Tag Archives: Fish passage

River Club Weir Removal

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No 12 on the Hogsmill obstruction hit list was the River Club weir.

With very shallow flows plunging over its crest, and a drop of 0.7 m between the upstream and downstream water levels, this weir on the Hogsmill near Tolworth was a complete barrier to fish passage under the majority of flow conditions. On each flank, the concrete and stone abutment walls were also structurally failing.

After lengthy discussions, in the course of which no justification for keeping or maintaining this weir could be established, we worked with the Environment Agency and Thames Rivers Trust to undertake a full weir removal.

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Once we had accessed and prepared the worksite, we began breaking up the main part of the weir using a long reach excavator with a hydraulic breaker attachment. It’s amazing how solid parts of these structures are, but our operator Roo’s patience eventually prevailed!

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Over 30 tonnes of concrete was removed from the channel and collected by a grab lorry.

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The stone walls were then broken up, and the new bank profiles were installed using a range of sustainable and biodegradable materials.  Native marginal plants and seed were then added to the new banks, which over time will provide erosion control and great habitat.

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It’s great to see the channel upstream already returning to a more natural state. Within a few days of removing the weir, a resident koi carp had moved from the upstream pool and was enjoying exploring some new habitat. This bodes well for all our native fish species we know are in other parts of the Hogsmill.

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We had fantastic weather throughout the duration of the project, although we could really do with some rain, as most of the rivers across the South East of the UK are experiencing extreme drought conditions.

A big thanks goes to to our local Environment Agency staff, Thames Rivers Trust, The River Club, two very supportive landowners and Roo Newby for his excavator operating skills.

We’ll now be focusing on the six remaining obstructions to fish passage with our aim of restoring this lovely (previously neglected) river to a more natural and improved ecological state.

Why remove weirs? Some excellent diagrams on how weirs affect river habitat and geomorphology have been produced by the Wild Trout Trust and you can also read our article on how weirs affect fish communities.

Before and After photos

BEFORE AND AFTER 1

The Timelapse Video

Hogsmill reconnected to the Thames with new fish pass in Kingston

The Hogsmill gauging station is an Environment Agency flow monitoring structure, essential for water resources planning and regulation.  It is the furthermost downstream weir in the catchment and poses a significant barrier to fish passage, preventing the recolonisation of fish to the river from the Thames below. Addressing passage at this key site has been discussed for several years but due to the sensitivity and importance in the recorded flow-gauging data, in combination with the unconventional structure it has resulted in a complicated and extensive process to identify and develop a suitable solution. JBA have helped inform the positioning of the upper baffle and through a programme of spot gaugings will update the rating of the gauging station.

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The impassable Hogsmill Kingston gauging station

The selected approach is a variation on the Low Cost Baffle (LCB) Solution as we have used elsewhere, whereby rigid baffles are put on the downstream weir face in a specific geometry and spacing to slow the water, deepen the flow and provide a distinct passage route on the weir face. However, due to the significantly steeper gradient at the site in comparison to that the design was developed for, the Trust and EA have modified the arrangement to help promote the correct hydraulics to enable fish to pass.  The maths aside, in essence this meant that each line of baffles is incrementally taller than the last, starting at 120mm and ending at 288mm at the downstream end.

With this novel approach the Trust and the Environment Agency are keen to establish how efficiently the pass/easement operates, as the principle could be adopted at other similar challenging sites. In order to get a comprehensive assessment, an exciting opportunity arose to work with Durham University who will use the project as part of a wider study looking at fish passage past human made barriers. That’s the good news. The bad news being that in order to be ready for the dace spawning season I was committed to delivering for three weeks through the bleak days of January and February. The summer would have been far too pleasant.

Due to the non-standard design the baffles were manufactured as a bespoke commission carried out by Northwood Forestry & Sawmills, made of oak as opposed to the standard recycled plastic. Once complete, myself and Norm spent the first week of the year huddled in his workshop drilling and bolting the stainless steel angle to each baffle which would enable them to be securely fixed to the concrete weir. All 36 pieces of the jigsaw were complete, coded and stacked in the lockup ready for installation.

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In the workshop with Norm fixing the stainless steel angle to the baffles

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Half of the baffles with angle attached and coded ready to go

The baffles would have to wait, as first we needed to help fish of the anguilliform variety, eels, to also be able to successfully navigate past the weir. With the eels having already made the 6000+km epic journey from the Sargasso Sea we were keen to ensure this weir would not be an abrupt dead-end. Although there was an eel pass on the weir, it has been largely ineffective for some time. A new and improved pass was called for, so out with the old and in with the new.

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Out with the old…

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and in with the new

With the eel pass installed, preparation to fix the baffles was underway. This primarily revolved around creating a dry working area, not necessarily an easy task when working in a river but with the temporary coffer dam supplied from RN Inspection Services this was achieved with unbelievable effectiveness.

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The coffer dam installed providing a manageable working area behind

For the next two weeks, Roo and I lugged baffles, drilled holes, spaced, chocked, clamped, injected resin, removed then re-erected the dam and battled the elements. As the jigsaw slotted together the theoretical schematic drawing I was now well familiar with became a reality.

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The baffles being clamped into position before the stud secured to the concrete with resin

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Getting into the flow of things

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Half way!

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Time to move the dam and install the second half of the baffles

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Finished! The notch bisecting the baffles to allow a deeper channel for fish to pass

Once complete, the time came to remove the dam and allow the water to flow over and through the baffles. With this action the Hogsmill became re-connected to the Thames once again.

The time lapse of the build conveniently summarises the full 3 weeks into 1 minute video can be seen here:

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About to ‘pull’ the dam

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Flow and fish passage returned to the channel

Over the coming year JBA (hydrology experts) will undertake a spot flow-gauging program, to ensure that the weir continues to gauge accurately with the modifications. Meanwhile Angus at Durham University and ourselves will monitor the LCB fish pass/easement aiming to understand its performance for coarse fishes such as chub, dace and roach.

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One aspect of the monitoring is a camera looking across the upper notch

A BIG thanks to: All those involved in the various teams at the Environment Agency, partners in the project especially those in Hydrometry and Telemetry and Fisheries.  Angus, Martyn and Jeroen at Durham University for bringing their wealth of monitoring experience and expertise to the project. JBA for carrying out the hydraulic assessments associated with the project. Bedelsford School who kindly agreed to us hooking up to their electricity supply to power the monitoring equipment. Rob Waite at the Royal Borough of Kingston for helping us to secure access and parking at the site. The Thames Anglers Conservancy, especially Will, for getting involved and helping to install the eel pass. Norm Fairey for your continued help with all things fishy and manufacturing. And my good mate Roo for the long hours, hard graft, permanently cold hands and near permanent good humour.

 

Improvement Works on the River Dour

By Chris Gardner

In 2015 the South East Rivers Trust was awarded a £31k grant through the Department of Environment Food and Rural Affairs (DEFRA) Catchment Partnership Action Fund (CPAF) to deliver fish passage and habitat improvements on the River Dour in Dover. All these improvements would be designed to increase the health and resilience of the river environment for fish and other species, to meet the requirements of the EU Water Framework Directive.

River Dour an Urban Chalkstream

The River Dour is a short (4 km) little chalkstream that rises in a rural setting but soon flows through the highly urban centre of Dover. This delightful little river boasts a healthy brown trout population, but the habitat is highly degraded due to urbanisation and impounded / fragmented due to a legacy of watermills.

A spotty brown trout

We assessed the urban part of the catchment by walkover survey, looking for potential projects. These were then prioritised on a cost-benefit basis, resulting in four sites where fish and eel passage could be addressed. Two were simple fish and eel passage easements and two were sites for eel passes.

The ruins of the old water mill at Minnis Lane.

Work began in October 2015 with a volunteer event at Minnis Lane near the aptly named village of River. An old watermill ruin at the site presented a complete barrier to fish and eel passage. The main part of the weir was very high and a fish pass here would have been way beyond the budget available. However, a parallel channel had one small step weir and an over wide shallow section of channel upstream that made this route impassable. To make this passable to fish and eels, we fitted a wooden box to the downstream end of the small step weir had a wooden box, to raise the water level up the step so fish could navigate it. We also used faggot bundles as flow deflectors to define a deeper channel through the over wide shallow section. Many thanks to Anita for helping to organise the volunteers and to the volunteers for their hard work! You know who you are!

Fixing in the faggot budles at Minnis Lane

In May 2016, we also fitted two eel passes to Halfords Weir and Lorne Road weirs. These are large weir structures that will require technical fish passes in the future to make them passable to fish, but eel passes were achievable with the budget we had available. The Halfords weir site was quite straightforward – the setup of the weir allowed a simple gravity-fed pass to be fitted. Eel passes allow eels to move over weirs through a piece of conduit which contains brush bristles that provide a crawling medium. Water passes through the pass to provide attraction flow and passes are located at the edge of the weir as studies have shown that eels predominantly use river margins as migration corridors.

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The Lorne Road site was more complicated, because this eel pass needed to go up and over the weir, so a gravity-fed option wasn’t appropriate. Instead, we designed this pass to be fed with a water pump to provide attraction flow. Many thanks to Malcolm and Mick at the Lorne Road Mill building who allowed us to tap into their power supply! Also many thanks to Paul and Simon for their work building the passes – your help was very much appreciated.

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We delivered the final aspect of the project later in spring 2016. Morrison’s weir (near Morrison’s supermarket) presented a barrier to juvenile life stages of trout and eel. To improve its pass-ability, the weir was notched in the middle to provide a streaming flow that fish can swim up, instead of a small drop and a thin plunging flow that fish can’t easily negotiate. Bristles were also fitted into the notch so eel could climb up.

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Wise words from Baton Path Mural

Three Weirs: Part 3

Author: Nick Hale, Project Officer

Read Part 1 and Part 2 first!

And so 3 weirs fell!

Gatehouse, Weir 1

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New Lodge, Weir 2

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and Dairyhouse, Weir 3

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Watch the video of their removal here:

A total of approximately 250 tonnes of concrete and steel was removed from the river.

Removing the three weirs has unimpounded over 1 km of channel. By reconnecting these previously isolated sections of river, just over 3 km of this part of the Lesser Teise is now fully passable for fish!

Increased flow velocity and diversity has already seen the return of natural processes such as erosion and deposition. The mix of deeper and shallower areas will create a range of habitat niches for invertebrates and fish etc.

Directly upstream of the former weirs, the flows have begun to rework the river bed and gravel berms until its natural gradient is restored (as before the weirs were introduced). A previously drowned-out ‘in channel’ meander sequence has exposed gravel riffles, berms and mid channel bars.

These gravels will be continually reworked in a downstream direction, being replaced by new material from upstream. The well-oxygenated gravel riffles will become ideal spawning habitat for both salmonid and coarse fish like brown trout and chub.

A number fish were identified during the works with species including brown trout, minnow, bullhead, stone loach, gudgeon, dace, roach, perch, pike, brook lamprey and barbel.

This stretch of historically dredged river is typically characterised by steep/vertical banks with little or no marginal habitat. This project has exposed an abundance of berms and marginal features that given time will be transformed into a diverse marginal habitat.

Removing the weirs has eliminated the landowners’ responsibility for the upkeep of the structures and saved costs on the twice a year maintenance visits from the EA to maintain the signage and clear blockages in the channel.

What’s next?

We will be monitoring the river over the next few years to record how the project develops. It will be interesting to see how the higher flows over the winter months rework all the gravels.

In the spring we will sow some native marginal plant seed mix on the lower banks of our structures and take some more photos for the next blog.

A BIG thanks to must go to the landowners, Ian Johnstone and the Kent High Weald Partnership and the Environment Agency for all their help!

The Three Weirs: Part 2

Author: Nick Hale, Project Officer

Read Three Weirs part 1 here.

After many months of planning the project we were finally able to start the fun part and get on the riverbank!

The first job was to take delivery of all the machines and equipment such as the 15m long reach excavator and wheeled dumpers.  Access to the site was quite tricky but having a good delivery driver is always good!

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Mike our excavator driver was raring to go but first we had to install our sediment control features, marker posts to measure changes in water levels/river bed level and also take a few photos for comparison.

It was great to finally get the breaker attached onto the machine and begin breaking out the concrete. The centre of the channel was broken up first, and then the sides.

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breaking-out-concrete

The weirs were certainly well built! Sheet piling had been installed at the up/downstream ends of each weir to key it into the riverbed and act as the formwork when casting the concrete. Lots of 6mm steel rebar had been used to provide extra strength, and in some places the concrete was almost 1m thick.

Removal of the sheet pilling was quite a challenge at the first weir, with the downstream section being 1.8m in length.

Mike gradually broke up the weirs into manageable pieces, which were loaded into the wheeled dumpers and transported to the collection site. A local company was contracted to collect the waste material using grab lorries. Each weir was made up of about 80 tonnes of concrete which was about 5 grab lorry loads per weir.

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Chestnut faggots were secured with chestnut posts (both locally sourced) to form the line of the new bank.

A marginal shelf of site-won soil was then encased in coir matting behind the faggots and secured to the banks with oak pegs. We will come back to plant these up in the spring.

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With high spirits and with the weather thankfully on our side, we tracked up the bank to do battle with the next two weirs. Although the construction of each was similar, each site posed its own challenges. However one by one they fell, with all three weirs being removed and banks reinstated in a three week period, reinstating flows and passage for the first time in over half a century.

The benefits of the work were made clear before we had even pulled out of the site. We saw numerous fish passing the previous barrier to explore the stretch upstream.

Look out for our third and final blog on the Three Weirs Project to see some before and after photos and the timelapse video!

The Three Weirs: Part 1

Author: Nick Hale, Project Officer

An Environment Agency assessment of the Teise and Lesser Teise, two tributaries of the Medway in Kent, indicated there is a problem with wild fish stocks in the river.  Generally there is an absence and/or low abundance of key expected species such as brown trout, chub, predatory perch and pike, roach and stone loach.

In fact, the situation is probably even worse than this assessment indicates as a large proportion of the fish caught here are derived from artificial stocking, masking the figures of the wild population.

So what can be done about it?

One major contributing factor to these poor wild fish populations is the presence of barriers to fish migration: weirs. Weirs fragment the available habitat, prevent fish from reaching spawning grounds and change the geomorphology of the river channel as well. (For a reminder about the pros and cons of weirs, our very own Dr Chris Gardner wrote an excellent article recently on how weirs affect fish communities).

The Medway Catchment Partnership have been looking at barriers to fish migration in a bid to address this on a catchment scale. As a start, last year we successfully removed Harper’s Weir, restoring fish passage to a 3.5 km stretch of the River Teise. However, if you travel downstream, you are confronted by more and more weirs.

The Three Weirs Project looked to tackle a set of three weirs on the Lesser Teise, continuing this great work.

Where are the Three Weirs?

The Three Weirs, named Gatehouse (1), New Lodge (2) and Dairyhouse (3) are located on a 1.2 km stretch of the Lesser Teise near Chainhurst, Kent. All three are concrete weirs, likely originally installed in the mid-20th century for the purposes of agricultural land management.

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What ecological benefits will result from removing the weirs?

The presence of a weir creates an impounding effect whereby the upstream water levels are much higher than normal.  This drowns out any natural features within the channel making it more like a canal than a river.

Impounded channel upstream of Dairyhouse Weir.

Impounded channel upstream of Dairyhouse Weir.

Removing the 3 weirs will increase the hydromorphological diversity (flow and river bed structure) of this stretch of river. Natural processes such as sediment transport and gravel mobilisation will return, exposing a range of original channel features like gravel berms and meanders. For example, the removal of Harpers Weir revealed 16 new riffles.

Ecological benefits will be seen both upstream and downstream of the existing weirs with salmonid and coarse fish being able to migrate through this previously unpassable stretch, improving their ability to carry out their full life cycle and ultimately increasing their chances of survival.

Localised improvements in water quality through faster, low flow conditions will reduce the build-up of ammonia and phosphates and increase dissolved oxygen to support a wider range of invertebrates. To capture these changes, an invertebrate survey has been undertaken by Robert Aquilina at specific locations along the proposed works area.  This process will be repeated in September 2017 to record any changes in diversity and abundance of the invertebrate community living within the channel.

In preparation for full removal, the wooden boards that were fixed to the crest of all 3 weirs were removed in 2015 with help of the Environment Agency. This reduced some of the impounding effect and allowed the channel upstream to adjust and stabilise prior to a complete removal of the structures in the future.

The next step now is to remove the weirs, but you’ll have to wait for part 2 to find out all about that!

Eel Passage on the Hogsmill

During the summer a collaborative project between Zoological Society of London (ZSL), Surrey Wildlife Trust Kingston Group and the South East Rivers Trust (SERT) was undertaken to improve elver and eel passage on one of the lower weirs of the Hogsmill, the Clattern Bridge weir.

The European eel is a critically endangered species and needs all the help we can give it.  Pollution, overfishing, global warming, disease and habitat loss have all contributed to the demise of this charismatic species. The eel has a fascinating and mysterious life cycle in which it starts life in the Sargasso sea as a larvae, migrates across the oceans via currents to European rivers, metamorphosing a couple of times on the way to become glass eels and then elvers.

Once in rivers, such as the Thames and the Hogsmill, they migrate upstream to find habitat in which to grow and develop into yellow eels. After 5-20 years of life in rivers like as the Hogsmill they metamorphose again into silver eels and travel back to the Sargasso Sea to complete their life cycle.  Weirs and habitat loss in our rivers are factors that cause issues for eels and stop their upstream migration to suitable habitat.

This project involved installing plastic tiles covered in regularly spaced plastic protrusions onto the weir face.  The weir at Clattern Bridge is smooth and has shallow fast water flowing over it.  The tiles allow eels to wriggle up the weir and into the river upstream enabling them to carry on their migration. Eels are not very good swimmers compared to other fish and prefer to ‘wriggle’ so increasing friction in this way is ideal for them!

Below is a great video of eels using a similar design on the Wandle:

Armed with a couple of battery drills, some long drill bits and various stainless steel fixings we attached a continuous line of tiles to the weir surface.

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It was a lovely sunny day and perfect for a day in the river!

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A big thanks to the Kingston local branch of SWT who provided funding for materials, drill bits and fixings and to ZSL for supplying manpower and eel tiles.

Elvers migrate upstream between April-September and so we are hoping they will appreciate our efforts when they arrive in 2017!

Author: Tim Longstaff

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How Weirs Affect Fish Communities     

By Dr Chris Gardner

River restoration describes a set of activities that help improve the environmental health of a river or stream. These activities aim to restore the natural state and functionality of the river system to promote improved fish populations, biodiversity, recreation, flood management and development. Restoration to improve fisheries tends to focus on increasing habitat quality and diversity, and a popular method of achieving this is by addressing barriers such as weirs.

Humans have adapted rivers for our own use, modifying them to facilitate land use for agriculture and development, navigation, water supply, power generation and other priorities. Fish evolved in rivers before humans had this influence and are therefore not adapted to the modifications we have made, such as dredged sections, straight uniform channels and weirs.

However, weirs and their associated downstream pools and upstream slow deep reaches are valuable and popular fishing spots as they provide habitats and daytime refuge areas for large adult fishes. So a balance needs to be struck between improving the river ecologically (more fish!) and providing areas where fish are likely to congregate in the daytime when anglers are on the banks (hot pegs!)

Weirs impact rivers in three main ways;

  1. Habitat fragmentation is frequently caused by human activities which disrupt the continuity of habitats used by wildlife and is a terrestrial conservation issue as well as an aquatic one. Habitats which were once continuous become divided into separate fragments, restricting the movements of organisms (g. fish) and separating them from habitats / resources / other fish required for their survival / the completion of their life-cycle. Fragmented habitats are also less resilient, preventing re-colonisation after pollution incidents and lowering genetic variability, due to the restricted effective population size, potentially placing populations at an evolutionary disadvantage.
  2. River habitat is degraded by the creation of an impounded reach upstream (river-like habitats become lake-like) drowning out natural features like riffles, causing important spawning and nursery habitats for river fishes to be lost, thus lowering recruitment and breeding success. Also, natural processes such as sediment transport are prevented. Rivers are naturally dynamic with erosion and deposition occurring in balance, creating a highly varied mosaic of micro-habitats for all life-stages of fishes. Weirs arrest this natural tendency for change, creating a uniform static environment. Upstream an over deep river channel akin to a lowland river is formed in the impounded reach which may be inappropriate for the fish community (for instance, barbel habitat may become bream habitat). Impoundments also alter the temperature regime, oxygen content and cause siltation in the upstream reach.
  3. Sediment transport is halted by weirs, reducing the supply of gravel to the downstream reach, leading to incised channels and reduced spawning and nursery habitats. River channels are natural systems with erosion and deposition occurring in balance. Sediment (e.g. gravel) is shaped and sorted by water flow patterns creating a large diversity of habitat types that support a high diversity of wildlife. Weirs stop natural processes and impact river channels in two main ways; Upstream – Sediment transport is interrupted by the weir, sediment (e.g. gravel) accumulates upstream. Due to the lack of energy in the impounded reach, sediment (e.g. gravel) is not shaped and sorted by the water flow and therefore creates a uniform habitat that supports a lower diversity of wildlife. Downstream – Sediment transport is interrupted by the weir, reducing the supply of sediment (e.g. gravel) to the downstream reach, which is vital for creating habitat features for wildlife. This lack of sediment from upstream leads to increased erosion of river banks and bed leading to channel incision (when a river has cut downward through its riverbed. The river begins at one elevation and incises downward through its bed while leaving its floodplain behind [higher]) throughout its course). Incised channels have knock-on impacts for;

Ecology: The incised channel only connects to its floodplain in extreme flood events, thus higher than normal water velocity is maintained in-channel during small to medium flood events. Aquatic wildlife (e.g. juvenile fishes) may become swept downstream during high flow events. Due to the steep banks there is also a lack of marginal transitional habitats, which is an important habitat in its own right, but also provide a refuge for wildlife in flood conditions.

Flooding downstream: Flood risk may have been increased downstream. Again, the incised channel only connects to its floodplain during extreme flood events, therefore the floodplain cannot store water in this location during small to medium flood events. This water that cannot be stored in the floodplain is quickly transported downstream, where it may contribute to flooding.

Addressing the impacts

Removal of the weir should always be the considered as the preferred option, which solves all the main issues described above. However, total removal is often not possible due to the way the landscape has developed since the weir was built, and there is also a need to take account of the wishes of the land owners and river users, like anglers, who may value the weir and its effect on the river channel. Dependant on this, the next best option may be a partial removal (lowering of the weir) and/or the implementation of a fish passage solution, which solves part of one problem (reducing the impounded reach) and all of the other (connecting the upstream and downstream habitats). Fish passage solutions include natural bypass channels (preferred as these create additional habitat), rock ramps and technical fish passes e.g. Larinier.

The design of a fish passage solution (e.g. a natural bypass channel) needs to take into account the wishes of other river users (e.g. anglers), provide free passage and attraction flow for a range of sizes of the species of fish present through a wide range of flow conditions, and be deliverable for the usually tight budgets available for such works. Modelling, using an existing locally specific Environment Agency flood model (a computer simulation of the rivers flow at a specific location), can be used to satisfy these needs and to ensure no increase in flood risk for any nearby residential properties. This approach is best practice and gives all concerned (residents, deliverer and the regulator) the confidence to implement schemes with the simplest, most cost effective design.

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Photo 1 – Harpers Weir on the River Teise (Medway catchment, Kent), which was recently removed revealing thirteen new gravel riffles, additional spawning habitat for the native brown trout.

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Photo 2 – A newly constructed nature like bypass channel around an impassable weir (visible to the left) on the River Darent, Kent.

Migratory fish? All fish!

So, just to reiterate, weirs affect fish communities by causing three main impacts. The first is habitat fragmentation, which prevents fish migration and separates fish from habitats and resources that they need to survive and reproduce. If fish do not have access to appropriate spawning habitat, successful recruitment will be reduced. The second is habitat degradation, as a barrier impounds or backs up a river, producing a lake-like habitat in the upstream section instead of a free flowing river; drowning out natural features like gravel riffles, that are important spawning and nursery areas. And the third is interrupting sediment transport that builds the habitats used by wildlife and fishes.

It is often thought that restricting the movement of fishes only affects ‘migratory’ species such as salmon, sea / brown trout and eels, as their migrations are relatively easy to observe and for many centuries man has been exploiting these behaviours to harvest these fishes from our rivers with fixed traps like eel racks and fishing weirs. However, all fish migrate to some extent, and all fish have life stage specific habitat requirements affected by habitat degradation. Salmonid species are not the only fish that spawn on gravel riffles: coarse species such as barbel, chub and dace are also affected by fragmentation and habitat degradation caused by impoundments. As with all fish species, recruitment success is dependent on the most limiting habitat requirement, potentially causing a population bottleneck. If a section of river can only support 100 juveniles of one year class as a result of the poor quantity or quality of juvenile habitat, this will limit the adult population size, as the adult population cannot be more than 100 of each year class.

Riffles provide cover in the form of weed growth, and are difficult places for predatory fish to hunt due to the fast current. They provide an abundant food supply of invertebrates, and overhead cover from a heavily rippled surface that hides them from predatory birds and other animals. Riffles are a good place to live if you are a baby trout or young salmon. They are also the preferred habitat for juvenile barbel. Riffles hidden under lake-like habitat upstream of a weir, lack the characteristics that make them great juvenile barbel habitats. If there are low numbers of small barbel, then there will be small numbers of big barbel and eventually no barbel at all.

Here, the trout and salmon fraternity are ahead of the game. The economic value of salmon (commercial and recreational) and the large declines in salmon populations since the 1980s have caused scientists and anglers alike to research and understand what the habitat requirements are for all life stages of these fishes. They have used this information to minimise potential population bottlenecks or limiting factors, due to available habitat. There are many things impacting our fish populations, but in-river habitat is the one thing that is relatively easy to address and benefits all wildlife. Organisations like the Wild Trout Trust have been encouraging progressive thinking and educating game anglers in fisheries management and river restoration. Lots of great work is going on out there, which is having a real positive effect on stocks. Coarse anglers need to take heed and follow this successful model, if we are to continue to enjoy high quality river fishing.

Coarse fish migrate and need to move around a river system to locate specialised habitats required at certain times and during certain conditions. Adult coarse fish such as chub, barbel, bream and roach migrate to spawn, often upstream, but also searching out the highest quality spawning habitat available to increase the survival of their eggs. Coarse fish lay their eggs on gravel (chub and barbel) and weed (bream and roach). The eggs take a few days to hatch into larvae and these larvae are then free-drifting with the flow of the river, until they find a suitable slack.

The movement of fishes

Modern telemetry tracking studies using implanted radio or acoustic tags have revealed these migrations. For example, in 2010 Dr Karen Twine, of the Environment Agency, radio tracked 20 adult barbel (6-15lb) in an 8.2 km reach (between two impassable weirs) of the Great Ouse for a year and a half. She demonstrated that the barbel utilised most of the river length available to them and made seasonal movements to spawning and over wintering habitats. Similarly, in 1993 Dr Martyn Lucas, of Durham University, radio tracked 31 adult barbel (2-6lb) over 15 months in a 7.2 km reach of the River Nidd, a tributary of the Yorkshire Ouse, with open access to the Ouse. Again these fish were highly mobile, ranging over sections of river from 2-20 km in length, and their movements were associated with seasonal shifts in habitat, upstream spawning migrations and downstream migrations to over wintering habitats in the lower reaches and main River Ouse. Fish will basically move as far as they are able, to fully exploit the best available habitat / resources: the more limited those resources, the further they will go.

Other studies on less fragmented rivers with more limited essential habitats have shown that fish have the potential to move over very long distances, where they are able to, due to absence of weirs that would otherwise restrict their movements. During my PhD in 2006, I tracked the movements of 80+ adult common bream (4-7lb) over four years in a long 40 km reach of the Lower River Witham, a very uniform fenland river in Lincolnshire. My bream were tightly shoaled and relatively immobile in a deep tributary off the main river at the upstream extent of the reach during the winter, moving short distances of, on average, about <5 km a month, up and downstream. In the spring, they became highly mobile moving on average 30 km a month utilising the entire length of the river available to them, with one individual moving over 120 km in a single month! At this time they were visiting shallow tributaries off the main river, before using these for spawning in late May / early June. Once they had spawned, they spread out and spent the rest of the summer in the main river foraging, again moving on average 20-30 km a month up and downstream. In the autumn they moved back upstream to the deep tributary for the winter. This same yearly pattern was observed throughout the study.

These studies demonstrate that adult fishes use different habitats at different times of the year and require free passage between them. Habitat requirements are different for adult and juvenile fishes, so during its life a fish will have many different habitat requirements. These requirements will be more crucial for juvenile fishes, due to their vulnerability to predators and therefore their need to find safe cover. If any one habitat type is lacking, limiting or inaccessible, there will be consequences for individual survival and therefore the population as a whole. Weirs often restrict populations to those reaches that have sufficient connectivity to enable life-cycle completion, with the availability of the most limiting habitat type being the controlling factor or bottleneck for the population.

Flood resilience

During my PhD I re-captured three of my bream that I’d been tracking for over a year and moved them 35 km downstream. They returned to their original point of capture within, on average, two weeks. Other fish species have also been shown to have this homing instinct and are therefore very aware of their surroundings. Floods often displace fish downstream, where there are no barriers to their movements, fish will follow their homing instinct to return to their previous area of residence once the flood has subsided. However, if the flood pushes them over an impassable weir they cannot return and as such fish can work their way further and further downstream.

One concern anglers and fishing clubs may sometimes have about weir removal is that club waters often only stretch for a few hundred yards, and anglers will want to keep ‘their’ fish in ‘their’ water. Weir removal presents a risk that fish may move to another section of river, free range fish! However, addressing degraded and fragmented habitats will improve the environment and lead to a healthier ecosystem with more fish in the longer term for all.

Drought resilience

A common misconception raised by anglers and members of the public, is that weirs delay river discharge and therefore make the river more resilient to drought. Weirs do hold back a quantity of water in the upstream impoundment, however on the river catchment scale this is generally a very insignificant amount, unless the weir is tens of metres high (i.e. a dam to create a reservoir). This impounded water does not slow the rate the river discharges at the catchment scale, they just store water in the upstream impoundment at the local scale and once the impoundment is full, the river flows over the weir at the same rate it enters the impoundment. If we use filling the kitchen kettle as an analogy, the kitchen tap runs and the kettle fills up. The tap is the river and the kettle the impoundment, once full the kettle overflows at exactly the same rate as the tap runs and the water bill ticks up exactly the same.

In the event of drought, rivers tend to dry from their upstream end first, impoundments upstream of weirs can and do provide refuge areas for fish in such an event, however these areas are likely to be heavily silted (as impoundments accumulate silt due to the lack of flow) and will quickly deoxygenate due to biological processes in the accumulated silt, given no freshening flow from upstream, leading to fish deaths. Fish will move downstream naturally in response to a drying river, using the rivers flow as a means of navigation (this is known as rheotaxis; negative rheotaxis for moving downstream with the flow and positive rheotaxis moving upstream against the flow). However if the fish encounter an impoundment upstream of a weir on this journey (a refuge migration), and there is no flow going over the weir, due to the drought, there will be no flow cues to navigate by. In this situation the fish will be unable to move any further as they simply will not be able to locate the exit. If no weirs exist, in the same drought situation, fish will move downstream just the same, seeking out deeper fresher water for refuge in the lower reaches of the river and once the drought has broken, be able to move back upstream to their previous haunts. When I worked in Dorset, there was a winterbourne stream (a stream that only flows when the aquifer is full, usually in the winter months), a tributary of the Dorset Stour, which had a number of weirs along its length. Every winter the stream would flow well and fish (usually trout and grayling) would enter to spawn. However, once summer arrived the springs that fed the river would stop and the river would dry. However, the weirs created impoundments that would trap fish and they would require rescue by the authorities and local residents to move them back to the main river and unfortunately mortalities would usually occur, even with such interventions. So in fact, perhaps counterintuitively, weirs actually reduce a rivers resilience to drought and can cause fish kills if fish become trapped in upstream impoundments instead of being able to move naturally in response to their environment.

Conclusion

The impacts caused by weirs are problems for coarse fish as well as salmon and trout: the principles may not be as well understood or as popular, but they are real. We need to address this and other factors that are limiting fish populations if our rivers are to fulfil their ecological potential.

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Photo 3. Juvenile barbel have similar habitat requirements to salmon and trout parr: riffles provide cover from weed growth, an abundant food supply of nymphs dislodged by the fast current and overhead cover, due to the heavily rippled surface, that hides them from predators.

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Photo 4. Male bream guarding marginal spawning sites in a shallow tributary off the main River Witham, Lincolnshire, in late May. Bream utilise oxbow lakes as spawning sites in natural ‘unmodified’ rivers: these shallow fen drain tributaries provide a surrogate habitat that allows them to reproduce. If these were unavailable or inaccessible due to impassable weirs, the bream population would suffer.

Award Winning Restoration on the Wandle

Our rehabilitation work on the River Wandle’s Carshalton Arm has won the Urban Category of the 2016 UK River Prize.

By opening up fish passage, enhancing river habitat, addressing urban diffuse pollution and reintroducing brown trout, we have attained ‘Good Ecological Potential’ for the Carshalton Arm and re-established trout for the first time in over 80 years!

We attended the Awards Ceremony in Blackpool this week at the River Restoration Centre‘s Annual Conference to collect our award and showcase our project to the wider river community.

We wouldn’t have been able to achieve this without all the people and organisations who helped us along way. To express our gratitude, we created this short film about the journey this project has taken us on.

Rustling up some Riffles

Our Project Officer Rosie has been out on the River Teise undertaking her first weir removal.

Why was this weir an issue?

Harpers Weir formed an impassable barrier to fish passage on the Teise, a tributary of the River Medway in Kent.

Harpers Weir

Weirs such as this impede the movement of fish upstream and downstream, preventing access to other habitats required for a healthy lifecycle.

They also impound the river upstream, slowing the flow of water and resulting in the deposition of silt on the channel bed. In the case of Harpers Weir, this impoundment was observed for several hundred metres upstream.

Impoundment

For all these reasons, we’ve been very keen to remove the weir and restore fish passage to this section.

In 2015, Rosie started the process by removing the wooden boards on top of the weir, reducing the height to see how the river would respond. You can read this blog here, but in summary it was looking good!

Then, in February this year, Rosie and our contractors Amenity Water Management arrived on site to start the full weir removal.

The concrete weir and flanks were broken up using a hydraulic breaker attachment, before being removed with an excavator. The concrete underneath the footbridge immediately upstream was also removed to let the river bed naturally re-grade back upstream (the footings would otherwise create a new barrier after weir removal).

Diggers on Site

Once the weir and bridge footings were removed, the banks where the weir once stood were regraded, using the excavator with a bucket attachment.  The toe of the banks was stabilised using coir geotextile to line the banks secured with wooden pegs, which in turn were held in place with faggots and pinned with untreated chestnut posts secured with high tensile fencing wire.

After Weir Removal

Once this weir was removed, the upstream impoundment disappeared, and a total of 16 new riffles emerged, with pools between them – all great habitat for many species of bugs, fish and water birds!

You can watch the whole project in our time-lapse footage below!

This project was funded through Defra’s Catchment Partnership Action Fund awarded to the Medway Catchment Partnership, focusing on the River Teise within the Medway Management Catchment.

Fish Passage on the Cuckmere

We have been working in partnership with the Cuckmere and Pevensey Levels Partnership to deliver a fish passage project at Sessingham Weir on the Cuckmere.

The weir currently causes an impassable barrier to fish, preventing them from accessing all the habitat required for a healthy lifecycle. This in turn can make fish less resilient to other pressures such as pollution and climate change.

Many fish species thought of as being non-migratory will in fact, given the opportunity, make substantial movements within a river system, particularly to find good spawning habitat. In unobstructed rivers, species such as roach may move 10’s of kilometers. By making the weir passable to fish, we will reconnect approximately 6.2 km of valuable habitat upstream and downstream.

Unfortunately due to the presence of a water pipe running underneath the weir it cannot be fully removed. In September we permanently lowered the weir blade and have already seen some fantastic habitat improvements such as increased flow diversity.

Before and After at Sessingham Weir

We are now planning to remove the redundant weir gate and are designing a solution to make the weir passable to fish.

Action on the River Teise!

We’ve started our project on the River Teise to make a weir passable for fish migration.

Weirs were introduced years ago to help control the flow of water, allowing our ancestors to operate mills. Nowadays many remain in rivers despite no longer being required.

Weirs are a barrier to fish passage and leave the habitat fragmented with fish populations isolated. In the event of pollution or other presures like climate change, these isolated populations are at greater risk with nowhere to escape to.

Although being a barrier to fish migration is a significant problem, weirs also interrupt the natural flow of rivers, resulting in a degradation of habitat. Upstream of weirs, water is slowed down which causes silt to drop out and accumulate in the channel.

This image from the Wild Trout Trust explains the effects of weirs on fish habitat:

WTT Weir

Funded through Defra’s Catchment Partnership Action Fund, we’ve started works on a weir on the River Teise – where Olly and Rosie carefully removed a number of boards from the top of the weir.

Weir Board Removal

After removing the boards, the impounded water levels upstream dropped noticeably, revealing many natural features of the river which had been drowned out when the weir was put in place.

Before

Before

AfterAfter

New Habitats

Some of the diverse new habitat revealed by lower water levels above the weir

The next phase of the project is to remove the impoundment and enable full fish passage and habitat restoration, so watch this space!

Delivery on the Dour

Last month, we ventured all the way to Dover to start delivery of our River Dour Restoration Project.

The Dour is a short river, roughly 4 km in length, which runs into the sea at Dover Harbour.

We were addressing fish passage at a weir near Kearsney Abbey at Minnis Lane. Here the Dour runs through a ruined building, causing blockages to fish passage with the presence of several small weirs. The river itself has started to create its own bypass channel and our aim was to enhance this channel to allow fish passage upstream of the site.

Minnis Lane Site

Installing Brash BundlesTo do this, we installed brash deflectors to concentrate the flow of water to a narrower section, ensuring there was enough flow for fish to swim up without becoming stranded.

We were joined by 6 local volunteers on the day to help us install 12 brash deflectors in the stream.

Each deflector was secured in place with hazel posts and wire to ensure they stayed in place in higher flows.

By placing the deflectors so that they’re pointing upstream, we directed the flow of water into the middle of the channel, creating a deeper section for fish and eels to swim up.

We will be back to finish the site off soon and see how our deflectors are working.

Many thanks to our volunteers who helped on the day: Bethany, Katharine, Lilian, Ray, Simon and Tom. And a big thank you to the local Scout Club for letting us park our van in their carpark.

So before….

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After!

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After..

After 2

The Hogsmill Fish Passage Mission Continues

The quest for complete fish passage along the Hogsmill takes another step forward. This time we have been addressing the A240 road bridge footing. The footing is wide resulting in a shallow flow of water, further exacerbated by it coming to a high point midway under the bridge. To finish the problem off, it comes to an abrupt end in the form of a vertical weir. All of these factors result in an obstruction to fish passage.

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The structure before work starts

So to the process… we surveyed the site, undertook the required calculations, came up with a design solution, applied for the relevant consents and permissions, spec’d the build, bought the materials, hired in the plant and brought in assistance from Aquamaintain with Roo and Jack.

The solution, a rock ramp similar to the one that we recently built at Kingston University but this time on a grander scale. The aim being to increase water depth over the bridge footing and then stepping the head down by producing a passable cascade with a slope of below 1:20. Six lines of boulder bars (bed check weirs) stabilise the structure with each boulder weighing between 400-900kg. These were positioned in upstream self supporting curves and configured to create a series of notches that provide varying passage options under different flow conditions.

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The first delivery of boulders arrive and are unloaded

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and some of the rock fill material

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The second line being placed into position

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Jack lining the void between boulder bars with geotextile before the rock fill goes in

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Setting the levels as another line is positioned

In total, 150 tonne of rock was used. Once each of the boulder lines were installed, the void space between them was filled with graded rock. This fill locks the structure together and armours it against erosion.

There is now a water depth of approximately 250-300mm over the footing. Water depths over the ramp are in excess of 250mm  in the form of passable streaming flows. In addition to providing fish passage for all species, the rock ramp creates interesting habitat that will rapidly be populated by an array of invertebrates.

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Before the works…

1 after

and after.

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Again, looking over the bridge, before…

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and after. This time without a weir.

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Various sizes and depths of notches set into each line

Once again, we had the great help from the guys from Aquamaintain to get the job done. Roo at the controls of the excavator, grappling with the grapple and Jack continually feeding us with rock . We also had the continued support from Epsom and Ewell Borough Council, of note from Stewart Cocker and Christopher Stone. A BIG thanks to you all.

 

 

What have we been up to?

We have had a busy start to 2015 – maybe it is time for you to catch up with what we have been up to?

River Restoration ~ Luke has been busy transforming the Ravensbury Park Back Channel on the River Wandle for both the local community and wildlife. Read all about his progress here.

Pollution Control ~ Olly has been working hard trialling new methods to mitigate against urban diffuse pollution. We have trialled Siltex in Carshalton Ponds and installed Mycofilters at problematic outfalls.

Looking Forward ~ We’ve got restoration projects this year on the Hogsmill and Beverley Brook so keep your eyes peeled for more updates!

First Class Degree in Fish Passage at Kingston Uni

When working in channel you really are dependent on dry weather conditions. This is especially so on the Hogsmill which acts like a spate river, with large flashy flows shortly after rain events.  I have been incredibly fortunate with the weather for a long run of things. Unfortunately, things had to change and change they did. I return from site after a wet and challenging month.

The Hogsmill splits just upstream of the Knights Park Campus at Kingston. There is a large head weir on the main channel and on the side channel, two lower head weirs at either end of a sloping 24m concrete spillway. Both channels were completely impassable to fish, with the exception of eels on the main weir thanks to the eel pass installed by the Zoological Society London (ZSL) and monitored by the University.

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The upstream weir flowing onto the sloping concrete spillway

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The downstream weir with shallow plunging flows

Addressing passage on the main weir would be both feasibly and financially unviable. Instead attention was focused on the side channel. The water flowed over both the side channel weirs in shallow, plunging flows and over the spillway it was fast and shallow. Kingston University kindly agreed to the Trust carrying out the work as continued partnership working following on from the habitat works carried out earlier on the year at the Campus.

The chosen solution was two-fold. A rock ramp would be built at the downstream weir to step the river down in the form of a passable channel. On the concrete spillway four lines of rock would be fixed to the bed. Varying sized gaps (notches) are left in each line so that water is retained in the channel even under low flow conditions and a variety of pass conditions are provided. These rock lines serve to back the water up, increasing depth and slowing velocities. This backing up then effectively ‘drowns out’ the top weir which would also be ‘notched’ to reduce the height.

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Notching the upper weir

Roo Newby from Aquamaintain was brought in to give a hand, whilst gracing me with his northern charm and banter. The first job was to create a dry working area. A cofferdam was set up at the top of the channel to direct all of the flow down the main channel.

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The cofferdam in place, in theory providing a dry working area

The next job, which took the best part of three weeks, was to place all of the 20t of stone into position in order to create the desired flow conditions. With incredibly limited machinery access to the channel, the chore of moving it all fell to Roo and I. We grunted and moaned our way through it, placing, moving and repositioning each stone several times before finally fixing them. By the end of the job we have grown muscles on our ears.

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The boulders placed into position on the spillway

Key stones on the spillway were fixed into position by drilling through them and into the bed. The hole in the bed was then cleaned and a section of steel rebar fixed with resin. The stone was then lifted and fitted snugly back onto the bar.

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Using resin to fix the rebar into the holes

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The line of rebar to secure the upper line of boulders of the rock ramp

The key stones and all other non-load bearing stones, were concreted into position. The key stones in the rock ramp were secured using multiple techniques. At the downstream end of the ramp, chestnut stakes were driven into the bed to stabilise the lower line. The second line relies on mass and by being keyed in by the rip rap rock on either side. The third line was concreted onto the existing weir apron. The upper line of stone sits on the weir sill and was secured by the resin and rebar technique.

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Roo ‘mucking in’ the third line of boulders in the rock ramp

The job was hampered by mizzly conditions to start followed by heavy rain. Roo and I then both succumbed to river lurgy, no doubt caused by the sewage in the river as a consequence of combined sewage outfalls flowing after the rain, in combination with the massive misconnected drains issue present throughout the catchment. The heavens continued to open with annoyingly frequent regularity resulting in several days having to be abandoned due to a flooded working area. The rain built up to a crescendo with apocalyptic volumes falling last Sunday, giving the work a true baptism of fire with the cement barely having time to dry. But it stood up to the battering.

With the exception of the upstream weir, the rest of the work would now be passable for elvers (young eel) and eels. To assist them past the upper weir, a ramp of concreted in stones was installed to provide climbing media for them to get up and over. They can now continue onto the next stretch of river. During the work, four elvers were found in the channel. This pleasingly and clearly demonstrates that the past work of installing eel passes on the weirs downstream is working.

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Eel passage through the rock ramp…

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and over the upper weir

Both weirs have been drowned out with average depths of 300mm through the rock ramp and up the length of the spillway. A diversity of flows throughout the work provides passable notches and importantly vital resting pools. Perturbation boulders break up the most intense flows, adding complexity as well as potential habitat.

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The lower weir before the work

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The rock ramp in place

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The rock ramp working under moderate flows

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Looking up the spillway at the upper weir

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and after where the upper weir has been drowned out (note the complexity of flows and slack resting pools

Before the work was even completed, we saw two fish using the rock ramp, giving real confidence that the design was working and demonstrating their real desire to push their boundaries. With the University stretch being the most prolific for fish in the whole river, the door has just been opened for them to begin to repopulate upstream.

Thanks to Kingston University, specifically Sivi Sivanesan for allowing and assisting us with this project. A BIG thanks for all the hard, back breaking work from Roo from Aquamaintain and to Jack for his days on site.

Of rock-ramps and fish

And the fish easement work on the Hogsmill continues. This time our work is focused on the river around the Thames Water Sewage Treatment works situated in Berrylands, Surbiton, specifically the stretch from the railway bridge down towards Kingston Cemetery.

The over-widened channel here reduces water depth which, along with three small weirs and a lot of concrete-lined channel in between, effectively prevents fish from moving upstream.

So these weirs will have to go. Or will they? Toby’s work connecting reaches of the Hogsmill to free fish movement has shown an alternative – slow the flow and over they go! Building rock ramps and pool passes drown out these structures enabling fish to navigate their way over them.

Illustration of a typical rock-ramp (Source: Thorncraft and Harris, 2000)

Illustration of a typical rock-ramp (Source: Thorncraft and Harris, 2000)

After a few essentials from the Environment Agency, Thames Water and Network Rail we ought to have this project going in the next couple of months. More to follow shortly…

Reference

Thorncraft G. and Harris, J.H. 2000. Fish passage and fishways in New South Wales: A status report, Technical Report 1/2000 prepared by NSW fisheries for Cooperative Research Centre for Freshwater Ecology.

A little light goes a long way..

Our work on the River Wandle continues with a habitat and fish passage project on the Ravensbury Park by-pass channel. This work will address habitat deficiencies, stream form and fish passage around the tilting weir which is a significant barrier to upstream fish movement. To read more about this project click here.

Toe-boarded and impounded nature of the back channel

Toe-boarded and impounded nature of the back channel

Concrete lined banks as well as over shaded character that limits aquatic vegetation  growth

Concrete lined banks as well as the over shaded character that limits aquatic vegetation growth

Obstruction at the A3 (but in the river, not on the road)

With barely enough time to draw breath, attention jumped from the headwaters in Ewell to the middle reaches near Tolworth, where the Hogsmill flows under the A3. The water through the culvert under the road is shallow. At the downstream end there is a 36 cm fall over a 30 metre long spillway, 34 cm of which is within the first 14 m. To top it off the concrete then comes to an abrupt end in the form of a 25 cm high weir. The cumulative effect of all of these factors result in, not only a completely impassable structure but also a concrete eye-sore.

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Looking downstream along the spillway.

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The weir at the downstream end.

With the budget unsurprisingly insufficient to stretch to the grand civil engineering feet of closing the A3, tearing up the road, breaking out all of the concrete, putting in a new bridge and then reinstating the road, we had to look for an alternative solution. A more cost effective and less disruptive easement was required. The chosen solution is two-fold. Firstly, the weir would be ‘drowned out’ by raising the water level downstream to back up over the spillway. Secondly, where the water depth tapers off up the spillway, a series of baffles will be secured to the concrete river bed to slow the flow and increase depth.

In order to drown out the weir, a ‘close-to-nature’ pool pass was designed in-house. In essence, the water level would be raised and then incrementally lowered by a series boulder bars. In each line a notch or gap is left in order to focus the flow and allow a place for fish to pass. Although this may sound a simple idea, plenty of calculations were required and carried out with the much appreciated help of colleague Tim Longstaff.

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Site set-up.

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Material being delivered.

This was not only a technical job but also one undertaken on a big scale.  In total 63 boulders of Purbeck stone supplied by Lovell Purbeck, each weighing between half and one tonne would need to be positioned into upstream facing, self supporting curved lines with their heights set to very fine tolerances. A 22 tonne excavator with a 15.5m reach complete with a grab were hired in complete with the very skilled hands of Barry Richards from Land & Water at the controls.

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The goliath.

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Barry at the helm.

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The first of 3 loads of Purbeck stone being unloaded.

In addition to the 45t of boulders that went into the construction of the boulder bars, a further 4t was used as 12 perturbation boulders and 25t of smaller Purbeck boulders were used as armouring.  A further 160t of Horsham stone was used to sit the upper boulder bar on, act as foundations and to help ‘waterproof’ the bars. The inclusion of all of this material encourages self-cleansing pools by increasing the flow thus reducing siltation and contaminant accumulation.

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The upper boulder bar complete.

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Steady progress.

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The boulders being placed into position with the long reach machine.

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The long reach living up to its name. The notch clearly visible in the upstream bar.

After the initial two days setting up the site and doing the required tree works, the build was completed in six days in which time we created a large scale water feature which now provides much needed fish passage.

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Before the work…

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and after from the same position.

The stretch has been completely transformed. Five large boulder bars now transect the channel; between each, deep pools provide vital resting areas for passing fish, connected by passable streaming flows. Each pool has a several large perturbation boulders to dissipate the energy of the water by breaking up the flow. These also serve as cover, habitat and to prevent the pools from being easily fished with nets. The weir has now disappeared, with 30cm of water now over the bottom 14m of the spillway and increased depth extending a total of 20m. Shortly the baffles will be fixed to the upper section of spillway to complete the solution.

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Complete, looking downstream…

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and upstream.

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Resting pools with boulders for refuge.

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Looking upstream at the alternating notches …

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with passable flows between each.

Formerly this stretch of river, although possessing a deep pool popular with fish, was dark and the bed littered with building debris, litter and rusting metal. There are now three good sized pools and two smaller ones. Light can now reach the river so vegetation will soon establish. The kingfisher, although initially a little disgruntled by our presence, is now successfully fishing for dace and roach that have already taken up residence in the pools and wagtails are now regularly seen running along each of the boulder bars.

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The weir, now you see it…

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and now you don’t (taken from the same position).

This project and the recent work at Green Lanes have both been the largest carried out in-house by the Trust and we are pleased to say both have gone very much to plan with great outcomes.

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We had a bit of time left over so we chiseled a Michelangelo’s David’esque sculpture out of one of the boulders. Oh no, its Tim.

A big thanks has to go to Tim, our hired in help from Aquamaintain for putting in some long hours, hard graft and welcome suggestions. Thanks to Barry, the digger driver, for making the large machine operate as if it were one of his own limbs, great skills! Thanks also to Rob Waite at the Royal Borough of Kingston for allowing us access to the site and for making the process so swift and effortless, Darryl-Clifton Day and the Environment Agency Fisheries Team who kindly contributed the recycled plastic baffles that will shortly be installed and Malcom Newson who gave advice and assistance with some calculations. Thanks to the EA for carrying out the utility search and thanks to all the others not mentioned who supplied us with materials or their time to enable to project to go to plan.

Fish passage and habitat works in the headwaters of the Hogsmill River

I am now back in the office and it’s time to update on the past six weeks which has been a busy time on the Hogsmill delivering a couple of projects in-house.  I will go back to the 11th of August when we started work at the confluence of the Hogsmill River and Green Lanes Stream. This is right up in the headwaters in Ewell within a kilometre of where the water flows out of the chalk to form the river.

In 1960, during the period when  concrete was considered the must-have look for most rivers, the entire confluence was dredged excessively deep, widened and entombed in a concrete cast. The best habitat within the Hogsmill River has since been cut off to fish by a sloping 35 metre length of uniformly wide concrete channel in conjunction with a small weir. Not wanting to do things by half, the banks were also cloaked in one metre high concrete and stone walls. The habitat was consequently considerably lacking, although nature has attempted to take a foothold with some sedge hanging precariously to cracks and crevices in the failing walls or with watercress clinging to silt deposits.

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The channel before work.

The aim of this project would address both these issues. Fish passage would be returned to the upper Hogsmill with the removal of the concrete bed whilst a 150 m cumulative length of river bank would be naturalised. With the scale of the work beyond that of our normal trusted volunteers, we brought in the great assistance of Aquamaintain Ltd. The work took three and a half weeks on site in which time 200 tonnes of concrete were removed from site in addition to a further 40 tonnes of soil.

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Concrete…

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concrete…

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followed by more concrete, 200 tonnes in total.

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You have to break a few eggs to make an omelette!

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The new channel being formed. Note the previously high banks on the left that have now been regraded as seen on the right.

Most of the large pieces of sandstone contained within the walls were recycled on site in various guises. Some were used to fill the gabion baskets forming the headwall for a surface water drain perched high on the bank. Other bits were used to make rock rolls that would stabilise the toe of exposed banks most susceptible to erosion. First though, the stone had to be broken up ‘convict style’ with a sledge hammer, a painstaking chore as Jack will vouch for. Finally, other pieces were placed in the completed channel to provide habitat diversity and create a variety of flow types.

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Jack at work breaking the stone up to fill the rock rolls

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The gabion headwall going in before the banks get regraded to match the front slope.

The banks have been naturalised with a combination of hazel faggots, site won brash, rock rolls and coir geotextile for temporary soil protection. A sinuous meandering channel form has now replaced the former straight and wide channel. Variations in depth, width and the inclusion of gravel, small boulders and multiple pieces of Large Woody Debris and root wads have all provided a diversity of habitat and complex flows within the channel. The low lying wetlands have been seeded with a mix of native species in addition to several hundred plants being introduced as a bit of kick start. Give it a year or two for the vegetation to establish and this stretch will be looking like the chalk stream that it should be.

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On completion. A new meandering and naturalised channel.

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The impounded stretch is returned to fast flowing water.

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Clean imported gravels, deeper holes, large wooded debris, root wads and planted marginal area.

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Looking upstream.

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The river already naturalising within a couple of weeks as watercress takes up a new residence.

The South East Rivers Trust has worked up this project with the continued support of Epsom and Ewell Borough Council who kindly contributed both financially and with their time, in particular that of Stewart Cocker and Christopher Stone.

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Finally, the before shot…

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and after.

This work is a continuation of the Catchment Restoration Fund Project funded by Defra, addressing fish passage along the length of the river. In combination with our two weir works last summer, also located in the Hogsmill Local Nature Reserve, this means that there is now a 1.5 km unobstructed stretch of river.

Without delay, work finished at the Green Lane on Wednesday and kicked off at Elmbridge Meadows on Thursday. Read on for what we are doing there.

A big thanks goes to the project partners, Epsom and Ewell Borough Council, especially to Stewart Cocker and Christopher Stone. Thanks to the EA for part funding the habitat element of the work and for undertaking the utility search. Thanks to Surrey County Council for permission to alter their asset. Finally, a BIG thanks to Ben, Jack and Tim from Aquamaintain for all the help, hard graft and good company.