South East Rivers Trust

Baffles and being baffled


As part of the continuing Defra funded Catchment Restoration Fund aimed at increasing fish passage on the Hogsmill River, this week saw us addressing the weir that we recently installed an eel pass onto in Kingston.

With removal of the weir being financially and logistically infeasible, an easement was required. The weir is formed by a 6.4 m sloping concrete structure with a vertical drop at the downstream end into the pool. The weir had previously been notched to allow fish to pass the structure, however, upon measuring the velocity of the flow the results indicate that it has remained largely impassable due to the high velocities experienced at the downstream end.

The weir structure
The weir structure with central deeper notch

Installing baffles within the notch was identified as being a viable solution. This would in theory slow the flow and increase water depth through the notch, therefore giving the fish an easier time of things. Calculations and designs were worked up with help and guidance from Darryl Clifton-Dey at the EA.

With the materials purchased, myself and Norman Fairey, our go to man that can, spent two days in his workshop cutting, drilling and sawing stainless steel and recycled plastic lumber to create the baffles and fixings, in addition to the batons that would increase the depth through the notch. The rounded baffles were very kindly donated to the project from Darryl and the EA. The results were great with the baffles being incredibly sturdy and robust.

Norm working his magic
Norm working his magic
The baffles pre fabricated and ready to be installed
The baffles pre fabricated and ready to be installed

With the pre-fabrication completed, it was time to get to site and undertake the installing. Four days had been allocated to do this and as the time drew nearer the weather forecast was that of Jekyll and Hyde. The Thursday and Friday were set to be hot, sunny and muggy and then the weekend would be torrential rain and thunderstorms. Working in-channel and with resin, it was vital that we made the most of the first couple of days. Donning black, thick PVC chest waders and lugging gear back and forth from the not so handy access route, sweating profusely was the order of the project.

Once again, the Thames Anglers’ Conservancy (TAC) came up trumps with enthusiastic and willing help rounded up by David Harvey. The TACs’ continued support with our projects is always appreciated. The first task was to create a dry(ish) working area. This involved creating a cofferdam using plastic sheet piles at the upstream end of the notch and the best part of two tonnes worth of sand bags which were bagged up on site. The result was great and for the first time the notch was unveiled and could be observed.

Cofferdam in place creating a dry (ish) working area
Cofferdam in place creating a dry (ish) working area

The next step was to fix the recycled plastic batons on either side of the notch to effectively increase the depth. This was done by drilling 150mm holes into the concrete and fixing stainless steel threaded bar using Zeroset Anchor Grout. The threaded bar was passed through the baton and secured with a countersunk stainless washer and nylon nut. Counter sinking was used as to prevent debris and litter accumulation. As was rounding the upstream end of the batons.

Fixing the side batons in place to increase  the depth of the notch
Fixing the side batons in place to increase the depth of the notch

Using a pre-cut MDF template stencil of the baffle layout, the positioning of each was established. Due to the rough cut nature of the notch, in places a cut-off saw was required to square up the edges and level off the bottom so that the baffles could sit securely, level and beneath the height of the baton. The concrete was sliced up and chunks were broken out with a lump hammer and bolster.

Dave breaking up the concrete
Dave breaking up the concrete

Again, the baffles were fixed with more Zeroset and studs and any voids beneath were filled with Underwater Mortar. With the rain due the following morning, with some hard graft, long hours and several gallons of sweat later we were ahead of schedule for completion.

Everything fixed into place
Everything fixed into place

Come the next morning the rains hadn’t materialised and we were again bathed in glorious hot sun for a further day of lugging things. Firstly, the best part of a tonne of gabion stone needed to be carted down the bank from our access point, down along the river, over the weir and into the pool at the base of the notch. The reason for doing so was to guide fish up into the notch. The larger pieces of stone, in combination with the ubiquitous lumps of concrete found in urban rivers stabilised the toe of the ramp and the rest of the stone formed the ramp itself. These were locked into position using the smaller gravel to dust found at the bottom of the bulk bag.

Stone ramp being created
Stone ramp being created

We were at the point of pulling the dam and seeing the moment of truth. The sand bags were piled to one side at the upstream end of the notch and the piles removed. The water poured into the notch, snaking its way down, swelling around the baffles and forming eddies, things were looking positive. It was upon closer inspection that it became apparent that in the last metre the water sped up and shallowed as it dropped into the pool. Water has a funny and rather frustrating habit of behaving as you wouldn’t necessarily predict and here it was doing just that. Conditions are improving, however, evidently our work here is not quite done with further tweaking required to remedy the bottom section.

The upper baffles working but with a drop at the lower end. Work in progress!
The upper baffles working well but with a drop at the lower end. Work in progress!
The band of merry men
The band of merry men

A massive thanks once again goes to Norman Fairey, to the TAC guys to include David Harvey, Neil Depledge, Keith Collett and Ian Clarke, as well as to Ben Tonkin and Andrew Woodhouse for volunteering and getting involved. Additional thanks goes to Darryl at the EA for advice and supplying the baffles and Rob Waite at the Royal Borough of Kingston for helping us to dispose of the sand bags.

Have any Question or Comment?

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.