Eel and elver passes - the River Restoration Centre

Elver and eel passes
A guide to the design and implementation of passage
solutions at weirs, tidal gates and sluices
The Eel Manual– GEHO0211BTMV-E-E
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Elver and eel passes
Prioritising Eel Pass Solutions
Climbing Substrates
Bristle and brush substrates
Other synthetic substrates
Gauging Weirs
Bristle Board Eel Passes
'Up and Over' Eel Passes
Non-gauging weirs
Dam wall
Natural falls
Culverts/Irish bridges and fords
Tidal flaps and gates
What are the issues?
Options for fish passage at tidal flaps
Self-regulating tidal flaps and gates
The Waterman SRT
The Williams SRT
Muted tidal-regulated (MTR) gate, Nehalem Marine.
Variable backflow flap gate (VBFGTM), Juel Tide Gates
Mouse holes, cat flaps and pet doors
Installation of light-weight gates
Counter-balanced flap gates
Automated gates
Siphon passes and piped passes
7.10 Revolving doors
In-river structures
Glossary of terms
Elver and eel passes
1 Introduction
There are around 26,000 obstructions in England and Wales that could prevent eel and
elver travelling freely upstream. Of these, about 16,000 are artificial. The obstructions
come in a large variety of forms, functions and sizes, and this can make it very hard to
identify the best solution for fish passage.
Figure 1: Duxbury Weir is an obstruction on the River Yarrow.
(Note: There is now a fish pass at this site to make it
easier for fish to travel upstream).
This Manual is intended to simplify that search by bringing together in one place all the
relevant examples, best practice, good ideas and tips from those who have designed
and built eel and elver passes. This builds upon previous Environment Agency R&D
Technical Reports “Fish Pass Design for Eel and Elver”, W2-070/TR1 and “Manual for
the provision of upstream migration facilities for eel and elver”, W2-070/TR2, published
in 2004 and attempts to answer all the basic questions about eel passage solutions.
These include:
What is the best solution for eel passage?
How do I go about making it happen?
Where do I get the materials?
Who will build it for me?
How much will it all cost?
This document is the result of a two-day eel workshop held in October 2009.It is the
work of: Jim Gregory, Diane Holland, Scott McKenzie, Dave Hunter, Pete Evoy,
Stephen Carter, Michael Clyde, Andy Don, Charles Crundwell, Adrian Fewings and
Chris Randall. The section on tidal flap gates and self-regulating tide gates is based on
work commissioned by the Environment Agency and produced by David Solomon.
For information on fish passes for other species and on the fish pass approval process,
refer to the “Environment Agency Fish Pass Manual: Guidance notes on the legislation,
selection and approval of fish passes in England and Wales”, V2.1 (2010).
Elver and eel passes
2 Prioritising Eel Pass Solutions
The Eels (England and Wales) Regulations 2009 Statutory Instrument (SI) came into
force on 15 January 2010 and supports Council Regulation (EC) No 1100/2007, which
requires Member States to develop Eel Management Plans (EMPs) for each river basin
district, with the objective of permitting the escapement to the sea of at least 40 % of
the historic silver eel biomass.
Under the 2007 Regulation Member States are required to report to the European
Commission on the implementation of the EMPs, with the first report to be presented
by 30 June 2012.
Part 4 of the SI – Passage of Eels:
makes 'provisions for the passage of eels through dams and other
obstructions' (regulations 12 to 16).
It is therefore essential that we develop accepted criteria, and a standard process, by
which barriers may be assessed and prioritised for eel passage and screening.
The eel workshop’s ‘passes group formulated the following list of factors to take into
consideration when formulating a prioritisation strategy for eel passage solutions.
Prioritisation will be led by Head Office.
Upstream Productivity
Distance to next barrier
Available Habitat to next barrier
Distance from head of tide
WFD status
Escapement potential
Structure Futures
Commercial fishery operation
Parasitic status
Pesticide status
Silver eel escapement compliance
Geographical location and spread
Designation (SAC, SSSI)
Elver pass present
Ownership (EA)
Predator status
Recreational benefit
Number of barriers below
Head drop
Upstream/downstream eel population
Obstruction type
River Flow conditions
Hydropower potential
National Dataset or local
WB or site
WFD Data
Obstructions Database
Obstructions Database
Obstructions Database
WFD Data
GIS derived
Local input
Local input
Local input
WFD Data
GIS derived
GIS derived
Local input
Local input
Local input
Local input
Obstructions Database
Obstructions Database
Local input
Local input
Local input
Local input
Elver and Eel passes
3 Climbing Substrates
3.1 Bristle and brush substrates
Tufts of bristles of various materials have been used to create substrates for eel
passes for many years. Early references record the use of brushes in an eel pass on
the Elbe as early as 1964 (O’Leary 1971, and Tesch 1977). These early installations
often arranged broom-heads in a suitable pattern. Now, brush mats are made
specifically for eel passes and come in a range of materials, dimensions and bristle
spacing to suit the site and size of eels. Typical is the range of bristle mats marketed by
the French company Fish-Pass.
Figure 3.1: Bristle substrate with nylon bristles fixed to a polypropylene sheet
Bristle mats are typically 1,000 mm by 400 mm. They are made from polypropylene
with clumps of bristles about 70 mm in length. Each clump comprises about 25 bristles.
The spacing of the bristle clumps is varied according to the size of eels that need to
pass – with a minimum gap of either 14 or 21 mm. These mats can be used in
installations regardless of whether the ramp has a lateral slope.
You can also buy panels with mixed spacing. These have a zone of more closely
spaced clumps up the centre and zones of wider spaced clumps to each side. You
would generally only use these if there were no lateral slope within the ramp.
You can cut the mats to fit your particular pass, and the current price from Fish-Pass is
€131 per 1,000 x 400 mm panel. The price is the same for all bristle spacing.
Many passes in England and Wales have mats that were made to a National Rivers
Authority (NRA) specification created in 1994. This specification requires:
backing boards made of black polypropylene, 9-10 mm thick, 1,000 mm long,
and 460 or 1,000 mm wide;
bristles made of 1 mm gauge green polyester – in clusters to fill 5 mm holes,
hand-drawn with stainless steel drawing wire or punch-filled;
bristle length to be 70 mm proud of board;
bristle spacing – 5mm holes drilled at 40 mm centres in staggered rows at 20
mm spacing (for eels over 150 mm);
Elver and eel passes
bristle spacing – 5mm holes drilled at 25 mm centres with 12.5 mm between
staggered rows (for elvers and small eels).
Section 7 lists the current contact details for the companies that provided quotes to this
specification in 1994. A number of installations using these substrates are detailed by
Solomon and Beach (2004), and we describe some of these through this manual.
Legault (1991) investigated the numbers of eels using three pass ramps that had
different bristle tuft spacing (7, 14 and 21 mm) at different slopes (15°, 30° and 45°).
The results were inconclusive (see table below).
Proportion of small eels (mean length 223 mm) using ramps with different bristle
substrates at three different slopes.
In two of the three ramps, this size-range of eels used the closest substrate spacing
(7mm) less than the wider-spaced ones. However, the variation with slope defies
simple explanation. Interestingly, the mean length of eels recorded at a fish lift at the
same site during the same period was 293 mm. At least one of the passage facilities
was clearly size selective. The fast current speeds in the approach to the fish lift may
have discouraged smaller eels from entering, or larger eels may have been less
inclined to enter the bristle substrate.
3.2 Other synthetic substrates
Many other synthetic substrates have been used for eel passes, including:
sacks sewn together (Tesch, 1977);
discarded trawl netting (Shotzberger and Strait 2002);
nylon garden netting and Astroturf (Knights and White 1998);
artificial vegetation, trade name Cassonia (Eckersley 1982);
geotextile matting – for example Enkamat 7020 (Dahl 1991), Enkamat 7220
(Wippelhauser 2001), and Tensar (Matthews and others 2001).
Enkamat is described by the manufacturer as ‘a dense three-dimensional permanent
erosion prevention mat, made of thick polyamide filaments fused where they cross’.
Various thicknesses are available. Types 7020 and 7220 are 20 mm thick.
Unfortunately geotextile matting limits the size of eel that can pass through the matrix.
Matthews and others mention that the larger ‘bootlace’ eels which passed through their
facility late in the season became tangled in the mesh. Dahl (1991) refers to problems
with Tensar matting when it was used in pipes: larger eels became jammed and died.
Voegtle and Larinier (2000) concluded that Enkamat was very ‘abrasive’, causing eels
to lose considerable amounts of mucus. They also found it to be size selective, only
allowing eels to pass if they were less than 260 mm. The main use of these substrates
maybe at sites where elvers and small eels predominate.
Elver and Eel passes
Figure 3.2: Milieu’s Eel-ladder substrate, for eels more than15 cm long
In recent years, some new synthetic substrates
have been developed. These are based on round,
solid shapes fixed to a flat bed. They are designed
for use without a lateral slope, in pumped-supply
passes and pass-traps.
Figure 3.3: Milieu’s experimental eel pass substrate, machined from solid polyurethane foam.
One type, used extensively in North America, is
called Eel-ladder. It was developed by Milieu Inc of
Quebec (see Figure 3.2). The Eel-ladder uses
open-topped cylinders 50.8 mm across. These are
placed in holes in the substrate bed so that the
tops project by 101.6 mm. The substrate comes as
a moulded modular channel and only needs a
frame to support it. This substrate is designed for
eels of 150 to 750 mm, so is best suited to passes
some distance up river. This design has been used
with great success in passes at Chambly Dam and
Beauharnois (both in Quebec), and a number of
other sites in Canada (see Solomon and Beach,
Milieu Inc also manufactures a smaller version of this substrate, for elvers and small
eels up to 150 mm long (see Figure 3.3). This has studs 25 mm across within a preformed channel 140 mm wide.
Figure 3.4: Plastic eel pass substrate developed by Fish­Pass in France. 6
Elver and eel passes
The French company Fish-Pass has also developed another solid plastic substrate
(Figure 3.4). It is made of acrylonitrile butadiene styrene (ABS) and supplied in sheets
that are designed to be fixed to sloping weir sills. The shapes are dome-topped
cylinders, 30mm in height with 14mm gaps. This shape minimises the build up of debris
which could block the pass. The optimal operating water depth within the substrate is
2-12 mm; the optimal slope is up to 35°.
Several eel passes in North America have
used a plastic substrate with the trade
name of Akwadrain. This is a plastic
moulding designed for vertical drainage
against underground walls or walls built
into banks. Details are shown in Figure
3.5. The main advantages of this material
are the very low cost and its physical
flexibility. This flexibility could allow it to
be draped over weir backs as a temporary
installation. The main limitation is its
delicate construction: it would need to be
regularly replaced in otherwise permanent
Figure 3.5: Akwadrain plastic
There have been experiments in France using concrete block substrates, including
some originally manufactured for car parks and walkways that allow grass to grow
Antoine Legault of Fish Pass has told us that he is experimenting with one such
substrate called Pelcar (see Figure 3.6).
Voegtle and Larinier (2000) have
examined the effectiveness of several
concrete block substrates. Most were
made specially but one was a car park
block, Evergreen. This is similar to the
Pelcar slab. They compared their
effectiveness with that of bristle
Voegtle and Larinier carried out tests at
three gradients:15°, 30° and 45°. For most
substrates the shallowest slope worked
best, with the highest level of successful
passage and the greatest tolerance to
Figure 3.6: Pelcar concrete substrate
variation in headwater level. Most
movement on this slope was achieved by swimming rather than crawling, as long as
there was an adequate depth of water (10-20mm). At steeper slopes, crawling was
more common and smaller eels in particular found ascent more difficult.
When crawling, an eel needs to support its body at several points. This means that
different sizes of eel benefit from different stud spacing. The most effective layout of
studs was found to be a quincunx, where four objects are set out in a square with a fifth
in the centre. This is the pattern formed by the staggered rows of brush bristles
described in the specification in Section 6.3.3.
For elvers, bristle substrates and a closely spaced concrete stud substrate worked
best. This was because of the level of support provided. For small eels (up to 150 mm
in length) these two substrates plus Evergreen gave the best results – provided the
depth of water was restricted (less than 20 mm at 15°, 10 mm at 30°, 5 mm at 45°).
Elver and Eel passes
For larger eels, the brush substrate and a larger concrete stud form were the least
selective, particularly on the steeper slopes. All substrates were also tested on a lateral
slope of 30° with good results.
Concrete substrates are very strong and most likely to be useful in places where their
great inherent strength is an advantage, such as sites subject to severe floods,
vandalism or heavy foot-traffic from canoeists or other river users.
Elver and eel passes
4 Gauging Weirs
4.1 Introduction
Powerful swimmers, including salmon and sea trout, find it easy to pass through many
hydrometric gauging structures such as crump-section weirs. However, it is widely
accepted that gauging weirs may make it more difficult for eels to migrate upstream.
There are two main factors:
The smooth nature of the weir surface provides no ‘crawling medium’.
Juvenile eels need this to stop themselves being washed downstream.
The nature of gauging weirs is such that they tend to have a fast-flowing,
shallow downstream face. Water speeds often exceed the swimming
capabilities of juvenile eels.
Eels and elver have very specific requirements for fish passage, justifying the
installation of dedicated facilities. Providing such facilities can be problematic. There is
likely to be resistance to any interference with the precision of flow gauging or the
installation of any structure which disturbs the smooth flow of water over the weir. The
Environment Agency has developed guidance (See Sections 4.2 and 4.3) on the type
of eel pass that is acceptable for installation at river flow measurement structures.
There are two types of eel pass that are acceptable for the different types of gauging
weir: bristle board passes and ‘up and over’ pump-fed passes. As a guide, these can
be installed at the following categories of gauging structure:
Crump weirs and rectangular weirs - bristle board pass
Compound crump weirs
- use ‘up and over’ pump-fed passes or bristle
board (where the lowest crest is in the centre section).
Flat v weirs , V shaped broad crested weirs and flumes - ‘up and over’ pump-fed
Non standard V, U shaped weirs and thin plate weirs (V shapes and rectangular) ‘up and over’ pump-fed or bristle board passes.
General design principles for both types of pass are described in Section 4.2 and 4.3.
Important note. At the time of writing, installation and maintenance of eel passes is
down to local arrangements on a site by site basis.
4.2 Bristle Board Eel Passes
General design principles
The base of the board will make a smooth joint with the upstream and
downstream slopes of the crump weir.
The boards shall run parallel to the wingwall and be mounted so that the
end of the bristle touches the wingwall The bristle tufts are 70mm long and
offset at 30mm intervals. The backing boards are 10mm thick – giving a
total board thickness of 80mm, from the tip of the bristles to the back of the
Elver and Eel passes
board. The outer face of the backing board is set 80mm out from the
The boards can only be installed on weirs that are wider than 4m in width.
The installation of the board has the effect of reducing the flow calculated
over a 4m wide weir of between 1.2 to 1.5% at a crump weir. This reduction
in flow becomes less on wider weirs.
On crump weirs between 2m and 4 m wide, boards with 30mm bristles and
spaced at 30mm may be installed. This gives the same % reduction in flow
as item 3 above.
Boards cannot be installed on weirs narrower than 2m.
The board shall extend downstream sufficiently far enough to ensure that the
end of the board is always below the lowest downstream water level
expected at the site. This would normally be at or beyond where the
downstream 1 in 5 slope of the crump weir terminates.
The board shall extend sufficiently far enough upstream to at least the point
where the upstream slope of the crump meets the river bed. Where possible
it is desirable to extend the board upstream as far as possible and if practical
extend around the wingwall return where it curves back into the bank. It is
important not to impede the stilling well inlet pipe.
The board shall extend to the top of the wingwall or above the modular limit
of the gauge above the weir crest which ever is the lesser.
The outer face of the board will be smooth with no external fixings or
fastenings protruding into to the flow of the river.
10. It should be mounted in such a manner that it can be taken off for cleaning if
11. Always undertake any detailed design in conjunction with the local
Hydrometry Team.
Figure 4.1 shows a prototype bristle board Eel Pass. The flow of water over a weir can
be modified sufficiently to allow eel and elver passage by bolting bristle boards to the
side walls of the weir. These are suitable to be used at flow measurement structures
that have vertical sidewalls and that have water running over the full width of the
channel .
Important note. The example in Figure 4.1 is a prototype bristle board eel pass.
Elver and eel passes
Figure 4.1: Crest section of installed pass
Vertical bristle boards can be used at flow measurement structures that have vertical
sidewalls known as “wingwalls” and that have water running over the full width of the
channel. These are predominantly crump type weirs but can also include compound
crump weirs, rectangular broad crested weirs, full width rectangular thin plate weirs and
non standard structures. See examples of these different weirs below.
Figure 4.2 Single crump weir
Figure 4.3 Compound Crump weir
In the compound crump example the pass must be on the lowest crest which is on the
right hand bank. Alternatively an up and over eel pass could be installed in the left
hand crump (see section 4.3)
Elver and Eel passes
Figure 4.4 Rectangular Weir
Bristle board eel passes are designed to be installed on crump type weirs. The eel
pass is constructed of boards that are supplied in 1m long and 0.4m wide sections with
bristle tufts 70mm long spaced at 30mm intervals set on a backing board 10mm thick.
They are mounted vertically along one side of the crump weir adjacent to the wing wall
(Figure 4.1).
Fixing boards to the wingwall
Figure 4.5: Bristle board with floor
Figure 4.6: Stand-off bracket
The boards illustrated in Figure 4.1 are 1m long by 40cm wide. The bristle tufts are
70mm long and offset at 30mm intervals. The backing boards are 10mm thick – giving
a total board thickness of 80mm, from the tip of the bristles to the back of the board.
Only fix brackets to the wing walls of the weir above the modular range of the flow
gauge. No fixings must be put into the weir face. Fit the boards to the wing walls as
shown in Figure 4.1. The boards are held in place with stainless steel brackets. (Those
shown in Figure 4.6 were produced by local steel fabricators.) The brackets have a
stand-off bend which allows the boards to be fixed without crushing the bristles.
Elver and eel passes
Figure 4.7: Stand-off bracket and spacer with board fixed
Fix the boards to the brackets using pre-drilled holes and stainless steel bolts. The
bolts pass through the bracket and board, and thread into a 70mm spacer nut (Figures
4.7 and 4.8). This nut prevents the bristles becoming crushed.
Figure 4.8: Bristle board with stainless steel spacer / nut
The crest of a crump section weir normally has an upstream slope of 1:2 and a
downstream slope of 1:5. The angle at such a crest is 38°, so cut each board at the
crest at an angle of 19°. The boards will then fit snugly.
Fit the boards with a floor to prevent an area of fast-moving water forming at the point
where elvers would enter the eel pass. This may be achieved by cutting strips of bristle
board 70mm wide and fixing them at 90° to the lower edge of the board. The main
Elver and Eel passes
bristles should face upward; overlapping bristles should face sideways (see Figures 4.5
and 4.9).
Figure 4.9: Pre-fabricated stainless steel bracket (showing stand-off)
Figure 4.10: Pre-cut floor section
Elver and eel passes
You may need to remove the bottom two rows of bristles from the main board, in order
to allow the bristles on the 70mm strip to mesh with those on the main board. Fix the
strip of board in place with stainless steel, self-tapping screws – no additional bonding
is required.
When fitting a bristle board pass, make sure you butt all of the boards tightly together
to provide a continuous crawling medium. Use enough boards to allow elvers and eels
to enter the downstream end of the pass in an area of slower-moving water. The
upstream end of the pass must be far enough upstream that eels can swim away from
the pass without being swept back over the weir.
You can use ‘thunderbolts’ to fix the brackets to the wing walls (see Figure 4.11).
Figure 4.11: The ‘thunderbolt’ – an 8mm,self-tapping masonry bolt used to fix
brackets to the wingwalls
‘Thunderbolts’ are self-tapping. They thread into an 8mm hole drilled into masonry
without the need for plugs or expanding bolts. This is a great benefit as the holes may
be drilled through the brackets. This allows the boards, brackets and spacers to be put
together on the bank and then fitted as a unit. We recommend that you use 8mm
washers between the bolt heads and brackets.
Where possible, install passes from the bank using cordless or hand tools.
Many crump weirs have curved wing walls upstream and downstream. On these
sections, you can bend boards by pulling them in with the brackets. The boards are
made from a plastic material and are reasonably flexible. Floors may not be necessary
in these areas of slower-moving water.
The installed pass is only 80mm wide and often occupies a very small portion of the
weir crest. We know from experience that you may therefore be able to calibrate the
weir to allow for the effect of the pass on flow measurement. Where possible, install the
pass during periods of lowest flow.
Take care when installing such weir enhancements. In particular, take note of the
general risks of working near water, working at height and using power tools. Use
generic risk assessments for cleaning and minor site modifications.
Elver and Eel passes
Examples of installation of the prototype bristle board eel pass.
Example 1: Frog Mill on the River Hamble (Southern region). NGR SU 5222 1491
4.12 Downstream view of pass installation at Frog Mill, River Hamble (NGR: 5222
1491). Note the area of slack water on left for elver and eel to enter pass.
Figure 4.13: Installed pass, showing brackets and fixings
Elver and eel passes
Figure 4.14: An elver utilising the eel board at Frog Mill
Example 2 : Drove Lane on the River Arle, Hampshire .
NGR: SU 5744 3262
Showing bristle-board pass on curved wing wall
Design: Bristle-board vertical pass. Attached to wing wall of a crump section gauging
Features: Low tech, maintenance free
Cost: Less than £500
Elver and Eel passes
Bristle boards: pre-fabricated.
Fabricated stainless steel brackets.
Stand-off spacers to prevent bristle crush.
Self-tapping fixing bolts
Fish-Pass France (
Cottam Brush Ltd (
Fabrication – local engineering company.
Literature: Fish pass design for eel and elver (Anguilla anguilla), R&D technical report
W2-070/TR1, Environment Agency.
Dave Hunter. Environment Agency, Southern Region.
Richard Iredale. For hydrometric enquiries. Environment Agency, Hydrometry &
Telemetry Monitoring, National Monitoring Service, Head Office Operations.
Tel: 0121 7084650
4.3 'Up and Over' Eel Passes
General design principles
Both entrance and exit need to be sited in a manner that is acceptable for
eel passage1 while not compromising the performance of the gauging
structure2. The lower entrance to the pass should be placed downstream of
the end of the gauging weir wing wall. For hydrometric reasons, the eel pass
should be fed by water pumped from downstream of the structure wherever
The pump should abstract no more than 0.5 litres of flow / second3. This
assumes that at least half of the 0.5 litre per second is discharged back
downstream. Less than half to be discharged upstream of the structure.
Such passes can only be installed where the minimum flow of the
watercourse exceeds 25 litres/second.
The siting of the pass entrance and exit should be near the margins of the
stream and not mid – channel.
At a site where a high flow rating extends beyond the top of the wing walls,
the H&T team will need to consider the impact of the eel pass on that high
flow rating.
Agreement will need to be achieved between local Fisheries and Hydrometry
teams on the design of the eel pass.
Elver and eel passes
The Hydrometry team can object to the installation of an eel pass if it
considers it has justifiable reasons. Adjudication of disputes will be resolved
by the Regional Hydrometry Client Panel.
Passes within the walls of a flow gauging weir may be acceptable at some
sites if the entire installation is mounted above the level of modular limit for
the weir.
Where high flows need to be accurately gauged, passes mounted in the
weir-channel at high level are not acceptable
10. In rivers where trash and debris could be a significant problem, eel passes
mounted within the gauging channel wing wall will be especially vulnerable to
damage during high flow events and should be avoided where possible.
1 Near the toe of the obstruction at the d/s; at the u/s where migrants are safe from risk
of wash-back
2 Avoids snagging debris or interfering with flow lines
3 Experience has shown that if correctly sited eels will find a very small attraction flow.
The nominated flow is sufficient for a bristle pass 200 mm wide that will allow
thousands of eels to pass per night. The pump only needs to be operating from dusk till
dawn. Key months for use of pass: April-September with some Regional variation
The pictures below show an example of a design for an 'Up and Over ' eel pass at
Bags Mill gauging weir, River Piddle (Wessex South). It is a pre-fabricated crawling
gutter elver friendly substrate with a pumped flow over it. Detailed plans and original
photos from Andy Don, Environment Agency.
Elver and Eel passes
Two types of ‘Up and Over’ Eel Passes are acceptable:
Within the area bounded by the gauging structure’s wing walls.
Where the eel pass is designed to be outside the confines of the structure.
This is necessary at sites such as flumes and some weirs with sloping side
Up and Over Eel Passes are suitable to be used at Hydrometric flow gauges where the
water does not permanently run across the full width of the weir. These sites are
predominantly Flat V weirs, V shaped broad crested weirs, non standard V or U shaped
weirs ,contracted thin plate weirs (V shapes and rectangular), flumes, or an alternative
approach at compound crump weirs with lowest crest in the centre section.
Structures suited to up and over type passes
Examples 1 to 3 below could have eel passes mounted within the confines of the
wingwall. Example 4 could have a bristle board pass attached to buttress end of
concrete support wall or up and over it.
Examples 5 and 6 would have to have eel passes outside the structure due to sloping
Elver and eel passes
1. Flat V weir
2. V Shaped broad crested weir
3. Compound crump. Central low section.
5. Flume
4.Contracted thin plate weir
6.Non standard shaped structures
Elver and Eel passes
Example of installation of the ‘up and over’ pump-fed eel pass. Bossington –
River Test (Southern). NGR: SU 3340 3133.
Design: Fabricated sectional UPVC drainage channel – pumped pass, with bristleboard substrate crawling medium and a 12v solar-powered pump. Water is pumped
from the tail water level back to the head of the pass where it descends by gravity.
Figures 4.15 and 4.16: Crawling gutter of fabricated steel with bristle medium
installed and lower lids fitted.
(Rule 12v bilge pump and pipe work can be seen in top right.)
Note. Hydrometric perspective. Figs 4.15 and 4.16 are for illustration purposes only.
The above eel pass and pump arrangement impact on the flow measurement
performance of the structure. The gutter should be located higher up the wing wall
beyond the modular range of the structure and the bilge pump pipe should be located
outside the flow measurement structure.
Figure 4.17: UPVC sectional channel
Elver and eel passes
Figure 4.18: Crawling gutter under construction. These curved wing walls are
found on many gauging structures. (Note the layer of fast-moving water on the
downstream weir face and the clean, debris-free surface. Elvers and eels would
be unable to swim through this structure.)
Figure 4.19: Gutter pass materials and bristle mat substrate being
assembled on site.
Elver and Eel passes
The pass is made up of light-weight materials that can be easily transported to the site.
The gutter sections lock together to form a continuous channel. The bristle-board
sections are inverted when fitted into the channel, so that the backing board forms the
Figure 4.20: UPVC sectional drainage channel, fixed to side wall.
(Hole for pumped water supply at near end, with pump tubing ready for fitting.)
The pass uses solar energy as well as mains and the control box has a timer to
conserve power. The timer allows the pass to run at night during periods of peak
migration. Energy generated from solar panels is stored in 12v deep cycle, leisure
batteries during daylight hours and used to run the pump at night.
Several types of pump have been tried at a number
of sites including the rule bilge pump 12v (500gph)
pictured. The rule pumps have been robust and
reliable. It is important that the pump does not
require more energy than the solar panel can
Features: Low tech, low cost and low
maintenance. Readily available components.
Easily fitted with catch box. Monitoring has shown that pass works well.
Elver and eel passes
Cost: Less than £1,500
Bristle boards: pre-fabricated.
UPVC drainage channel used for block paving drainage. Supplied with lid.
10cm x 5 cm FSC-certified wooden plank bolted to side wall. UPVC channel
fixed to this plank.
12v rule submersible pump, with 2.5cm internal outlet. Delivers 2000 litres
per hour, at 1.5m head differential.
Suitable hosing and wiring.
Power supply, through transformer and timer (run at night).
Alternative power: solar panel with controller.
Suitable sealants and adhesives (as used in aquariums).
Fish-Pass France (
tam Brush Ltd (
Local builders merchants for drainage channel.
Fabrication – local engineering company.
Qualified electrician
Sealant supplied by aquatic suppliers.
Contact for design type is Andy Don, Environment Agency.
Contact point for pass approvals is Greg Armstrong, Environment Agency
Contact for hydrometric enquiries is Richard Iredale, Environment Agency, Hydrometry
& Telemetry Monitoring, National Monitoring Service, Head Office Operations.
Tel: 0121 7084650
References for constraints at gauging stations:
Environment Agency (2004). Manual for Provision of Upstream Migration Facilities for
Eel and Elver – Science Report SC020075/SR2, P50 – 51
Elver and Eel passes
5 Non-gauging weirs
5.1 Dam wall
These are found where large volumes of water are being impounded, for example for
drinking water supply. They are mostly associated with large structures, normally at the
head of catchments. They are often impassable without a special fish lift.
Refer to Fish Pass Design for Eel and Elver, Section 3.4 (Solomon and Beach, 2004).
5.2 Natural falls
In the past, we have not considered improving fish passage over and around natural
obstructions. However, the growing interest in hydropower generation may lead to
developers looking at these sites again.
5.3 Culverts/Irish bridges and fords
These can create problems with blockage and perching and interrupt gravel movement.
Fast-moving water can prevent fish and eel migration. Other problems can be caused
by long culvert lengths, steep gradients and smooth sides.
The example below is in the Hafren forest, Wales, on the River Severn.
For further details, see the SEPA/Forestry Commission R&D document, Culverts and
Elver and eel passes
Another example is on the River Liza in the Lake District. Culverts in forest were
identified as limiting salmonid migration and preventing gravel movement downstream.
The problem was solved by installing a box-section culvert.
We recommend addressing the problem at the consent/planning stage for new
applications. There is then an opportunity to push for a design that prevents scour and
which provides a ramp/substrate for eel passage at the entrance. We should also
check that the nature of the culvert and base eases eel passage.
5.4 Weirs
The large diversity in form, size, function and location of weirs means that any eel
passage solution is going to be site-specific, requiring individual technical design and
Example 1: Crosthwaite Dam
River/catchment: River Gilpin, Lake District
NGR: SD4386691012
Type of obstruction: Stone-pitched mill weir
Issues: Grade 2listed structure. Old corn mill has been on site for more than 350
years. Salmon and sea trout migration are impeded at this weir. Gravels upstream are
under-utilised. There have been previous attempts to improve fish passage – see
photograph. For example, concrete pillows have been built onto the weir apron in an
attempt to form a deeper channel. This has been largely ineffective.
Design solution: The original design solution (illustrated) was for a single section
Larinier fish pass to improve salmonid access. This design was subsequently altered to
include an additional notch alongside the salmonid pass. This provides passage for
eels. The notch will be lined with bristle board on one side and pelcar studs on the
other. The eel pass will then be suitable for eel of different age classes.
Cost: Approximately £19,000 in 2009
Contact: Environment Agency, North West Region, North Area. Penrith office.
Elver and Eel passes
Eel pass materials: The bristle board used in the eel notch, as illustrated in the
Example 2: Blackweir Fish Pass
River/catchment: River Taff, South Wales
NGR: ST 1707 7805
Type of obstruction: Concrete abstraction weir
Issues: The weir is the main abstraction supply for Cardiff docks. It has a large 30metre wide sill, with an apron 15 m long and a 2.5 metre head drop. Fish passage is
required for migratory salmonids and eels.
Design solution. The design solution illustrated is a Larinier fish pass for salmonids
with a bristle board bolted on for elvers.
Cost: Approximately £19,000 in 2009.
Elver and eel passes
Contact: Mike Clyde, Environment Agency Wales, South East Region, Cardiff office.
Phone 02920 245224.
Elver and Eel passes
Example 3: Head weir
River catchment: River Mole, Taw catchment
NGR: SS 6656 1850
Type of obstruction: Concrete abstraction weir for former mill, now use as a fish farm.
Original design solution: Larinier fish pass with adjacent eel pass.
Cost: £250,000 (total includes both passes/designs etc).
Contact: Kelvin Broad, Environment Agency, South West Region, Devon area.
Exminster office. Phone 01392 316032.
Elver and eel passes
Elver and Eel passes
Example 4: Bude, Devon
River/catchment: Lower River Neet
NGR: SS2070506479
Type of obstruction: Adjustable, tidal, amenity weir
Issues: An existing weir on the river was due to be removed. However, the local
council was concerned that water levels upstream would drop significantly during the
summer months. The river is used by canoeists and is popular with holiday-makers. An
adjustable weir was installed to ensure water levels remain high during the summer
months. However, when the weir is raised during the spring/summer months, it
becomes almost impossible for eels and elvers to move upstream. The summer is the
main period for elver migration.
Design solution: We attached a boxed-in metal ramp to the concrete wall, and
installed a pump to provide water to the brush substrate inside. A camera was also
installed as part of the design. Eels and elvers have been monitored using the pass.
The operation of the weir meant that the pass had to be350mm above the existing
Cost: £6,750
Contact: Kelvin Broad, Environment Agency, South West Region, Devon and Cornwall
area. Phone 01208 265012.
Elver and eel passes
Elver and Eel passes
Example 5: Stapleford Mill Weir
River/catchment: River Gowy, Mersey catchment
NGR: SJ 48218 64790
Type of obstruction: Steep concrete weir
Issue: Steep, fast-flowing water, a concrete apron
Eel pass at Stapleford Mill weir, on the River Gowy: under construction and
Design solution: A pre-fabricated eel gutter was installed on both sides of the weir.
This was lined with a brush substrate. This allows a constant flow of water through the
gutter– encouraging eel movement. The pass was easily installed.
Cost: £3,000
Contact: Environment Agency, North West Region, South area
Elver and eel passes
Example 6: Botley Mill
River/catchment: River Hamble, Southampton
NGR: SU 5156 1316
Issue: Milling has taken place on this site for many years. Parts of the existing building
date back to 1536. The site retains a large head of water of more than three metres.
The large fall of water hampered the upstream migration of all fish, which could only
enter the upper catchment at times of very high flow or high tide.
Design solution: A rock ramp was installed in a natural channel of the river. This
diverts the water around the obstruction at a flow and level which eels and elvers can
use to ascend. The unusual feature of this innovative German design is the use of a
regular pattern of upstanding rocks, nearly one metre high, embedded in the pass. This
arrangement ensures that the water velocities in the pass are not too high. It also
allows fish in the pass to retreat downstream without becoming stranded if the water
levels upstream should fall. The pass is 28m long with a 1:22 slope. As it will operate
with flows as low as 50 l/sec, the pass works for more than 75% of the year. The pass
has opened up 15km of chalk stream habitat upstream of the mill.
Cost: £70,000 (2009)
Contact: Hannah Wright, Environment Agency, Southern Region, Colvedene Court.
Phone 01962 764952.
Elver and Eel passes
Example 7: Dursley
River/catchment: River Cam
Issue: This section of the Cam had previously been
in a culvert through an industrial site. When the site
came up for redevelopment as housing in 2007, the
developers were persuaded to remove the culverts
and return the river to an open channel.
Design solution: As this section had a large change
in gradient, six rock ramps were installed – each with
a three-metre drop. The rocks were tipped randomly
into the prepared channel rather than being placed
Contact: Charles Crundwell, Environment Agency,
Midlands Region, West area. Phone 01684 864374.
Elver and eel passes
6 Tidal flaps and gates
Many terms describe structures that allow water to flow seawards by gravity, but not
landwards. These terms include tidal flaps, tidal gates, tide gates, flap gates, flap
valves and tidal sluices. In this report, ‘tidal flap’ describes a top-hinged flap, and ‘tidal
door’ a side-hung flap. ‘Tidal sluice’ describes a structure where gates are lifted or
lowered according to the relative levels of water on each side, under either manual or
automatic control.
There are of course sites where these structures are located away from tidal influence.
Here they are used to isolate the area to be drained from high river levels in the
channel to which they drain. The issues for eel passage are similar to those in tidal
The term ‘level equalisation’ refers to making the water levels each side of the tidal
device the same. There is then no tendency for water to flow in either direction. With
tidal flaps, eels can only pass for short periods when levels equalise. This window of
opportunity may last for just minutes. Typically, level equalisation will occur twice in
each 12-hour tidal cycle – once on the ebbing tide, and once on the flood.
The design of tidal flaps allows run-off to flow seawards when the landward water level
is higher than the tide level. But it prevents landward flow of tidal water. Their use has
allowed the development of large areas of very productive farm land in areas that were
previously flooded by the tide, or were at least poorly drained. Large areas of England
and Wales lie below the high-tide level. Much is drained by the use of tidal flaps, tidal
doors, tidal sluices or by pumping.
Tidal flaps range greatly in size: from tens of centimetres to several metres in width and
depth. They are generally rectangular or circular in shape (see Figure 6.1).Being tophinged, they tend to close under their own weight. The seating face may be sloped
back towards the top to encourage positive seating.
For hundreds of years, tidal flaps were made of wood. Larger installations were
reinforced with iron straps. From Victorian times onwards, cast iron was commonly
used for small and medium-sized gates. It is still much-used. However a wider range of
materials is increasingly common, including stainless steel, cast aluminium, highdensity polyethylene (HDPE), co-plastix and other plastic materials. Many flaps are
fabricated rather than cast, especially where non-standard sizes or designs are
Elver and Eel passes
- t iron tidal flaps from a 1910 catalogue from Glenfield
Figure 7.1. Drawings of cas
- ble hinge link arrangement that allows
and Kennedy of Kilmarnock. Note the dou
self-seating of the flap, the lifting eyes, and the -split flap arrangement of the
circular flap.
The size of the flap – or more precisely the dimensions of the culvert that the flap
controls – is determined by the highest flow that may need to pass through the culvert.
This may be the flood flow that follows heavy rain. However, even greater capacity may
be needed if the culvert has to drain flooding caused by overtopping of the barrier by
storms or exceptional tides (Thorn, 1959). This generally means that the flap nearly
always lets through a very small fraction of its maximum capacity –the flap will be open
just a crack even at low tide.
Generally, fabricated wooden flaps and frames were imperfect seals. They allowed
some landward flow when outside water levels were higher than the levels landwards
of the structure. Thorn (1959) described the construction of large wooden flaps: ‘On the
larger outfalls they usually comprise an outer skin of vertical timber, felt, and an inner
skin of horizontal timber, the whole strengthened by mild steel angle or channel bracing
with steel suspensions.’
Figure 6.2 shows a large wooden flap being removed from a site in Washington
State, US. It is clear that in this condition the seal would have been far from
perfect – even though the gate was still effective at minimising back-flow when
Elver and eel passes
Cast-iron flaps and surrounds fitted better, but still often provided an imperfect seal.
The latest generation of flaps have purpose-ground seating faces and neoprene seals.
When closed, they are effectively waterproof.
Figure 6.3: The estuary of the River Lymington in Hampshire. These tidal doors
are in an open position. The tide has started to flood. The picture shows
noticeable landward flow through the structure (towards the right of the picture).
A few minutes later, as the landward flow increased, the doors slammed shut.
Tidal doors, often referred to as pointing doors,
tend to be larger than tidal flaps. Generally of
wooden construction, they look very like lock
gates. However, there has been a recent trend
for side-hung smaller gates, and in some cases
top-hung flaps have been converted to sidehung gates (Figure 6.13).
Gates tend to remain in the position to which
they were last pushed by the flow, and may
remain open after water levels have equalised.
As a result, the gates may slam shut once the
tide really starts to flow landwards. They may
also tend to swing with wave action when there
is little flow in either direction. For this reason,
gates are often only used in more sheltered
Figure 6.4: Round top hinged
tidal flap gate
The top-hinged tidal flap (round or square) is the
most common design found in tidal and fluvial
situations in England and Wales. The flap can
be made of many \different types of material.
Traditionally, they were made of wood or cast
iron. More recently, lighter weight plastic, fibreElver and Eel passes
glass or rubber compounds have been used. The flap can be attached directly onto the
pipe. However on many outfalls the pipe comes complete with:
a concrete apron or basin to prevent erosion and build-up of sediments;
a winch to hold the gate open for clearing debris.
This set-up further restricts fish migration by leaving a vertical drop (perched) from the
pipe exit to the concrete platform. This drop acts a total barrier to fish migration until
covered by the incoming tide or by raised fluvial water levels.
Figure 6.5: Square, top-hinged tidal flap gate
Flap-gate technology for flood prevention and land drainage has improved significantly
in recent years. A wooden flap gate would warp and not seal completely. Today’s flap
gates can achieve almost an almost complete seal. When closed, they create a total
fish barrier. The heavier the gate, the greater the hydraulic head difference that is
needed to open it. So when the gate is open, the water passes through at high speed.
This makes fish passage very difficult, particularly for poor swimmers such as eels.
When hydraulic head drops, the gate closes very quickly.
6.1 What are the issues?
Tidal flaps prevent fish and other biota from moving freely. They are a particular issue
for small eels migrating landwards because of their limited swimming ability. Solomon
and Beach have reviewed the information on the swimming ability of elvers (2004a).
They found that the burst speed of an 80 mm elver is generally about 0.5 m/sec,
varying with temperature and between individuals. (Burst speed is a speed that can
typically be maintained for 20 seconds.)
The theoretical relationship between hydraulic head and the velocity of water passing
through a gap is shown in Figure 6.6. On this basis, a head difference of only 12.7 mm
would generate a velocity of 0.5 m/sec. In practice, edge effects would reduce the
velocity through a narrow gap. However elvers would still be unable to overcome the
water speeds generated by even very small head differences. These figures tally with
Elver and eel passes
the conclusions of Wood and Blennerhassett (undated), who concluded that heads in
excess of 15-20 mm would defeat elvers.
Velocity m/sec
Head difference mm
Figure 6.6: Theoretical relationship between head difference and the speed
water travels through a gap.
As already described, the commonest form of tidal flap is a circular or square top-hung
gate. This closes under its own weight, and is held open by the seaward flow when the
landward level exceeds the seaward.
The seating face is often sloped to aid self-closing. This means that the door closes
while there is still a positive head on the landward side. As the gate closes, the flow
around the gate is moving quickly – beyond the swimming ability of a small eel. The
situation is further complicated by the route that water takes during low freshwater
Tidal flap apparatus has to be large enough to cope with the highest flow that is likely
to occur. So it is nearly always handling only a very small fraction of its maximum
capacity. Low flows are of course common during the months when small eels migrate
landward – typically April to September. The flap will only open to a small extent, with
the flow ‘squirting’ sideways through a small gap (Figure 6.7). This is in fact a worse
situation for larger fish of all species than it is for small eels.
In recent years, there has been a move towards installing structures with two tidal flaps
in series (Figure 6.8). This offers greater protection to industrial or residential property
that would be at risk if a single control device failed. However the design is not helpful
to eels: it rules out the possibility of a flap being occasionally held slightly ajar for a tide
or two by a stick or plastic bottle. This would allow water to flow inland and eels to pass
(see Figures 6.9 and 6.14).
Elver and Eel passes
Figure 6.7: Water squirting through the gap of an almost-closed rectangular tidal
flap at the mouth of a marsh drainage channel on the Thames Estuary. Even
when the tide rises to the bottom of the door, this flow is too fast for elvers to
overcome. As the velocity starts to fall, the gate shuts.
Tidal flaps are now a major impediment to the landward movement of elvers and small
eels. However small elvers are remarkably adept at exploiting small leakage flows and
occasional failures in defences – such as debris holding a flap slightly open for a tide or
two. Since tidal flaps used to be imperfect seals, they probably allowed regular
passage. These factors may explain why there have been few observations of a
complete lack of eels within areas protected by tidal gates. The presence of eels may
be the result of a short window of opportunity once every several years. However it is
important to be aware that modern designs for tidal flaps may effectively be fish proof.
The ideal situation would be for elvers and young eels to have some opportunity of
landward passage for at least a short time on each tide – the rest is up to them.
Elver and eel passes
Figure 6.8: Section through the tidal control structure at the outlet of a marsh
drainage system at Barking Creek in Essex. There are two 1,200 mm squaresection cast-iron flaps. These are in series (highlighted in red, held wide open)
with a penstock between.
Elver and Eel passes
Figure 6.9: The outer flap in the structure shown in Figure 6.8. Note the two
plastic bottles jammed in the flap which prevent the flap from closing
completely. Were this a single flap, the jammed bottles would allow some
landward flow and the passage of elvers. However, the presence of a second flap
a couple of metres behind this one prevents this.
Tidal doors are generally considered benign for eel and elver passage, and indeed for
other species of fish. They tend to remain fairly wide open throughout the ebb tide and
ensuing slack water. This allows strong swimmers, such as salmon and sea trout, to
pass inland throughout the ebb tide. It also allows weak swimmers such as small eels
and elvers to travel inland for a significant period: towards the end of the ebb, over
slack water, and for a short time on the flood. Firth (undated) studied 59 tidal structures
on waterways draining to the Humber Estuary, between Spurn Head and Boothferry
Bridge on the Ouse and Keadby on the Trent. He concluded that, in most situations,
tidal doors had little effect on fish movement, when compared with tidal flaps.
6.2 Options for fish passage at tidal flaps
6.2.1 Replace with tidal gates
One option is to rotate the hinges hanging the gate from the horizontal to near the
vertical. Do not make them entirely vertical as the weight of the gate would not then
help to shut it. You would have to modify the hinge in order for the structure to cope
with the weight of the gate, and to physically prevent the gate from opening too far. The
change will allow the gate to open far more easily, to a greater extent, and for longer. In
addition, fish can enter from the side more easily than they can from below.
Elver and eel passes
Figure 6.9: Large side-hinged gates are angled inward, providing a small
closing force, and are typically mounted over large rectangular culverts.
Side-hinged gates are now available commercially. Large, side-hinged rectangular
doors made of aluminium or stainless steel are attached to square or rectangular
concrete culverts (see Figures6.10 and 6.11). Although the gates can weigh more than
a tonne, they open easily under relatively little water pressure from upstream. Some
side-hinged gates may require only one inch of water level difference to open up to 45°
(Coos Watershed Association, unpublished data). Such wide opening is expected to
allow much better fish passage than would be normal with top-hinged tide gates.
You must install the top hinge closer to the culvert opening than the bottom hinge. This
gives the tide gate a slight downward tilt, which restores the tide gate to the closed
(default) position at the end of the ebb tide (Charland 1998, 2001).
Side-hinged tide gates do have a major disadvantage: it is more difficult and costly to
build the support structure needed to hang such gates. The gate must be installed at
precise angles from the vertical. It must therefore be placed in a structure that will not
change its orientation over time. It also needs to be suspended using strong hinges
that are resistant to corrosion (Charland 2001). If the angle of the support structure
does change, the door will either not open properly or will not close during flood tide.
Great care must be taken to install the gate at the correct angle of tilt, because the
angle will be very difficult to change later on.
Side-hinged tide gates are reported to provide better fish passage, upstream water
quality, and estuarine connectivity than the traditional top-hinged gates. However,
neither design is entirely fish or environmentally friendly. The basic problem is that both
types are very good at doing what they were designed for, namely removing the
influence of high tides on upland water levels.
Figures 6.10 and 6.11: Side-hung gate on the River Avon, Hampshire.
Elver and Eel passes
Figure 6.12: Side-hinged, rectangular tide gate.
Side-hung tidal doors overcome many of the problems for the passage of elvers and
small eels. And they do not appear to have any inherent disadvantages compared to
tidal flaps. We recommend the use of this simple alternative wherever it is viable.
Figure 6.13: Side-hinged gates are fitted to replace top-hung flaps at Schneider
Creek, Washington State, US.
Photograph courtesy of Tom Slocum, Washington Conservation Districts North West
Region Engineering Program.
The issue here was the passage of coho salmon. The right-hand gate is fitted with a
muted tidal regulator.
6.2.2 Pegging flaps
Pegging flaps causes them to remain slightly open throughout the tidal cycle. This
‘unofficial’ practice was once fairly widespread on the Essex marshes (John Claydon,
Elver and eel passes
personal communication). The practice involved placing a small piece of wood to hold
the door ajar (see Figure 6.14). The aim was to keep the field ditches wetted in dry
weather by allowing some landward flow. These ditches often acted as stock fences
between adjacent fields. When major rainfall opened the flap further, the piece of wood
would be released and washed away.
This practice could be a way of improving elver passage during the relevant months of
the year, provided the relevant permissions were obtained. However, caution must be
exercised: if you peg just one side of the flap, rather than the bottom or both sides, the
peg may cause damage to the gate structure – especially where the head on the tidal
side of the flap is considerable.
Pegging may also cause the build up of silt. This can be an undesirable side-effect of
several of the options considered here. In many cases, the tidal water outside the flap
may be turbid with a high silt load. If some tidal water is allowed to pass, silt may be
carried landwards and deposited in the quieter flows there.
Figure 6.14: A pegged tidal flap:a piece of wood at the 3 o’clock position holds
the flap ajar.
6.2.3 Naturally open flaps
Most tidal flaps close naturally close under neutral conditions, by virtue of their own
weight or a backward slope to the sealing face. However, there are ways to ensure the
gate can remain in a naturally open position under neutral conditions, closing only
when a seaward head builds up and/or there is significant landwards flow:
One approach is to position the sealing face with a forward slope, such that
the gate is slightly open when hanging in a vertical position. Any significant
flow landwards will cause the gate to close.
Elver and Eel passes
A simpler approach is to use a chain or cable rigged to an eye on the flap,
and supported in some way in front of the flap (see Figure 6.15). This does
not require any significant modification to the installation. The weight of the
chain, or a weight attached to the cable, can be adjusted until the gate is just
held open under neutral conditions by a catenary action. Again, any
significant landward flow would shut the gate. This option is probably only
realistic where the structure readily allows the cable to be held well in front of
the gate, for example where the flap lies within a channel as in Figure 6.15.
Figure 6.15:Twometre square section tidal flap at Havering, Essex. This drains an
area of Havering Marshes to the Thames Estuary.
(The weight of the chain and cable reduces the closing weight of the door, delaying
closure. Additional weight would hold the flap open after water levels have equalised
on the rising tide. Note that the chain and cable were not installed for this purpose.
6.2.4 Permanent gap
A small gap, permanent or seasonal, allows elvers and small eels to pass when water
levels allow. It recreates the effects of the imperfect seal that was common to most
flaps before modern materials and manufacturing methods created good seals.
Elver and eel passes
Figure 6.16. The nearer flap is fitted with a mitigator fish passage device, which
holds the door ajar until the rising tide lifts the floats. The further culvert has a
side-hinged gate, converted from a top-hinged flap – the remains of the flap
hinges can be seen on top of the concrete bulkhead. Photograph courtesy of
Guillermo Giannico, Oregon State University.
There is a range of options for creating such a gap. They include holes drilled in gates,
and devices to hold the door open a small amount. For elvers, a gap of just a few
millimetres is likely to allow passage for a short period when water levels equalise. And
if the gap were fitted with a crawling substrate, it could allow elvers and small eels to
make progress against a stronger flow – and thus at greater heads and for longer on
each tide. An arrangement of rigid pins may be more effective than conventional
flexible substrates. These pins could vary in diameter and spacing for eels of different
A good approach might be to develop a device that can be readily fixed and removed
from a flap. This is basically an extension of the pegging principle and would allow
seasonal deployment (see Section 6.2.3).
A significant drawback with permanent gaps is that they allow water through for much
longer than is actually useful for migration. Elvers are likely to be attracted to an outfall
by the seaward flow of fresh water. Once the tide starts to flood, the small gaps left
around the flap would be unlikely to attract elvers from very far a field. This is in
contrast to the strong landward flow that would occur if the flap were fully open or not
installed at all. A ‘permanent gap’ would really only allow elver passage for a relatively
short time around level equalisation. A better approach may be to use slow-closing
flaps. See below.
Elver and Eel passes
6.2.5 Slow-closing flaps
Slow-closing flaps are a logical extension of permanent gaps. The gap would be open
only when eels and elvers could make use of it, rather than throughout the tidal cycle.
This would greatly reduce the extent of tidal intrusion and may allow the gap to be
somewhat larger when passage is possible.
It may be possible to arrange for the flap to have ‘stiff’ hinges for the last (say) 10° of
closure. A positive pressure would be required to close it. The engineering options for
this have not been explored, and it may prove difficult to make such a device reliable or
A better approach may be to have a device set away from the hinges which delays
closure. Possibilities include coil springs, or rubber ball set into a cup mounting. The
increasing pressure as the tide rose would compress the spring or ball, closing the
gate. The same mechanism would open the gate slightly just before equalisation on the
falling tide, again giving elvers access.
6.2.6 Mitigator fish passage device, Nehalem Marine
This device is similar in design to self-regulating tidal flaps (see Section 7). It is
included here because the device is intended to allow fish passage rather than
significant tidal intrusion. Floats are mounted on a lever system attached to the gate.
These operate cams which bear upon the bulkhead onto which the gate is mounted,
holding the gate open a short way for part of the tidal cycle. As the tide rises to lift the
floats, the cams are turned. This allows the water pressure to shut the gate.
6.2.7 Elver passes
An elver pass may be a viable option at some sites, as an alternative to passage
through a flap culvert (see Section 4). This may be a preferred option where any risk of
landward flow is unacceptable. This landward flow is likely to happen with many of the
options discussed so far.
Elver and eel passes
7 Self-regulating tidal flaps and
In recent years, tidal flaps have been developed that allow controlled tidal intrusion.
They allow a degree of tidal interchange and let salt water enter the area draining to
the structure, usually for conservation purposes.
Several studies have examined the ecological changes that happen if tidal and salt
waters are excluded. See for example Johnston et al. (2003), Giannico and Souder
(2004) and Kroon and Ansel (2006).
There is increasing interest in
allowing some tidal intrusion
into wetlands which are at
present cut-off from tidal
influence by tidal flaps and
gates. Such intrusion in usually
referred to as regulated tidal
exchange, or RTE. Rupp and
Nicholls (2007) published a
map showing proposed sites
which could benefit from this
approach throughout north
west Europe. The map included
several sites in England and
Figure 7.1: Drawing from US Patent 3,974,654,
‘Self regulating tide gate’, dated 17 August
Where the gates are under
manual or automatic control,
the operating regime can be
modified to manage RTE.
Several self-regulating tidal
gates have been developed.
These are basically modified
tidal flaps that allow RTE
without the need for power or
supervision. Most designs are
fitted with floats which hold the
gate open for part of the tidal
cycle, but close the gate at
some stage during the flood.
The earliest reference found to
such a device is shown in
Figure 7.1.
Figure 7.2 shows an Australian development of this type (Green and Pease 2007). The
gate is held open during the first part of the flood tide by the weight of the float and its
associated metalwork. As the tide rises it lifts the float, gradually shutting the flap. On
the falling tide the weight of the float opens the flap wider than normal, even at very low
seaward flows. You can adjust the water level at which the gate closes by re-arranging
the alignment of the float.
Elver and Eel passes
Figure 7.2: Automatic tidal flap in New South Wales, Australia.
7.1 The Waterman SRT
The Waterman SRT is manufactured by Waterman Industries in the US. Two of these
gates have been installed in the UK: one at Goosemoor on the estuary of the Exe; the
other at Cone Pill, a small stream draining into the Severn Estuary (Figure 7.3).
Figure 7.3: Waterman SRT
The Cone Pill structure was the first of its type to be installed, in 2004. Matthews and
Crundwell (2004) describe the installation, operation and lessons learned.
Elver and eel passes
7.2 The Williams SRT
A different style of SRT device has been developed by Mike Williams from the South
West Region of the Environment Agency.
The requirements were exacting. The gate had to be closed at high and low tides, but
open at an intermediate stage – so that the water passing landwards was saline rather
than backed-up fresh water.
The device is fundamentally a steel plate that rotates across the mouth of a circularsection culvert (Figure 7.4). A weighted float rotates the plate. The operating sequence
is shown in Figure 7.5.
The prototype was
installed on an outfall on
the estuary of the River
Axe in January 2009, and
has so far operated
without significant
A second device is shortly
to be installed in
Hampshire, where it will
replace one of the tidal
flaps in the Lymington
causeway. This will allow
RTE into the reed bed
area which had been part
of the tidal estuary of the
Lymington River until the
causeway was built in
Figure 7.4: Williams SRT during installation on the
Axe Estuary, Devon
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Figure 7.5: Design details for the Williams SRT
Elver and eel passes
Figure 7.6: Operating sequence of the SRT under both ‘normal’ and ‘flood’
conditions. (Courtesy of Waterman Industries Inc)
Stoneman Engineering
Address: Park Works, Station Road, Willand, Cullompton, Devon, EX15 2QA.
Phone: 01884 820369
Watermans, US
Address: Watermans, P.O. Box 458, Exeter, California 93221.
Phone: 559-562-4000
Fax: 559-562-2277
Elver and Eel passes
Advantages and disadvantages
Requires no maintenance of electrical
power supply
Relatively expensive
Adjustable manually to operate over a
specified range
Not tested to Flood Risk Management
Robust, requires no more maintenance
than a standard flap gate
Floats can collect debris and need to be
Opens fully, allowing easy fish passage
Can be fitted as the total flap gate,
alongside a normal gate or within an
existing flap gate
What might you need to do to build this?
Flood modelling
Permission of landowner
Flap gate design
Fabrication firm to build and fit
Development control consent
Local authority planning consent (change of land use)
Table of contacts: name, contact details and experience
Mike Williams
Specialist, FRB)
Contact details
Exminister House, Exeter
Internal 7 24 6033
External 01392316033
Charles Crundwell
Senior Technical
Riversmeet House,Tewkesbury
Internal 7 22 4374
External 01684 864374
Science report:
Regulating tidal
exchange. Installation
of fish-friendly flap
gate on River Axe,
SRT Cone Pill
Improving connectivity
between the North Sea
and the tidal Trent
7.3 Muted tidal-regulated (MTR) gate, Nehalem Marine.
In this interesting example, the gate is controlled by the water level on the landward
side of the structure. This is usually the target of regulation. A float on the landward
side operates the gate using a series of levers and a rod that projects through the
structure (see Figures 7.7 and 7.8).
Elver and eel passes
Figure 7.7: Float mechanism for an MTR gate at Schneider Creek, Washington
State, US. Photograph courtesy of Tom Slocum, Washington Conservation
District’s North West Region Engineering Program.
Figure 7.8: Gate actuating mechanism for an MTR gate at Schneider Creek,
Washington State, US. The door is being held open. Photograph courtesy of
Tom Slocum, Washington Conservation District’s North West Region
Engineering Program.
Elver and Eel passes
7.4 Variable backflow flap gate (VBFGTM), Juel Tide Gates
This variable backflow flap gate (VBFG™) builds on the principle of the slow-closing
flap described in Section 6.2.5. In the original design, the gate was held open by a
hydraulic cylinder until the flow of water landwards – and the head acting upon the
open gate – exerted enough force to overcome the cylinder. The gate then closed.
Current models use a shock-cord rigging arrangement to create the same effect. When
the rigging is correctly balanced, the tension increases as the gate closes. This
prevents the gate from slamming shut. A major advantage of this design is that the
gate is either fully open, or is closed. The rigging can be adjusted to effect closing with
almost any level of tidal intrusion.
Figure 7.9: Juel Variable Backflow Flap Gate (VBFGTM). The gate is fully open, on
the ebb tide. Photos reproduced with permission from Juel Tide Gates of
Seattle, Washington, US (
The gate is made from heavy-duty, 316 stainless steel and copolymer. It is designed to
require minimal maintenance.
Elver and eel passes
7.5 Mouse holes, cat flaps and pet doors
There has been a lot of work done on using a ‘cat flap’ approach to allow some water
to flow through a larger flap. The idea is that the flow of water through the smaller flap
is very much less than the capacity of the larger ‘parent’ gate.
The smaller flap is light and can be held open much wider, and for a longer period, than
the larger gate. Depending on its size, the cat flap can be made of very lightweight
material. There is also the scope for a mechanism that holds the cat flap open for much
longer than it would stay open naturally. This is rather like an SRT which delays closure
of the whole of a large flap (see Section 8), but the mechanism can be very much
lighter and cheaper. The consequences of failure are also very much reduced in
comparison with what would happen if the main gate remained open when it should be
A float-operated cat flap has been installed in a massive tidal flap on the River Gilpin in
Cumbria. The gate was manufactured by Aquatic Control Engineering and installed in
2009. Some details are shown in Figure 7.13. The cat flap is relatively large (1,000 mm
wide and 400 mm tall) and is designed for the passage of sea trout. It is top-hinged and
held open by a sliding float arrangement. When the tide rises to the limit of travel of the
float, water flows down a pipe into the float, causing it to fall. This allows the cat flap to
close. The float drains on the next low tide, commencing the cycle once more. The
performance of this system has been monitored by Ben Bayliss from the Environment
Agency’s North West Region.
There has also been work done in the US on the scope for using a bottom-hinged cat
flap, which is naturally open until lifted by a float. The idea appears to have been
originally proposed by Charland (1998). According to Jeff Juel of Juel Tide Gates, only
a few devices were installed and these subsequently failed and were removed. No
details of the problems are available. A similar design is to be installed in the tide gate
on the River Stiffkey in Norfolk, to allow access for sea trout. Details are shown in
Figures 7.11 and 7.12.
Elver and Eel passes
Figure 7.10: Tidal flap at Maydays Farm, Essex. This is an interesting installation
as the flap is separately hinged in two halves. The ,lower half is fitted with a
small ‘cat-flap’, presumably designed to operate at very low flows.
However this cat flap, which is made of cast iron, is too heavy to make a useful
contribution to elver passage.
Figure 7.11: Section through tidal flap on the River Stiffkey showing the
proposed bottom-hinged cat-flap in an open position. Drawing shown with
permission of Sandy Cowie.
Elver and eel passes
Figure 7.12: Bottom-hinged cat flap, to be installed in the tidal flap in the River
Stiffkey. The door is in the closed position, with the float in the high-tide
position. The dimensions of the opening are 600 x 300 mm. Photograph shown
with permission of Sandy Cowie.
These solutions have been used primarily in the US and Australia. They allow fish
migration but also re-establish tidal regimes and water quality. They are particularly
helpful for reducing mosquito and pH problems. There is no reason why they could not
be widely installed in England and Wales where they could improve connectivity
cheaply. You would of course need to first check their suitability for a specific location
by modelling the impacts of the gate.
Figure 7.14: Design for a bottom-hinged pet door. It features a small hole
in the standard door. The pet door is mounted over the hole with a float
and arm assembly attached.
Elver and Eel passes
Figure 7.15: Design for atop-hinged pet door. It features a small hole in the
standard door with a small pet door mounted over the hole.
Figure 7.16: A bottom-hinged pet door.
Elver and eel passes
Figure 7.17: Two mouse holes on a tidal sluice on the River Medway.
The holes are only reached for a short duration on incoming tide. Note the roped open
flap on the top left hand side. This is required as a fall-back option if the mouse hole
needs to be closed.
Figure 7.18: Mouse hole on sluices on the River Test. As the downstream water
levels rise, fish can pass through hole.
Elver and Eel passes
Figure 7.19: Closed up mouse hole
on tidal flaps on the River Lyd,
Figure 7.20: A flap gate at Warth
Brook, Gloucestershire. The gate’s
correct invert level allows
continuous opportunity for fish
Advantages and disadvantages
Requires no electrical power supply
Adjustable manually to operate over a
specified range
Opens fully, allowing easy fish passage
Relatively inexpensive
Easy to remove or close if an unexpected
problem occurs
Can be retrofit
Can allow the build up of silt upstream
Not currently tested to Flood Risk
Management specification
Mouse hole or pet door could cause
Small hole can create fast-moving water
that prevents fish migration
FRM prefer flap gates to have drop as
this helps to remove sediment
Any local fabricator.
Elver and eel passes
What do you need to build this?
Flood-risk modelling
Tidal or fluvial level data
Level data
Fabricator to adapt existing gate
Local consents (agricultural payment schemes)
EA Team to arrange gate removal and refixing
Tables of contacts: name, contact details and experience
Dave Hunter
Contact details
Environment Agency 01794832732
Adrian Fewings
Environment Agency 01962764952
Pet door on River Yar,
Yarmouth, Isle of
Mouse hole, tidal, on
River Medway, Mouse
hole on River Test
7.6 Installation of light-weight gates
The lighter the flap, the further it will open, and the longer it will remain partially open
towards level equalisation. This may give elvers a few extra minutes of access on each
tide. Aquatic Control Engineering manufactures a range in high-density polyethylene
(HDPE), with stainless steel reinforcement, which are much lighter than their cast-iron
Bates (1992) compared the opening of two 1.2 m diameter flaps: one made of cast iron
and the other of aluminium. With a head differential of 300 mm, the aluminium gate
opened to about 750 mm, the cast-iron one to 150 mm. Light-weight flaps may be most
useful for eels when used in a cat-flap approach (see Section 7.4).
Elver and Eel passes
Figure 7.21: A light-weight HDPE tidal flap, manufactured by Aquatic Control
Equipment (ACE) Ltd. Photograph courtesy of ACE.
The main reasons why flap gates prevent elvers and eels from migrating are that:
the weight of the steel flap generally causes the gate to seal;
when there is sufficient upstream hydraulic head to open the flap, the water
velocity around the flap’s edges is too great.
If the standard steel flap is replaced by a lighter design, the hydraulic head required to
open the flap is reduced. The flap will open wider for longer, lengthening the time when
eels can pass and reducing the water velocities around the edges.
In most cases you can replace a cast-iron, steel or hard-wood flap with one made of
aluminium, plastic or fibre glass.
Figure 7.22: Double-hung flap valve (plastic door)
Elver and eel passes
Advantages and disadvantages
Requires no electrical power supply or
additional maintenance
Opens wider than standard flap gate,
which makes it easier for more fish to
Relatively inexpensive. Light-weight
gates cost less than their heavy-weight
Easy to remove or replace if unexpected
problem occurs
Can be retrofit, cost effective.
Some resistance from FRM in sites with
a lot of floating debris. Their view is that
damage to flap and vandalism may be
more likely.
Often only up to 600mm in size
Electrolysis corrodes aluminium more
easily than cast iron or steel (especially if
a different metal is used in the hinges)
Cast iron is still the industry norm, so
finding UK suppliers can be tricky
May improve fish passage, but do not
significantly improve water quality or
connectivity to a stream or an estuary
Ham Baker
Midland Valves
What do you need to build this?
EA Operational Delivery team to arrange gate removal and replacement
Landowner agreement
7.7 Counter-balanced flap gates
Putting counter-weights on flap gates, as in Figure 7.23, effectively reduces the gate’s
tendency to close. For any given flow of water, the gate is likely to open further, and
remain open longer. By adjusting the size and position of the weights, you may be able
to arrange for the gate to remain slightly open under neutral conditions.
Elver and Eel passes
Figure 7.23: Counter-weighted tidal flaps on the River Lymington, Hampshire.
These tend to open further and remain open longer than conventional flaps of
similar weight.
Figure 7.24: Counter-weighted tidal flaps on the River Lymington. Shown fully
open during a spate.
Counter-balances have been installed on very large and heavy flap gates in order to
help the gates open and discharge once the influence of tide has ceased. Without
these counter-balances, the weight of the gates would prevent them from opening and
the trapped water would pond until there was a sufficient head to open the gates. For
land drainage reason this is inefficient – or even counter–productive. Counter-weights
Elver and eel passes
fixed to the top of the gate aid easy opening. These balances can be so effective that it
is possible for one person to open the gates fully even if they weigh several tonnes.
The counter-balances were generally installed in tidal situations on large rivers where
the sheer volume of tidal water would close the gates. After the tide lock it was
important for water to escape as quickly as possible to prevent fluvial flooding. The
counter-balances allow the gates to open much wider than a standard flap gate would
and therefore they are not such a barrier to fish as other types of flap gates.
Counter-balanced flap gates are now rarely installed. The preference now is for flap
gates with more advanced designs and materials and also for automated penstocks.
Where counter-balanced flaps are still used, it is possible to alter the counter-balances
to make them stay open for longer. This increases the opportunity for fish to pass.
Figure 7.25: Counter-balanced gates on the River Leadon, Gloucestershire. Note
that the gates are shut because of tidal influence. They soon open wide with the
ebbing tide.
Elver and Eel passes
Figure 7.26: Counter-balanced gates on the Bottesford Beck, Scunthorpe.
Advantages and disadvantages
Requires no electrical power supply
Adjustable manually to improve fish
passage opportunity
Can allow the build of silt upstream
Rarely installed now. Generally made to
order, so more expensive than other
Opens wider than a standard flap gate,
allowing easier fish passage
Less prone to blockage
Suitable for large river
Local fabricator or Environment Agency’s Operations Delivery team to arrange.
What do you need to build this?
Flood-risk modelling if gates are altered
Tidal or fluvial level data
Installation of continuous monitoring of water quality
Water level data
Local consents (agricultural payment schemes)
EA Operations Delivery team to arrange testing of counter balances
Elver and eel passes
Tables of contacts: name, contact details and experience
Martin Hayes, Alison
Charles Crundwell
Contact details
Environment Agency 01543404842
Bottesford Beck
Environment Agency 01684864374
River Leadon and
Bottesford Beck
7.8 Automated gates
Another option is to install powered and automated sluices, with their operation linked
to the tidal cycle. The benefits are that the sluice can be opened fully, and the time that
it remains open can be maximised. The main disadvantages are that they are
expensive to build and maintain. They are also of little use in isolated locations where
there is no power.
7.9 Siphon passes and piped passes
In some cases it is not desirable to let any water through a sluice gate. This may be
because of the risk from flooding or for ecological reasons. The only solution for elver
and eel passage will then be a closed siphon pass or pumped pass.
Siphon passes specifically for fish are new. Currently the only company developing
them are Fish Flow Innovations based in the Netherlands
( There have not yet been any installations in the UK.
The Fish Flow siphon fish ladder is based on the principle of a poorly functioning
siphon: a siphon with an air bubble. The volume of the bubble defines the flow rate
through the system. This volume is controlled by a vacuum pump in order to prevent a
change in flow rate due to the import or export of gas from the water. The pass is only
suitable for sites where the free downstream passage of silver eels is achievable.
Pictures and designs
Figure 7.27: Fish siphon for all species
Elver and Eel passes
Figure 7.28: The inside of an all-species fish siphon.
Figures 7.29 and7.30: Eel-only fish siphon. Pipe encloses eel brushes only. This
siphon was installed on the River Roggebotsluis near Kampen in February 2007.
The ladder enables migration of eels between Lake IJsselmeer and Lake Veluwe.
Elver and eel passes
Figure 7.31: Newly constructed siphon pass at Hertogswetering near Berghem
(Brabant) in May 2006.
Figure 7.32:The top of the fish ladder. Note the eel brushes which
aid eel migration.
Advantages and disadvantages
Can be used where no ingress of water
is allowed
Can be designed for all fish species or
eel only
Fully adjustable flow rates to suit different
fish species
Low power requirements
Small footprint, so land purchase is not
Low maintenance
Requires electrical power to start siphon
Installation costs
Not widely available
Elver and Eel passes
What do you need to build this?
Planning permission
Abstraction licence
Revolving doors
No information on revolving doors was available for this version of the manual.
Elver and eel passes
8 Sluices
8.1 In-river structures
Generally, in-river structures fall into two categories:
undershot – where surplus water passes under the structure;
overshot – where water weirs over a facet of the structure.
In some places, both types of structures are used in combination.
Examples of the main types of undershot structures:
A. Radial gate, right.
B. Large penstock sluice
C. Small penstock sluice
D. Twin leaf gate (above)
E. Large undershot gate (left)
Elver and Eel passes
Examples of the main types of overshot structures:
F. Penning weir (door down)
The same structure (door up)
G. Stop log weir
H. Large tilting weirs
I. Pre-fabricated tilting weir
J. Older style tilting weir
Elver and eel passes
Examples of structures that are both under and overshot:
In extreme conditions, the radial gate (A), pictured left, and the
penstock Sluice gate (C), middle picture, can act as overshot
structures. The twin leaf gate (E), pictured far right, can be
used in either mode irrespective of flow conditions.
Initial assessment
When you assess whether a structure is suitable for an eel pass, you must understand
not only the mechanics of the structure’s operation, but also how it is managed. An eel
passage may not be necessary. For example, a penning weir would only obstruct
passage during the winter months. Throughout spring and summer, it is flat. So the
weir would not prevent upstream eel migration as this happens in the warmer months.
You should also investigate the mechanics of the river and its biology, specifically eel
populations. Look at any previous survey data, including catch returns where available.
The location of the candidate site within its catchment is also important as this often
has a bearing on the most common size ranges of the eel likely to use a pass. This
should inform key aspects of the design, such as the size (width) of the channel and
the density of the bristle substrate.
Eel passes for these structures fall into two main categories: those that are gravity fed
and those that are pump fed. Under these two broad headings, there are many
variations which are tailored to each structure type. However all eel passes for these
structures will have some things in common. These include:
bristle substrate in some format;
a small flow of water to the downstream and upstream elements;
some form of cover.
Two problems are common to both types of pass:
ensuring the eels are ‘delivered’ far enough away from the weir crest not to
be swept downstream again;
safe-guarding eels that emerge from an upstream pipe from predation or
It is possible to choose either gravity or pumped solutions at most sites. However,
pumped passes tend to be used at larger sites that already have electrical supply.
Both solutions have advantages and disadvantages and we look at these in the case
studies which follow.
Elver and Eel passes
There is a case study for each structure type and we have included a component list
with suppliers for the key elements.
Case study 1: Large multi-structure site, with flood control and penning, at the tidal
limit of a river with an eel fishery.
This site has an array of control structures
including large penning doors. These are shown
in the open position.
The doors are open during flooding and from the
beginning of November to the end of March.
The river is therefore unobstructed at these
At all other times, the penning doors are closed
and the river flows down an adjacent bypass
channel. The channel itself has two control
structures: a tilting weir and a set of undershot
sluice gates.
The same structures look very different at low tide and with low freshwater flows. This
illustrates the need to identify the constraints of a site and design eel passes that will
operate in very different conditions.
A pumped ‘up and over’ system was chosen for this site. A standard arrangement for
this type of system can be seen in Figures 8.1 and 8.2.
Elver and eel passes
A stainless steel channel was mounted on the inside of the sluice gate channel. This
forms the downstream element of the pass. The channel was lined with twin density
substrate which continues up to the apex. Here a ‘splitter box’ splits the pumped flow
into the downstream channel and the upstream eel delivery pipe.
The term ‘up and over’ refers to the fact that the apex (where the splitter box and
transition to the upstream pipe are mounted) is higher than the height at which water
weirs over the structure.
The approximate dimensions for a pass of this size and importance would normally be:
stainless steel channel: 300mm width x 150mm depth
delivery pipe: 150mm diameter;
substrate: 300mm width, 70mm tufts set in two pitches of 30mm and 20mm,
cut and fitted onsite.
This installation formed part of a capital refurbishment of the flood defence structure.
The contract to carry out these works and install the eel pass was awarded to:
W.S. Atkins and May Gurney
This type of solution would be effective for structures A, B, C, F, G, H.
Substrate: The one in this example was supplied by: Fish-Pass France,
Channel: Stainless steel, bespoke, fabricated to fit by Sub-contractor to
W.S.Atkins/May Gurney
Pump: Electric, continuously rated, 50 litres/ minute. Supplied as part of contract.
Installation: May Gurney
Other information
The pass is within an enclosed structure. There was therefore no need for any form of
anti-desiccation/predation lid or cover.
In order to assess how well the pass was working, a camera was installed above the
downstream channel. This records any eels ascending through the bristle substrate.
The camera has worked well. It has not only proved that the pass is functioning
correctly, but is now used to record eel numbers in this catchment.
Elver and Eel passes
Figure 8.1: The route of a typical downstream channel with substrate, the flow
splitter box (blue) and the upstream delivery pipe (red).
Figure 8.2: The position of the pass entrance. Note that the pass will be cranked
towards the bank. This is to maximise the opportunity for eels to find the flow
coming down the pass –eels migrating upstream routinely stay close to the
Elver and eel passes
Case study 2: Fixed pass with pump on undershot gate
This eel pass is at South Ferriby
on the Humber. It crosses a tidal
sluice with a head difference of
approximately 2.5m.
The pass is permanently fixed to
the door, but is engineered not to
foul on any other part of the
structure when the door is lifted
clear of the water during flood
The pass is bolted to the rear of
the door. It lifts clear of the water
as the door is raised. The flow of
water through the downstream
bristle channel is jetted onto the
head of the channel. Eels ascending the pass simply drop into the upstream side of the
structure. A technical drawing can be seen below.
In this instance, a bespoke fabrication was commissioned. However, the pass works on
the same principle as all the other passes in this section: it has a small flow of water
that feeds a bristle-lined channel.
The Environment Agency is developing a standardised channel that will be suitable for
most sites. The channel will accept the most common permutations of bristle substrate.
It will be prefabricated out of GRP (Glass Reinforced Plastic).The channel will be
delivered in 2.4m lengths that can be added together. The units can be supplied with or
Elver and Eel passes
without a pre-attached hinged lid. We hope that, in many cases, this modular approach
will simplify installation and reduce costs.
At the sites for case studies 1 and 2,there was existing electrical and security
infrastructure. And for these particular examples it would have been difficult to maintain
a constant water flow by gravity alone. A pumped pass was therefore the better option.
The disadvantage of any pumped system is that it consumes electricity. Not only is
there a cost element, but at some point the pump will stop. This may be due to a power
outage or to the pump’s strainer becoming clogged with weed and debris. Careful
design and installation can improve reliability, but a failed pump might severely impact
eel migration.
This pass solution would be effective for structure D.
Substrate:20mm gap bristle substrate supplied by Cottam Brush Ltd. Phone: 0845 434
84 36
Channel: Stainless steel, bespoke, fabricated to fit. Supplied by local contractor.
Pump: Electric, continuously rated, 110v, 50 litres/ minute. From internet supplier.
Installation: Carried out by the contractor refurbishing sluices for Environment Agency
Flood Risk Management.
Case study 3: Twin leaf gate
This twin leaf gate posed a similar problem to that in case study 2. An eel pass had to
be fixed to the uppermost leaf in such a way that it could not foul on either the lower
leaf or on any other part of the structure – in any gate position.
A further constraint was the proximity of the headwall. This meant that this fixed eel
pass could not extend very far downstream. The only option was to increase the angle
of the downstream channel.
A bespoke channel unit was made that bolts directly to the upper surface of the top leaf
and moves up and down with the structure. The final angle of the downstream channel
Elver and eel passes
was nearly 80°. However, the short run of the pass allows eels to manage this steep
incline. A simple bag trap placed on the upstream element of the pass confirmed that
eels are using this pass successfully.
The advantages of a gravity-fed pass are generally:
there are no running costs;
the pass is completely reliable if appropriately maintained.
One disadvantage can be a lack of flow if the pass becomes completely blocked by
debris. However, this is rare in a well-installed pass, where the bristles act as a strainer
for surface weed but still allow water down the channel.
The installer must also be confident that the eels are getting safely beyond the
influence of the weir crest. This can depend on flow.
This solution is appropriate for structures C, E, F and G.
Substrate: Bristle board with 30 mm spacing between clumps supplied by Fish-Pass
Channel: Bespoke stainless steel channel, 200mm wide. Supplied by ACE Fabrications
Installation: ACE contractors.
Case study 4: Flood defence
This flood defence structure has two
large tilting weirs. It is at a
freshwater site some eight miles
from the tidal limit.
One obvious constraint was that any
eel pass must not interfere with the
tilting action of the weirs.
Due to the width of the river at this
point, it was decided to have an eel
pass on each side. These were
mounted as near to the bank side as
possible in order to intercept
upwardly migrating eels. The flow
down the eel passes is relatively tiny
compared with the river flow over the
weirs. However, the eels found and
used the passes in large numbers
within 24 hours of installation.
Elver and Eel passes
The tilting weir used at this structure is a
bucket type. This made it possible to
attach the eel pass channels directly to
the mass concrete side walls of the
structure. The stainless steel channel
was prefabricated offsite then cut to fit
on site. It had various cranked elements
to clear the workings of the tilting weir
and was also pitched at a steeper angle
of around 55°. This angle was needed to
avoid the lower end of the pass fouling
on the telemetry sensors for the site. A
CCTV system was installed to monitor
the performance of the pass.
This picture shows the bespoke flow splitter box. It is
basically an interceptor which sits at the apex of the
downstream bristle channel and the upstream pipe. It
distributes the pumped flow to these two elements of
the eel pass.
The upstream delivery pipe terminates into broken,
rocky substrate. This provides the eels with more than
one exit so that they are not an easy target for
This pass solution would be effective for structures A,
B, C, F, G and H.
This installation was part of a capital refurbishment
carried out by W.S. Atkins and May Gurney. The
substrate was supplied by Fish-Pass France.
Case study 5: Pre-fabricated tilting weir
This pre-fabricated
tilting weir replaced
a leaking stop log
structure. The
previous structure
would probably not
have hindered
upstream eel
migration: the plant
growth that often
colonises such weirs
acts as a substrate.
Eels were able to
climb up it, through
it and into the
penned water level.
This new structure is
typical of penning
weir upgrades.
Elver and eel passes
The key problems with these weirs are:
Modern ‘clean’ materials such as stainless steel and plastics offer little
opportunity for secondary weed growth.
There is a ‘confusing’ overflow: eels would naturally use falling water to
locate a climbing surface, but the discharge point from the weir crest is
beyond the rest of the structure.
These structures are small and nearly always in remote locations. So there is
seldom any electrical supply for a pump.
The angle of tilt – and therefore the head difference – can change on a daily
basis. Any solution must be dynamic and able to cope without compromising
the effectiveness of the penning structure.
The solution is to use a gravity-fed
pass, shown left with the lid open. The
pass is fed water from an integral
element that augments the weir crest
along its width except for where the
pass sits. This design encourages
water to flow down the eel pass.
This self-adjusting pass is deceptively
simple. There is a float at the lower
end and two densities of substrate,
arranged in a ‘V’ formation, allow for
differing flow rates.
A simple gear box at its connection point to the weir crest transmits changes in the
weir’s angle to the upstream element. This element projects beyond the influence of
the weir crest, ensuring that the eels are not swept back downstream. This upstream
element has a flexible bristle substrate which continues to point down into the upstream
water level at around 15°regardless of the angle of the tilting weir. The CAD
illustrations below (Figures 8.3 and 8.4) show how this works: at two different penning
heights and weir angles, the upstream element is maintained at a constant downwards
angle via the gearbox. Also note the weir augmentation elements which force water
down the pass.
Figure 8.3: Tilting weir
upright position
Figure 8.4: Tilting weir,
lowered position
Elver and Eel passes
It was essential that this pass did not affect the structural integrity of the tilting weir. To
this end, no holes or fixings were drilled into the weir. Instead, the pass was clamped in
place. This makes it easier to remove the eel pass at the end of the summer. The
augmentation elements stay fixed to the weir crest throughout the year.
The picture below shows the same eel pass with its integral hinged lid closed. Note the
level of the channel and substrate. This is maintained by the downstream float. Trial
and error established the appropriate level for this size of eel pass at the range of
angles at which the penning structure would operate. On this site it was found that
submersion was appropriate up to the underside of the lid. A simple bag trap showed
that eels started to use the pass immediately.
Weed has never blocked the pass because the upstream element sieves the weed and
prevents it from entering the pass. This also has the advantage of forming a localised
floating raft of weed that creates some cover for the emerging eels. Wherever possible,
the pass should be made from recycled materials.
The pass can be adapted and installed on structures such as C,F,G and J.
Substrate: Bristles spaced at 20mm and 30mm on 100mm width baseboards. Also
used flexible, rubber-backed substrate. The rigid substrate boards are removable from
the pass and were supplied by: Cottam Brush Ltd. For more details see or phone 0845 434 84 36
Channel/pass: Supplied by Berry and Escott Engineering, Bridgwater, Somerset, TA6
5LT. Phone01278 444861. Email
Installation: Berry and Escott Engineering and Environment Agency staff.
Elver and eel passes
Lessons learned
Practical experience has shown that laying a bristle substrate with a 20mm gap
between bristle clumps alongside another with 30mm spacing will allow the passage of
eels/elvers ranging in size from 80mm to 750mm. This narrower spacing is favoured by
smaller eels and may therefore not be appropriate for sites higher up catchments –
where larger eels may be more prevalent.
As ramp length increases, the angle of the slope should decrease. However practical
experience has shown that even with steep slopes (up to 60°) eels are still able to
ascend quite long ramps. In exceptional circumstances, it may be appropriate to have
steeper passes up to 75°.
The channel element of the pass will need relatively little water to be effective. For a
channel 200 mm wide and lined with bristle substrate, a flow of 0.5 litres/second will
feed both the downstream element and the upstream delivery pipe.
Elver and Eel passes
Modular eel pass channel components
GRP channel in modular sections with
male/female ends
240 cm x 20 cm (internal) x 10cm
Each section has full-length hinged lid
Bristle substrate (100mm width x 2)
Fixed with stainless steel,
self-tapping screws
Flow-splitter for pumped passes
Channel with flow-splitter attached
Elver and eel passes
Pre-formed angles make installation easier
Each has own hinged lid
30o and 45o but bespoke angles can
be made
Can be used in combination
Elver and Eel passes
Channel for gravity-fed passes
Same dimensions but with taller sides
Sample substrate inserted. Triangular
void blanked off so that all water flows ~
through substrate
Self-adjusting eel passes for tilting weirs
Elver and eel passes
Elver and Eel passes
S+D Plastics of Burnham-on-Sea,
Phone: 01278 781853
Elver and eel passes
Glossary of terms
Apron. Usually made from concrete or rock-filled gabion baskets. The apron is
installed upstream and downstream of the gauging structure to prevent erosion of the
river bed.
Bootlace eel. Non-technical term used for juvenile eels in the first and second year
fresh-water life stage.
Brackets. Flat stainless steel bar bent and drilled to allow fixing of bristle boards to the
wing walls.
Bristle boards. Plastic boards drilled at intervals with bunches of monofilament bristles
inserted into the holes. Provides a medium for eels and elvers to gain purchase and
crawl upstream.
Bristle tufts. Bunches of monofilament strands inserted into backing boards.
Crawling gutter. Artificial channel filled with a crawling medium. Supplied with a water
flow to allow eels to pass an obstruction.
Crest. The highest point on a gauging structure. Maintains the upstream water level.
Crump weir. Gauging structure with a horizontal crest and angular profile, The
upstream slope is 1 in 2; the downstream slope is 1 in 5.
Eel. If you need a definition for eel, you are perhaps starting with the wrong manual.
But they are a catadromous fish which migrate as juveniles from the sea to fresh water,
where they spend a proportion of their life.
Elver. Juvenile, pigmented eel (0+), often entering fresh water in late spring and early
summer. This term is only used in the UK. For the rest of Europe the term yellow eel is
used for all immature pigmented eel.
Flat V crump weir. An angular profile weir – similar to the crump weir but with a Vshaped crest rather than a horizontal one. V crump weirs are usually found on rivers
with a low summer discharge.
Glass eel. Juvenile, non-pigmented eel which migrate to coastal and fresh waters.
Head differential. The difference between the water level upstream of a structure and
the water level downstream.
Head water level. The water level above a structure.
Tail water level. The water level downstream of a structure.
Wing walls. Vertical walls usually made from brick concrete or gabion baskets to
protect banks from erosion at a gauging structure and provide a stable gauging
dimension. They run parallel to the river flow.
Yellow eel. A pigmented eel resident in fresh or coastal waters. In the UK the term is
used for >0+ eel.
Elver and Eel passes
The following manufacturers have products described or mentioned in this manual:
Aquatic Control Engineering Ltd (ACE)
Hall Farm, Main Street, Rampton, Nottinghamshire, DN22 0HR
Phone: 01777 249080
Contact: Marcus Widdison
Fish Flow Innovations
vanTwickelostraat 2, PO Box 423, 7400 AK Deventer, Netherlands.
Phone: +31 570 619292.
Check out product information at:
8 Allée de Guelédan, ZA Parc Rocade Sud, 35135 Chantepie, France.
Phone: +33 (0)2 99 77 32 11
Contact: Dr Antoine Legault
Fish-Pass is a small company that undertakes research and consultancy on freshwater
fisheries. It also manufactures and supplies complete systems for eel and elver
passage facilities. Their products include:
bristle substrate mats;
plastic moulding substrates;
prefabricated passes;
design and fabrication of eel lifts;
design of standard passes for eels.
Ham Baker
Garner Street, Etruria, Stoke-on-Trent, ST4 7BH
Phone: 01782 202300
Contact: Malcolm Sargeant
Juel Tide Gates
9732 12thArea SW, Seattle, Washington 98117, USA
Phone: +1 206 300 4204
Contact: Jeff Juel.
Elver and eel passes
LandustrieSneek BV
PO Box 199, NL-8600 AD Sneek, Netherlands.
Phone: +31 515 486888.
Contact: WabeJager.
Represented in by UK by:
Deritend Group Ltd
Cyprus Street, off Upper Villiers Street, Wolverhampton WV2 2PB.
Contact: Jamie Wesley (mobile no: 07795 007853)
Milieu Inc.
188 Henrysburg, Saint-Bernard-de-Lacolle, Quebec, Canada J0J 1V0
Phone: +1 514 247 2878.
Contact: Denis Desrochers
Milieu Inc is an environmental consultancy and supplier of the Eel-ladder substrate
ramps. Their products and services include:
eel-ladder plastic substrate ramps;
eel-ladder elver substrate;
design, fabrication and evaluation of eel passes.
NijhuisPompen BV
Parallelveg 4, 7102 DE Winterswijk, Netherlands
Phone: +31 543 547474
Stoneman Engineering (SW) Ltd
Park Works, Station Road, Willand, Cullompton, Devon EX15 2QA
Tauw BV
Handelskade 11, PO Box 133, 7400 Deventer, Netherlands
Phone: +31 570 699328
Contact: Anne Bosma
UK partners are W S Atkins. Contact John Sheppard in their Peterborough Office on
01733 366917.
Waterman Industries
25500 Road 204, Exeter, California 93221, USA
Phone: +1 559 562 4000
Brie Comte Robert, BP 95, 77253 Coubert Cedex, France
Phone: +33 1 64 06 76 05
Manufacture Pelcar and Evergreen concrete blocks.
Elver and Eel passes
MMG Civil Engineering Systems Ltd
Vermuyden House, Wiggenhall St Germans, Kings Lynn, Norfolk PE34 3ES
Phone: 01553 85791
Manufacture the Enkamat geotextile.
American Wick Drain Corporation
1209 Airport Road, Monro, NC 28110, USA
Phone: +1 704 238 9200
Manufacture the Akwadrain substrate.
Bristle substrate suppliers
Cottam Brothers Ltd
Sheepfolds Industrial Estate, Sunderland, SR5 1BB
Phone: 0191567 1091.
Dawson and Son Ltd
Eldon Brush Works, Clayton Wood Rise, West Park Ring Road, Leeds, LS6 6RH.
Phone: 0113 275 9321
Cooks of Norwich
9 Concorde Road, Norwich NR6 6BH
Phone: 01603 484444.
W S Read and Sons Ltd
554 Green Street, London E13 9DA.
Phone: 020 8472 0825.
Elver and eel passes
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