The Trust has been busy undertaking work on the Hogsmill River recently.
To see what has been happening please click here to read various blogs.
The Trust has been busy undertaking work on the Hogsmill River recently.
To see what has been happening please click here to read various blogs.
The South East Rivers Trust and Kingston University are undertaking extensive habitat improvement works on the Hogsmill River in Kingston. The plans are BIG so we need lots of people to get involved.
To find out more, please click here.
That’s another two obstructions ticked off the Hogsmill list with fish passage successfully being restored to this stretch of the river. Since the last blog, when the downstream weir was in the process of being removed, things have moved on dramatically. Before I explain what has been done, first a quick good news story. At the top of the stretch a surface water outfall discharges into the river next to the bridge footing. Every day, large quantities of fat and food matter were entering into the river from this due to a clearly misconnected pipe up the system. You can see the severity of this from the photo, although without scratch and sniff you will have to take our word on the pungent aroma. I reported this to our local Environment Agency Pollution Officer and in double quick time they had been out to site and identified the source as being the kitchen sinks in the pub over the road. The pub was unaware of the misconnection and has agreed to rectify the issue by connecting the sinks into the foul sewer and not the grey water. Misconnections are a big problem and cause poor water quality in urban rivers. Click here for help to identify if your house is misconnected.
Back to the main works, with the weir gone, the impounded water drained away revealing the river bed that lay beneath the deep, dark and sluggish water for so many decades. What became immediately clear was the impact that excessive historical dredging has had on the river. The true scale of the river at this point was made apparent. The facade of being a large, deep river was lifted and the reality of the stream which the Hogsmill really is at this point in its journey unveiled. A river, no more than a metre wide snaked its way in the bottom of the 10 metre wide channel. Admittedly the chalk fed source of the river 3.5km upstream will be running low at this time of year but the river looked markedly out of place in the over-sized channel.
Nine rows of chestnut posts were driven in, crossing the channel in shallow curves at regular(ish) intervals down the 90m stretch of river. In total 700 posts were used as I am sure the guys on site will testify to. The introduction of gravel then commenced. Large flints were hand placed behind the posts to prevent washout and maintain the head across each pool. As the deliveries of gravel came in, quantities of these large flints were irregular and lacking. A decision was made to substitute these for more uniform, larger gabion stones which would assure the longevity of the structures. This was also banked up at the outside edge of each line of posts to prevent the river from eroding the banks.
The gravel orders were repeatedly made and delivery after delivery entered the yard. These were loaded onto the tracked dumper and moved to where they were needed in the river before being placed with the excavator. Trip after trip, tonne after tonne, the gravels were moved and introduced. The dredged channel appeared to eat the gravel up without as much as a burp. It rapidly became clear that our 200 tonne allocation was looking decidedly insufficient. Without more gravels the river would be passable, however the habitat would be lacking with wide, deep, sluggish pools similar to that of before. The budget was checked and with a groan it relinquished more funds. By the close of play, 360 tonnes of stone have been introduced and the transformation now complete, although further narrowing and planting would help this stretch further but with budgets as they are this will have to wait.
The upper line of posts has drowned out the bridge footing, providing a depth of approximately 300mm for fish to pass. From here, each ramp reduces the one metre head over the length of the site in approximately 100-120mm increments. The water now flows through the low flow channel in the form of a beautiful, streaming, passable flow.
With the works completed last Friday and with no time to settle in the site was brutally tested by the apocalyptic weather that we experienced at the weekend. With bated breath I pulled up on-site this morning and was pleased to see that it had passed, everything was where it should have been. The river has started to react positively to the works. Deep pools have been scoured out, kicking the gravels up to form shoals and riffles. The varied positioning of the low flow notches in each line of posts are creating a sinuous, meandering channel. Already a mosaic of habitats are being created which will provide the required niches for various inhabitants of the river to occupy. The rubbish strewn, brick filled bed has now been replaced with nice clean gravels. Fish can now freely migrate up and downstream as they choose. A few tweaks will be required to the levels as the river adjusts and settles into its new form but essentially the job is done.
A big thank you to Chris at Woodland Environmental for kindly allowing us to access the river through their site, without this we really would have struggled. Thank you also to Cain Bio-Engineering http://www.cainbioengineering.co.uk/ for helping with the design solution. And a huge thank you to the guys from Aquamaintain (http://aquamaintain.com/), Ben Kavanagh, John Mudd akaNorthern, Nick Gibbo Gibbs’on’ and Keith. They stoically took on every one of the 700 posts, laughed in the face of each of the 370 tonnes of stone and worst of all had to put up with me for a week on site (it beats being in the office). They took pride in their work and have carried out a fantastic job with great effect. If you listen carefully enough you can hear the faint sound of fins clapping in gratitude.
The Trust is continuing with our mission to improve connectivity for fish passage along the length of the Hogsmill River which flows from Ewell to Kingston.
As a brief recap to the project, the Wandle Trust has received funding from Defra, through the Catchment Restoration Fund, to improve connectivity along the Hogsmill River. This is because fish passage has been identified as being a major contributing factor to the river’s current failing status under the European Union’s Water Framework Directive (WFD). At the start of the project there were 15 obstructions along the 6 mile length of the Hogsmill, resulting in the available habitat, of which much is heavily degraded, being highly fragmented and therefore limiting. This causes bottlenecks at varying life stages, ultimately leading to the survival of the fish species present being compromised and extremely vulnerable.
Following on from the recently completed project in the Hogsmill Local Nature Reserve in the headwaters of the river, where two weirs were removed and the channel renaturalised, we have now moved our focus for the time being to the middle section of the river where two obstructions lie near to the Toby Carvery pub off the Old Malden Lane/Worcester Park Road.
The problem here is twofold. Firstly, as the river flows under a bridge, the concrete footing which is 18m long and 9m wide causes the water to flow incredibly shallow. At the downstream end of this there is a lip with a head drop of approximately 200mm. The combination of these factors forms an obstruction, especially during low to moderate flow conditions.
The second issue is that 60m further downstream there is a weir very similar to those that we recently removed. The weir is 7.5m wide with a concrete apron and sill with a head drop of 0.7m. This is a complete barrier at all times. The river through this stretch has historically been excessively dredged and as a result is over-wide, deep and with all of the gravels having been removed leaving a barren, lifeless clay bed. The project will also address these habitat issues.
The solution that we have come up with, with design advice from Cain Bio-Engineering http://www.cainbioengineering.co.uk/, is to once again completely remove the downstream weir, including the abutment walls. This alone would not rectify the problem. Firstly, the bridge footing would still be impassable but secondly the fall from the bridge footing to the downstream bed level is too large. If left to its own devices, erosion would cause the footing to become undercut and a series of large, impassable cascades being formed.
To remedy this, a series of 8-9 rock ramps will be installed to stabilise the bed and drop the river in 100-120mm increments. This will be achieved by driving in untreated chestnut posts to form a curve from one bank to the other. Narrow gaps will be left between each post and large gravel rejects will be placed upstream of these to essentially create a mini step. The posts will be driven deeper to create a discharge point which will be approximately one metre wide. The positioning of the discharge point will vary from one rock ramp to another to help create a sinuous channel through the reach. The positioning of which will be governed by the current topography which will become uncovered as the downstream weir is removed. The top ramp will be positioned to drown out the bridge footing by approximately 150-200mm to allow fish to pass.
Creating a low flow channel will help to build the river’s resilience to potential climate change. Once the rock ramps are in, gravels will be introduced to narrow the over-wide channel, also creating a low flow channel between each rock ramp. The gravels will additionally provide vital habitat for varying levels through the food chain from plankton to invertebrates to fish.
The Trust has brought in the help of Aquamaintain (http://aquamaintain.com/), a local contracting firm who specialise in all things fishy and watery. The company is run and site managed by Ben Kavanagh, with Northern John, Nick Gibson and Keith making up the team.
One week into the project, things are going very well. The preliminary tree clearance to gain access has been undertaken, a track way has been cut into the bund along the length of the site to transport materials back and forth, and as I left site this morning, the fun bit, the first half of the weir was being broken out. A big thanks also needs to go to Chris of Woodland Environmental who has kindly agreed to grant us access to get to the river through his land.
Further updates to follow shortly.
The work in the Hogsmill Local Nature Reserve has now been completed! The weirs have gone, the abutment walls are no longer and the roar of water plunging over nearly a vertical metre onto concrete has now been replaced with the relaxing, idyllic burbling sound as it flows over clean, imported gravels . The river now boasts naturalised, re-graded banks populated with an array of native riverine plants. All we now need are the fish to start pushing forth into uncharted grounds.
Mystery Weir, as it was affectionately known, is a memory of the past (photos taken from the same point).
And Cracked Weir…
The weirs are rapidly turning into memories of the past. Whilst out on the banks, many people have commented that it is hard to imagine that these stretches have ever been different from their current state. This I believe is a great testament to the great job that Land and Water (http://www.land-water.co.uk/) carried out, lead by Site Manager and all round good egg, Tom Cartmel and his crew of merry men (Tony, Chris, Lee, Kevin, Lorne, Dave, Pete & Buta).
Yesterday, the Wandle Trust and Epsom and Ewell Council’s Countryside Team Volunteers (Sue, Mike, Linda, Roger, Jim, Duncan, Clive and Epsom and Ewell Borough Council (EEBC) staff Stewart and Lindsay) had a very enjoyable day planting the banks with plug plants to include greater sedge, lesser sedge, pendulus sedge, meadowsweet, hemp agrimony and purple loosestrife. With permission in place with the Environment Agency, we also trans-located a variety of large, mature plants from the river nearby which had the ‘Changing Rooms’ effect of instantly finishing the job. With a bit of sun (now that the tree canopy has been opened up) and rain, it is hoped that the vegetation will grow like wildfire. This will not only help to soften and naturalise the sections whilst offering a diversity of habitats but the root structure will be integral to the longevity of the banks.
Volunteers hard at work….
“An army marches on its stomach”
Oh, and as a bonus, Tim and I bagged some handy materials for future projects…
Who would have thought that so much hard work and effort would go into such a project? It certainly has been a journey with various twists and turns, consents to gain, flood risk modelling to model, surveys to measure, plans to design, logistics to sort, bureaucracy to tick, historic ruins to uncover, relationships to build, weirs to remove and plants to plant but dare I say it, job done!
To all those who have helped along the way (in approximate chronological order), Tim, Bella and all others at the Trust, all those at the EA to include Paul Stewart, Peter Ehmann and Ruth Hanniffy, Matt Horritt, Cain Bio-Engineering, Chris Stone and John Adamson at EEBC, Rikki Hill at Surrey County Council for lending us Traffic Cones and in-turn to all the residents of Crosslands Road to whom we caused a great deal of inconvenience by enforcing parking restrictions, Steven Nelson at Epsom & Ewell Historical and Archaeological Society. A big thanks once again to Tom and the rest of the Land and Water crew, the Countryside Team Volunteers, Lindsay Coomber and a huge thank you to Stewart Cocker at EEBC who really has helped above and beyond to get us to where we are today. Thank you! (I really do apologise if I have missed anyone out).
No time to sit back and rest on my laurels, in a week and half we begin the whole process once again on two further weirs one mile downstream. The techniques will vary somewhat, so keep watching this space for updates.
We are now three weeks into the removal of two weirs on the Hogsmill as it flows through the Hogsmill Local Nature Reserve, Ewell. The weather could not have been better for this project. With barely a drop of rain over this period with the dry spell set to continue this week, ground and river conditions have allowed our contractors Land & Water to crack on at a great pace.
The downstream weir, or Mystery weir as it is affectionately known, is nearing completion with all concrete and stone work removed from site. The banks have been lowered, re-profiled and protected with two different techniques, one on each bank. The first, which has been installed on the more isolated bank, is a traditional brash bundle (faggot) approach created from material won by the tree works that have taken place (foreground in the photos below). Behind these is a line of coir matting to prevent wash-out before the banks have vegetated and are self supporting.
On the other bank, which is subject to more footfall due to the nearby public footpath, a more robust technique is being implemented. This involves a three layer method of a polysester Geogrid on the front face with a coir matting acting as the sandwich filling with a geotextile behind, again to prevent washout (far bank in photos).
The upstream weir is coming on leaps and bounds. With significantly less space in the vicinity, work for the guys on site has been a little more convoluted but none-the-less progress is looking great. The brash bundle approach on the far bank is nearing completion and then work on the near bank can commence.
The project has also had an interesting historical twist. Being aware that historic gunpowder mills may be in the area, we had contacted Epsom & Ewell Historical and Archaeological Society in advance. Sure enough, at the first weir the brick remains have been uncovered. Steven Nelson keenly came out to look into the findings equipped with trowel. He has dated them back to circa 1700-1750 and believes that the walls form what would have been the ‘incorporating’ house where the powder was ground before sending to other installations for further processing.
Then, on the upstream weir a further structure was uncovered. This time Steven believes that it is the remains of a Corning Mill building. The remains of this will be left on view as a reminder of the history of the river.
So work will continue this week which, all things going to plan, should see most of the work completed, including the introduction of the gravels which will form riffles and spawning habitat.
Once again a big thanks to the continued hard work of all the guys at Land & Water who are doing a first class job.
After months of involved planning, the first two weirs on the Hogsmill have been broken out, returning natural flows to the upper river. Land and Water, the contractors undertaking the work, started on site last Monday (5th August) with preliminary preparations. This involved establishing the site compound and tree clearance around both weirs to allow access for the large excavators.
The extra light will also assist in vegetation establishing, which is key in the success of the chosen bioengineered bank solutions. The bank on the downstream weir was also benched to provide a stable platform for the machines to sit on. The footpath adjacent to the upstream weir was diverted into the recreation grounds and a temporary footpath laid so that the regular cyclists, pram pushers and dog walkers do not have to get their feet dirty.
Next came the fun bit. Armed with a breaker, the excavator was set loose on breaking out the downstream weir’s abutment walls, shortly followed by the weir itself. The tough, re-inforced concrete put up some resistance but ultimately it succumbed with satisfaction as lump after lump fell. The ponded section of river upstream quickly drained through the new channel.
The same process was then repeated on the upstream weir.
Where the large, vertical concrete and stone abutment walls once stood forming a distinct barrier between river and banks, these are now being replaced by low lying, soft engineered banks which gradually slope down to the river. These will shortly be planted with a variety of riverine species.
In total approximately 350 metres of river has been returned to a more natural flow regime providing potentially key spawning and juvenile fish habitat. The removal of the weirs now means that there is a 1.5km uninterrupted stretch of river.
A big thank you for all of those that have helped to get the project this far, especially Stewart Cocker at Epsom and Ewell Borough Council, Matt Horritt who helped us with the flood risk modelling, Cain Bioengineering for the designs, all the bods at the EA , Tom Cartmel at Land and Water, and all the guys’ hard at work on site.
Watch this space for further updates.
The third Downstream Defender is in the road! Despite hitting some groundwater, progress to date has been very good (touching plenty of wood). We are currently in the process of building the monitoring chambers up and downstream of the silt trap and connecting up the pipes before it will come live early next week. R.J. Dance/Emerald Construction have done a sterling job and a big thanks to them. We also have to thank the patience of all those that the works have affected, in particular the residents of Mill Pond Place who have very kindly put up with our disruption for some time now. We are now into the final leg!
The installation of the third and final silt trap is underway on Butter Hill.
Work started on Monday with R.J.Dance being the Principal Contractor. Progress to date has been great: the large surface water drain has been tapped into, a large deep hole has been excavated, and we are awaiting delivery of the chamber on Friday (21st June) morning.
Next week will see the pipes being connected and the Downstream Defender becoming active. Exciting times!!
The Public Notices published in the Sutton Guardian on 14 March 2013 included details of some road closures proposed for Butter Hill, Mill Lane and Denmark Road in Carshalton.
These road closures are proposed so that the Wandle Trust can install silt traps (hydrodynamic vortex chambers) into the surface water drains as part of a vital project in order to intercept sediment and other harmful pollution which is currently getting washed off the roads and into the river.
Unfortunately, the details given in the Sutton Guardian were inaccurate, stating that the closures would last for a duration of 12 weeks.
In reality, the worst case scenario is that each road will be closed for 2 weeks.
Radar surveys are currently being undertaken to establish if we have the room to install the silt traps between the utilities in the road. We aim to know the answer to this by the end of the week (Friday 5th April).
With this information we will be able to identify if and where the silt traps can be installed. Where possible, if the roads can remain open they will be. If closures are required due to the only space being in the centre of the carriageway, we will give all concerned plenty of advanced warning of the disruption that may be caused.
The Carshalton arm of the Wandle is currently struggling to establish a sustainable population of fish, particularly trout. We believe that a very significant reason for this is the amount of road run-off entering the river. It is reported that between 20-40% of sediment in an urban river derives from road run-off. This has many negative impacts. Trout require clean, uncompacted gravels to spawn in. With so much sediment entering the river, these gravels can get clogged up, meaning it is difficult for the trout to dig redds (nests) and even if they are successful, the sediment limits oxygen from reaching the eggs, causing them to suffocate.
There is also the impact of contaminants being washed from the road into the river, including a wide range of organic contaminants (so called persistent organic pollutants, POPs) which include polyaromatic hydrocarbons (PAHs), polychlorinated biphenols (PCBs), hydrocarbons, dioxins, pesticides, herbicides and many heavy metals. This is a list of things that have no place in a river! The full impact of pollutants are yet unknown on most aquatic life but unsurprisingly they are considered detrimental, not only to fish but on the whole food chain from invertebrates upward.
The advanced design of the silt traps we’re proposing will help to limit the amount of these contaminants entering the river, and once they’ve been installed, the only evidence of their presence being a couple of manholes.
Installing these silt traps will help to greatly improve the condition of the Carshalton arm of the Wandle, making it a much more pleasant place for the local community to spend time around.
The Wandle Trust has been working closely with Sutton Council to draw up the Streetworks Application and other plans, giving careful consideration to limit the disruption caused by works.
We apologise for any inconvenience caused but hope that you will support us with these actions which will greatly help to improve the Wandle!