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.
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.
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.
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.
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.
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.
Complete, looking downstream…
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.
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.
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.