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.
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.
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.
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.
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.
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.
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.
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.
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.