Tag Archives: Eels

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.

 

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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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.

Continued effort at making the Hogsmill a little more welcoming for eels

A joint effort between the Zoological Society of London (ZSL), the Thames Anglers Conservancy (TAC) and South East Rivers Trust continued the ongoing efforts to assist elvers/eels with the final stages of their long 3500 mile migration from the Sargasso Sea up into the Hogsmill catchment. In August last year, the same partners installed an eel pass onto the most downstream obstruction in the catchment at Clattern Bridge, Kingston, which showed immediate signs of success with the first elvers ever being recorded further upstream at the ZSL eel monitoring site the following week.

Upstream of Clattern Bridge the next obstacle, the Environment Agency’s gauging weir, has already an eel pass installed for several years and so this time we turned out attention to the next weir upstream. Although a few incredibly determined eels have successfully made it past this obstacle, the weir  does without doubt prove to be the end of the line for many. With its vertical face, smooth sloping concrete surface and fast flows, the elvers generally have little chance of clinging on and and making it to the calm of the river bed upstream.

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The sloping weir with vertical face

In order to provide purchase and slower the flows, yesterday eel tiles were installed to the weir, guiding them in at the downstream end, helping them up and over and feeding them safely into the slack water at the bed upstream. A custom built debris deflector  and bracket to mount the tiles to the wall adjacent to the vertical face were manufactured by Norm Fairey, our regular go-to handy man.

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Eel tiles fixed to the concrete slope

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Vertical tiles to help the eels over the weir crest

Once completed we wandered down to have a look at the eel pass on the gauging station. It appears that the brush material makes a perfect media for plants to establish in, especially our arch nemesis Himalayan balsam. Concerned that the root network would be clogging the pass we set about unbolting it from the wall.

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Is it an eel pass or a hanging basket?

What we found was a densely matted root system which would have prevented even the smallest elver from getting through. Each section of the pass was removed, cleared and reattached ready for the first visitor to utilise. On the whole a very satisfying day.

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No way through!

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All clear and ready for the first migratory visitor

It will be interesting to see the effects that these actions will have on this years monitoring undertaken by ZSL and the keen volunteers. We will keep you posted with any news. This project was funded by Defra through the Catchment Restoration Fund.

A big thanks to David Harvey and Neil Depledge from TAC and Joe Pecorelli from ZSL. 

To see updates from the Wandle, please check out the website

Hogsmill eels get a helping hand

Clattern Bridge eel tiles

Working in close partnership with the Zoological Society of London and Thames Anglers’ Conservancy, we’ve recently been delivering a small but important project to improve eel passage on the little Hogsmill River through Kingston.

Like the Wandle and the Mole, the Hogsmill should be an important stronghold for eels, which are now considered critically endangered. But recent monitoring efforts by volunteers from Kingston University have found no elvers in the river at all, leading to suspicions that they find the concrete sill under Kingston’s historic Clattern Bridge an impassable barrier.

By installing specially-designed eel tiles, we hope we’ve now solved this problem, and that eels will now be found swimming up the Hogsmill.

Click here to read Tim’s full report, with more photos.