these
larger resource protection efforts.
Supp. 1
5.16
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Section 5.2
Addressing the Unified Sizing Criteria
To accomplish the goal of maintaining predevelopment runoff characteristics, there must be a
reasonable standard that is easily recognized, reproducible, and applied without opportunity for
misrepresentation. The simplest and most effective solution is to eliminate the need for
evaluating predevelopment conditions on a site-by-site basis and apply the same standard to all
sites. For rainfall amounts less than two to three inches, there is little difference in the amount of
runoff from most sites in undeveloped conditions although runoff amounts are lowest for woods.
To best maintain predevelopment runoff characteristics, the target for ESD implementation
should be “woods in good condition”.
The Act requires the implementation of ESD to the MEP to maintain predevelopment runoff
characteristics. While ESD may be used to address Rev and WQv, limiting it to these criteria
alone may not provide sufficient treatment to mimic natural hydrology for wooded conditions or
address Cpv. It may be necessary to increase the size of single ESD practices and/or connect
them in series to decrease the volume of runoff to that expected from a naturally forested area.
Implementing ESD to that extent may not be practicable on all projects and a minimum standard
is needed. Sizing ESD practices to capture and treat both Rev and WQv is a practical minimum
requirement for all projects.
5.2.1
Performance Standards for Using Environmental Site Design
¾
The standard for characterizing predevelopment runoff characteristics for new
development projects shall be woods in good hydrologic condition;
¾
ESD shall be implemented to the MEP to mimic predevelopment conditions;
¾
As a minimum, ESD shall be used to address both Rev and WQv requirements; and
¾
Channel protection obligations are met when ESD practices are designed according to
the Reduced Runoff Curve Number Method described below.
5.17
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Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
5.2.2
Environmental Site Design Sizing Criteria
The criteria for sizing ESD practices are based on capturing and retaining enough rainfall so that
the runoff leaving a site is reduced to a level equivalent to a wooded site in good condition as
determined using United States Department of Agriculture (USDA) Natural Resource
Conservation Service (NRCS) methods (e.g., TR-55). The basic principle is that a reduced
runoff curve number (RCN) may be applied to post-development conditions when ESD practices
are used. The goal is to provide enough treatment using ESD practices to address Cpv
requirements by replicating an RCN for woods in good condition for the 1-year rainfall event.
This eliminates the need for structural practices from Chapter 3. If the design rainfall captured
and treated using ESD is short of the target rainfall, a reduced RCN may be applied to postdevelopment conditions when addressing stormwater management requirements. The reduced
RCN from Table 5.3 is calculated by subtracting the runoff treated by ESD practices from the
total 1-year 24-hour design storm runoff.
Table 5.3 was developed using the “Change in Runoff Curve Number Method” (McCuen, R.,
MDE, 1983) to determine goals for sizing ESD practices and reducing RCNs if those goals are
not met. During the planning process, site imperviousness and soil conditions are used with
Table 5.3 to determine a target rainfall for sizing ESD practices. Table 5.3 is also used to
determine the reduced RCNs for calculating additional stormwater management requirements if
the targeted rainfall cannot be met using ESD practices.
ESD Sizing Requirements:
PE = Rainfall Target from Table 5.3 used to determine ESD goals and size practices
QE = Runoff depth in inches that must be treated using ESD practices
= PE x Rv; Rv = the dimensionless volumetric runoff coefficient
= 0.05 + 0.009(I) where I is percent impervious cover
ESDv = Runoff volume (in cubic feet or acre-feet) used in the design of specific ESD practices
=
(PE )(Rv )(A)
12
where A is the drainage area (in square feet or acres)
5.2.3
Addressing Stormwater Management Requirements Using ESD
¾
Treatment: ESD practices shall be used to treat the runoff from 1 inch of rainfall (i.e.,
PE = 1 inch) on all new developments where stormwater management is required.
ESD practices shall be used to the MEP to address Cpv (e.g., treat the runoff from the 1year 24-hour design storm) in accordance with the following conditions:
o Cpv shall be addressed on all sites including those where the 1-year postdevelopment peak discharge (qi) is less than or equal to 2.0 cfs.
Supp. 1
5.18
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
o Cpv shall be based on the runoff from the 1-year 24-hour design storm calculated
using the reduced RCN (see Table 5.3). If the reduced RCN for a drainage area
reflects “woods in good condition”, then Cpv has been satisfied for that drainage
area.
o When the targeted rainfall is not met, any remaining Cpv requirements shall be
treated using structural practices described in Chapter 3.
ESD practices may be used to treat the Overbank Flood Protection and Extreme Flood
Volumes (i.e., Qp2, Qp10, Qf) using the tables found in Appendix E.2 and the procedures
described below.
¾
Practices: The runoff, QE, shall be treated by acceptable practices from the lists
presented in this Chapter (see Sections 5.3 and 5.4). QE may be treated using an
interconnected series or “treatment train” of practices.
¾
Multiple Drainage Areas: ESD requirements shall be addressed for the entire limit of
disturbance. When a project is divided into multiple drainage areas, ESD requirements
should be addressed for each drainage area.
¾
Off-Site Drainage Areas: ESD requirements shall be based on the drainage area to the
practices providing treatment. It is recommended that runoff from off-site areas be
diverted away from or bypass ESD practices. However, if this is not feasible, then ESD
practices should be based on all pervious and impervious areas located both on-site and
off-site draining to them.
¾
Reduced RCNs: When using reduced RCNs, the following conditions apply:
o ESD practices should be distributed uniformly within each drainage area.
o Where multiple ESD practices are used within a drainage area, individual
practices may be oversized on a limited scale to compensate or over manage for
smaller practices. The size of any practice(s) is limited to the runoff from the 1year 24-hour storm, QE, draining to it.
5.2.4
Basis for Using Table 5.3 to Determine ESD Sizing Criteria
¾
Application: Table 5.3 shall be used to determine both the rainfall targets for sizing
ESD practices and the additional stormwater management requirements if those targets
are not met.
¾
Hydrologic Soil Groups: Each chart in Table 5.3 reflects a different hydrologic soil
group (HSG). Designers should use the charts that most closely match the project’s soil
conditions. If more than one HSG is present within a drainage area, a composite RCN
may be computed based on the proportion of the drainage area within each HSG (see
examples below).
5.19
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
¾
Measuring Imperviousness: The measured area of a site that does not have vegetative
or permeable cover shall be considered total impervious cover. Estimates of proposed
imperviousness may be used during the planning process where direct measurements of
impervious cover may not be practical. Estimates should be based on actual land use and
homogeneity and may reflect NRCS land use/impervious cover relationships (see Table
2.2a in TR-55, USDA-NRCS, 1986) where appropriate. The percent imperviousness
(%I) may be calculated from measurements of site imperviousness.
¾
RCN*: RCN* is an alternate method to estimate PE when alternative surfaces (e.g.,
permeable pavements, green roofs) are used to reduce runoff. RCN* is a composite value
for the limit of disturbance using the effective RCNs identified in Section 5.3 for each
alternative surface.
¾
Reduced RCNs: Areas shown in green (right hand side) on Table 5.3 show the target
RCN for “woods in good condition” for the respective HSG. Areas shown in yellow (left
hand side) show the reduced RCN for each HSG that is applied to stormwater
management calculations if the design rainfall is below the target.
¾
Rainfall (Inches): Target rainfall (PE) amounts for sizing ESD practices to mimic
wooded conditions for each respective HSG are located across the top of Table 5.3.
These rainfall amounts are also used to determine the reduced RCNs for calculating
additional stormwater management requirements if the targeted amounts cannot be met.
Supp. 1
5.20
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Table 5.3 Rainfall Targets/Runoff Curve Number Reductions used for ESD
%I
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
RCN*
40
43
46
48
51
54
57
60
61
66
69
72
74
77
80
84
86
89
92
95
98
PE = 1"
%I
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
RCN*
61
63
65
67
68
70
72
74
75
78
80
81
83
85
87
89
91
92
94
96
98
PE = 1"
38
40
41
42
44
44
48
51
54
57
61
66
71
73
77
81
85
89
55
60
64
65
66
66
68
70
71
73
75
77
79
81
82
84
87
89
Hydrologic Soil Group A
1.2"
1.4"
1.6"
1.8"
38
40
41
42
42
46
48
50
52
55
61
67
70
74
78
82
86
38
39
39
40
40
41
42
42
44
47
55
62
65
70
74
78
83
38
39
39
40
41
41
42
44
50
56
60
65
70
75
80
38
39
40
42
45
48
52
58
65
70
76
Hydrologic Soil Group B
1.2"
1.4"
1.6"
1.8"
55
61
62
63
63
66
67
68
70
72
74
76
78
79
81
84
86
55
58
59
60
60
62
64
65
67
69
71
73
75
76
78
81
83
55
56
56
58
60
61
63
65
67
69
71
72
74
77
80
55
58
60
62
65
66
67
70
73
76
2.0"
2.2"
2.4"
2.6"
38
40
40
40
44
49
58
65
72
38
40
42
48
57
66
38
42
50
59
38
39
40
2.0"
2.2"
2.4"
2.6"
55
57
59
61
62
65
69
72
55
59
63
66
55
57
59
55
Cpv Addressed (RCN = Woods in Good Condition)
RCN Applied to Cpv Calculations
5.21
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Table 5.3 Runoff Curve Number Reductions used for Environmental Site Design (continued)
%I
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
%I
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
RCN*
74
75
76
78
79
80
81
82
84
85
86
86
88
90
91
92
93
94
95
97
98
RCN*
80
81
82
83
84
85
85
86
87
88
89
90
91
92
93
94
94
95
96
97
98
PE = 1"
70
72
73
74
77
78
78
78
80
82
82
83
84
85
86
88
89
PE = 1"
77
78
78
79
82
82
83
84
85
85
86
86
86
86
87
88
89
Hydrologic Soil Group C
1.2"
1.4"
1.6"
1.8"
70
72
73
75
76
76
76
78
80
80
81
82
82
83
85
86
70
71
72
73
74
74
74
76
77
78
79
79
79
80
82
83
70
71
71
71
71
73
75
75
75
76
76
77
79
80
70
71
72
72
72
72
72
73
75
76
Hydrologic Soil Group D
1.2"
1.4"
1.6"
1.8"
77
78
81
81
82
82
83
83
84
84
84
84
84
85
86
77
78
79
79
80
80
81
81
81
81
92
82
82
82
83
77
78
78
78
78
78
78
78
79
79
79
80
80
77
78
78
Cpv Addressed (RCN = Woods in Good Condition)
RCN Applied to Cpv Calculations
Supp. 1
5.22
2.0"
2.2"
70
71
72
70
2.0"
2.2"
77
2.4"
2.6"
2.4"
2.6"
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
5.2.5
Design Examples: Computing ESD Stormwater Criteria
Design examples are provided only to illustrate how ESD stormwater sizing criteria are
computed for hypothetical development projects. These design examples are also utilized
elsewhere in the manual to illustrate design concepts.
Design Example No. 5.1: Residential Development – Reker Meadows
The layout of the Reker Meadows subdivision is shown in Figure 2.6.
Site Data:
Location:
Site Area:
Drainage Area:
Soils:
Impervious Area:
Frederick County, MD
38.0 acres
38.0 acres
60% B, 40% C
13.8 acres
Step 1: Determine ESD Implementation Goals
The following basic steps should be followed during the planning phase to develop initial targets
for ESD implementation.
A. Determine Pre-Developed Conditions:
The goal for implementing ESD on all new development projects is to mimic forested runoff
characteristics. The first step in this process is to calculate the RCN for “woods in good
condition” for the project:
•
Determine Soil Conditions and RCNs for “woods in good condition”
Soil Conditions
Area
Percent
HSG
RCN†
‡
A
38
0
0%
B
55
22.8 acres
60%
C
70
15.2 acres
40%
D
77
0
0%
†
RCN for “woods in good condition” (Table 2-2, TR-55)
‡
Actual RCN is less than 30, use RCN = 38
•
Determine composite RCN for “woods in good condition”
RCN woods =
(55 × 22.8 acres) + (70 × 15.2 acres)
= 61
38 acres
The target RCN for “woods in good condition” is 61.
5.23
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
B. Determine Target PE Using Table 5.3:
PE =
Rainfall used to size ESD practices
During project planning and preliminary design, site soils and proposed imperviousness are used
to determine the target PE for sizing ESD practices to mimic wooded conditions.
•
Determine Proposed Imperviousness (%I)
Proposed Impervious Area (as measured from site plans): 13.8 acres
%I = Impervious Area / Drainage Area
= 13.8 acres / 38 acres
= 36.3%;
Because %I is between 35% and 40%, both values should be checked and the more conservative
result used to determine target PE.
For this example, assume imperviousness is distributed proportionately (60/40) in B and C soils.
•
Determine PE from Table
Using %I = 35% & 40% and B Soils:
PE ≥ 1.8 inches will reduce the RCN to reflect “woods in good condition” for %I = 35% & 40%
Using %I = 35% & 40% and C Soils:
Supp. 1
5.24
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
For %I = 35%, PE ≥ 1.6 inches will reduce the RCN to reflect “woods in good condition”
For %I = 40%, PE ≥ 1.8” to achieve the same goal.
For this project, PE happens to be the same for both soil groups, therefore use PE = 1.8 inches of
rainfall as the target for ESD implementation.
C. Compute QE :
QE =
Runoff depth used to size ESD practices
QE = PE x Rv, where
PE = 1.8 inches
Rv = 0.05 + (0.009)(I); I = 36.3
= 0.05 + (0.009 x 36.3) = 0.38
QE =
=
1.8 inches x 0.38
0.68 inches
ESD targets for the Reker Meadows project:
PE =
QE =
1.8 inches
0.68 inches
By using ESD practices that meet these targets, Rev, WQv, and Cpv requirements will be
satisfied. Potential practices could include swales or micro-bioretention to capture and treat
runoff from the roads. Likewise, raingardens and disconnection of rooftop runoff could be used
to capture and treat runoff from the houses.
Step 2: Determine Stormwater Management Requirements After Using ESD
For this example, it is assumed that ESD techniques and practices were implemented to treat
only 1.2 inches of rainfall (e.g., PE = 1.2 inches) over the entire project. After all efforts to
implement ESD practices have been exhausted, the following basic steps should be followed to
determine how much additional stormwater management is required.
A. Calculate Reduced RCNs
PE =
Rainfall used to size ESD practices
During the planning and design processes, site soils, measured imperviousness, and PE are used
to determine reduced RCNs for calculating Cpv requirements.
•
Determine Reduced RCN for PE = 1.2 inches
5.25
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Using %I = 35% & 40%, B Soils, and PE = 1.2 inches:
For B Soils, PE = 1.2 inches, and %I = 35% & 40%, reduced RCN = 63
Using %I = 35% & 40%, C Soils, and PE = 1.2 inches:
For C Soils, PE = 1.2 inches, and %I = 35% & 40%, reduced RCN = 73 & 75, respectively
Use the more conservative value, 75, for calculating a composite RCN for the site.
A composite RCN may be calculated as follows:
For PE = 1.2 inches:
RCN =
(63 × 22.8 acres) + (75 × 15.2 acres)
= 67.8
38 acres
Use 68
B. Calculate Cpv Requirements
The composite RCN for “woods in good condition” is 61 (see Step 1A above).
The design RCN (68) does not reflect the composite RCN for “woods in good condition” (61)
and, therefore Cpv must be addressed. However, PE ≥ 1.0 inches and Cpv requirements are based
on the runoff from the 1-year 24-hour design storm calculated using the reduced RCN (68).
Supp. 1
5.26
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
•
Compute Cpv Storage Volume
When PE ≥ 1.0 inches, Cpv shall be the runoff from the 1-year 24-hour design storm calculated
using the reduced RCN. If the reduced RCN for a drainage area reflects “woods in good
condition”, then Cpv has been satisfied for that drainage area.
Calculate Cpv using design PE = 1.2 inches (RCN = 68):
Cpv = Q1 x A
where: Q1 is the runoff from the 1-year 24-hour design storm
Q1 =
(P − 0.2S) 2
(P + 0.8S)
(Equation 2.3, TR-55, USDA NRCS 1986)
where: P = 1-year 24-hour design storm
S = (1000/RCN) – 10 (Equation 2-4, TR-55)
= (1000/68) – 10
= 4.7
Q1 =
[2.6 − (0.2 x 4.7)]2 2.76
=
= 0.43 inches
[2.6 + (0.8x 4.7)] 6.36
Cpv = 0.43 inches x 38 acres
= 1.36 ac. – ft. or 59, 240 cubic feet
Cpv Storage Requirements for Reker Meadows
Rainfall (PE)
Cpv (ac-ft)
Cpv (cu. ft.)
PE ≥ 1.8 inches
NA
NA
PE = 1.2 inches
1.36
59,240
Conventional Design
1.65
71,875
Notes:
Target PE for RCN = woods
Design PE
from Chapter 2 (see page 2.18)
Stormwater management requirements for the Reker Meadows project include using ESD
practices to treat 1.2 inches of rainfall and structural practices from Chapter 3 (e.g., shallow
wetland) to treat the Cpv of 59,240 cubic feet.
5.27
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Design Example No. 5.2: Commercial Development - Claytor Community Center
The layout of the Claytor Community Center is shown in Figure 2.9.
Site Data:
Location:
Site Area:
Drainage Area:
Soils:
Impervious Area:
Dorchester County
3.0 acres
3.0 acres
100% B
1.9 acres
Step 1: Determine ESD Implementation Goals
The following basic steps should be followed during the planning phase to develop initial targets
for ESD implementation.
A. Determine Pre-Developed Conditions:
The goal for implementing ESD on all new development projects is to mimic forested runoff
characteristics. The first step in this process is to calculate the RCN for “woods in good
condition” for the project.
•
Determine Soil Conditions and RCNs for “woods in good condition”
Soil Conditions
HSG
RCN†
Area
Percent
‡
A
38
0
0%
B
55
3.0 acres
100%
C
70
0 acres
0%
D
77
0
0%
†
RCN for “woods in good condition” (Table 2-2, TR-55)
‡
Actual RCN is less than 30, use RCN = 38
The site is entirely located in HSG B, and the target RCN for “woods in good condition” is 55.
B. Determine Target PE Using Table 5.3
PE =
Rainfall used to size ESD practices
During the project planning and preliminary design, site soils and proposed imperviousness are
used to determine target PE for sizing ESD practices to mimic wooded conditions.
Supp. 1
5.28
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
•
Determine Proposed Imperviousness (%I)
Proposed Impervious Area (as measured from site plans): 1.9 acres
%I = Impervious Area / Drainage Area
= 1.9 acres / 3.0 acres
= 63.3%
Because %I is closer to 65% than 60%, use the more conservative value, 65%.
•
Determine PE from Table
Using %I = 65% & B Soils:
PE ≥ 2.0 inches will reduce the RCN to reflect “woods in good condition” for %I = 65%
For this project, use PE = 2.0 inches
C. Compute QE :
QE =
QE =
=
=
QE =
=
Runoff depth used to size ESD practices
PE x Rv, where
PE = 2.0 inches
Rv = 0.05 + (0.009)(I); I = 63.3%
0.05 + (0.009 x 63.3)
0.62
2.0 inches x 0.62
1.24 inches
5.29
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
ESD targets for the Claytor Community Center project:
PE =
QE =
2.0 inches
1.24 inches
By using ESD practices that meet these targets, Rev, WQv, and Cpv requirements will be
satisfied. Potential practices could include permeable pavements, micro-bioretention, or
landscape infiltration to capture and treat runoff from the rooftops, parking lots, and drive aisles.
Step 2. Determine Stormwater Management Requirements After Using ESD
For this example, it is assumed that ESD techniques and practices were implemented to treat
only 1.6 inches of rainfall (e.g., PE = 1.6 inches) over the entire project. After all efforts to
implement ESD practices have been exhausted, the following basic steps should be followed to
determine if any additional stormwater management is required.
A. Calculate Reduced RCNs
PE =
Rainfall used to size ESD practices
During the design process, site soils, measured imperviousness, and PE are used to determine
reduced RCNs for calculating Cpv requirements.
•
Determine Reduced RCN for PE = 1.6 inches
Using %I = 65%, B Soils, and PE = 1.6 inches:
For B Soils, PE = 1.6 inches, and %I = 65%, reduced RCN = 65
B. Calculate Cpv Requirements
The RCN for “woods in good condition” = 55 (see Step 1A above).
Supp. 1
5.30
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
The design RCN (65) does not reflect “woods in good condition” (55) and therefore Cpv must be
addressed. However, PE ≥ 1.0 inches, and Cpv is based on the runoff from the 1-year 24-hour
design storm calculated using the reduced RCN (65).
•
Compute Cpv Storage Volume
When PE ≥ 1.0 inches, Cpv shall be the runoff from the 1-year 24-hour design storm calculated
using the reduced RCN. If the reduced RCN for a drainage area reflects “woods in good
condition”, then Cpv has been satisfied for that drainage area.
Calculate Cpv using design PE = 1.6 inches (RCN = 65)
Cpv = Q1 x A
where: Q1 = runoff from the 1-year 24-hour design storm
(P − 0.2S) 2
Q1 =
(P + 0.8S)
(Equation 2.3, TR-55, USDA NRCS 1986)
where: P = 1-year 24-hour design storm
S = (1000/RCN) – 10 (Equation 2-4, TR-55)
= (1000/65) – 10
= 5.4
[2.8 − (0.2x 5.4)]2 2.96
Q1 =
=
= 0.42 inches
[2.8 + (0.8x 5.4)] 7.12
Cpv = 0.42 inches x 3.0 acres
= 0.105 ac. – ft. or 4,574 cubic feet
Cpv Storage Requirements for Claytor Community Center
Rainfall (PE)
Cpv (ac-ft)
Cpv (cu. ft.) Notes:
PE ≥ 2.0 inches
NA
NA
Target PE for RCN = woods
PE = 1.6 inches
0.105
4,574
Design PE
Conventional Design
0.21
9,150
See Note Below*
*NOTE: Prior to 2009, Cpv was not required on the Eastern Shore. However, an estimated 0.21
ac.-ft (9,150 cubic feet) would have been needed to address Cpv in Design Example No. 2 in
Chapter 2.
Stormwater management requirements for the Claytor Community Center project include using
ESD practices to treat 1.6 inches of rainfall and structural practices from Chapter 3 (e.g., shallow
wetland) to treat the Cpv of 4,574 cubic feet.
5.31
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Design Example No. 5.3: Multiple Drainage Areas – Pensyl Pointe
The layout of the Pensyl Pointe subdivision is shown in Figure 2.12.
Site Data:
Location:
Montgomery County, MD
Site Area:
38.0 acres
Drainage (DA) 1
Area:
7.6 acres
Soils:
60% B, 40% C
Impervious Area:
2.25 acres
Drainage (DA) 2
Area:
Soils:
Impervious Area:
30.4 acres
60% B, 40% C
11.55 acres
Step 1: Determine ESD Implementation Goals
The following basic steps should be followed during the planning phase to develop initial targets
for ESD implementation.
A. Determine Pre-Developed Conditions:
The goal for implementing ESD on all new development sites is to mimic forested runoff
characteristics. The first step in this process is to calculate the RCNs for “woods in good
condition” for the project.
•
Determine Soil Conditions and RCNs for “woods in good condition”
DA 1
Soil Conditions (DA 1)
HSG
RCN†
Area
Percent
‡
A
38
0
0%
B
55
4.6 acres
60%
C
70
3.0 acres
40%
D
77
0
0%
†
RCN for “woods in good condition” (Table 2-2, TR-55)
‡
Actual RCN is less than 30, use RCN = 38
•
Determine Composite RCN for “woods in good condition” for DA 1
RCN woods =
Supp. 1
(55 × 4.6 acres) + (70 × 3.0 acres)
= 61
7.6 acres
5.32
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
The target RCN for “woods in good condition” is 61
DA 2
Soil Conditions (DA 2)
HSG
RCN†
Area
Percent
A
38‡
0
0%
B
55
18.2 acres
60%
C
70
12.2 acres
40%
D
77
0
0%
†
RCN for “woods in good condition” (Table 2-2, TR-55)
‡
Actual RCN is less than 30, use RCN = 38
Determine Composite RCN for “woods in good condition” for DA 2
(55 × 18.2 acres) + (70 × 12.2 acres)
= 61
30.4 acres
The target RCN for “woods in good condition” is 61
RCN woods =
B. Determine Target PE Using Table 5.3:
PE =
Rainfall used to size ESD practices
During the planning and preliminary design processes, site soils and proposed imperviousness
are used to determine target PE for sizing ESD practices to mimic wooded conditions.
•
Determine Proposed Imperviousness (%I)
DA 1
Proposed Impervious Area (as measured from site plans): 2.25 acres;
%I = Impervious Area / Drainage Area
= 2.25 acres / 7.6 acres
= 30.0%
DA 2
Proposed Impervious Area (as measured from site plans): 11.55 acres;
%I = Impervious Area / Drainage Area
= 11.55 acres / 30.4 acres
= 38.0%
Because %I is closer to 40% than 35%, use the more conservative value , 40%, to determine
target PE.
5.33
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
For this example, assume imperviousness in DA 1 & DA 2 is distributed proportionately (60/40)
in B and C soils.
•
Determine PE from Table
DA 1
Using %I = 30% and B Soils:
P ≥ 1.6 inches will reduce RCN to reflect “woods in good condition”
Using %I = 30% and C Soils:
PE ≥ 1.6 inches will reduce the RCN to reflect “woods in good condition”.
For DA 1, PE happens to be the same for both soil groups, therefore use PE = 1.6 inches of
rainfall.
Supp. 1
5.34
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
DA 2
Using %I = 40% and B Soils:
PE ≥ 1.8 inches will reduce the RCN to reflect “woods in good condition”.
Using %I = 40% and C Soils:
PE ≥ 1.8 inches will reduce the RCN to reflect “woods in good condition”.
For DA 2, PE happens to be the same for both soil groups, therefore use PE = 1.8 inches of
rainfall.
C. Compute QE:
DA 1
QE =
QE =
Runoff depth used to size ESD practices
PE x Rv, where
PE = 1.6 inches
Rv = 0.05 + (0.009)(I); I = 30.0%
= 0.05 + (0.009 x 30.0)
= 0.32
QE =
=
1.6 inches x 0.32
0.51 inches
5.35
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
DA 2
QE =
QE =
Runoff depth used to size ESD practices
PE x Rv, where
PE = 1.8 inches
Rv = 0.05 + (0.009)(I); I = 38.0%
= 0.05 + (0.009 x 38.0)
= 0.39
QE =
=
1.8 inches x 0.39
0.70 inches
ESD targets for the Pensyl Pointe project:
DA 1
PE =
QE =
DA 2
1.6 inches
0.51 inches
PE =
QE =
1.8 inches
0.70 inches
By using ESD practices that meet these targets, Rev, WQv, and Cpv requirements will be
satisfied. Potential practices could include swales or micro-bioretention to capture and treat
runoff from the roads. Likewise, raingardens and disconnection of runoff could be used to
capture and treat runoff from the houses.
Step 2. Determine Stormwater Management Requirements After Using ESD
For this example, it is assumed that ESD techniques and practices were implemented to treat
only 1.6 inches of rainfall (e.g., PE = 1.6 inches) over the entire project. After all efforts to
implement ESD practices have been exhausted, the following basic steps should be followed to
determine if any additional stormwater management is required.
A. Calculate Reduced RCNs
PE =
Rainfall used to size ESD practices
During the planning and design processes, site soils, measured imperviousness, and PE are used
to determine reduced RCNs for calculating Cpv requirements.
•
Determine Reduced RCNs for PE = 1.6 inches
Supp. 1
5.36
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
DA 1
Using %I = 30%, B Soils, and PE = 1.6 inches:
For B Soils, PE = 1.6 inches, and %I = 30%, reduced RCN = 55 (woods in good condition)
Using %I = 30%, C Soils, and PE = 1.6 inches:
For C Soils, PE = 1.6 inches, and %I = 30%, reduced RCN = 70 (woods in good condition)
Composite RCNs may be calculated as follows:
For PE = 1.6 inches:
RCN =
(55 × 4.6 acres) + (70 × 3.0 acres)
= 60.9
7.6 acres
Use 61
5.37
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
DA 2
Using %I = 40%, B Soils, and PE = 1.6 inches:
For B Soils, PE = 1.6 inches, and %I = 40%, reduced RCN = 56
Using %I = 40%, C Soils, and PE = 1.6 inches:
For C Soils, PE = 1.6 inches, and %I = 40%, reduced RCN = 71
Composite RCNs may be calculated as follows:
For PE = 1.6 inches:
RCN =
(56 × 18.2 acres) + (71 × 12.2 acres)
= 62
30.4 acres
Reduced RCNs for the Pensyl Pointe project:
PE = 1.6 inches
Supp. 1
DA 1
DA 2
RCN = 61
RCN = 62
5.38
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
B. Calculate Cpv Requirements
DA 1
The composite RCN for “woods in good condition” is 61 (see Step 1A above).
The design RCN (61) for PE = 1.6 inches reflects “woods in good condition” and therefore Cpv is
addressed.
Cpv Storage Requirements for Pensyl Pointe - DA 1
Rainfall (PE)
Cpv (ac-ft)
Cpv (cu. ft.)
PE ≥ 1.6 inches
NA
NA
PE = 1.6 inches
NA
NA
Conventional Design
0.30
13,070
Notes:
Target PE for RCN = woods
Design PE
From Chapter 2 (see page 2.32)
DA 2
The composite RCN for “woods in good condition” is 61 (see Step 1A above).
The design RCN (62) does not reflect the composite RCN for “woods in good condition” (61)
and Cpv must be addressed. However, PE ≥ 1.0 inches, and Cpv is based on the runoff from the
1-year 24-hour design storm calculated using the reduced RCN (62).
Calculate Cpv using design PE = 1.6 inches (RCN = 62)
Cpv = Q1 x A
Where Q1 is the runoff from the 1-year 24-hour design storm
Q1 =
(P − 0.2S) 2
(P + 0.8S)
(Equation 2.3, TR-55, USDA NRCS 1986)
where: P = 1-year 24-hour design storm
S = (1000/RCN) – 10 (Equation 2-4, TR-55)
= (1000/62) – 10
= 6.1
Q1 =
[2.6 − (0.2 x 6.1)]2 1.90
=
= 0.25 inches
[2.6 + (0.8x 6.1)] 7.48
Cpv = 0.25 inches x 30.4 acres
= 0.63 ac. – ft. or 27,440 cubic feet
5.39
Supp.1
Chapter 5. Environmental Site Design...................................................................... Sizing Criteria
Cpv Storage Requirements for Pensyl Pointe – DA 2
Rainfall (PE)
Cpv (ac-ft)
Cpv (cu. ft.)
PE ≥ 1.8 inches
NA
NA
PE = 1.6 inches
0.63
27,440
Conventional Design
1.31
57,065
Notes:
Target PE for RCN = woods
Design PE
From Chapter 2 (see page 2.33)
Stormwater management requirements for the Pensyl Pointe project include using ESD practices
to treat 1.6 inches of rainfall and structural practices from Chapter 3 (e.g., shallow wetland) to
treat the Cpv of 27,440 cubic feet.
Supp. 1
5.40
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Section 5.3
Alternative Surfaces
An effective method to reduce imperviousness in residential, commercial, and industrial
applications is to use more permeable alternatives. Roofs and pavements are often overlooked
areas that may be replaced with more permeable surfaces. Green roofs are particularly useful
alternatives for reducing impervious cover and provide much needed green space in ultra-urban
or high-density developments. Whether made from porous asphalt or concrete, interlocking
pavers, or reinforced turfs, permeable pavements are a cost-effective alternative for parking lot
and roadway surfaces.
Alternative surface variants include:
¾ A-1.
¾ A-2.
¾ A-3.
Green Roofs
Permeable Pavements
Reinforced Turf
5.41
Supp.1
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
A-1. Green Roofs
Green roofs are alternative surfaces that replace conventional construction materials and include
a protective covering of planting media and vegetation. Also known as vegetated roofs, roof
gardens, or eco-roofs, these may be used in place of traditional flat or pitched roofs to reduce
impervious cover and more closely mimic natural hydrology. Green roofs produce less heat than
conventional systems. Therefore, they may be used to help mitigate stormwater impacts and
temperature increases caused by new development.
There are two basic green roof designs that are distinguished by media thickness and the plant
varieties that are used. The more common or “extensive” green roof is a lightweight system
where the media layer is between two and six inches thick. This limits plants to low-growing,
hardy herbaceous varieties. An extensive green roof may be constructed off-site as a modular
system with drainage layers, growing media, and plants installed in interlocking grids.
Conventional construction methods may also be used to install each component separately.
“Intensive” green roofs have thicker soil layers (eight inches or greater) and are capable of
supporting more diverse plant communities including trees and shrubs. A more robust structural
loading capacity is needed to support the additional weight of the media and plants. Intensive
green roofs are more complex and expensive to design, construct, and maintain, and are
therefore, less commonly used.
Applications:
Green roofs may be used to replace most conventional roofs in both new and redevelopment
applications in residential, commercial, and industrial projects. Green roofs are particularly
useful for reducing impervious cover in ultra-urban or high-density areas as well. Green roofs
may also mitigate temperature increases on projects located in thermally sensitive watersheds.
Performance:
When designed according to the guidance provided below, the rooftop area covered by a green
roof will have runoff characteristics more closely resembling grassed or open space areas. The
capacity of a green roof to detain runoff is governed by planting media thickness and roof slope
or “pitch.” However, the RCNs shown in Table 5.4 below are used to determine how green roofs
contribute to addressing the ESD Sizing Criteria.
Table 5.4 Effective RCNs for Extensive Green Roofs
2
3
4
Roof Thickness (in.):
94
92
88
Effective RCN:
6
85
8
77
Because impermeable liners are an integral component in all systems, green roofs do not provide
groundwater recharge. Therefore, additional treatment is needed to compensate for the loss of
recharge from rooftop areas. This is equal to Rev for the rooftop area and may be provided in
separate infiltration practices or as additional storage within downstream ESD practices.
Supp. 1
5.42
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Constraints:
The following constraints are critical when considering the use of green roofs to treat stormwater
runoff:
¾ Infrastructure: The location of existing and proposed utilities (e.g., HVAC, gutters,
downspouts, electricity) will influence the design and construction of green roofs.
¾ Structure: Green roofs are not suitable for use on steep roofs (> 30% or 4:12). Sloped
roofs may require additional measures to prevent sliding and ensure stability. The structure
must also be capable of supporting the additional weight (live and dead load) of a green roof.
Typical dead load ranges from 8 to 36 lbs/ft2. Live load is a function of rainfall retention
(e.g., 1 inch of rain or 10 inches of snow equals 5.2 lbs/ft2). For redevelopment projects and
existing buildings, additional measures (e.g., trusses, joists, columns) may be needed for
support.
¾ Waterproofing: Materials should be durable under the conditions associated with vegetated
covers. Supplemental barrier layers may be required with waterproofing membranes that
may be damaged by plant roots.
¾ Drainage: Building drainage (e.g., gutters, deck drains, scuppers) must be capable of
managing large rainfall events without inundating the roof.
Design Guidance:
The following conditions should be considered when designing green roofs:
¾ Conveyance: Runoff shall flow through and exit green roof systems in a safe and nonerosive manner. Overflow structures shall be capable of passing the 2-year 24-hour design
storm without inundating the roof. A semi-rigid, plastic geocomposite drain or mat layer
should be included to convey runoff to the building drainage system. Flat roof applications
may require a perforated internal network to facilitate drainage of rainfall. Additionally,
roof flashing should extend six inches above the media surface and be protected by counterflashing.
Runoff from adjacent roofs should not drain to the green roof. If bypassing a green roof is
impractical, an overflow device (e.g., gutter, deck drain) should be used.
All green roofs shall include a waterproofing system or membrane. Materials used should be
durable under vegetated cover conditions and resistant to biological and root attack. A
supplemental barrier may be needed to protect the waterproofing from plant roots.
5.43
Supp.1
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
¾ Treatment: Green roof systems shall meet the following conditions:
o Planting media shall be non-soil engineered mixes conforming to the specifications
found in Appendix B.4. Media layers should be between two to six inches thick.
Dual media systems may be applied where green roof assemblies are four inches or
thicker.
o Individual layers (e.g., root barriers, drainage mats, separation geotextiles) shall
conform to the specifications found in Appendix B.4.
Figure 5.2 Cutaway of a Typical Green Roof
¾ Structure:
o The roof structure shall be capable of bearing the maximum predicted dead and live
loads associated with green roof systems. Standardized media weights and
procedures (e.g., ASTM E-2397-05, E-2399-05) shall be used to establish the dead
load bearing capacity of the roof.
o Green roofs with pitches steeper than 2:12 shall include supplemental measures (e.g.,
slope bars, rigid stabilization panels, reinforcing mesh) to enhance stability and
prevent media sliding.
¾ Landscaping: Vegetation is critical to the function and appearance of any green roof.
Therefore landscaping plans should be provided according to the guidance in Appendix B.4.
Supp. 1
5.44
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
A vigorous, drought-tolerant vegetative cover should be established using varieties of sedum,
delosperma, or similar varieties native or suitable for growth in Maryland.
Construction Criteria:
The following items should be addressed during construction of projects with green roofs:
¾ Waterproofing Installation: Measures shall be taken to prevent membrane damage during
green roof installation. Any flaws, irregularities, or conditions that may cause leaks or roof
damage shall be identified and repaired. The waterproofing membrane should be visually
inspected and tested for water tightness prior to installation of the planting mix.
¾ Slope Stabilization Measures: Where required, slope stabilization measures should be
placed prior to green roof installation. In some situations, slope stabilization may be
integrated into the roof structure.
¾ Green Roof Installation: Green roof systems should be installed according to the
manufacturer’s instructions. Generally, root-barrier layers, walkways, and irrigation systems
should be installed first.
Inspection:
¾ The following certifications shall be required during construction:
o Prior to placement of the waterproofing, drainage, and treatment materials,
certification that the constructed roof meets the load bearing capacity specified on
the approved plans.
o After its installation and prior to placement of the planting media and stock,
certification regarding the water tightness of the waterproofing membrane.
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
o
o
During placement of the waterproofing membrane.
During placement of the drainage system.
During placement of the planting media.
Upon installation of the plant material.
Before issuing use and occupancy approvals (new construction only).
During the second growing season to ensure adequate vegetation survival.
Maintenance Criteria:
Green roofs require annual maintenance to ensure optimum performance. Typically, eighteen
months are needed to establish adequate initial plant growth. Periodic irrigation may be needed
during this time and basic weeding, fertilizing, and in-fill planting may be required as well.
After plants are established, the roof should be inspected and light weeding performed once or
twice per year.
5.45
Supp.1
Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
A-2. Permeable Pavements
Permeable pavements are alternatives that may be used to reduce imperviousness. While there
are many different materials commercially available, permeable pavements may be divided into
three basic types: porous bituminous asphalt, porous concrete, and interlocking concrete paving
blocks or grid pavers. Permeable pavements typically consist of a porous surface course and
uniformly graded stone or sand drainage system. Stormwater drains through the surface course,
is captured in the drainage system, and infiltrates into the surrounding soils. Permeable
pavements significantly reduce the amount of impervious cover, provide water quality and
groundwater recharge benefits, and may help mitigate temperature increases.
Applications:
Permeable pavements are effective for reducing imperviousness in parking lots, driveways,
plazas, and access roads in both new and redevelopment applications in residential, commercial,
and industrial projects. They are particularly useful in high-density areas where space is limited.
Rainwater passes through the permeable surface, is temporarily stored in the subbase material,
and slowly infiltrates into the underlying soils.
Performance:
When designed according to the guidance provided below, areas covered by permeable
pavements will have runoff characteristics more closely resembling vegetated areas. The
capacity of permeable pavements to capture and detain runoff is governed by the storage
capacity, compaction of the subbase, and in-situ soil properties. Consequently, RCN’s applied to
these systems vary with individual design characteristics. The effective RCN’s shown in Table
5.5 are used when addressing the ESD Sizing Criteria.
Constraints:
The following constraints are critical when considering the use of permeable pavements to
capture and treat stormwater runoff:
¾ Space: Permeable pavements work best when designed in a series of narrow strips. The size
and distribution of paved surfaces within a project must be considered early during planning
and design. Permeable pavements should not be used in areas where there are risks for
foundation damage, basement flooding, interference with subsurface sewage disposal
systems, or detrimental impacts to other underground structures.
¾ Topography: Runoff should sheetflow across permeable pavements. Pavement surfaces
should be gradual (≤ 5%) to prevent ponding of water on the surface and within the subbase.
¾ Soils: Sandy and silty soils are critical to successful application of permeable pavements.
The HSG should be A, B or C.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Subsurface water conditions (e.g., water table) will help determine the stone reservoir
thickness used. The probability of practice failure increases if the reservoir intercepts
groundwater. Therefore, subbase inverts should be above local groundwater tables.
¾ Drainage Area: Permeable pavements are an at-source practice for reducing the effects of
impervious cover and addressing ESD criteria. As the impervious area draining to each
practice increases, practice effectiveness weakens. Therefore, runoff from adjacent areas (or
“run-on”) should be limited.
¾ Hotspot Runoff: Permeable pavements should not be used to treat hotspots that generate
higher concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
¾ Structure: Most permeable alternatives have a lower load bearing capacity than
conventional pavements. Therefore, applications should be limited to locations that do not
receive heavy vehicle traffic and where sub soils are not compacted.
¾ Operation: Permeable pavements are highly susceptible to clogging and subject to owner
neglect. Individual owners need to be educated to ensure that proper maintenance and winter
operation activities will allow the system to function properly.
Design Guidance:
The following conditions should be considered when designing permeable pavements:
¾ Conveyance: Runoff shall flow through and exit permeable pavements in a safe and nonerosive manner. Permeable pavements should be designed off-line whenever possible.
Runoff from adjacent areas should be diverted to a stable conveyance system. If bypassing
these areas is impractical, then runoff should sheetflow onto permeable pavements.
Pavement surfaces shall have a permeability of eight inches per hour or greater to convey
water into the subbase rapidly. The slope of the permeable pavement shall be at least 1% but
no greater than 5%. Any grade adjustments requiring fill should be accomplished using the
subbase material. Permeable pavements may be placed in sloped areas by terracing levels
along existing contours.
Pavement systems should include an alternate mode for runoff to enter the subbase reservoir.
In curbless designs, this may consist of a two-foot wide stone edge drain. Raised inlets may
be required in curbed applications.
The bottom of the subbase shall be level to enhance distribution and reduce ponding within
the reservoir. A network of perforated pipes may be used to uniformly distribute runoff over
the bed bottom. Perforated pipes may also be used to connect structures (e.g., cleanouts,
inlets) located within the permeable pavement section.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
All permeable pavements shall be designed to ensure that water surface elevations for the
10-year 24 hour design storm do not rise into the pavement to prevent freeze/thaw damage to
the surface. Designs should include overflow structures like overdrains, inlets, edge drains,
or similar devices that will convey excess runoff safely to a stable outfall.
¾ Treatment: All permeable pavement systems shall meet the following conditions:
o Applications that exceed 10,000 ft2 shall be designed as infiltration practices using
the design methods outlined in Appendix D.13 for infiltration trenches. A porosity (n)
of 30% and an effective area of the trench (At) equal to 30% of the pavement surface
area shall be used.
o A subbase layer of a clean, uniformly graded aggregate with a porosity (n) of 30%
(1.5” to 2” stone is preferred) shall be used below the pavement surface. The
subbase may be 6”, 9” or 12” thick.
o Filter cloth shall not be used between the subbase and sub soils. If needed, a 12”
layer of sand or pea gravel (⅛” to ⅜” stone) may be used to act as a bridging layer
between the subbase reservoir and subsurface soils.
Table 5.5 Effective RCNs for Permeable Pavements
Hydrologic Soil Group
Subbase
A
B
C
6”
9”
12”
761
623
40
841
653
55
932
773
70
D
─
─
─
1.
Design shall include 1 - 2” min. overdrain (inv. 2” below pavement base) per 750 s.f. of pavement area.
Design shall include 1 - 2” min. overdrain (inv. 2” below pavement base) per 600 s.f. of pavement area
3.
Design shall include 1 - 3” min. overdrain (inv. 3” below pavement base) and a ½” underdrain at subbase
invert.
2.
¾ Soils:
o Permeable pavements shall not be installed in HSG D or on areas of compacted fill.
Underlying soil types and condition shall be field-verified prior to final design.
o For applications that exceed 10,000 ft2, underlying soils shall have an infiltration
rate ( f ) of 0.52 in/hr or greater. This rate may be initially determined from NRCS
soil textural classification and subsequently confirmed by geotechnical tests in the
field as required in Chapter 3.3.1.
o The invert of the subbase reservoir shall be at least four feet above (two feet on the
lower Eastern Shore) the seasonal high water table.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Figure 5.3 Examples of Permeable Pavements
Typical Section
Typical Section w/Overdrain & Underdrain
Permeable Pavement w/Micro-Bioretention - Plan View
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
¾ Setbacks:
o Permeable pavements shall be located down gradient of building structures and be
setback at least 10 feet from buildings, 50 feet from confined water supply wells, 100
feet from unconfined water supply wells, and 25 feet from septic systems.
o Permeable pavements should also be sized and located to meet minimum local
requirements for underground utility clearance.
¾ Structure: All permeable pavement systems shall be capable of bearing the anticipated
vehicle and traffic loads. Pavement systems conforming to the specifications found in
Appendix B.4 should be structurally stable for typical (e.g., light duty) applications.
¾ Landscaping: Permeable pavement shall be identified on landscaping plans. Trees and
shrubs should not be located adjacent to asphalt and concrete where damage by root
penetration and clogging from leaves are a concern.
Construction Criteria:
The following items should be addressed during construction of projects with permeable
pavement:
¾ Erosion and Sediment Control: Final grading for installation shall not take place until the
surrounding site is stabilized. If this cannot be accomplished, runoff from disturbed areas
shall be diverted around proposed pavement locations.
¾ Soil Compaction: Sub soils shall not be compacted. Construction should be performed with
lightweight, wide tracked equipment to minimize compaction. Excavated materials should
be placed in a contained area.
¾ Distribution Systems: Overdrain, underdrain, and distribution pipes shall be checked to
ensure that both the material and perforations meet specifications (see Appendix B.4). The
upstream ends of pipes should be capped prior to installation. All underdrain or distribution
pipes used should be installed flat along the bed bottom.
¾ Subbase Installation: Subbase aggregate shall be clean, washed, and free of fines. The
subbase shall be placed in lifts and lightly rolled according to the specifications (see
Appendix B.4).
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
o
During excavation to subgrade.
During placement and backfill of any drainage or distribution system(s).
During placement of the subbase material.
During placement of the surface material.
Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following procedures should be considered essential for maintaining permeable pavement
systems:
¾ Pavements should be used only where regular maintenance can be performed. Maintenance
agreements should clearly specify how to conduct routine tasks to ensure long-term
performance.
¾ Pavement surfaces should be swept and vacuumed to reduce sediment accumulation and
ensure continued surface porosity. Sweeping should be performed at least twice annually
with a commercial cleaning unit. Washing systems and compressed air units should not be
used to perform surface cleaning.
¾ Drainage pipes, inlets, stone edge drains, and other structures within or draining to the
subbase should be cleaned out at regular intervals.
¾ Trucks and other heavy vehicles can grind dirt and grit into the porous surfaces, leading to
clogging and premature failure. These vehicles should be prevented from tracking and
spilling material onto the pavement.
¾ Deicers should be used in moderation. When used, deicers should be non-toxic and organic
and can be applied either as blended magnesium chloride-based liquid products, or as
pretreated salt. Snow plowing should be done carefully with blades set one-inch higher than
normal. Plowed snow piles and snowmelt should not be directed to permeable pavement.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
A-3. Reinforced Turf
Reinforced turf consists of interlocking structural units with interstitial areas for placing gravel
or growing grass. These systems are suitable for light traffic loads and are commonly used for
emergency vehicle access roads and overflow or occasionally used parking.
Applications:
Reinforced turf is effective for reducing imperviousness in parking lots, driveways, plazas, and
access roads in both new and redevelopment applications in residential, commercial, and
industrial projects. It is particularly useful in high-density areas where space is limited. Because
reinforced turf is an open load-bearing matrix within a vegetated or gravel surface, runoff
characteristics are similar to open space in good condition or gravel.
Performance:
When designed according to the guidance provided below, reinforced turf areas are considered as
permeable surfaces. Post development RCN’s for reinforced turf applications may be assumed
to be “open space in good condition” or “gravel” depending on the surfacing material used.
Constraints:
The following constraints are critical when considering the use of reinforced turf to capture and
treat stormwater runoff:
¾ Space: Reinforced turf works best when designed as small areas or in a series of narrow
strips. The size and distribution of these surfaces within a project must be considered early
during planning and design.
¾ Topography: Runoff should sheetflow onto and across reinforced turf. Contributing
drainage slopes should be moderate (≤ 5%). If slopes are too steep, then level-spreading
devices may be needed to redistribute flow. Turf surfaces should be gradual (≤ 4%) to
prevent ponding of water within the subbase.
¾ Soils: Reinforced turf may be used in all soils but works best in sandy soils.
¾ Drainage Area: Reinforced turf is an at source practice for reducing impervious cover. As
the impervious area draining to each application increases, effectiveness weakens.
Therefore, runoff from adjacent areas should be limited.
¾ Hotspot Runoff: Reinforced turf should not be used to treat hotspots that generate higher
concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
¾ Structure: Most reinforced turf has a lower load bearing capacity than conventional
pavements. Therefore, applications should be limited to locations that do not receive heavy
vehicle traffic and where sub soils are not compacted.
¾ Operation: Reinforced turf is susceptible to owner neglect. Individual owners need to be
educated to ensure that proper maintenance and winter operation activities will allow the
system to function properly.
Design Guidance:
The following conditions should be considered when designing reinforced turf:
¾ Conveyance: Runoff shall enter, flow through, and exit reinforced turf in a safe and nonerosive manner. Reinforced turf should be designed off-line whenever possible.
The slope of reinforced turf shall be at least 1% but no greater than 5%. Reinforced turf
applications may be placed in sloped areas by terracing levels along existing contours.
¾ Treatment: All reinforced turf systems shall meet the following conditions:
o A subbase layer of clean, uniformly graded stone or sand with a porosity (n) of 30%
(1.5” to 2” stone is preferred) shall be used below the turf surface. The subbase may
be 6” to 12” thick.
¾ Soils:
o Reinforced turf shall not be placed on areas of compacted fill.
o Reinforced turf should be installed in HSG A. B, or C for maximum effiectiveness.
¾ Setbacks:
o Reinforced turf should be sized and located to meet minimum local requirements for
underground utility clearance.
¾ Structure: Reinforced turf shall be capable of bearing the anticipated vehicle and traffic
loads. Systems conforming to the specifications found in Appendix B.4 should be
structurally stable for typical (e.g., light duty) applications.
¾ Landscaping: Reinforced turf shall be identified on landscaping plans. Trees and shrubs
should not be located adjacent to reinforced turf where damage by root penetration is a
concern.
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Chapter 5. Environmental Site Design.............................................................Alternative Surfaces
Construction Criteria:
The following items should be addressed during construction of projects with reinforced turf:
¾ Erosion and Sediment Control: Final grading for installation shall not take place until the
surrounding site is stabilized. If this cannot be accomplished, runoff from disturbed areas
should be diverted around proposed locations.
¾ Soil Compaction: Sub soils shall not be compacted. Construction should be performed with
lightweight, wide tracked equipment to minimize compaction. Excavated materials should
be placed in a contained area.
¾ Filter Cloth: Filter cloth shall not be used between the subbase and sub soils.
¾ Subbase Installation: The subbase shall be placed in lifts and lightly rolled according to
the specifications (see Appendix B.4). Subbase aggregate should be clean, washed, and free
of fines.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
During excavation to sub grade.
During placement of the subbase material.
During placement of the surface material.
Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following procedures should be considered essential for maintaining reinforced turf:
¾ Reinforced turf should be used only where regular maintenance can be performed.
Maintenance agreements should clearly specify how to conduct routine tasks to ensure longterm performance of these systems.
¾ Drainage pipes, inlets, stone edge drains, and other structures within or draining to the
subbase should be cleaned out at regular intervals.
¾ Trucks and other heavy vehicles can damage the interlocking matrix, leading to premature
failure. These vehicles should be prevented from driving onto the turf.
¾ Reinforced turf should be mown regularly and clippings removed from the application area.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Section 5.4
5.4.1
Treatment Using Nonstructural and Micro-Scale Practices
Introduction
Disconnecting impervious cover and treating urban runoff closer to its source are the next steps
in the design process for implementing ESD. Using nonstructural techniques (e.g., disconnection
of rooftop runoff, sheetflow to conservation areas) and micro-scale practices (e.g., rain gardens,
bio-swales) throughout a development is an effective way to accomplish this goal. Nonstructural
practices may be used to disconnect impervious cover and direct runoff over vegetated areas to
promote overland filtering and infiltration. Micro-scale practices are useful for capturing and
treating runoff near the source. Whether runoff is directed over permeable areas or captured in
small water quality treatment practices, there are reductions in both volume and pollutants
delivered to receiving streams. Accordingly, these practices may be used to address the ESD
sizing criteria when designed and implemented properly.
Nonstructural and micro-scale practices are an integral part of the ESD stormwater management
plans. Therefore, the use of these practices shall be documented at the concept, site
development, and final design stages and verified with “as-built” certification. If practices are
not implemented as planned, then volumes used to design structural practices shall be increased
appropriately to meet the ESD sizing criteria.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
5.4.2
Nonstructural Practices
Nonstructural practices combine relatively simple features, grading, and landscaping to divert
runoff into vegetated areas and away from conventional storm drain systems. Runoff flows over
these areas, filters through the vegetation, and soaks into the ground. Runoff should be conveyed
as sheetflow into and through these areas. As depth and velocity of flow increase, runoff
concentrates and the ability of vegetation to filter and detain runoff diminishes rapidly.
Consequently, requirements and conditions for nonstructural practices reflect the need to
maintain sheetflow conditions.
Nonstructural practices include:
¾ N-1.
¾ N-2.
¾ N-3.
Supp. 1
Disconnection of Rooftop Runoff
Disconnection of Non-Rooftop Runoff
Sheetflow to Conservation Areas
5.56
Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
N-1. Disconnection of Rooftop Runoff
Rooftop disconnection involves directing flow from downspouts onto vegetated areas where it
can soak into or filter over the ground. This disconnects the rooftop from the storm drain system
and reduces both runoff volume and pollutants delivered to receiving waters. To function well,
rooftop disconnection is dependent on several site conditions (e.g., flow path length, soils,
slopes).
Applications:
There are many opportunities for disconnecting rooftops in both new and redevelopment designs.
Runoff may be directed to undisturbed natural areas (e.g., vegetated buffers) or landscaped areas
(e.g., lawns, grass channels). Rooftop disconnection is possible in commercial, industrial, and
residential settings given the constraints listed below.
Performance:
The PE values shown in Table 5.6 may be applied to the ESD sizing criteria when the
contributing rooftop area is adequately disconnected. Rev requirements (see Chapter 2) are also
addressed when the PE from Table 5.6 meets or exceeds the soil specific recharge factor listed in
Section 2.2.
Constraints:
The following constraints are critical when considering the use of rooftop disconnection to
capture and treat stormwater runoff:
¾ Space: A permeable, vegetated treatment area equal to the flow path length must be
available down gradient from the downspout to effectively disconnect rooftop runoff.
Additional treatment using micro-scale practices may be used to fully meet PE requirements.
¾ Topography: Runoff must be conveyed as sheetflow from the downspout and across open
areas to maintain proper disconnection. Level spreaders may be needed at the downspout to
dissipate flow. Additionally, disconnected downspouts should be located on gradual slopes
(≤ 5%) and directed away from buildings to both maintain sheetflow and prevent water
damage to basements and foundations. If slopes are too steep (> 5%), a series of terraces or
berms may be required to maintain sheetflow. These terraces may be readily constructed of
landscaping stones, timber, or earthen berms.
¾ Soils: Downspout disconnections work best in undisturbed, sandy soils that allow runoff to
infiltrate. Clayey soils or soils that have been compacted by construction equipment greatly
reduce the effectiveness of this practice.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Figure 5.4 Disconnection of Rooftop Runoff
Plan View
Profile
Supp. 1
5.58
Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Drainage Area: The rooftop area to each downspout should be small enough to prevent
concentration of flow within the permeable treatment area. Disconnections may not be
feasible for large rooftops or those with a limited number of downspouts.
¾ Reconnections: Disconnections are ineffective if runoff flows onto impervious areas located
directly below the downspout. This practice may not be feasible if there are large areas of
imperviousness close to downspouts.
Design Guidance:
The following conditions should be considered when designing rooftop disconnections:
¾ Conveyance: Runoff from disconnected downspouts shall drain in a safe and non-erosive
manner through vegetated areas to the property line or downstream BMP.
¾ Treatment: Disconnections shall meet the following conditions:
o A pervious area at least 15 feet long (12 feet for Eastern Shore projects) shall be
available down gradient of disconnected downspouts. The length of the
disconnection flow path may be increased up to 75 feet to address larger values of PE
as shown in Table 5.6.
o Disconnections shall be located on an average slope of 5% or less. Terraces, berms,
or similar grade controls may be used where average slopes exceed 5%.
o The drainage area to each disconnected downspout shall be 500 ft2 or less.
o Disconnected downspouts shall be at least 10 ft. from the nearest impervious surface
of similar or lower elevation to prevent reconnection.
Table 5.6. ESD Sizing Factors for Rooftop Disconnection
Disconnection Flow Path Length (ft.)
Western
15
30
45
60
Shore
Eastern
12
24
36
48
Shore
PE (in.) =
0.2
0.4
0.6
0.8
75
60
1.0
¾ Landscaping: Areas receiving disconnected rooftop runoff shall be identified and notations
related to grading and construction operations included on the landscaping plans.
Disconnections should be directed over HSG A, B, or C (e.g., sands, sandy loams, loams).
HSG D or soils that are compacted by construction equipment may need to be tilled and/or
amended to increase permeability. Groundcover should be provided after any soil
amendments are used. Turf grass is the most common groundcover in residential
applications. However, trees and shrubs as well as other herbaceous plants will enhance
infiltration and evapotranspiration of runoff.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Construction Criteria:
The following items should be addressed during the construction of projects with planned
rooftop disconnections:
¾ Erosion and Sediment Control: Erosion and sediment control practices (e.g., sediment
traps) should not be located in vegetated areas receiving disconnected runoff.
¾ Site Disturbance: Construction vehicles and equipment shall avoid areas receiving
disconnected runoff to minimize disturbance and compaction. Should areas receiving
disconnected runoff become compacted, scarifying the surface or rototilling the soil to a
depth of four to six inches shall be performed to ensure permeability. Additionally,
amendments may be needed for tight, clayey soils.
Inspection:
A final inspection shall be conducted before use and occupancy approval to ensure that sizing
for treatment areas have been met and permanent stabilization has been established.
Maintenance Criteria:
Maintenance of areas receiving disconnected runoff is generally no different than that required
for other lawn or landscaped areas. The areas receiving runoff should be protected from future
compaction (e.g., by planting trees or shrubs along the perimeter). In commercial areas, foot
traffic should be discouraged as well.
Supp. 1
5.60
Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
N-2. Disconnection of Non-Rooftop Runoff
Non-rooftop disconnection involves directing flow from impervious surfaces onto vegetated
areas where it can soak into or filter over the ground. This disconnects these surfaces from the
storm drain system, reducing both runoff volume and pollutants delivered to receiving waters.
Non-rooftop disconnection is commonly applied to smaller or narrower impervious areas like
driveways, open section roads, and small parking lots and is dependent on several site conditions
(e.g., permeable flow path length, soils, slopes, compaction) to function well.
Applications:
There are many opportunities for disconnecting impervious surfaces in both new and
redevelopment designs. Runoff may be directed as sheetflow to undisturbed natural areas (e.g.,
vegetated buffers) or landscaped areas (e.g., lawns, grass channels). Non-rooftop disconnection
is possible in commercial, industrial, and residential settings given the constraints listed below.
Performance:
The PE values shown in Table 5.7 below may be applied to the ESD sizing criteria when the
contributing developed area is adequately disconnected. Rev requirements (see Chapter 2) are
also met when the PE from Table 5.7 meets or exceeds the soil specific recharge factor listed in
Section 2.2.
Constraints:
The following constraints are critical when considering the use of non-rooftop disconnection to
capture and treat stormwater runoff:
¾ Space: A permeable, vegetated treatment area equal to the minimum flow path length
needed for treatment must be available down gradient of the impervious cover to effectively
disconnect runoff. If the flow path length is insufficient, additional treatment may be
provided using micro-scale practices.
¾ Topography: Runoff must be conveyed as sheetflow onto and across open areas to maintain
proper disconnection. Additionally, disconnections should be located on gradual slopes (≤
5%) and directed away from buildings to both maintain sheetflow and prevent water damage
to basements and foundations. If slopes are too steep (> 5%), a series of terraces or berms
may be required to maintain sheetflow. These terraces may be readily constructed of
landscaping stones or timber.
¾ Soils: Non-rooftop disconnection works best in undisturbed, sandy soils that allow runoff to
infiltrate. Clayey soils or soils that have been compacted by construction greatly reduce the
effectiveness of this practice.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
¾ Drainage Area: The impervious area to each discharge location should be small enough to
prevent flow concentration onto permeable treatment areas. Disconnections may not be
feasible for large blocks of impervious cover or areas with limited discharge points.
¾ Hotspot Runoff: Disconnections should not be used to treat hotspots that generate higher
concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
Design Guidance:
The following conditions should be considered when designing non-rooftop disconnections:
¾ Conveyance: Runoff from disconnected areas shall drain in a safe and non-erosive manner
through vegetated areas to the property line or downstream BMP.
A 1 to 2 foot wide gravel (typ. No. 67 stone) transition strip should be provided from the
disconnected area to the vegetated area to assure that runoff will flow in a safe and nonerosive manner.
¾ Treatment: Disconnections shall meet the following conditions:
o The flow path or “disconnection” through vegetated areas shall be at least 10 feet
and shall not exceed 75 feet. The flow path may be increased to address larger values
of PE to a maximum of 1 inch as shown in Table 5.7.
o The maximum contributing impervious flow path length shall be 75 feet, and the
maximum contributing pervious flow path shall be 150 feet.
o Disconnections shall be located on an average slope of 5% or less. Terraces, berms,
or similar grade controls may be used where average slopes exceed 5%.
o The drainage area to each disconnection shall be 1,000 ft2 or less.
o Disconnections shall be at least 10 ft. from the nearest impervious surface of similar
or lower elevation to prevent reconnection.
Table 5.7. ESD Sizing Factors for Non-Rooftop Disconnection
Ratio of Contributing Length to Disconnection Length
Impervious
0.2:1
0.4:1
0.6:1
0.8:1
Ratio
Pervious
0.4:1
0.8:1
1.2:1
1.6:1
Ratio
PE (in.) =
0.2
0.4
0.6
0.8
1:1
2:1
1.0
¾ Landscaping: Areas receiving disconnected runoff shall be identified and notations related
to grading and construction operations included on the landscaping plans.
Disconnections should be directed over HSG A, B, or C (e.g., sands, sandy loams, loams).
HSG D and soils that are compacted by construction equipment may need to be tilled and/or
amended to increase permeability. Groundcover vegetation should be provided after any soil
Supp. 1
5.62
Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
amendments are used. Turf grass is the most common groundcover in residential
applications. Trees and shrubs as well as other herbaceous plants will enhance infiltration
and evapotranspiration of runoff.
Construction Criteria:
The following should be addressed during construction of projects with non-rooftop
disconnections:
¾ Erosion and Sediment Control: Erosion and sediment control practices (e.g., sediment
traps) should not be located in areas designated for non-rooftop disconnections.
¾ Site Disturbance: To minimize disturbance and compaction, construction vehicles and
equipment shall avoid areas receiving disconnected runoff. Should areas receiving
disconnected runoff become compacted, scarifying the surface or rototilling the soil to a
depth of four to six inches shall be performed to ensure permeability. Additionally,
amendments may be needed for tight, clayey soils.
Inspection:
A final inspection shall be conducted before use and occupancy approval to ensure that adequate
treatment areas and permanent stabilization has been established.
Maintenance Criteria:
Maintenance of areas receiving disconnected runoff is generally no different than that required
for other lawn or landscaped areas. The areas receiving runoff should be protected from future
compaction (e.g., by planting trees or shrubs along the perimeter). In commercial areas, high
foot traffic should be discouraged as well.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Fig. 5.5 Non-Rooftop Disconnection
Plan View
Profile
Supp. 1
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.6 Non-Rooftop Disconnection
Plan View
Isometric
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
N-3. Sheetflow to Conservation Areas
Stormwater runoff is effectively treated when flow from developed land is directed to adjacent
natural areas where it can soak into or filter over the ground. To function well, this practice is
dependent on several site conditions (e.g., buffer size, contributing flow path length, slopes,
compaction).
Applications:
Sheetflow to conservation areas can be used in most development situations provided that site
conditions allow implementation. This practice may be used wherever existing stream buffers
and other natural areas are protected, expanded, or created during project planning and
stormwater runoff may be directed into them, given the constraints listed below.
Performance:
The PE values shown in Table 5.8 may be applied to the ESD sizing criteria when runoff from
developed areas is directed into a conservation area meeting the criteria below. Rev requirements
(see Chapter 2) are also met for the contributing drainage area.
Constraints:
The following constraints are critical when considering the use of sheetflow to conservation
areas to treat stormwater runoff:
¾ Space: Conservation areas need to be wide enough to effectively treat runoff and protect
natural resources. Flow path lengths from impervious and pervious areas should be
minimized to prevent concentration and erosive conditions.
¾ Topography: Runoff should enter conservation areas as sheetflow to enhance performance
and prevent erosion. If slopes are too steep to maintain sheetflow (> 5%), then levelspreading devices will be needed to redistribute flow prior to entering designated buffers.
¾ Hotspot Runoff: Conservation areas should not be used to treat hotspots that generate
higher concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
¾ Easements: Public maintenance access and formal, legal protection are essential for longterm viability of conservation areas. Acceptable conservation easements, vegetation
management plans, or other enforceable instruments are required to prevent encroachment by
surrounding landowners minimize invasive or noxious plant growth, and protect conservation
areas.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Design Guidance:
The following conditions should be considered when designing sheetflow to conservation areas:
¾ Conveyance: Runoff from contributing areas shall sheetflow into conservation areas.
Either the average contributing overland slope should be 5% or less or a level-spreading
device must be used. A boundary spreader, gravel diaphragm, or infiltration berm should be
located along the downstream perimeter of the conservation area to diffuse flows from larger
storms.
¾ Treatment: Designs using sheetflow to conservation areas shall meet the following
conditions:
o Conservation areas shall be 20,000 square feet or larger to be accepted for ESD
purposes.
o The minimum effective width for conservation areas shall be 50 feet. Conservation
area widths may be increased to address larger values of PE as shown in Table 5.8.
o The maximum PE applied to conservation areas shall be 1.0 inch.
o Conservation areas may include existing natural resources, created or restored
resources, or a combination of both.
Table 5.8. Sheetflow to Conservation Area Sizing Factors
50
75
100
Min. Width (ft) =
PE (in.) =
0.6
0.8
1.0
Example: An existing wooded area (60 ft. wide by 250 ft. long) is placed in a
conservation easement and identified as a possible area for treating stormwater
runoff. While the effective width (60 ft.) is sufficient to treat the area, 15,000 square
feet is less than the 20,000 square foot minimum.
To meet the minimum area requirement, the conservation area will then be expanded
an additional 20 feet in width through reforestation. This increases the conservation
area to 20,000 square feet. Expanding the width by 20 feet through reforestation also
increases the effective width to 80 feet. Therefore, a PE = 0.8 may be applied to the
contributing drainage area.
¾ Landscaping: Landscaping plans should clearly specify how vegetation within buffers will
be established and managed. These plans should include plants that are native or adapted to
Maryland and procedures for preventing noxious or invasive plants. Managed turf (e.g.,
playgrounds, regularly mown and maintained open areas) is not an acceptable form of
vegetation management.
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Figure 5.7 Sheetflow to Conservation Areas
Profile
Plan View
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Easements: Conservation areas shall be protected by an acceptable easement or other
enforceable instrument that ensures perpetual protection of the area. The easement must
clearly specify how the natural area vegetation shall be managed and boundaries will be
marked.
Construction Criteria:
The following should be addressed during construction of projects with sheetflow to
conservation areas:
¾ Erosion and Sediment Control: Erosion and sediment control plans shall clearly indicate
where conservation areas are located and what measures will be used for protection during
construction. Buffers shall be clearly marked in the field and not receive sediment-laden
runoff prior to project completion. Erosion and sediment control practices shall not be
located within buffers.
¾ Site Disturbance: Buffers shall not be disturbed (i.e.; cleared or graded) during
construction except for temporary impacts associated with incidental utility construction or
mitigation and afforestation projects. Any temporary impacts shall be immediately repaired
and stabilized.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o During initial grading operations to ensure that buffers are clearly marked in the
field.
o Before use and occupancy approval to verify area measurements andensure that
permanent stabilization has been established.
Maintenance Criteria:
Conservation areas shall remain unmanaged other than routine debris removal and repairing
areas of concentrated flow. Invasive and noxious plant removal and bi-annual mowing for
meadow areas may be needed. Signs should be maintained and supplemental plantings
performed as needed.
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5.4.3
Micro-Scale Practices
Micro-scale practices are small water quality treatment devices used to capture and treat
stormwater runoff from discrete impervious areas (e.g., less than one acre). These practices
typically include natural systems, vegetation, and soils and may be interconnected to create a
more natural drainage system. In many cases, they may resemble the larger structural practices
(e.g., infiltration, filters, dry swales) described in Chapter 3. However, the design variants listed
below can be distributed throughout a project to provide stormwater management at the source
unlike their structural relatives that were typically used as “end-of-pipe” treatment for larger
drainage areas.
Micro-scale practice variants include:
¾
¾
¾
¾
¾
¾
¾
¾
¾
M-1.
M-2.
M-3.
M-4.
M-5.
M-6.
M-7.
M-8.
M-9.
Rainwater Harvesting
Submerged Gravel Wetlands
Landscape Infiltration
Infiltration Berms
Dry Wells
Micro-Bioretention
Rain Gardens
Swales
Enhanced Filters
Performance Standards for Micro-Scale Practices
¾
Micro-scale practices used for new development shall promote runoff reduction and
water quality treatment through infiltration, filtration, evapotranspiration, rainwater
harvesting, or a combination of these techniques.
¾
Micro-scale filters used for new development shall be designed to promote recharge (e.g.,
enhanced filter) and be planted as part of the landscaping plans.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
M-1. Rainwater Harvesting (Cisterns and Rain Barrels)
Rainwater harvesting practices intercept and store rainfall for future use. Stored water may be
used for outdoor landscaping irrigation, car washing, or non-potable water supply. The capture
and re-use of rainwater promotes conservation, as well as reduces runoff volumes and the
discharge of pollutants downstream.
Applications:
Rainwater harvesting can be applied on residential, commercial, municipal, or industrial sites.
For small-scale residential applications, rain barrels are typically used to provide storage of
rooftop runoff. These systems are generally designed for outdoor use. However, because water
demand varies seasonally, other treatment practices may be needed for dewatering during winter
months.
Larger storage tanks or cisterns are used in commercial or industrial applications. These systems
use the captured rainwater for non-potable water supply, providing a year-round source. The
complexity of the sizing, installation, and accessories of this type of application make it more
realistic for commercial operations. Separate plumbing, pressure tanks, pumps, and backflow
preventers are necessary for indoor applications.
Performance:
The pollutant removal capability of rainwater harvesting systems is directly proportional to the
amount of runoff captured, stored, and re-used. Therefore, PE for the contributing drainage area
is based on the volume captured in the rainwater harvesting design. In addition, Rev
requirements may be met only when stored water is used on landscaped areas.
Constraints:
The following constraints are critical when considering the use of rainwater harvesting
techniques to capture and re-use stormwater runoff:
¾ Space: Lack of space and the presence of surrounding trees can limit the opportunities for
rain barrels and cisterns. Leaves and woody debris from overhead trees can clog the storage
tanks or attract birds whose droppings may contaminate the tank. Space limitations can be
overcome if storage is provided on the roof or underground. The proximity to building
foundations also needs to be considered for dewatering and overflow conditions.
¾ Topography: Locating storage tanks in low areas may increase the volume of rainwater
stored but will require pumping for distribution. To prevent erosion on steeply sloped
surfaces, a bermed or concave holding area down gradient can store water for landscape
irrigation.
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¾ Drainage Area: The drainage area to each storage tank needs to consider year-round water
demands. The drainage area to each rain barrel needs to be small enough to prevent
concentrated flow during dewatering operations.
¾ Operation: Rain barrels and other storage tanks must be operated and maintained
throughout the year. This includes any necessary dewatering in between rain events so that
the required storage volume is available. Where freezing and ice formation are concerns,
rainwater harvesting systems should be located underground or indoors.
Rain barrels are subject to elimination and/or neglect by homeowners. Education is needed
to ensure that captured runoff will flow to pervious surfaces and overall system function is
sustained.
Design Guidance:
The following conditions should be considered when designing rainwater harvesting systems:
¾ Conveyance: A stable discharge shall be provided to pervious areas for any necessary
dewatering between storm events. An overflow shall be provided to pass larger storm events.
Conveyance to rainwater harvesting storage tanks consists of gutters, downspouts, and pipes.
The overflow should be near the top of the storage unit and may consist of plastic hoses or
similar materials to direct runoff safely to a stable outfall to down gradient properties.
¾ Treatment: Rainwater harvesting systems shall meet the following conditions:
o Screens and filters shall be used to remove sediment, leaves, and other debris from
runoff for pretreatment and can be installed in the gutter or downspout prior to
storage.
o Rain barrels and cisterns shall be designed to capture at least 0.2 inches of rainfall
from the contributing rooftop area. A PE value based on the ESDv captured and
treated shall be applied to the contributing rooftop area.
o Where rainwater harvesting systems are connected to indoor plumbing, the Rev
requirement shall be addressed separately.
o The design shall plan for dewatering to vegetated areas.
o The design of large commercial and industrial storage systems shall be based on
water supply and demand calculations. Stormwater management calculations shall
include the discharge rate for distribution and demonstrate that captured rainwater
will be used prior to the next storm event.
o Large capacity systems shall provide dead storage below the outlet and an air gap at
the top of the tank. Gravity-fed systems should provide a minimum of six inches of
dead storage. For systems using a pump, the dead storage depth will be based on the
pump specifications.
¾ Distribution System: Most outdoor distribution is gravity fed or can be operated with a
pump. For underground tanks or cisterns, a pump, pressure tank, and backflow preventer
will be needed.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Dewatering: During the non-growing season, irrigation systems are typically turned off and
may need to be dewatered.
¾ Observation Wells: An observation well consisting of an anchored, perforated pipe (4”
min.) shall be provided on all below-ground installations. The top of the observation well
shall be at least six inches above grade.
¾ Safety: Above ground home storage tanks shall have secured openings small enough to
prevent child entry. For underground systems, manholes shall be secured to prevent
unauthorized access.
¾ Operation: Rainwater storage designs need to consider the potential for freezing. These
systems may need to be located indoors or underground below the frost line if freezing
conditions are expected.
¾ Mosquitoes: Screens should be provided to prevent mosquitoes and other insects from
entering the tanks.
¾ Setbacks: Overflow devices shall be designed to avoid ponding or soil saturation within 10
ft. of building foundations.
Construction Criteria:
The following should be addressed during construction of projects with rainwater harvesting
systems:
¾ Site Disturbance: Underground storage tanks shall be placed on or in native soils. If
placement on fill material is necessary, a geotechnical analysis may be needed.
¾ Storage Tanks:
o Storage tanks shall be designed to be watertight and all materials should be sealed
with a water safe, non-toxic substance.
o Storage tanks shall be protected from direct sunlight and shall be opaque to prevent
the growth of algae.
o The top of underground tanks shall be beneath the frost line.
o Cisterns may be ordered from a manufacturer or constructed on-site. Typical
materials used to construct cisterns are fiberglass, wood, metal, or reinforced
concrete.
o Rain barrels can be purchased or custom made from large, plastic (e.g., 55-gallon)
drums.
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Figure 5.8 Rain Barrels
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.9 Cistern – Plan View
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
¾ Pressurization: Depending on the use of stored water, pressurization may be required. To
add pressure, a pump or pressure tank can be used.
Inspection:
Prior to operation, certification shall be required that the constructed system meets the
conditions specified on the approved plans. Additionally, certification regarding the water
tightness of the underground storage tank shall be required after its installation.
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of rainwater harvesting systems:
¾ Access shall be provided for cleaning, inspection, and maintenance in all cisterns. A drain
plug shall also be provided to allow the system to be completely emptied if needed.
¾ Leaf screens, gutters, and downspouts should be cleaned to prevent clogging. Built-up debris
can also foster bacterial growth in gutters and downspouts.
¾ Storage tank lids and mosquito screens should be inspected and cleaned.
¾ Damaged components should be replaced as needed.
¾ To avoid freezing of components, above ground systems should be disconnected, drained,
and cleaned at the start of the Winter season.
¾ Underground system connections should be checked for frozen lines and ice blockages
during Winter.
¾ Indoor systems may require more specific maintenance.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
M-2. Submerged Gravel Wetlands
A submerged gravel wetland is a small-scale filter using wetland plants in a rock media to
provide water quality treatment. Runoff drains into the lowest elevation of the wetland, is
distributed throughout the system, and discharges at the surface. Pollutant removal is achieved
in a submerged gravel wetland through biological uptake from algae and bacteria growing within
the filter media. Wetland plants provide additional nutrient uptake and physical and chemical
treatment processes allow filtering and absorption of organic matter.
Applications:
A submerged gravel wetland can be located in limited spaces, typically set aside for site
landscaping such as traffic islands or roadway medians. These systems are best suited for
Maryland’s Eastern Shore or areas where a high water table or poorly drained soils are present.
This practice is not recommended for individual lots in a residential subdivision. Depending on
individual site soil characteristics, a larger drainage area may be required to maintain saturated
conditions within the wetland.
Performance:
When designed according to the guidance provided below, PE for the contributing drainage area
is based on the volume captured by submerged gravel wetlands. In some cases Rev may be met.
Constraints:
The following constraints are critical when considering the use of submerged gravel wetlands to
capture and treat stormwater runoff:
¾ Space: Additional space is needed for pretreatment measures to prevent sediment or debris
from entering and clogging the gravel bed.
¾ Topography: While surrounding local slopes should be relatively flat (<2%), there needs to
be sufficient elevation drop to maintain positive drainage to and through the filter media.
¾ Soils: The HSG should be C or D, or a high groundwater table, hard pan, or other confining
layer should be present to maintain submerged flow conditions.
¾ Drainage Area: The drainage area should be large enough (e.g., one acre) to maintain
submerged flow conditions.
¾ Hotspot Runoff: Submerged gravel wetlands should not be used to treat hotspots that
generate higher concentrations of hydrocarbons, trace metals, or toxicants than are found in
typical stormwater runoff and may contaminate groundwater.
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¾ Wetland Vegetation Establishment: Use of native wetland plant stock obtained from a
local aquatic plant nursery is recommended for establishing vegetation. Design variations
may use wetland mulch or topsoil on top of the gravel, which may allow for successful seed
germination. However, use of the rock media for establishing wetland conditions requires
specific planting stock. Frequent inspection and maintenance will be necessary until wetland
plantings are well established.
Design Guidance:
The following conditions should be considered when designing submerged gravel wetlands:
¾ Conveyance: Pretreated stormwater enters via piped or overland flow and discharges into
the gravel-filled chamber. A perforated pipe (4 to 6-inch preferred) at the base of the gravel
layer allows for flow-through conditions and maintains a constant water surface elevation.
Discharges that exceed the ESDv exit to a stable outfall at non-erosive velocities. These
systems should be located off-line.
¾ Treatment: Submerged gravel wetlands shall meet the following conditions:
o Pretreatment shall be provided for 10% of the total ESDv. An above ground forebay
area or below ground pretreatment chamber may be used.
o Storage for 75% of ESDv for the entire drainage area contributing to the wetland
shall be provided. A PE value based on the ESDv captured and treated shall be
applied to the contributing drainage area. Temporary ponding depth shall not be
greater than the tolerance levels of the wetland vegetation. Temporary storage of
ESDv may be provided above the gravel bed.
o Storage calculations shall account for the porosity of the gravel media.
o The gravel substrate shall be no deeper than four feet.
o Surface area requirements for stormwater wetlands in Chapter 3 do not apply to this
practice because pollutant removal primarily takes place within the rock media.
¾ Flow Splitter: A flow splitter should be provided to divert the ESDv to the submerged
gravel wetland (see Details No. 5 and No. 6, Appendix D.8).
¾ Treatment Cells: Multiple treatment cells are optional and may be separated by earth
berms.
¾ Observation Wells: An observation well consisting of an anchored, six-inch diameter
perforated pipe shall be required. The top of the observation well shall be at least six inches
above grade.
¾ Landscaping: A minimum of three different types of wetland species shall be provided.
Replacement plantings may be necessary.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.10 Submerged Gravel Wetland
Section
Section
Plan View
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Construction Criteria:
The following items should be addressed during the construction of projects with submerged
gravel wetlands:
¾ Site Disturbance: All on-site disturbed areas shall be stabilized prior to allowing runoff to
enter the newly constructed wetland.
¾ Erosion and Sediment Control: The proposed location of a submerged gravel wetland
shall be protected during construction. Surface runoff shall be diverted away from the
practice during grading operations. Flow splitters and other conveyance infrastructure shall
be blocked.
Wetland construction shall be performed with lightweight, wide-tracked equipment to
minimize disturbance and compaction. Excavated materials shall be placed in a contained
area. Any pumping operations shall discharge filtered water to a stable outlet.
¾ Gravel Media: The aggregate shall be composed of an 18 to 48 inch layer of clean washed,
uniformly graded material with a porosity of 40%. Rounded bank run gravel is
recommended.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
During excavation to subgrade.
During placement of backfill of perforated inlet pipe and observation wells.
During placement of geotextiles and all filter media.
During construction of any appurtenant conveyance systems such as diversion
structures, inlets, outlets, and flow distribution structures.
o Upon completion of final grading and establishment of permanent stabilization, and
before allowing runoff to enter the wetland.
o
o
o
o
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of submerged gravel wetlands:
¾ During the first year of operation, inspections should be conducted after every major storm
and poorly established areas revegetated.
¾ Sediment accumulation in the pretreatment areas should be removed as necessary.
¾ Signs of uneven flow distribution within the wetland may mean that the gravel or underdrain
is clogged. The gravel and/or underdrain may need to be removed, cleaned, and replaced.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ A dense stand of wetland vegetation should be maintained through the life of the facility with
plantings replaced as needed.
¾ Inlets and outlets to each submerged gravel wetland cell should be free from debris to
prevent clogging.
¾ Erosion at inflow points should be repaired. Flow splitters should be functional to prevent
bypassing of the facility.
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M-3. Landscape Infiltration
Landscape infiltration utilizes on-site vegetative planting areas to capture, store, and treat
stormwater runoff. Rainwater is stored initially, filters through the planting soil and gravel
media below, and then infiltrates into native soils. These practices can be integrated within the
overall site design by utilizing a variety of landscape features for storage and treatment of
stormwater runoff. Storage may be provided in constructed planters made of stone, brick,
concrete, or in natural areas excavated and backfilled with stone and topsoil.
Applications:
Landscape infiltration can be best implemented in residential and commercial land uses.
Residential areas with compact housing such as clustered homes and townhouses can utilize
small green spaces for landscape infiltration. Because space in these instances prevents
structural pretreatment, the drainage area to these practices should be limited to less than 10,000
ft2. Larger drainage areas may be allowed where soil testing is performed and pretreatment
forebays can be implemented. Successful application is dependent upon soil type and
groundwater elevation.
Performance:
The PE values determined by Equation 5.1 may be applied to the ESD sizing criteria when
landscape infiltration systems are designed according to the guidance provided below. Rev
requirements are also met when the PE from Equation 5.1 meets or exceeds the soil specific
recharge factor listed in Section 2.2.
Constraints:
The following constraints are critical when considering the use of landscape infiltration to
capture and treat stormwater runoff:
¾ Space: Landscape infiltration should not be used in areas where operation may create a risk
for basement flooding, interfere with subsurface sewage disposal systems, or other
underground structures. The initial site planning process shall consider landscaping
opportunities where these practices may be implemented.
¾ Topography: Steep terrain affects the successful performance of landscape infiltration.
These practices should be constructed without a slope. If slopes entering these practices are
too steep, then level-spreading devices such as check dams, terraces, or berms may be needed
to maintain sheetflow.
¾ Soils: Permeable soils are critical to the successful application of landscape infiltration.
The HSG should be A or B.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Drainage Area: Drainage areas less than 10,000 ft2 are most appropriate for landscape
infiltration. Larger drainage areas may require pretreatment and soils testing to verify the
infiltration rates.
¾ Hotspot Runoff: Landscape infiltration should not be used to treat hotspots that generate
higher concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
¾ Infrastructure: Landscape designers should consider overhead electrical and
telecommunication lines when selecting plant materials.
Design Guidance:
The following conditions should be considered when designing landscape infiltration:
¾ Conveyance: Stormwater runoff is collected in landscaped areas where water will sheetflow
across the facility, percolate through the planting media, and infiltrate into underlying soils.
A flow splitter should be used to divert runoff in excess of the ESDv away from the facility at
non-erosive velocities to a stable, downstream conveyance system. If bypassing the practice
is not feasible, an internal overflow devise such as an elevated yard inlet may be used.
¾ Treatment: Landscape infiltration shall meet the following design criteria:
o The drainage area to any individual practice shall be 10,000 ft2 or less.
o The surface area (At) of landscape infiltration practices shall be at least 2% of the
contributing drainage area. A PE value based on Equation 5.1 shall be applied to the
contributing drainage area.
PE =
At
0.05 × DA
(Equation 5.1)
o Landscape infiltration facilities located in HSG B (i.e., loams, silt loams) shall not
exceed 5 feet in depth. Facilities located in HSG A (i.e., sand, loamy sand, sandy
loam) shall not exceed 12 feet in depth.
o Landscape infiltration facilities shall be designed to fully dewater the entire ESDv
within 48 hours. Temporary storage of the ESDv may be provided above the facility.
o A 12 to 18-inch layer of planting soil shall be provided as a filtering media at the top
of the facility.
o A minimum 12-inch layer of gravel is required below the planting soil.
o A 12-inch layer of clean sand shall be provided at the bottom to allow for a bridging
medium between the existing soils and stone within the bed.
o The storage area for the ESDv shall be determined for the entire system and includes
the temporary ponding area, the soil, and the sand and gravel layers in the bottom of
the facility. Storage calculations shall account for the porosity of the gravel and soil
media.
o Pretreatment measures shall be implemented along the main stormwater runoff
collection system where feasible. These include installing gutter screens, a
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removable filter screen on rooftop downspout pipes, a sand layer or pea gravel
diaphragm at the inflow, or a two to three-inch surface mulch layer.
¾ Soils: Landscape infiltration shall be installed in HSG A or B. The depth from the bottom of
the facility to the seasonal high water table, bedrock, hard pan, or other confining layer shall
be greater than or equal to four feet (two feet on the lower Eastern Shore).
¾ Flow Splitter: A flow splitter should be provided to divert excess runoff away from
landscape infiltration. An elevated yard inlet may also be used in the facility for this
purpose.
¾ Setbacks:
o Landscape infiltration shall be located down gradient of building structures and shall
be setback at least 10 feet from buildings, 50 feet from confined water supply wells,
100 feet from unconfined water supply wells, and 25 feet from septic systems.
o Landscape infiltration shall be sized and located to meet minimum local requirements
for clearance from underground utilities.
¾ Observation Wells: An observation well consisting of an anchored, perforated pipe (4” to
6” diameter) shall be provided. The top of the observation well shall be at least six inches
above grade.
¾ Landscaping: Landscaping plans shall be provided according to the guidance in Appendix
A. Plant tolerance to saturated and inundated conditions shall be considered as part of the
design. A dense and diverse planting plan will provide an aesthetically pleasing design,
which will enhance property value and community acceptance.
Construction Criteria:
The following items should be addressed during construction of projects with landscape
infiltration:
¾ Erosion and Sediment Control: Final grading for landscape infiltration should not take
place until the surrounding site is stabilized. If this cannot be accomplished, runoff from
disturbed areas should be diverted around the proposed location of the facility.
¾ Soil Compaction: Construction of the landscape infiltration facility should be performed
with lightweight, wide-tracked equipment to minimize disturbance and compaction.
Excavated materials should be placed in a contained area.
¾ Planter Boxes: Planter boxes may be made of stone, brick, or concrete.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.11 Landscape Infiltration
Section
Plan View
¾ Filter Cloth: Filter cloth shall not be installed on the bottom of any landscape infiltration
practice.
Landscape infiltration may be constructed as an excavated trench in natural ground and
backfilled with sand, gravel, and planting soil. These applications should use non-woven
filter cloth to line the sides of the facility to prevent clogging.
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¾ Gravel and Filter Media: See Appendix B.4.B. for material specifications for the sand,
gravel, and planting soil media.
¾ Landscape Installation: The optimum planting time is during the autumn months. Spring
is also acceptable but may require watering.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
o
During excavation to subgrade.
During placement of backfill and observation well.
During placement of filter fabric, soil, and gravel media.
During construction of appurtenant conveyance structures.
Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of landscape infiltration:
¾ During the first year of operation, inspection frequency should be after every major storm
and poorly established areas revegetated.
¾ Sediment accumulation on the surface of the facility should be removed and the top two to
three inches of surface layer replaced as needed.
¾ The top few inches of the planting soil should be removed and replaced when water ponds
for more than 48 hours or there is algal growth on the surface of the facility.
¾ If standing water persists after filter media has been maintained, the gravel, soil, and sand
may need to be cleaned and/or replaced.
¾ Occasional pruning and replacement of dead vegetation is necessary. If specific plants are
not surviving, more appropriate species should be used. Watering may be required during
prolonged dry periods.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
M-4. Infiltration Berms
An infiltration berm is a mound of earth composed of soil and stone that is placed along the
contour of a relatively gentle slope. This practice may be constructed by excavating upslope
material to create a depression and storage area above a berm or earth dike. Stormwater runoff
flowing downslope to the depressed area filters through the berm in order to maintain sheetflow.
Infiltration berms should be used in conjunction with practices that require sheetflow (e.g.,
sheetflow to buffers) or in a series on steeper slopes to prevent flow concentration.
Applications:
Infiltration berms may be used on gently sloping areas in residential, commercial, open space, or
wooded land use conditions. They must be installed along the contour in order to perform
effectively. The purpose of this practice is to augment natural stormwater drainage functions in
the landscape by promoting sheetflow and dissipating runoff velocities.
Performance:
Infiltration berms may be incorporated into the design with other practices such as disconnection
of rooftop and non-rooftop runoff, sheetflow to conservation areas, or grass swales to enhance
pollutant removal.
Constraints:
The following constraints are critical when considering the use of infiltration berms to treat
stormwater runoff:
¾ Space: The presence of large trees may limit the use of infiltration berms along a hillside.
Berms may be threaded carefully along the contour of wooded slopes in order to avoid
disturbing existing vegetation.
¾ Topography: Infiltration berms should not be installed on slopes greater than 10% to
prevent erosion at the upstream toe of the berm.
¾ Soils: Infiltration berms should not be installed on slopes where soils have low shear
strength (or identified as “slough prone” or “landslide prone”).
¾ Drainage Area: The drainage area should be small enough to prevent flow concentration
upslope of the berm.
¾ Hotspot Runoff: When infiltration berms are designed in conjunction with other infiltration
practices, they should not be used to treat hotspots that generate higher concentrations of
hydrocarbons, trace metals, or toxicants than found in typical stormwater runoff and may
contaminate groundwater.
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¾ Storage Capacity: Infiltration berms have relatively limited capacity to meet ESDv
requirements as a stand-alone practice. They may provide storage for pretreatment, address
Rev, or be incorporated within the design of other practices.
Design Guidance:
The following conditions should be considered when designing infiltration berms:
¾ Conveyance: Stormwater discharges greater than the two-year, 24 hour design storm shall
flow over the berm at non-erosive velocities. Stormwater runoff from impervious areas is
intercepted by infiltration berms that are placed on the contour to prevent erosive,
concentrated runoff patterns. Runoff flows to a depressed area immediately above the berm
where velocities are reduced, stormwater flows through the berm, and sheetflows downslope.
¾ Treatment: Infiltration berms shall meet the following conditions:
o Berms shall be installed along the contour at a constant elevation and be level.
o When used in a series along a slope, the elevation at the downstream toe of each
berm shall be the same elevation as the crest of the next berm downslope.
o The berm shall be asymmetric in shape. The crest should be two feet wide.
o The berm shall be graded so that a concave shape is provided at the up gradient toe.
o The design shall consider soils suitable to resist slope failure and slumping. Side
slopes should be very shallow and a ratio of 3:1 is recommended for mowed berms.
o A berm will consist of a six-inch layer of compacted topsoil with a gravel or
aggregate interior.
o The storage volume created behind and up to the crest of the berm may be used to
address pretreatment, or Rev, or contribute to ESDv requirements.
¾ Soils: Subsurface soils shall be uncompacted and may need to be scarified in order to
encourage infiltration.
¾ Plant Materials: Berms should be planted with native meadow vegetation and shrubs. Turf
grass may be used on berms that are to be mown.
Construction Criteria:
The following items should be addressed during construction of projects with infiltration berms:
¾ Erosion and Sediment Control: Final grading for infiltration berms should not take place
until the surrounding site is stabilized. If this cannot be accomplished, runoff from disturbed
areas should be diverted around proposed locations.
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Figure 5.12 Infiltration Berms
Section
Plan View
¾ Soil Compaction: Construction shall be performed with lightweight, wide-tracked
equipment to minimize disturbance and compaction. Existing soils in the location of
proposed berms should be scarified to maximize infiltration.
¾ Gravel and Soil Media: See Appendix B.4.B for material specification for the gravel and
planting soil media.
¾ Landscape Installation: The optimum planting time is during the Fall. Spring is also
acceptable but may require watering.
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¾ Implementation with Other Practices: When infiltration berms are incorporated into a
system using other practices (e.g., Disconnection of Non-Rooftop Runoff), the Construction
Criteria for that practice shall also be considered.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o During placement of gravel media, and soil.
o Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of infiltration berms:
¾ Berms should be inspected regularly to ensure that ponding water does not create nuisance
conditions.
¾ Signs of concentrated flow and other surface erosion should be repaired to promote
sheetflow.
¾ A dense mat of vegetation should be present at all times. Vegetation should be replaced as
needed.
¾ When infiltration berms are incorporated in a system using other practices, the Maintenance
Criteria for that practice shall also be considered.
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M-5. Dry Wells
A dry well is an excavated pit or structural chamber filled with gravel or stone and provides
temporary storage of stormwater runoff from rooftops. The storage area may be constructed as a
shallow trench or a deep well. Rooftop runoff is directed to these storage areas and infiltrates
into the surrounding soils prior to the next storm event. The pollutant removal capability of dry
wells is directly proportional to the amount of runoff that is stored and allowed to infiltrate.
Applications:
Dry wells can be used in both residential and commercial sites and are best suited for treating
runoff from small drainage areas such as a single rooftop or downspout. Dry wells are not
appropriate for treating runoff from large impervious areas such as a parking lot. Successful
application is dependent upon soil type and groundwater elevation.
Performance:
When designed according to the guidance provided below, dry wells will provide treatment for
the required ESDv and Rev.
Constraints:
The following constraints are critical when considering the use of dry wells to capture and
infiltrate stormwater runoff:
¾ Space: Dry wells should not be used in areas where their operation may create a risk for
basement flooding, interfere with subsurface sewage disposal systems, or affect other
underground structures. There are limited opportunities for dry well implementation in highdensity neighborhoods.
¾ Topography: Steep terrain affects the successful performance of a dry well. Installation on
slopes greater than 20% should be avoided.
¾ Soils: Permeable soils are critical to the successful application of dry wells. The HSG
should be A or B.
¾ Drainage Area: Small drainage areas (e.g., 500 ft2) are most appropriate for dry well
applications. Larger non-residential areas may be treated provided the dry well is sized
according to the requirements for infiltration practices found in Section 3.3.
¾ Hotspot Runoff: Dry wells should not be used to treat hotspots that generate higher
concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
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¾ Operation: Dry wells are subject to neglect by homeowners. Education is needed to ensure
that proper maintenance will allow the system to continue to function properly.
Design Guidance:
The following conditions should be considered when designing dry wells:
¾ Conveyance: Discharge from the overflow shall be directed to an above ground splash pad
and conveyed in a non-erosive manner to a stable outfall. Rooftop runoff is collected
through gutters and downspouts and discharged directly into a dry well. The downspout
extends underground and across the entire length of a dry well. An overflow pipe is also
installed to pass excess runoff generated from larger storms.
¾ Treatment: Dry wells shall meet the following conditions:
o Pretreatment measures shall be installed to allow filtering of sediment, leaves, or
other debris. This may be done by providing gutter screens and a removable filter
screen installed within the downspout pipe. The removable filter screen should be
installed below the overflow outlet and easily removed so that homeowners can clean
the filter.
o A dry well shall be designed to capture and store the ESDv. A PE value based on the
ESDv captured and treated shall be applied to the contributing drainage area. The
storage area for the ESDv includes the sand and gravel layers in the bottom of the
facility. Storage calculations shall account for the porosity of the gravel and sand
media.
o The drainage area to each dry well shall not exceed 1,000 square feet. Drainage
areas should be small enough to allow infiltration into the ground within 48 hours
(e.g., 500 ft2 to each downspout). Infiltration trenches may be used to treat runoff
from larger drainage areas (see Section 3.3).
o Dry wells located in HSG B (i.e., loams, silt loams) shall not exceed 5 feet in depth.
Dry wells located in HSG A (i.e., sand, loamy sand, sandy loam) shall not exceed 12
feet in depth.
o The length of a dry well should be longer than the width to ensure proper water
distribution and maximize infiltration.
o A one-foot layer of clean sand shall be provided in the bottom of a dry well to allow
for bridging between the existing soils and trench gravel.
¾ Soils: Dry wells shall be installed in HSG A or B. The depth from the bottom of a dry well to
the seasonal high water table, bedrock, hard pan, or other confining layer shall be greater
than or equal to four feet (two feet on the lower Eastern Shore).
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Figure 5.13 Dry Well
Section
Gutter Drain Filter (Typical)
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¾ Setbacks:
o Dry wells shall be located down gradient of building structures and shall be setback
at least 10 feet from buildings, 50 feet from confined water supply wells, 100 feet
from unconfined water supply wells, and 25 feet from septic systems.
o Dry wells shall be setback a minimum of 100 feet from slopes of 15% and 200 feet
from slopes of 25%.
¾ Observation Wells: An observation well consisting of an anchored, 4 to 6-inch diameter
perforated pipe shall be required. The top of the observation well shall be at least six inches
above grade.
¾ Underground Distribution Pipe: This pipe (4 to 6 inch diameter) will be perforated to fill
the trench along its entire length.
¾ Landscaping: A minimum one-foot of soil cover shall be provided from the top of the trench
to the ground surface elevation. The soil should be stabilized with a dense cover of
vegetation. In areas where frost heave is a concern, soil cover may need to be as much as
four feet. In these cases, a geotechnical engineer should be consulted.
Construction Criteria:
The following items should be addressed during construction of projects with dry wells:
¾ Erosion and Sediment Control: Final grading for proposed dry wells should not take place
until the surrounding site is completely stabilized. If this cannot be accomplished, runoff
from disturbed areas should be diverted.
¾ Soil Compaction: Construction of a dry well shall be performed with lightweight, widetracked equipment to minimize disturbance and compaction. Excavated materials shall be
placed in a contained area.
¾ Underground Chamber: A subsurface prefabricated chamber may be used.
¾ Dry Well Bottom: The bottom shall be as level as possible to minimize pooled water in
small areas that may reduce overall infiltration and longevity.
¾ Filter Cloth: Filter cloth shall not be installed on the bottom of the well. Non-woven filter
cloth should be used to line the top and sides of the dry well to prevent the pore space
between the stones from being blocked by the surrounding native material.
¾ Gravel Media: The aggregate shall be composed of an 18 to 48-inch layer of clean washed,
uniformly graded material with 40% porosity (e.g., ASTM No. 57 stone or equal).
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Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
o
During excavation to subgrade.
During placement of backfill of perforated inlet pipe and observation well.
During placement of geotextiles and all filter media.
During construction of appurtenant conveyance.
Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of dry wells:
¾ Privately owned practices shall have a maintenance plan and shall be protected by easement,
deed restriction, ordinance, or other legal measures preventing its neglect, adverse
alteration, and removal.
¾ Dry wells shall be inspected and cleaned annually. This includes pipes, gutters, downspouts,
and all filters.
¾ Ponding, standing water, or algal growth on the top of a dry well may indicate failure due to
sedimentation in the gravel media. If water ponds for more than 48 hours after a major storm
or more than six inches of sediment has accumulated, the gravel media should be excavated
and replaced.
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M-6. Micro-Bioretention
Micro-bioretention practices capture and treat runoff from discrete impervious areas by passing it
through a filter bed mixture of sand, soil, and organic matter. Filtered stormwater is either
returned to the conveyance system or partially infiltrated into the soil. Micro-bioretention
practices are versatile and may be adapted for use anywhere there is landscaping.
Applications:
Micro-bioretention is a multi-functional practice that can be easily adapted for new and
redevelopment applications in commercial and industrial projects. Stormwater runoff is stored
temporarily and filtered in landscaped facilities shaped to take runoff from various sized
impervious areas. Micro-bioretention provides water quality treatment, aesthetic value, and can
be applied as concave parking lot islands, linear roadway or median filters, terraced slope
facilities, residential cul-de-sac islands, and ultra-urban planter boxes.
Performance:
The PE values determined by Equation 5.2 may be applied to the ESD sizing criteria when microbioretention systems are designed according to the guidance provided below. Rev requirements
are also met when the PE from Equation 5.2 meets or exceeds the soil specific recharge factor
listed in Section 2.2.
Constraints:
The following constraints are critical when considering the use of micro-bioretention to capture
and treat stormwater runoff:
¾ Space: The surface area of a typical micro-bioretention filter is dependent on the area of the
contributing imperviousness. The size and distribution of open areas within a project (e.g.,
parking lot islands, landscaped areas) must be considered early during a project’s planning
and design if these practices are considered.
¾ Topography: Slopes of contributing areas and filter beds should be gradual (< 5%). If
slopes are too steep, then level-spreading devices may be needed to redistribute flow prior to
filtering. If slopes within micro-bioretention practice are too steep, then a series of check
dams, terraces, or berms may be needed to maintain sheetflow internally.
There should also be an elevation difference between the inflow and outflow of a microbioretention practice to allow flow through the filter. This difference is critical when
designing downstream conveyance systems (e.g., grass channels, storm drains).
¾ Soils: Soil conditions are a crucial determining factor for micro-bioretention because
specific applications will be affected. When located in sandier soils, these practices may be
used to promote recharge (see M-3, Landscape Infiltration). If clayey soils are encountered,
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
an underdrain system may be needed to convey water downstream. Also, elevated
groundwater may limit filter bed thickness and excavated applications.
Subsurface water conditions (e.g., water table) will help determine the thickness of filter beds
used. The probability of practice failure increases if the filter bed intercepts groundwater.
Therefore, micro-bioretention practice inverts should be above local groundwater tables.
¾ Drainage Area: The drainage area to micro-bioretention practices should be limited. As the
impervious area draining to each practice exceeds ½ acre, practice effectiveness weakens and
larger systems designed according to Chapter 3 should be considered.
¾ Hotspot Runoff: Micro-bioretention practices that are designed to promote infiltration of
runoff into the ground should not be used to treat hotspots that generate higher concentrations
of hydrocarbons, trace metals, or toxicants than are typically found in stormwater runoff and
may contaminate groundwater.
¾ Infrastructure: The location of existing and proposed buildings and utilities (e.g., water
supply wells, sewer, storm drains, electricity) will influence the design and construction of
micro-bioretention. Landscape designers should also consider overhead electrical and
telecommunication lines when selecting trees to be planted.
Design Guidance:
The following conditions should be considered when designing micro-bioretention practices:
¾ Conveyance: Micro-bioretention systems should be designed off-line whenever possible. A
flow splitter should be used to divert excess runoff away from the filter media to a stable,
downstream conveyance system. If bypassing a micro-bioretention practice is impractical,
an internal overflow device (e.g., elevated yard inlet) may be used.
Runoff shall enter, flow through, and exit micro-bioretention practices in a safe and nonerosive manner. Inflow may be through depressed curbs with wheel stops, curb cuts, or
conveyed directly using downspouts, covered drains, or catch basins. Depending on site
layout and the size and shape of the impervious area being treated, overflow structures
should be located to maximize internal flow paths through the filter media. An underdrain
system may be necessary to discharge treated stormwater safely downstream. Underdrains
may be interconnected to other micro-scale practices as part of a treatment system or directly
to the storm drain.
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¾ Treatment: Micro-bioretention practices shall meet the following conditions:
o The drainage area to any individual practice shall be 20,000 ft2 or less.
o Micro-bioretention practices shall capture and store at least 75% of the ESDv.
o The surface area (Af) of micro-bioretention practices shall be at least 2% of the
contributing drainage area. A PE value based on Equation 5.2 shall be applied to the
contributing drainage area. Temporary storage of the ESDv may be provided above
the facility with a surface ponding depth of 12 inches or less.
PE =
Af
0.075 × DA
(Equation 5.2)
o Filter beds shall be between 24 and 48 inches deep.
o Filter beds shall not intercept groundwater. If designed as infiltration practices,
filter bed inverts shall be separated at least four feet vertically (two feet on the lower
Eastern shore) from the seasonal high water table.
o A surface mulch layer (maximum 2 to 3 inches thick) should be provided to enhance
plant survival and inhibit weed growth.
o The filtering media, mulch, and underdrain systems shall conform to the
specifications found in Appendix B.4.
¾ Setbacks:
o Micro-bioretention practices should be located down gradient and setback at least 10
feet from structures. Micro-bioretention variants (e.g., planter boxes) that must be
located adjacent to structures should include an impermeable liner.
o Micro-bioretention practices shall be located at least 30 feet from water supply wells
and 25 feet from septic systems. If designed to infiltrate, then the practice shall be
located at least 50 feet from confined water supply wells and 100 feet from
unconfined water supply wells.
o Micro-bioretention practices shall be sized and located to meet minimum local
requirements for clearance from underground utilities.
o Any trees planted in micro-bioretention practices shall be located to avoid future
problems with overhead electrical and telecommunication lines.
¾ Landscaping: Landscaping plans shall be provided according to the guidance in Appendix
A. Vegetation is critical to the function and appearance of any micro-bioretention system.
Native and adapted plants are preferred, hardier, and usually require minimal nutrient or
pesticide application. Also, aesthetically pleasing landscape designs generally enhance
property value and community acceptance.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.14 Micro-Bioretention (Variation 1 - Parking Lot)
Plan View
Section
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Figure 5.15 Micro-Bioretention (Variation 2 - Parking Lot)
Plan View
Section
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.16 Micro-Bioretention (Variation 3)
Profile
Plan View
Construction Criteria:
The following items should be addressed during construction of projects with microbioretention:
¾ Erosion and Sediment Control: Micro-bioretention practices should not be constructed
until the contributing drainage area is stabilized. If this is impractical, runoff from disturbed
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areas should be diverted away and no sediment control practices should be used near the
proposed location.
¾ Soil Compaction: Excavation should be conducted in dry conditions with equipment
located outside of the practice to minimize bottom and sidewall compaction. Only
lightweight, low ground-contact equipment should be used within micro-bioretention
practices and the bottom scarified before installing underdrains and filtering media.
¾ Underdrain Installation: Gravel for the underdrain system should be clean, washed, and
free of fines. Underdrain pipes should be checked to ensure that both the material and
perforations meet specifications. The upstream ends of the underdrain pipe should be capped
prior to installation.
¾ Filter Media Installation: Bioretention soils may be mixed on-site before placement.
However, soils should not be placed under saturated conditions. The filter media should be
placed and graded using excavators or backhoes operating adjacent to the practice and be
placed in horizontal layers (12 inches per lift maximum). Proper compaction of the media
will occur naturally. Spraying or sprinkling water on each lift until saturated may quicken
settling times.
¾ Landscape Installation: The optimum planting time is during the Fall. Spring planting is
also acceptable but may require watering.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o
o
o
o
During excavation to subgrade and placement and backfill of underdrain systems.
During placement of filter media.
During construction of appurtenant conveyance.
Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of micro-bioretention practices:
¾ The top few inches of filter media should be removed and replaced when water ponds for
more than 48 hours. Silts and sediment should be removed from the surface of the filter bed
when accumulation exceeds one inch.
¾ Where practices are used to treat areas with higher concentrations of heavy metals (e.g.,
parking lots, roads), mulch should be replaced annually. Otherwise, the top two to three
inches should be replaced as necessary.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Occasional pruning and replacement of dead vegetation is necessary. If specific plants are
not surviving, more appropriate species should be used. Watering may be required during
prolonged dry periods.
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M-7. Rain Gardens
A rain garden is a shallow, excavated landscape feature or a saucer-shaped depression that
temporarily holds runoff for a short period of time. Rain gardens typically consist of an
absorbent-planted soil bed, a mulch layer, and planting materials such as shrubs, grasses, and
flowers. An overflow conveyance system is included to pass larger storms. Captured runoff
from downspouts, roof drains, pipes, swales, or curb openings temporarily ponds and slowly
filters into the soil over 24 to 48 hours.
Applications:
Rain gardens can be primary or secondary practices on residential, commercial, industrial, or
institutional sites. This practice is typically used to treat runoff from small impervious areas like
rooftops, driveways, and sidewalks. Rain gardens can also be used in retrofitting and
redevelopment applications and in series where existing slopes require energy dissipation.
Performance:
The PE values determined by Equation 5.3 may be applied to the ESD sizing criteria when rain
gardens are designed according to the guidance provided below. Rev requirements are also met
when the PE from Equation 5.3 meets or exceeds the soil specific recharge factor listed in Section
2.2.
Constraints:
The following constraints are critical when considering the use of rain gardens to capture and
treat stormwater runoff:
¾ Topography: Rain gardens require relatively flat slopes (< 5%) to accommodate runoff
filtering through the system. Some design modifications can address this constraint through
the use of infiltration berms, terracing, and timber or block retaining walls on moderate
slopes.
¾ Soils: Clayey soils or soils that have been compacted by construction equipment greatly
reduce the effectiveness of this practice. Loosening of compacted soils may improve
drainage capability.
¾ Drainage Area: The drainage area to a rain garden should be relatively small, typically less
than 2,000 square feet.
¾ Infrastructure: The location of existing and proposed buildings and utilities (e.g., water
supply wells, sewer, storm drains, electricity) will influence rain garden design and
construction. Landscape designers should also consider overhead electrical and
telecommunication lines when selecting trees to be planted.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Location:
o Lot-by-lot use of rain gardens is not recommended in residential subdivisions due to
removal by homeowners. If used on a lot-by-lot basis, educating the homeowners
will be needed to prevent removal.
o Rain garden excavation in areas with heavy tree cover may damage adjacent tree root
systems.
Design Guidance:
The following conditions should be considered when designing rain gardens:
¾ Conveyance: Runoff shall enter, flow through, and exit rain gardens in a safe and nonerosive manner. Energy dissipation shall be provided for downspout discharges using a
plunge area, rocks, splash blocks, stone dams, etc. Runoff shall enter a rain garden at the
surface through grass swales and/or a gravel bed. A minimum internal slope of one percent
should be maintained and a shallow berm surrounding the rain garden is recommended to
avoid short-circuiting. For sloped applications, a series of rain gardens can be used as
“scalloped” terraces to convey water non-erosively.
¾ Treatment: Rain gardens shall meet the following conditions:
o The drainage area to a rain garden serving a single lot in a residential subdivision
shall be 2,000 ft² or less. The maximum drainage area to a rain garden for all other
applications shall be 10,000 ft2. Micro-bioretention (M-6) or bioretention (F-6)
should be considered when these requirements are exceeded.
o The surface area (Af) of rain gardens shall be at least 2% of the contributing
drainage area. A PE value based on Equation 5.3 shall be applied to the contributing
drainage area. Temporary storage of the ESDv may be provided above the facility
with a surface ponding depth of 6 inches or less.
PE =
Af
0.10 × DA
(Equation 5.3)
o Excavated rain gardens work best where HSG A and B are prevalent. In areas of
HSG C and D, at-grade applications should be considered.
o A minimum six to twelve-inch layer of planting soil shall be provided.
o A mulch layer two to three inches deep shall be applied to the planting soil to
maintain soil moisture and to prevent premature clogging.
o The planting soil and mulch shall conform to the specifications found in Appendix
B.4.
¾ Landscaping: Landscaping plans shall clearly specify how vegetation will be established
and managed. A rain garden should be located in full to partial sun, at least two feet above
the seasonal high water table and be 12 to 18 inches deep. Plants selected for use in a rain
garden should tolerate both saturated and dry conditions and be native or adapted to
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Maryland. Neatly trimmed shrubs, a crisp lawn edge, stone retaining walls, and other
devices can be used to keep a rain garden neat and visually appealing.
Construction Criteria:
The following items should be addressed during the construction of projects with rain gardens:
¾ Erosion and Sediment Control: Rain gardens shall not be constructed until the
contributing drainage area is stabilized. During construction, runoff should be diverted and
the use of heavy equipment avoided to minimize compaction.
¾ Planting Soil: Planting soil should be mixed on-site prior to installation. If poor soils are
encountered beneath the rain garden, a four-inch layer of washed gravel (⅛ to ⅜ inch gravel
preferred) may be used below the planting soil mix.
¾ Landscape Installation: The optimum planting time is during the Fall. Spring planting is
also acceptable but may require watering.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o During excavation to subgrade and placement of planting soil.
o Upon completion of final grading and establishment of permanent stabilization.
Maintenance Criteria: The following items should be addressed to ensure proper maintenance
and long-term performance of rain gardens:
¾ Rain garden maintenance is generally no different than that required of other landscaped
areas.
¾ Privately owned practices shall have a maintenance plan and be protected by easement, deed
restriction, ordinance, or other legal measures preventing its neglect, adverse alteration, and
removal.
¾ The top few inches of the planting soil should be removed and replaced when water ponds
for more than 48 hours. Silts and sediment should be removed from the surface of the bed as
needed.
¾ Where practices are used to treat areas with higher concentrations of heavy metals (e.g.,
parking lots, roads), mulch should be replaced annually. Otherwise, the top two to three
inches should be replaced as necessary.
¾ Occasional pruning and replacement of dead vegetation is necessary. If specific plants are
not surviving, more appropriate species should be used. Watering may be required during
prolonged dry periods.
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Figure 5.17 Rain Garden
Section
Plan View
Section
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M-8. Swales
Swales are channels that provide conveyance, water quality treatment, and flow attenuation of
stormwater runoff. Swales provide pollutant removal through vegetative filtering,
sedimentation, biological uptake, and infiltration into the underlying soil media. Three design
variants covered in this section include grass swales, wet swales, and bio-swales.
Implementation of each is dependent upon site soils, topography, and drainage characteristics.
Applications:
Swales can be used for primary or secondary treatment on residential, commercial, industrial, or
institutional sites. Swales can also be used for retrofitting and redevelopment. The linear
structure allows use in place of curb and gutter along highways, residential roadways, and along
property boundaries. Wet swales are ideal for treating highway runoff in low-lying or flat terrain
with high groundwater. Bio-swales can be used in all soil types due to the use of an underdrain.
Grass swales are best suited along highway and roadway projects.
Performance:
The PE values determined by the equations 5.3 and 5.2 (reprinted below) may be applied to the
ESD sizing criteria when grass swales and bio-swales are designed according to the guidance
provided below. For wet swales, PE for the contributing drainage area is based on the volume
captured. Rev requirements are also met when the applicable PE meets or exceeds the soil
specific recharge factor listed in Section 2.2.
Swales should not be designed to meet Qp or Qf requirements except under extremely unusual
conditions. Swales may be used to convey runoff for these larger storm events however, the
ESDv should be treated separately. This can be accomplished with a flow splitter or diversion so
that the entire design storm is passed safely.
Constraints:
The following constraints are critical when considering the use of swales to capture and treat
stormwater runoff:
¾ Topography: Care should be taken when using swales in rugged terrain. Steep slopes will
increase velocity, erosion, and sediment deposition thus shortening the design life of the
swale.
¾ Soils: Design variants are dependent upon soil types. Grass swales work best in HSG A, B,
or C and wet swales are best suited for HSG C or D. Bio-swales typically include an
underdrain and may be installed in all soil types. Extreme temperatures and frozen ground
need to be considered when calculating design volumes.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Drainage Area: The drainage area contributing to all design variants should be less than
one acre. Practices in Chapter 3 should be considered for larger drainage areas.
¾ Hotspot Runoff: Swales should not be used to treat hotspots that generate higher
concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
¾ Location: The location of swales needs to be considered carefully. Wet swales are not
recommended for residential developments due to the potential nuisance or mosquito
breeding conditions. Swales along roadways can be damaged by off-street parking and are
susceptible to winter salt applications. Also, the choice of vegetation and landscaping can be
limited in adjacent areas.
Design Guidance:
The following conditions should be considered when designing swales:
¾ Conveyance: Stormwater discharged into and through swales needs to be non-erosive.
Sheetflow should be promoted wherever possible using precise grading, level earthen weirs,
or pea gravel diaphragms. If concentrated flow is delivered from curb cuts or storm drain
pipes, some form of energy dissipation (e.g., plunge pools or rip-rap) is needed.
¾ Treatment: All swales shall meet the following criteria:
Swales shall have a bottom width between two and eight feet.
The channel slope shall be less than or equal to 4.0%.
The maximum flow velocity for the ESDv shall be less than or equal to 1.0 fps.
Swales shall be designed to safely convey the 10-year, 24-hour storm at a non-erosive
velocity with six inches of freeboard.
o Channel side slopes shall be 3:1 or flatter.
o A thick vegetative cover shall be provided for proper function.
o
o
o
o
The following criteria apply to each specific design variant:
Grass swales: Grass swales shall be used for linear applications (e.g., roadways) only, and
shall be as long as the treated surface. The surface area (Af) of the swale bottom shall be at
least 2% of the contributing drainage area, and a PE value based on Equation 5.3 shall be
applied to the contributing drainage area. The maximum flow depth for ESDv treatment
should be 4 inches, and the channel should have a roughness coefficient (Manning’s n) value
of 0.15. This can be accomplished by either maintaining vegetation height equal to the flow
depth or using energy dissipaters like check dams, infiltration berms, or riffle/pool
combinations.
PE =
Af
0.10 × DA
(Equation 5.3)
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Chapter 5. Environmental Site Design............................Nonstructural and Micro-Scale Practices
Bio-swales: The surface area (Af) of the bio-swale bottom shall be at least 2% of, and a PE
value based on Equation 5.2 shall be applied to the contributing drainage area. Bio-swales
shall be designed to temporarily store at least 75% of the ESDv. A two to four-foot deep
layer of filter media shall be provided in the swale bottom. Underdrains shall be provided in
HSG C or D and shall conform to the specifications found in Appendix B.4. The use of
underdrains is recommended for all applications,
PE =
Af
0.075 × DA
(Equation 5.2)
Wet swales: Wet swales shall be designed to store at least 75% of the ESDv. A PE value
equivalent to the volume captured and treated shall be applied to the contributing drainage
area. Wet swales should be installed in areas with a high groundwater table and check dams
or weirs may be used to enhance storage.
¾ Check Dams: Check dams or weirs may be used to enhance storage and channel roughness
or provide grade control in steeper applications. Where used, these structures should be
anchored into the swale wall and notched to allow passage of larger design storms with a
minimum six-inch freeboard. Plunge pools or other energy dissipation may be required
where the elevation difference between the tops of weirs to the downstream channel invert is
a concern.
¾ Landscaping: Landscaping plans shall specify proper grass or wetland plantings based on
the design variant chosen and anticipated hydrologic conditions along the channel (see
Appendix A). Native species are best for survival and enhancing bio-diversity and wildlife.
Construction Criteria:
Construction specifications for swales can be found in Appendix B.3. In addition, the following
items should be addressed during the construction of projects with swales:
¾ Erosion and Sediment Control: Swales are often used for conveying runoff to sediment
trapping devices during site construction. Care should be taken to ensure proper construction
where stormwater management swales are used for this purpose. After the drainage area is
completely stabilized, accumulated sediment should be removed and the swale excavated to
the required dimensions. Any required infrastructure (e.g., check dams, underdrains) may
then be installed, the bottom and side slopes scarified, and a good stand of vegetation
established.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o During placement and backfill of underdrains and the installation of diaphragms,
forebays, check dams, or weirs.
o Upon completion of final grading and establishment of permanent stabilization.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
Figure 5.18 Bio-Swale
Section
Plan View
Maintenance Criteria:
The following items should be addressed to ensure proper maintenance and long-term
performance of swales:
¾ For grassed swales, regular mowing (at least bi-annually) is critical in order to reduce
competition from weeds and irrigation may be needed during dry weather to establish
vegetation. Sparsely vegetated areas need to be re-seeded to maintain dense coverage.
¾ If water does not drain within 48 hours, the bottom soil should be tilled and revegetated.
¾ Inspections should be performed once a year to assess slope integrity, vegetative health, soil
stability, compaction, erosion, ponding, and sedimentation. Periodic removal of sediment,
litter, or obstructions should be done as needed. Eroded side slopes and the swale bottom
should be repaired and stabilized where needed.
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Figure 5.19 Wet Swale
Profile
Plan View
Section
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
M-9. Enhanced Filters
An enhanced filter is a modification applied to specific practices (e.g., micro-bioretention) to
provide water quality treatment and groundwater recharge in a single facility. This design
variant uses a stone reservoir under a conventional filtering device to collect runoff, remove
nutrients, and allow infiltration into the surrounding soil.
Applications:
The structural stormwater filtering systems in Chapter 3 and the micro-filtering structures above
can be modified relatively easily for most development projects. Depending on soil conditions, a
stone reservoir can be sized appropriately to provide Rev for the drainage area to the system.
These practices are subject to the same constraints and design requirements as conventional and
micro-scale filters.
Performance:
When designed according to the guidance provided below, enhanced filters may be used to
address Rev for the contributing impervious area using the percent volume method. When
coupled with other properly designed structural or micro-scale practices, the combined system
will address the ESD sizing criteria.
Constraints:
The following constraints are critical when considering the use of enhanced filters to capture and
treat stormwater runoff:
¾ Space: The surface area of a typical enhanced filter is dependent on the design of the
practice above. Similarly, the size and distribution of open areas within a project (e.g.,
parking lot islands, landscaped areas) must be considered early during a project’s planning
and design if these practices are used.
Enhanced filters should not be used in areas where their operation may create a risk for
basement flooding, interfere with subsurface sewage disposal systems, or affect other
underground structures.
¾ Soils: Soil conditions are important when designing enhanced filters. Local soil type is a
primary factor for determining Rev and in sizing the stone reservoir.
Subsurface water conditions (e.g., water table) will help determine the stone reservoir
thickness used. The probability of practice failure increases if the reservoir intercepts
groundwater. Therefore, enhanced filter practice inverts should be above local groundwater
tables.
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¾ Hotspot Runoff: Enhanced filters should not be used to treat hotspots that generate higher
concentrations of hydrocarbons, trace metals, or toxicants than are found in typical
stormwater runoff and may contaminate groundwater.
¾ Infrastructure: The location of existing and proposed buildings and utilities (e.g., water
supply wells, sewer, storm drains, electricity) will influence the design and construction of
enhanced filters.
Design Guidance:
The following conditions should be considered when designing enhanced filters:
¾ Conveyance: Runoff shall enter the stone reservoir in a safe and non-erosive manner.
Typically, runoff flows through the upper scale practice, into the stone reservoir and
infiltrates into the ground. As the reservoir fills, an underdrain system is used to discharge
treated stormwater safely downstream. Underdrains may be connected to other micro-scale
practices or open or closed storm drain systems.
All structural and micro-scale filters should be designed off-line whenever possible. A flow
splitter should be used to divert excess runoff away from the filter media to a stable,
downstream conveyance system. If bypassing these practices is impractical, internal
overflow devices (e.g., elevated yard inlet) may be used.
¾ Treatment: Enhanced filters shall meet the following conditions:
o Enhanced filters shall be coupled with properly designed filters to address both ESD
and Rev requirements.
o At a minimum, enhanced filter reservoirs shall be designed to store the Rev. The
stone reservoir volume is equal to the surface area multiplied by depth divided by the
porosity (n) of the stone [Volume = Surface Area (ft2) x Depth (ft) ⁄ 0.4].
o The stone reservoir (#57 stone preferred) shall be at least 12 inches thick below the
underdrain.
o A 12-inch layer of sand or pea gravel (⅛ to ⅜ inch stone) may be used to act as a
bridging layer between the stone reservoir and subsurface soils.
o The invert of the stone reservoir shall be separated at least four feet (two feet on the
lower Eastern Shore) from the seasonal high water table.
¾ Setbacks:
o Enhanced filters shall be located at least 25 feet from septic systems, 100 feet from
unconfined water supply wells, and 50 feet from confined water supply wells.
o Enhanced filters shall be sized and located to meet minimum local requirements for
clearance (both vertical and horizontal) from sewer and water lines. Designs may
need to include special protection if underground utilities cross through enhanced
filters.
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Chapter 5. Environmental Site Design.............................Nonstructural and Micro-Scale Practices
¾ Observation Wells: An observation well consisting of an anchored, 4 to 6-inch diameter
perforated pipe shall be provided. The top of the observation well shall be at least six inches
above grade.
Construction Criteria:
The following items should be addressed during the construction of projects with enhanced
filtering practices:
¾ Erosion and Sediment Control: Enhanced filters shall not be used as sediment control
practices (e.g., sediment traps). Construction runoff should be directed away after initial
rough grading. Enhanced filters should not be constructed until the contributing drainage
area is stabilized.
¾ Soil Compaction: Existing soils in the location of enhanced filters should be scarified to
maximize infiltration. Construction shall be performed with lightweight, wide-tracked
equipment to minimize disturbance and compaction.
¾ Reservoir Installation: Stone for the reservoir system should be clean, washed, and free of
fines. Stone should be placed in horizontal layers (six inches per lift maximum) over the
entire area of the practice using excavators or backhoes operating adjacent to the practice.
Inspection:
¾ Regular inspections shall be made during the following stages of construction:
o During excavation to subgrade.
o During placement of gravel, and installation of underdrain systems and observation
wells.
o At all stages required for the ESD practice located above the enhanced filter.
Maintenance Criteria:
Enhanced filters require minimal maintenance in addition to that needed for the practice above to
ensure optimum performance. Generally, maintenance is the same as that used to keep the
primary practice in good condition. Additional measures include making sure there is no water
in the observation well. The presence of water 48 hours after a rain event indicates that the
enhanced filter may be clogged and need replacement.
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Figure 5.20 Enhanced Filters
Section -Variation 1
Section – Variation 2
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Chapter 5. Environmental Site Design.................................................................... Redevelopment
Section 5.5
5.5.1
Redevelopment
Introduction
Redevelopment is defined as any construction, alteration, or improvement that disturbs 5,000
square feet or more of impervious areas where the existing land use is commercial, industrial,
institutional, or multifamily residential. In order to determine whether a project is subject to new
development or redevelopment criteria, the existing site imperviousness shall be calculated.
Sites that are already substantially developed with more than 40% existing impervious surface
coverage will be subject to redevelopment requirements.
The term “site” is defined as any combination or single tract, lot, or parcel of land in one
ownership, or is contiguous and in diverse ownership, where development is to be performed as
part of a unit, subdivision, or project. Therefore, when the total site impervious area under
existing conditions exceeds the 40% threshold, redevelopment requirements may apply. When
calculating site imperviousness, the local approving agency may allow lands protected by forest
preservation, conservation easements, or other mechanism may be subtracted from the total site
area. This will create incentive to preserve and protect natural resources in redevelopment
projects.
5.5.2
Redevelopment Policy
As described above, the 40% site impervious area threshold will determine whether a project will
be regulated as new development or redevelopment. When redevelopment requirements apply,
all existing impervious areas located within a project’s limit of disturbance (LOD) are required
for management. Because redevelopment projects present a wide range of constraints and
limitations, the policy below allows for flexibility and an evaluation of options that can work in
conjunction with broader watershed goals and local initiatives.
1.
Stormwater management shall be addressed for redevelopment according to the following
criteria:
a. Reduce existing impervious area by at least 50%;
b. Implement ESD practices to the MEP to provide water quality treatment for at
least 50% of existing impervious areas; or
c. Use a combination of impervious area reduction and ESD implementation for at
least 50% of existing impervious areas.
2.
Alternative stormwater management measures may be used to meet the requirements
above provided that the developer satisfactorily demonstrates to the approving authority
that impervious area reduction and ESD have been implemented to the MEP. Alternative
stormwater management measures include but are not limited to:
a. An on-site structural BMP to provide water quality treatment for at least 50% of
existing impervious areas;
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Chapter 5. Environmental Site Design.................................................................... Redevelopment
b. An off-site structural BMP to provide water quality treatment for an area equal to
or greater than 50% of existing impervious areas; or
c. A combination of impervious area reduction, ESD implementation, and on-site or
off-site structural BMP for an area equal to or greater than 50% of existing
impervious areas.
3.
An approving agency may develop separate programmatic policies for providing water
quality treatment for redevelopment projects when the above requirements cannot be met.
These policies shall be reviewed and approved by MDE and may include but are not
limited to:
a.
b.
c.
d.
e.
Retrofitting existing structural BMPs;
Stream restoration;
Trading policies that involve other pollution control programs;
Watershed management plans; or
Fees paid to the approving agency that are dedicated to stormwater management
program implementation.
4.
Stormwater management shall be addressed according to new development requirements
when existing site impervious area is less than or equal to 40% and for any net increases
in impervious area.
5.5.3
Impervious Cover
Stormwater management requirements for redevelopment will apply to impervious areas within
the project LOD. Impervious area is defined as any surface that does not allow stormwater to
infiltrate into the ground. As a matter of policy, if gravel is compacted to the point where it will
no longer infiltrate, then it is impervious. Any gravel driveway or parking area that is regularly
used is likely to become impervious over time. However, a gravel road that is infrequently used
may be considered pervious. These determinations should be done on a case-by-case basis.
Alternative surfaces may be used to meet redevelopment requirements. However, when
designing to meet runoff reduction requirements the appropriate curve number should be used
according to the design specifications in this Chapter. These practices however, are not
considered permeable surfaces, and may be regulated differently by other State and local
programs
5.5.4
Design Process for Redevelopment
All redevelopment projects shall be subject to the Design process for Redevelopment as outlined
in the step wise procedures in Figure 5.21.
Section 5.1 of this chapter describes the design process for all development in Maryland that
includes a comprehensive review and approval of concept, site development, and final plans by
the local review agencies. These procedures will also apply to redevelopment projects and the
guidance provided in Section 5.1 of this chapter should be referenced for more specific detail at
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Chapter 5. Environmental Site Design.................................................................... Redevelopment
each step and for a check list of items required for interim reviews. The process described below
outlines the steps in Figure 5.21 and will highlight considerations specific to the design of a
redevelopment project. Approving agencies shall use the process outlined in Figure 5.21 as an
enforceable mechanism during review of the plan. Documentation that all steps were followed
during project development and specific rationale to support the proposed design shall be
required.
Step 1. Concept Phase: Develop a site map and assess existing natural resources as described
in Section 5.1.3.1. Existing buildings, impervious areas, utilities, storm drain systems,
neighboring properties, and all environmental and infrastructure constraints are identified.
Opportunities to reduce existing and proposed impervious cover by using site design techniques
and alternative surfaces are evaluated. The approximate locations of proposed impervious areas
are identified and opportunities for using ESD practices are evaluated. Additionally the
developer shall provide a narrative to the local review agencies to support the design for concept
approval.
Step 2. Submit Concept Plan: Local agencies provide review and comment back to the
developer. Concept plan approval must be given by the local review agencies before proceeding
to the site development phase.
Step 3. Site Development Phase: Incorporate comments from review agencies and finalize
proposed site layout indicating how existing and proposed impervious areas are hydrologically
connected to landscaped features (e.g., islands, vegetated planters, and green spaces). Evaluate
opportunities for implementing ESD practices on open space and landscaped areas for storage,
filtration, infiltration, and water quality treatment of stormwater runoff. Develop an erosion and
sediment control plan and an overlay plan. Provide a narrative to the local review agencies to
support the design for site development approval.
Step 4. Submit Site Development Plan: Local agencies provide review and comment on the
site development plan back to the developer. All options for meeting stormwater management
requirements using ESD planning techniques and practices have been explored. Local agencies
will provide comments and suggestions for final design. These may include potential
management strategies in the event that stormwater requirements cannot be met using ESD. Site
development plan approval must be given by the local review agencies before proceeding to final
design
Step 5A. Final Design Phase – A: After all ESD options have been explored, evaluate
alternative management strategies including on-site and off-site structural BMPs and design
according to Chapter 3. Local jurisdications should provide guidance on acceptable stormwater
treatment measures that may include retrofit projects, stream restoration, pollution trading,
watershed management plans, a fee-in-lieu, or other approved practices.
Step 5B. Final Design Phase – B: Finalize plans and address any remaining comments from
the local review agencies.
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Figure 5.21 Design Process for Redevelopment
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Chapter 5. Environmental Site Design.................................................................... Redevelopment
Step 6. Submit Final Plans: Final stormwater management and erosion and sediment control
plans are submitted for approval. The designer needs to demonstrate that on-site ESD practices
have been implemented to the MEP.
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Section 5.6
5.6.1
Special Criteria for Sensitive Waters
Introduction
In Maryland, there are several different types of sensitive watersheds, each with unique features
or regulatory requirements. In some watersheds, enhanced pollutant removal may be needed to
protect drinking water supply or shellfish harvesting. In others, temperature increases caused by
new development may need to be mitigated to preserve coldwater habitat. In addition to these
special needs, there are numerous State programs (e.g., Critical Areas, Wetlands and Waterways,
Forest Conservation) that regulate activities within receiving waters. This section presents
additional criteria that should be considered when designing projects in sensitive watersheds.
This section also identifies requirements from other State regulatory programs that will influence
ESD implementation.
5.6.2
Water Quality Standards
The purpose of Maryland’s water quality standards is to protect, maintain, and improve surface
water quality. Two of the components of these standards are the Designated Uses and water
quality criteria to protect them. Each major stream segment in Maryland is assigned one of the
following Designated Uses:
•
•
•
•
USE I & I-P: Water Contact Recreation and Protection of Nontidal Warmwater Aquatic Life
where P indicates public water supply or reservoir protection areas.
USE II & II-P: Support of Estuarine and Marine Aquatic Life and Shellfish Harvesting
USE III & III-P: Nontidal Cold Water
USE IV & IV-P: Recreational Trout Waters
For each designated use, specific water quality criteria are designed to protect aquatic life and
human health. Typically, there are numeric criteria for toxics, dissolved oxygen, bacteria, and
temperature (e.g., 5 mg/l for dissolved oxygen). There are also narrative standards that are used
for other pollutants (e.g., oil, grease, odor) where specific values are impractical. For the
majority of Maryland’s waters, these criteria represent minimum standards for the support of
balanced indigenous populations and contact recreation commonly known as
"fishable/swimmable." For higher quality waters that exceed fishable/swimmable standards, the
existing water quality conditions must be maintained.
5.6.3
ESD Implementation and Watershed Use
Stormwater management decisions are influenced by the nature and quality of the receiving
waters. Therefore, Designated Uses should be identified during the initial site and resource
mapping steps. In most cases, the majority of water quality concerns in a given watershed can be
addressed through the use of ESD to the MEP. For example, maximizing the use of ESD is a
critical component of any approval for additional discharges to higher quality waters identified in
Maryland’s Tier II Antidegradation Policy. However, in Use III and IV, ESD implementation
alone may not be sufficient to maintain critical in-stream temperatures.
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Chapter 5. Environmental Site Design.................................................................... Special Criteria
5.6.4
At-Source Techniques for Mitigating Thermal Impacts
Temperature increases caused by development impact the quality of coldwater streams.
Temperatures should not exceed 68º F in Use III and 75º F in Use IV streams or the ambient
water temperature, whichever is greater. The lethal temperatures for brook, and brown and
rainbow trout are 72º and 82º F, respectively. Therefore, one of the primary design objectives is
to prevent stream warming and maintain habitat quality for coldwater aquatic life. Implementing
ESD to the MEP, including using infiltration where appropriate, will help mitigate many of the
thermal impacts associated with development. However, additional techniques may be needed to
limit thermal impacts at the source.
In a study prepared for MDE in 1990 by the Metropolitan Washington Council of Governments,
it was determined that “[i]mperviousness together with local meteorological conditions had the
largest influence on urban stream temperatures” (Thermal Impacts Associated with Urbanism
and Stormwater Best Management Practices, John Galli, 1990). This study reported that as
watershed imperviousness increased, progressively smaller rainfall depths were needed to
produce large stream temperature fluctuations. Clearly, reducing imperviousness will help
reduce thermal impacts and techniques for accomplishing this are listed in Section 5.1.
The color of impervious surfaces also affects temperature increases. Darker surfaces like asphalt
pavement or shingles absorb solar radiation, elevating temperatures as this energy is transferred
as heat to surrounding areas, including stormwater runoff. Lighter colored materials like grey or
white concrete reflect solar radiation resulting in less elevated temperatures. A material’s ability
to reflect solar heat is measured as its Solar Reflectance Index or “SRI” and varies from 0 (a
black surface) to 100 (a white surface) and above. In thermally-sensitive watersheds, designers
should consider using materials with SRI values greater than 29 (see Table 5.9) for paving and
steep-sloped (≥2:12) roofing, and materials with SRI values greater than 78 for low-sloped
(≤2:12) roofing.
Table 5.9 Solar Reflectance Indices (SRI) for Typical Paving & Roofing Materials
Paving Materials:
Condition
SRI
Asphalt
New
0
Weathered
6
Gray Concrete
New
35
Weathered
29
White Concrete
New
86
Weathered
45
Roofing Materials:
Gray Asphalt Shingles
22
Gray EPDM (Rubber)
21
Light Gravel on Built-Up Roof
37
White-Coated Gravel on a Built-Up Roof
79
White EPDM (Rubber)
84
White PVC
104
Source: LEED®-NC for New Construction Reference Guide Ver. 2.2 (USGBC, October 2005)
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Another option for mitigating thermal impacts at the source is to shade buildings and paved areas
from the sun. Trees, large shrubs, and non-invasive vines on trellises can be used to screen these
areas from the sun and cool the air through evapotranspiration. The full benefits of shading may
not be realized until the trees and shrubs mature. Depending on the age and type of plants used,
this may be several years. In the interim, any receiving waters may be degraded and resources
lost as a result of temperature increases. When using this technique, designers should strive to
provide shade within five years of project completion.
5.6.5
Additional Techniques for Mitigating Thermal Impacts
While thermal impacts are primarily caused by increases in impervious area, stormwater
management practices, including ESD techniques, may contribute to the problem. When
designing these techniques for use in coldwater areas, minimizing temperature increases is a
primary concern. The following techniques will help reduce thermal impacts associated with
ESD practices:
¾ Maximize the infiltration capacity of each practice. Increasing infiltration reduces the
amount of surface runoff and lowers the thermal energy flowing into coldwater streams.
¾ Design filtering practices (e.g., micro-bioretention) so that underdrains are at least four feet
below the surface. Soil temperatures at this depth are cooler and fluctuate little in response
to surface weather conditions. As runoff flows through, thermal energy is dissipated and
effluent temperatures are decreased.
If overflow and conveyance connection constraints limit underdrain depth, use the enhanced
filter option 2 (see M-9, Section 5.4.3). In this variant, the perforated underdrain is located at
the bottom of a stone reservoir and below the outlet pipe. As the water surface elevation
within the reservoir rises above the invert of the outlet pipe, cooler water is siphoned from
the bottom.
¾ Use shade-producing plants in landscaped practices. As discussed above, trees, shrubs, and
non-invasive vines on trellises can be used to screen impervious areas from the sun.
5.6.6
Other Resources
In addition to the various Designated Uses, designers must also consider sensitive conditions and
design requirements associated with other programs that regulate development activities related
to critical resources. Similar to water quality concerns, most of these may also be addressed
through the use of ESD to the MEP. However, there are additional concerns like buffer widths,
construction materials used, or wetland types that may need to be considered. This section
identifies some of these program-specific requirements.
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Wetlands & Waterways
Wetlands are essential natural resources that provide fish and wildlife habitat, flood protection,
and water quality enhancement. These sensitive areas are impacted by even the smallest of
changes in hydrology or water quality. For this reason, stormwater management measures
should not be located within nontidal wetlands and their buffers, tidal wetlands, and 100-year
floodplains. This includes many of the ESD techniques listed in this Chapter. If stormwater
management facilities must be located within these areas, State and federal permits are required.
In addition to the above restrictions, runoff from new development and redevelopment must be
treated prior to discharging directly into jurisdictional wetlands or waters of the State. In most
cases, using ESD to MEP will provide adequate treatment and meet this requirement. Where
discharges are permitted, there are additional concerns. When implementing ESD within areas
of sensitive wetlands with unusual or unique plant communities like bogs, Delmarva bays, or
Wetlands of Special State Concern, designers should incorporate features and materials that
complement or mimic local natural conditions. For example, bogs are nutrient-deficient, acidic
environments where runoff pH is critical. In these areas, designers should specify the use of
native or locally available materials that are acidic (pH < 7) like granite or sandstone instead of
limestone or marble (pH > 7) for riprap in conveyance channels and energy dissipaters.
Likewise, landscaping should promote native plants that match both the conditions found within
ESD practices and local wetland communities.
In addition to using local or native materials and plants, designers should consider how runoff is
conveyed to wetlands. Storm drainage systems are usually designed to discharge directly into
wetlands and/or floodplains. This approach minimizes the ecological interaction that occurs
between wetland areas and adjacent buffers. Using more natural channel designs (e.g., coastal
plains outfalls, step/pool systems, bioswales) or promoting sheetflow to convey runoff from ESD
practices into wetlands connects and promotes interaction within these areas
Maryland’s Critical Areas
Maryland’s Critical Area Act recognizes that the land immediately surrounding the Chesapeake
and Atlantic Coastal Bays and their tributaries has the greatest potential to affect water quality
and wildlife habitat. Therefore, all land within 1,000 feet of tidal waters or adjacent tidal
wetlands is designated as the “Critical Area.” While the State Critical Area Commission
provides oversight and reviews some development projects, each appropriate County and
municipality enforces this law.
All development located within the Critical Area must address additional criteria. Some
provisions of these criteria, like those relating to the protection of habitat, are applied uniformly
throughout the Critical Area. Others provisions that may impact ESD implementation are related
to water quality and site imperviousness and are specific to land classifications discussed below.
5.125
Supp.1
Chapter 5. Environmental Site Design.................................................................... Special Criteria
Within the Critical Area, land is designated as either Intensely Developed Area, Limited
Development Area, or Resource Conservation Area (IDA, LDA, & RCA, respectively) based on
uses that existed at the time the local programs were adopted. The IDAs are those areas of
concentrated development where there is little natural habitat. Any new development and
redevelopment projects within the IDA must include stormwater management practices to reduce
post-development phosphorus loads to at least 10% below pre-developed levels. Commonly
known as the 10% Rule, this requirement may be addressed using many of the ESD practices
described in this Chapter or by using structural practices found in Chapter 3. While
implementing ESD to the MEP should meet or exceed phosphorus reduction requirements in
most cases, applicants may be required to use the Critical Areas methodology to demonstrate
compliance with the 10% Rule as part of the plan approval process. Additional guidance for
addressing the 10% Rule within the IDA may be found in the Critical Area 10% Rule
Guidance Manual (MDNR, 2003).
LDAs are those regions where development density is low to moderate and wildlife habitat is not
dominated by agriculture, wetlands, forests, or other natural areas. Similarly, RCAs are
characterized by the dominance of agriculture or protected resources like forests or wetlands.
Within these areas, new development or redevelopment must address standard water quality
requirements, conserve natural areas, and incorporate corridors to connect wildlife and plant
habitat. To accomplish these goals, imperviousness, alternative surfaces, or “lot coverage” is
generally limited to 15% of the property or project area. There are also strict limits on clearing
of existing woodland or forests. All clearing of these areas requires at least a 1:1 replacement.
To protect habitat, a forested buffer is required on all new development in all three land
designations. Extending a minimum of 100 feet from the Mean High Water Line of tidal waters
or the landward edge of tidal wetlands and tributary streams, this buffer acts as a water quality
filter and protects important riparian habitat within the Critical Area. This distance may be
expanded to include adjacent sensitive areas like hydric or highly erodible soils or steep slopes.
If it is within a subdivision in the RCA, the minimum width of the buffer is 200 feet.
Disturbance associated with new development is generally prohibited within the buffer, and,
accordingly, stormwater practices (e.g., micro-scale practices, structural facilities) cannot be
located within it.
Forest Conservation Act
The Maryland Forest Conservation Act (FCA) was enacted in 1991 to minimize the loss of
forests during land development. As a result, identifying and protecting forests is an integral part
of the development process. The primary areas targeted for protection include forests adjacent to
streams or wetlands, located on steep slopes, or within or adjacent to forest blocks or wildlife
corridors. Any activity requiring a subdivision application, grading permit, or erosion and
sediment control plan approval on areas exceeding 40,000 square feet is subject to the FCA and a
Forest Conservation Plan may be required. The Forest Conservation Plan includes a map and
narrative that describes how existing forested and sensitive areas will be protected, if
afforestation will be required, and how any replanted trees will be protected. While
implementation is not directly affected by the FCA, trees may be planted within ESD practices
and associated buffers located adjacent to critical habitat, steeply sloping ground and highly
Supp. 1
5.126
Chapter 5. Environmental Site Design.................................................................... Special Criteria
erodible soils, large forest tracts, 50-foot stream buffers, or similar areas. Additionally,
landscaping within ESD practices may be used to meet afforestation requirements when it
exceeds 2,500 square feet, is at least 35 feet wide and protected by an approved landscape
management plan.
5.127
Supp.1
Construction Specifications for ESD Practices
Appendix
B.4
Appendix B.4. Construction Specifications for Environmental Site Design Practices
B.4.A Green Roof Specifications
1. Material Specifications
Because there is significant variation in green roof assemblies and methods, providing
comprehensive specifications is not feasible. Material specifications for green roofs will vary
based on each roofing system and specific information should be obtained from the appropriate
manufacturer or retailer. The following information and specifications, which include acceptable
materials for generic applications, is not exclusive or limiting.
2. Planting Media
Planting media should be a soil-like mixture with an organic content of 15% or less (wet
combustion or loss on ignition methods). The grain size distribution is necessary for to attain
proper moisture content, permeability, nutrient management and non-capillary porosity, and soil
structure. Grain size guidelines vary for single and dual media green roof assemblies.
The planting media shall be tested and meet the following criteria:
•
•
•
•
•
•
•
•
•
•
Non-capillary Pore Space at Field Capacity, 0.333 bar
(TMECC 03.01, A)
Moisture Content at Field Capacity
(TMECC 03.01, A)
Maximum Media Water Retention (FLL)
Alkalinity, CaCO3 equivalents (MSA)
Total Organic Matter by Wet Combustion (MSA)
pH (RCSTP)
Soluble Salts (DTPA saturated media extraction –
RCSTP)
Cation Exchange Capacity (MSA)
Saturated Hydraulic Conductivity (FLL):
o Single Media Assemblies
o Dual Media Assemblies
≥ 15% (volume)
≥ 12% (volume)
≥ 30% (volume)
≤ 2.5%
≤ 3–15% (dry wt.)
6.5 – 8.0
≤ 6 mmhos/cm
≥ 10 meq/100 g
≥ 0.05 in/min
≥ 0.30 in/min
Mineral Fraction Grain Size Distribution (ASTM D422):
o
o
o
o
o
o
Single Media
0
≤ 5%
≤ 10%
5 – 50%
20 – 70%
75 – 100%
Clay Fraction (2 micron)
% Passing #200 Sieve
% Passing # 60 Sieve
% Passing #18 Sieve
% Passing ⅛ inch Sieve
% Passing ⅜ inch Sieve
B.4.1
Dual Media
0
5 – 15%
10 – 25%
20 – 50%
55 – 90%
90 – 100%
Supp. 1
Appendix B.4. Construction Specifications for Environmental Site Design Practices
3. Green Roof Layers
Root Barriers – should be thermoplastic membranes with minimum thickness of 30 mils.
Membranes certified for use as root barriers are recommended. However, only FLL currently
offers a recognized certification test. Many FLL-certified materials are locally available.
Granular Drainage Media – should be a non-carbonate mineral aggregate meeting the following
specifications:
•
•
•
•
•
•
•
Saturated Hydraulic Conductivity
Total Organic Matter (by wet combustion)
Abrasion Resistance (ASTM C131-96)
Soundness (ASTM C88 or T103 or T103-91)
Porosity (ASTM C29)
Alkalinity, CaCO3 equivalents (MSA)
Grain Size Distribution (ASTM C136)
o Percent Passing #18 Sieve
o Percent Passing ¼ inch Sieve
o Percent Passing 3/8 inch Sieve
≥ 25 inches/minute
≤ 1%
≤ 25% loss
≤ 5% loss
≥ 25%
≤ 1%
≤1%
≤ 30%
≤ 80%
Separation Fabric – should be a lightweight, non-woven geotextile that is easily penetrated by
roots while providing a durable separation between drainage and growth media layers.
Separation fabrics should meet the following:
•
•
•
•
Unit Weight (ASTM D3776)
Grab Tensile Strength (ASTM D4632)
Mullen Burst Strength (ASTM D4632)
Permittivity (ASTM D4491)
Supp. 1
B.4.2
≤ 4.25 ounces per square yard
≤ 90 lbs.
≥ 135 lbs/inch
≥ 2 sec-1
Appendix B.4. Construction Specifications for Environmental Site Design Practices
B.4.B Specifications for Permeable Pavements & Reinforced Turf
These specifications include information on acceptable materials for typical applications and are
not exclusive or limiting. The designer is responsible for developing detailed specifications for
individual projects and specific conditions.
1.
Pervious Concrete Specifications
Design Thickness - Pervious concrete applications shall be designed so that the thickness of the
concrete slab shall support the traffic and vehicle types that will be carried. Applications may be
designed using either standard pavement procedures (e.g., AASHTO, ACI 325.9R, ACI 330R) or
using structural values derived from flexible pavement design procedures.
Mix & Installation – Traditional Portland cements (ASTM C 150, C 1157) may be used in
pervious concrete applications. Phosphorus admixtures may also be used. Materials should be
tested (e.g., trial batching) prior to construction so that critical properties (e.g., settling time, rate
of strength development, porosity, permeability) can be determined.
Aggregate – Pervious concrete contains a limited fine aggregate content. Commonly used
gradations include ASTM C 33 No. 67 (¾ in. to No. 4), No. 8 (⅜ in. to No. 16) and No. 89 (⅜ in.
to No. 50) sieves. Single-sized aggregate (up to 1 inch) may also be used.
Water Content – Water-to-cement ratios between 0.27 and 0.30 are used routinely with proper
inclusion of chemical admixtures. Water quality should meet ACI 30a. As a general rule,
potable water should be used although recycled concrete production water meetingASTM C 94
or AASHTO M 157 may also be used.
Admixtures – Chemical admixtures (e.g., retarders or hydration-stabilizers) are used to obtain
special properties in pervious concrete. Use of admixtures should meet ASTM C 494 (chemical
admixtures) and ASTM C 260 (air entraining admixtures) and closely follow manufacturer’s
recommendations.
Base Course – The base course shall be AASHTO No. 3 or 4 course aggregate with an assumed
open pore space of 30% (n = 0.30).
2.
Permeable Interlocking Concrete Pavements (PICP)
Paver Blocks – Blocks should be either 3⅛ in. or 4 in. thick, and meet ASTM C 936 or CSA
A231.2 requirements. Applications should have 20% or more (40% preferred) of the surface
area open. Installation should follow manufacturer’s instructions, except that infill and base
course materials and dimensions specified in this Appendix shall be followed.
Infill Materials and Leveling Course – Openings shall be filled with ASTM C-33 graded sand or
sandy loam. PICP blocks shall be placed on a one-inch thick leveling course of ASTM C-33
sand.
B.4.3
Supp. 1
Appendix B.4. Construction Specifications for Environmental Site Design Practices
Base Course - The base course shall be AASHTO No. 3 or 4 course aggregate with an assumed
open pore space of 30% (n = 0.30).
3.
Reinforced Turf
Reinforced Grass Pavement (RGP) – Whether used with grass or gravel, the RGP thickness shall
be at least 1¾” thick and a load capacity capable of supporting the traffic and vehicle types that
will be carried.
B.4.D Specifications for Micro-Bioretention. Rain Gardens, Landscape Infiltration &
Infiltration Berms
1.
Material Specifications
The allowable materials to be used in these practices are detailed in Table B.4.1.
2.
Planting Soil
The soil shall be a uniform mix, free of stones, stumps, roots or other similar objects larger than
two inches. No other materials or substances shall be mixed or dumped within the microbioretention practice that may be harmful to plant growth, or prove a hindrance to the planting or
maintenance operations. The planting soil shall be free of Bermuda grass, Quackgrass, Johnson
grass, or other noxious weeds as specified under COMAR 15.08.01.05.
The planting soil shall be tested and shall meet the following criteria:
•
•
•
•
Soil Component - Loamy Sand or Sandy Loam (USDA Soil Textural Classification)
Organic Content - Minimum 10% by dry weight (ASTM D 2974). In general, this can be
met with a mixture of loamy sand (60%-65%) and compost (35% to 40%) or sandy loam
(30%), coarse sand (30%), and compost (40%).
Clay Content - Media shall have a clay content of less than 5%.
pH Range – Should be between 5.5 - 7.0. Amendments (e.g., lime, iron sulfate plus sulfur)
may be mixed into the soil to increase or decrease pH.
There shall be at least one soil test per project. Each test shall consist of both the standard soil
test for pH, and additional tests of organic matter, and soluble salts. A textural analysis is
required from the site stockpiled topsoil. If topsoil is imported, then a texture analysis shall be
performed for each location where the topsoil was excavated.
3.
Compaction
It is very important to minimize compaction of both the base of bioretention practices and the
required backfill. When possible, use excavation hoes to remove original soil. If practices are
Supp. 1
B.4.4
Appendix B.4. Construction Specifications for Environmental Site Design Practices
excavated using a loader, the contractor should use wide track or marsh track equipment, or light
equipment with turf type tires. Use of equipment with narrow tracks or narrow tires, rubber tires
with large lugs, or high-pressure tires will cause excessive compaction resulting in reduced
infiltration rates and is not acceptable. Compaction will significantly contribute to design
failure.
Compaction can be alleviated at the base of the bioretention facility by using a primary tilling
operation such as a chisel plow, ripper, or subsoiler. These tilling operations are to refracture the
soil profile through the 12 inch compaction zone. Substitute methods must be approved by the
engineer. Rototillers typically do not till deep enough to reduce the effects of compaction from
heavy equipment.
Rototill 2 to 3 inches of sand into the base of the bioretention facility before backfilling the
optional sand layer. Pump any ponded water before preparing (rototilling) base.
When backfilling the topsoil over the sand layer, first place 3 to 4 inches of topsoil over the sand,
then rototill the sand/topsoil to create a gradation zone. Backfill the remainder of the topsoil to
final grade.
When backfilling the bioretention facility, place soil in lifts 12” to 18”. Do not use heavy
equipment within the bioretention basin. Heavy equipment can be used around the perimeter of
the basin to supply soils and sand. Grade bioretention materials with light equipment such as a
compact loader or a dozer/loader with marsh tracks.
4.
Plant Material
Recommended plant material for micro-bioretention practices can be found in Appendix A,
Section A.2.3.
5.
Plant Installation
Compost is a better organic material source, is less likely to float, and should be placed in the
invert and other low areas. Mulch should be placed in surrounding to a uniform thickness of 2”
to 3”. Shredded or chipped hardwood mulch is the only accepted mulch. Pine mulch and wood
chips will float and move to the perimeter of the bioretention area during a storm event and are
not acceptable. Shredded mulch must be well aged (6 to 12 months) for acceptance.
Rootstock of the plant material shall be kept moist during transport and on-site storage. The plant
root ball should be planted so 1/8th of the ball is above final grade surface. The diameter of the
planting pit shall be at least six inches larger than the diameter of the planting ball. Set and
maintain the plant straight during the entire planting process. Thoroughly water ground bed
cover after installation.
B.4.5
Supp. 1
Appendix B.4. Construction Specifications for Environmental Site Design Practices
Trees shall be braced using 2” by 2” stakes only as necessary and for the first growing season only.
Stakes are to be equally spaced on the outside of the tree ball.
Grasses and legume seed should be drilled into the soil to a depth of at least one inch. Grass and
legume plugs shall be planted following the non-grass ground cover planting specifications.
The topsoil specifications provide enough organic material to adequately supply nutrients from natural
cycling. The primary function of the bioretention structure is to improve water quality. Adding
fertilizers defeats, or at a minimum, impedes this goal. Only add fertilizer if wood chips or mulch are
used to amend the soil. Rototill urea fertilizer at a rate of 2 pounds per 1000 square feet.
6.
Underdrains
Underdrains should meet the following criteria:
•
•
•
•
•
•
Pipe- Should be 4” to 6” diameter, slotted or perforated rigid plastic pipe (F 758, Type PS 28, or
AASHTO-M-278) in a gravel layer. The preferred material is slotted, 4” rigid schedule 40 PVC or
SDR35 pipe.
Perforations - If perforated pipe is used, perforations should be ⅜” diameter located 6” on center
with a minimum of four holes per row. Pipe shall be wrapped with a ¼” (No. 4 or 4x4) galvanized
hardware cloth.
Gravel – The gravel layer (No. 57 stone preferred) shall be at least 3” thick above and below the
underdrain.
The main collector pipe shall be at a minimum 0.5% slope.
A rigid, non-perforated observation well must be provided (one per every 1,0000 square feet) to
provide a clean-out port and monitor performance of the filter.
A 4” layer of pea gravel (⅛” to ⅜” stone) shall be located between the filter media and underdrain
to prevent migration of fines into the underdrain. This layer may be considered part of the filter
bed when bed thickness exceeds 24”.
The main collector pipe for underdrain systems shall be constructed at a minimum slope of 0.5%.
Observation wells and/or clean-out pipes must be provided (one minimum per every 1000 square feet
of surface area).
7.
Miscellaneous
These practices may not be constructed until all contributing drainage area has been stabilized
Supp. 1
B.4.6
Appendix B.4. Construction Specifications for Environmental Site Design Practices
Table B.4.1 Materials Specifications for Micro-Bioretention, Rain Gardens & Landscape InfiltrationMaterial
Plantings
Planting soil
[2’ to 4’ deep]
Organic content
Mulch
Pea gravel diaphragm
Curtain drain
Geotextile
Gravel (underdrains and
infiltration berms)
Specification
see Appendix A, Table A.4
loamy sand (60 - 65%) &
compost (35 – 40%)
or
sandy loam (30%),
coarse sand (30%) &
compost (40%)
Min. 10% by dry weight
(ASTM D 2974)
shredded hardwood
pea gravel: ASTM-D-448
ornamental stone: washed
cobbles
AASHTO M-43
Size
n/a
n/a
Notes
plantings are site-specific
USDA soil types loamy sand or sandy loam; clay content < 5%
aged 6 months, minimum; no pine or wood chips
NO. 8 OR NO. 9
(1/8" TO 3/8”)
stone:
2” to 5”
n/a
NO. 57 OR NO. 6
AGGREGATE
(3/8" to 3/4")
4” to 6” rigid schedule 40
PVC or SDR35
Underdrain piping
F 758, Type PS 28 or AASHTO
M-278
Poured in place concrete (if
required)
MSHA Mix No. 3; f’c = 3500
psi @ 28 days, normal weight,
air-entrained; reinforcing to
meet ASTM-615-60
n/a
Sand
AASHTO-M-6 or ASTM-C-33
0.02” to 0.04”
B.4.7
PE Type 1 nonwoven
Slotted or perforated pipe; 3/8” perf. @ 6” on center, 4 holes per
row; minimum of 3” of gravel over pipes; not necessary
underneath pipes. Perforated pipe shall be wrapped with ¼-inch
galvanized hardware cloth
on-site testing of poured-in-place concrete required:
28 day strength and slump test; all concrete design (cast-in-place
or pre-cast) not using previously approved State or local
standards requires design drawings sealed and approved by a
professional structural engineer licensed in the State of Maryland
- design to include meeting ACI Code 350.R/89; vertical loading
[H-10 or H-20]; allowable horizontal loading (based on soil
pressures); and analysis of potential cracking
Sand substitutions such as Diabase and Graystone (AASHTO)
#10 are not acceptable. No calcium carbonated or dolomitic sand
substitutions are acceptable. No “rock dust” can be used for sand.
Supp. 1
Stormwater Credits for Innovative Site Planning
Appendix
E.1
Appendix E.1. Stormwater Credits ...............................................................................Introduction
E.1.0 Stormwater Credits
In Maryland, there are many programs at both the State and local level that seek to minimize the
impact of land development. Critical Areas, forest conservation, and local stream buffer
requirements are designed to reduce nonpoint source pollution. Non-structural practices can play
a significant role in reducing water quality impacts and are increasingly recognized as a critical
feature of every stormwater BMP plan, particularly with respect to site design. In most cases,
non-structural practices must be combined with structural practices to meet stormwater
requirements. The key benefit of non-structural practices is that they can reduce the generation
of stormwater from the site; thereby reducing the size and cost of stormwater storage. In
addition, they can provide partial removal of many pollutants. Non-structural practices have
been classified into six broad groups and are designed to mesh with existing state and local
programs (e.g., forest conservation, stream buffers etc.). To promote greater use, a series of six
stormwater credits are provided for designers that use these site planning techniques.
Credit 1.
Credit 2.
Credit 3.
Credit 4.
Credit 5.
Credit 6.
Natural Area Conservation
Disconnection of Rooftop Runoff
Disconnection of Non Rooftop Runoff
Sheet Flow to Buffers
Open Channel Use
Environmentally Sensitive Development
This chapter describes each of the credits for the six groups of non-structural practices, specifies
minimum criteria to be eligible for the credit, and provides an example of how the credit is
calculated. Designers should check with the appropriate approval authority to ensure that the
credit is applicable to their jurisdiction. Clearly both of the site designs used to illustrate the
credits could be more creative to provide more non-structural opportunities.
In general, the stormwater sizing criteria provide a strong incentive to reduce impervious cover
at development sites (e.g., Rev, WQv, Cpv or Qp and Qf). Storage requirements for all five
stormwater sizing criteria are directly related to impervious cover. Thus, significant reductions
in impervious cover result in smaller required storage volumes and, consequently, lower BMP
construction costs.
These and other site design techniques can help to reduce impervious cover, and consequently,
the stormwater treatment volume needed at a site. The techniques presented in this chapter are
considered options to be used by the designer to help reduce the need for stormwater BMP
storage capacity. Due to local safety codes, soil conditions, and topography, some of these site
design features will be restricted. Designers are encouraged to consult with the appropriate
approval authority to determine restrictions on non-structural strategies.
NOTE: This chapter contains archived material and is presented here for historical purposes only.
E.1.1
Supp. 1
Appendix E.1. Stormwater Credits ...............................................................................Introduction
These credits are an integral part of a project’s overall stormwater management plan and BMP
storage volume calculation. Therefore, use of these credits shall be documented at the initial
(concept) design stage, documented with submission of final grading plans, and verified with
“as-built” certifications. If a planned credit is not implemented, then BMP volumes shall be
increased appropriately to meet Rev, WQv, Cpv, and Qp where applicable.
Table E.1.1 Summary of Stormwater Credits
Stormwater Credit
WQv
Rev
No credit. Use as
receiving area
w/Percent Area
Method.
Cpv or Qp
Natural Area
Conservation
Reduce Site Area
Reduced Rv
No credit. Use with
Percent Area
Method.
Longer tc
(increased flow path).
CN credit.
Reduced Rv
No credit. Use with
Percent Area
Method.
Longer tc (increased
flow path)
CN credit
Sheet Flow to
Buffers
Subtract
contributing site
area to BMP
Reduced Rev
CN credit
Open Channel Use
May meet WQv
Meets Rev
Longer tc
(increased flow path)
No CN credit
Environmentally
Sensitive
Development
Meets WQv
Meets Rev
No CN credit
tc may increase
Disconnection of
Rooftop Runoff
Disconnection of
Non-Rooftop
Runoff
Supp. 1
E.1.2
Forest/meadow CN for
natural areas
Chapter E.1. Stormwater Credits for Innovative Site Planning ............ Natural Area Conservation
Section E.1.1 Natural Area Conservation Credit
Natural Area Conservation Credit
A stormwater credit is given when natural areas are conserved at development sites, thereby
retaining pre development hydrologic and water quality characteristics. A simple WQv
credit is granted for all conservation areas permanently protected under conservation
easements or other locally acceptable means. Examples of natural area conservation include:
¾
¾
¾
¾
forest retention areas
non-tidal wetlands and associated buffers
other lands in protective easement (floodplains, open space, steep slopes)
stream systems
Under the credit, a designer can subtract conservation areas from total site area when
computing the water quality volume. The volumetric runoff coefficient, Rv, is still
calculated based on the percent impervious cover for the entire site.
As an additional incentive, the post development curve number (CN) used to compute the
Cpv or Qp2, and Qp10 for all natural areas protected by conservation easements can be
assumed to be woods in good condition when calculating the total site CN.
As an example, the required WQv for a ten acre site with three acres of impervious area and three
acres of protected conservation area before the credit would be:
WQv = [(P)(Rv)(A)]!12; where P= 1”, Rv= 0.05+0.009(30%)
WQv = [(1”) (0.32)(10 acres)]!12 = 0.266 acre-feet.
Under the credit, three acres of conservation are subtracted from total site area, which yields a
smaller storage volume:
WQv =[(P)(Rv)(A)]!12; where P=1”, Rv=0.05+0.009(30%)
WQv =[(1”)(0.32)(10-3 acres)]!12 = 0.187 acre-feet.
The recharge requirement (Rev) is not reduced using this credit.
E.1.3
Supp. 1
Appendix E.1. Stormwater Credits ....................................................... Natural Area Conservation
Criteria for Natural Area Credit
To receive the credit, the proposed conservation area:
•
Shall not be disturbed during project construction (e.g., cleared or graded) except for
temporary impacts associated with incidental utility construction or mitigation and
afforestation projects,
•
Shall be protected by having the limits of disturbance clearly shown on all construction
drawings and delimited in the field except as provided for above,
•
Shall be located within an acceptable conservation easement or other enforceable instrument
that ensures perpetual protection of the proposed area. The easement must clearly specify
how the natural area vegetation shall be managed and boundaries will be marked [Note:
managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form
of vegetation management], and
•
Shall be located on the development project.
Supp. 1
E.1.4
Chapter E.1. Stormwater Credits for Innovative Site Planning ............ Natural Area Conservation
Example of Calculating Natural Area Credit
Site Data - 51 Single Family
Lots
Area = 38 ac.
Conservation Area = 7.0 ac
Impervious Area = 13.8 ac
Rv = .38, P= 0.9”
Post dev. CN = 78
Original WQv = 1.08 ac-ft.
Original Rev = .25 ac-ft.
Original Cpv = 1.65 ac-ft.
Original Qp10 = 2.83 ac-ft.
Computation of Stormwater Credits
WQv = [(P)(Rv)(A)]!12
= [(0.9)(.38)(38.0 - 7.0 ac.)]!12
= 0.89 ac-ft
Rev = Same as original
(However, area draining to Natural Area may used with the Percent Area Method)
Cpv and Qp10 (total site): CN reduced from 78 to 75
E.1.5
Supp. 1
Appendix E.1. Stormwater Credits ................................................. Rooftop Runoff Disconnection
Section E.1.2 Disconnection of Rooftop Runoff Credit
Disconnection of Rooftop Runoff Credit
A credit is given when rooftop runoff is disconnected and then directed to a pervious area
where it can either infiltrate into the soil or filter over it. The credit is typically obtained by
grading the site to promote overland filtering or by providing bioretention areas on single
family residential lots.
If a rooftop is adequately disconnected, the disconnected impervious area may be deducted
from total impervious cover (therefore reducing WQv). In addition, disconnected rooftops
can be used to meet the Rev requirement as a non-structural practice using the percent area
method (see Chapter 2).
Post development CN’s for disconnected rooftop areas used to compute Cpv and Qp can be
assumed to be woods in good condition.
Criteria for Disconnection of Rooftop Runoff Credit
The credit is subject to the following restrictions:
•
•
•
•
•
•
•
•
•
•
Rooftop cannot be within a designated hotspot,
Disconnection shall cause no basement seepage,
The contributing area of rooftop to each disconnected discharge shall be 500 square feet or
less,
The length of the "disconnection" shall be 75’ or greater, or compensated using Table E.1.2,
Dry wells, french drains, rain gardens, or other similar storage devices may be utilized to
compensate for areas with disconnection lengths less than 75 feet. (See Table E.1.2 and
Figure E.1.1, dry wells are prohibited in “D” soils),
In residential development applications, disconnections will only be credited for lot sizes
greater than 6000 sq. ft.,
The entire vegetative "disconnection" shall be on an average slope of 5% or less,
The disconnection must drain continuously through a vegetated channel, swale, or through a
filter strip to the property line or BMP,
Downspouts must be at least 10 feet away from the nearest impervious surface to discourage
"re-connections”, and
For those rooftops draining directly to a buffer, only the rooftop disconnection credit or the
buffer credit may be used, not both.
Supp. 1
E.1.6
Chapter E.1. Stormwater Credits .................................................... Rooftop Runoff Disconnection
Figure E.1.1 Schematic of Dry Well
Table E.1.2 Rooftop Disconnection Compensation Storage Volume Requirements
(Per Disconnection Using Drywells, Raingardens, etc.)
Disconnection
0 - 14 ft.
15 - 29 ft. 30 - 44 ft. 45 - 59 ft. 60 - 74 ft.
Length Provided
% WQv Treated
by Disconnect
% WQv Treated
by Storage
Max. Storage
Volume*
≥ 75 ft.
0%
20%
40%
60%
80%
100%
100%
80%
60%
40%
20%
0%
40 cu-ft.
32 cu-ft.
24 cu-ft.
16 cu-ft.
8 cu-ft.
0 cu-ft.
36 cu-ft.
28.8 cu-ft.
21.6 cu-ft.
14.4 cu-ft.
7.2 cu-ft.
0 cu-ft.
(Eastern Rainfall
Zone)
Max. Storage
Volume*
(Western Rainfall
Zone)
*Assuming 500 square feet roof area to each downspout.
E.1.7
Supp. 1
Appendix E.1. Stormwater Credits ................................................. Rooftop Runoff Disconnection
Example of Using the Rooftop Disconnection Credit
Site Data - 51 Single Family Lots
Area = 38 ac., ½ acre lots
Original Impervious Area = 13.80
ac.
Original Rv = .38
Post dev. CN = 78
# of Disconnected Rooftops = 22
Original WQv = 1.08 ac-ft
Original Rev = 0.25 ac-ft
Original Cpv = 1.65 ac-ft
Original Qpv = 2.83 ac-ft
60% B Soils
40% C Soils
Composite S=0.208 (20.8%)
22 Lots Disconnected w/5
Downspouts each.
∴ 2500 sq. ft. each lot
Net impervious area reduction =
(22)(2500)/43560 = 1.3 ac
Net Impervious Area =
13.8 - 1.3 = 12.5 acres
Computation of Stormwater Credit:
New Rv= 0.05+.009 (12.5 ac/38 ac) = .35
∴ WQv= [(0.9)(.35)(38 ac)]!12 = 1.00 ac-ft.
Required Rev (Percent Area Method)
Rev = 20.8%× 13.8 ac. =2.87 acres
Rev treated by disconnection =1.3 acres
Rev remaining for treatment = 1.57 acres non structurally or 0.14 acre-feet structurally
Cpv and Qp (total site): CN reduced from 78 to 76
Supp. 1
E.1.8
Chapter E.1. Stormwater Credits ............................................ Non Rooftop Runoff Disconnection
Section E.1.3 Disconnection of Non Rooftop Runoff Credit
Disconnection of Non Rooftop Runoff Credit
Credit is given for practices that disconnect surface impervious cover runoff by directing it to
pervious areas where it is either infiltrated into the soil or filtered (by overland flow). This
credit can be obtained by grading the site to promote overland vegetative filtering or
providing bioretention areas on single family residential lots.
These "disconnected" areas can be subtracted from the impervious area when computing
WQv. In addition, disconnected surface impervious cover can be used to meet the Rev
requirement as a non-structural practice using the percent area method (See Chapter 2).
Criteria for Disconnection of Non Rooftop Runoff Credit
The credit is subject to the following restrictions:
•
•
•
•
•
•
•
•
•
Runoff cannot come from a designated hotspot,
The maximum contributing impervious flow path length shall be 75 feet,
The disconnection shall drain continuously through a vegetated channel, swale, or filter strip
to the property line or BMP,
The length of the "disconnection" must be equal to or greater than the contributing length,
The entire vegetative "disconnection" shall be on an average slope of 5% or less,
The surface impervious area to any one discharge location cannot exceed 1,000 ft2.
Disconnections are encouraged on relatively permeable soils (HSG’s A and B),
If the site cannot meet the required disconnect length, a spreading device, such as a french
drain, rain garden, gravel trench or other storage device may be needed for compensation,
and
For those areas draining directly to a buffer, only the non rooftop disconnection credit or the
stream buffer credit can be used, not both.
E.1.9
Supp. 1
Appendix E.1. Stormwater Credits ......................................... Non Rooftop Runoff Disconnection
Example of Calculating the Non Rooftop Disconnection Credit
Site Data -Community Center
Area = 3.0 ac
Original Impervious Area =
1.9 ac. = 63.3%
Original Rv = .62
Post dev. CN = 83
B Soils, S = 0.26
Original WQv = 6752 ft3
Original Rev = 1688 ft3
Original Cpv = N/A
Original Qp2 = 10,630 ft3
0.33 ac of surface
imperviousness disconnected
Net impervious area reduction
1.9 - 0.33 = 1.57 ac.
Computation of Stormwater Credit:
New Rv = 0.05+.009 (1.57 ac/3.0 ac)= .52
∴WQv = [(1.0)(0.52)(3.0 ac)]!12 = 0.13 ac-ft (5662.8 cf)
Required Rev (Percent area method)
Rev = (S)(Ai) = (0.26)(1.9 ac.) = 0.49 acres
Rev treated by disconnection = 0.33 acres
Rev remaining for treatment = 0.16 acres non structurally or 551.2 cf structurally
Cpv and Qp Post developed CN may be reduced
Supp. 1
E.1.10
Chapter E.1. Stormwater Credits .................................................................... Sheetflow to Buffers
Section E.1.4 Sheetflow to Buffer Credit
Sheetflow to Buffer Credit
This credit is given when stormwater runoff is effectively treated by a natural buffer to a
stream or forested area. Effective treatment is achieved when pervious and impervious area
runoff is discharged to a grass or forested buffer through overland flow. The use of a filter
strip is also recommended to treat overland flow in the green space of a development site.
The credits include:
1.
2.
3.
The area draining by sheet flow to a buffer is subtracted from the total site area in the
WQv calculation.
The area draining to the buffer contributes to the recharge requirement, Rev.
A wooded CN can be used for the contributing area if it drains to a forested buffer.
Criteria for Sheetflow to Buffer Credit
The credit is subject to the following conditions:
•
•
•
•
•
•
The minimum buffer width shall be 50 feet as measured from bankfull elevation or centerline
of the buffer,
The maximum contributing length shall be 150 feet for pervious surfaces and 75 feet for
impervious surfaces,
Runoff shall enter the buffer as sheet flow. Either the average contributing overland slope
shall be 5.0% or less, or a level spreading device shall be used where sheet flow can no
longer be maintained (see Detail No. 9 in Appendix D.8),
Not applicable if rooftop or non rooftop disconnection is already provided (see Credits 2 &
3),
Buffers shall remain unmanaged other than routine debris removal, and
Shall be protected by an acceptable conservation easement or other enforceable instrument
that ensures perpetual protection of the proposed area. The easement must clearly specify
how the natural area vegetation shall be managed and boundaries will be marked [Note:
managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form
of vegetation management].
Figure E.1.2 illustrates how a buffer or filter strip can be used to treat stormwater from adjacent
pervious and impervious areas.
E.1.11
Supp. 1
Appendix E.1. Stormwater Credits ................................................................. Sheetflow to Buffers
Figure E.1.2 Example of Sheetflow to Buffer Credit
Supp. 1
E.1.12
Chapter E.1. Stormwater Credits .................................................................... Sheetflow to Buffers
Example of Using the Sheetflow to Buffer Credit
Site Data - 51 Single Family
Area = 38.0 ac
Original Impervious Area =
13.8 ac = 36.3%
Original Rv = .38
Post-dev. CN = 78
Original WQv = 1.08 ac-ft
Original Rev = 0.24 ac-ft
Original Cpv = 1.65 ac-ft
Original Qpv = 2.83 ac-ft
Credit
5.0 ac draining to
buffer/filter strip
Rooftops represent 3% of
site imperviousness = 0.41
acres
Computation of Stormwater Credits
New drainage area = 38 ac.– 5 ac.= 33.0 acres
Rv remains unchanged to BMP; Rv=0.05+0.009(36.3)=0.38
WQv =[(P)(Rv)(A)]!12
=[(0.9)(0.38)(33.0 ac.)]!12
= 0.94 ac-ft
Required Rev (Percent Area Method)
Rev = 20.8%×13.8 ac. = 2.87 acres
Rev treated by disconnection = 0.41 acres
Rev remaining for treatment = 2.46 acres non structurally or 0.214 ac-ft structurally
Cpv and Qp (total site): CN is reduced slightly.
E.1.13
Supp. 1
Appendix E.1. Stormwater Credits .......................................................................... Grass Channel
Section E.1.5 Grass Channel Credit
Grass Channel Credit (in lieu of Curb and Gutter):
Credit may be given when open grass channels are used to reduce the volume of runoff and
pollutants during smaller storms (e.g., < 1 inch). The schematic of the grass channel is
provided in Figure 5.3.
Use of a grass channel will automatically meet the Rev for impervious areas draining into the
channel. However, Rev for impervious areas not draining to grass channels must still be
addressed. If designed according to the following criteria, the grass channel will meet the
WQv as well.
CNs for channel protection or peak flow control (Cpv or Qp) will not change.
Criteria for the Grass Channel Credit
The WQv credit is obtained if a grass channel meets the following criteria:
•
•
•
•
•
•
The maximum flow velocity for runoff from the one-inch rainfall shall be less than or equal to
1.0 fps (see Appendix D.10 for methodology to compute flowrate),
The maximum flow velocity for runoff from the ten-year design event shall be non erosive,
The bottom width shall be 2 feet minimum and 8 feet maximum,
The side slopes shall be 3:1 or flatter,
The channel slope shall be less than or equal to 4.0%, and
Not applicable if rooftop disconnection is already provided (see Credit 2).
An example of a grass channel is provided in Figure E.1.3.
Supp. 1
E.1.14
Chapter E.1. Stormwater Credits .............................................................................. Grass Channel
Figure E.1.3 Example of Grass Channel
E.1.15
Supp. 1
Appendix E.1. Stormwater Credits .......................................................................... Grass Channel
Example of Grass Channel Credit
Site Data - 51 Single Family
Residences
Area = 38.0 ac
Original Impervious Area =
13.8 = 36.3%
Rv = .38
CN = 78
Original WQv = 1.08 ac-ft
Original Rev = 0.25 ac-ft
Original Cpv = 1.65 ac-ft
Original Qpv = 2.83 ac-ft
Credit
12.5 acres meet grass channel
criteria
Computation of Stormwater Credits
New WQv Area = 38 ac - 12.5 ac = 25.5 ac
WQv = [(0.9)(0.38)(25.5 ac.)]!12
= 0.74 ac-ft
Required Rev (Percent Area Method)
Rev =20.8%×13.8 ac. =2.87 acres
4.5 acres of imperviousness lie within area drained by grass channels, and
4.5 acres > 2.87 acres
∴ Rev requirement is met.
Cpv and Qp: No change
Supp. 1
E.1.16
Chapter E.1. Stormwater Credits ................................................................ Sensitive Development
Section E.1.6 Environmentally Sensitive Development Credit
Environmentally Sensitive Development
Credit is given when a group of environmental site design techniques are applied to low
density or residential development. The credit eliminates the need for structural practices to
treat both the Rev and WQv and is intended for use on large lots.
Criteria for Environmentally Sensitive Development Credit
These criteria can be met without the use of structural practices in certain low density residential
developments when the following conditions are met:
For Single Lot Development:
• total site impervious cover is less than 15%,
• lot size shall be at least two acres,
• rooftop runoff is disconnected in accordance with the criteria outlined in Section E.1.2, and
• grass channels are used to convey runoff versus curb and gutter.
For Multiple Lot Development:
• total site impervious cover is less than 15%,
• lot size shall be at least two acres if clustering techniques are not used,
• if clustering techniques are used, the average lot size shall not be greater than 50% of the
minimum lot size as identified in the appropriate local zoning ordinance and shall be at least
one half acre,
• rooftop runoff is disconnected in accordance with the criteria outlined in Section E.1.2,
• grass channels are used to convey runoff versus curb and gutter,
• a minimum of 25% of the site is protected in natural conservation areas (by permanent
easement or other similar measure), and
• the design shall address stormwater (Rev, WQv, Cpv, and/or Qp10) for all roadway and
connected impervious surfaces.
E.1.17
Supp. 1
Appendix E.1. Stormwater Credits ............................................................ Sensitive Development
Example of Environmentally Sensitive Development
Site Data - 1 Single Family Lot
Area = 2.5 ac
Conservation Area = 0.6 ac
Impervious Area = .35 ac (includes
adjacent road surface) = 14%
B soils
Eastern Rainfall Zone for WQv
Rv = 0.05+0.009(14) = .18
CN = 65
WQv : Use P=0.2 as I<15%
WQv = [(0.2)(A)]!12
= [(0.2)(2.5)]!12×(43560 ft/ac.)
= 1,815 ft3
Rev = [(S)(Rv)(A)]!12
= [(0.26)(0.18)(2.5)]!12×(43,560ft/ac.)
= 424.7 ft3
Computation of Stormwater Credits:
WQv is met by site design
Rev is met by site design
Cpv and Qp: No change in CN, tc may be longer which would reduce Qp requirements
Supp. 1
E.1.18
Chapter E.1. Stormwater Credits ............................................................................Other Strategies
Section E.1.7 Dealing with Multiple Credits
Site designers are encouraged to utilize as many credits as they can on a site. Greater reductions
in stormwater storage volumes can be achieved when many credits are combined (e.g.,
disconnecting rooftops and protecting natural conservation areas). However, credits cannot be
claimed twice for an identical area of the site (e.g. claiming credit for stream buffers and
disconnecting rooftops over the same site area).
Section E.1. 8 Other Strategies to Reduce Impervious Cover
Definition: Site planning practices that reduce the creation of impervious area in new residential
and commercial development and therefore reduce the WQv for the site.
Examples of progressive site design practices that minimize the creation of impervious cover
include:
• Narrower residential road sections
• Angled one way parking
• Shorter road lengths
• Subdivisions with open space
• Smaller turnarounds and cul-de-sac radii
• Smaller front yard setbacks
• Permeable spill-over parking areas
• Shared parking and driveways
• Smaller parking demand ratios
• Narrower sidewalks
• Smaller parking stalls
It should be noted that most site designers may have little ability to control these requirements,
which are typically enshrined in local subdivision, parking and/or street codes.
Where these techniques are employed, it may be possible to reduce stormwater storage volumes.
For example, because the WQv is directly based on impervious cover, a reduction in impervious
cover reduces WQv. For Cpv and Qp, the designer can compute curve numbers (CN) based on
the actual measured impervious area at a site using:
CN =
(98)I + ∑ (CN )(P )
A
where:
CN = curve number for the appropriate pervious cover
I = impervious area at the site
P = pervious area at the site
A = total site area
E.1.19
Supp. 1
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