Trout in the Town: Weir notching and rock berms in Bruton

Theo Pike describes the Brue CREW’s recent project: 

If you’ve read this year’s issue of our journal Salmo Trutta, you may recall Caitlin Hafferty’s fascinating feature about the dynamics of community-led river management initiatives. 

Caitlin’s article focused on one particular case study: the Somerset town of Bruton and its very active community group, the Brue CREW. This group is enthusiastically led by the town’s Mayor Ewan Jones: it aims to restore the natural balance of Bruton’s stretch of the little River Brue as a haven for wildlife, and promote the riverside as a relaxing place for local people to spend time with nature.

WTT has supported the Brue CREW’s development for several years, and links have also been forged with other local Trout in the Town groups like CATCH in Wincanton. Responding to residents’ concerns about low summer flows and reeds encroaching into the river channel, Mike Blackmore conducted an Advisory Visit in 2015, and identified problems caused by the weir on the downstream side of the town’s historic bridge…

“At Church Bridge, the channel becomes abruptly wider and is completely exposed to open sunlight. The river also becomes impounded (held up) by the weir immediately downstream of the bridge. Both the widening of the channel and the impounding effect of the weir causes a significant slowing of flow through this section. This creates the perfect conditions for plants that would otherwise be confined to the margins of the river to encroach into the channel. To exacerbate the issue further, the weir has trapped bed material and nutrients upstream causing the bed to rise and become uniformly shallow, and ideal growing media for emergent vegetation.

Given enough sunlight and protection from grazing, marginal vegetation will generally encroach into a channel as far as flow velocity and depth allow. In many cases, this is a beneficial process that effectively helps rivers to adjust to lower flow conditions. The channel essentially self-narrows until an equilibrium is achieved between the vegetation and flow speed. However, if a channel is uniformly wide and shallow, low flows can be so diffuse that this equilibrium is not readily achieved, and the channel can become choked.”

In order to protect the bridge footings, completely removing the weir wouldn’t be a realistic option, but strategically notching this structure could draw a stronger flow down one part of the channel, creating a low-flow ​‘thalweg’ and encouraging the river to take a more meandering course between the high retaining walls. Building berms in the river above the bridge could also accelerate this effect, as well as helping to scour silt out of the bridge’s northern arch under particularly high flows. At the same time, here in the headwaters of the Brue’s catchment, a more sinous channel would help to ​‘slow the flow’ in high water conditions, and reduce flood risk downstream.

With all these benefits in mind, the Brue CREW successfully secured £1,000 funding from Somerset Rivers Authority through the Hills to Levels project, and we started the first phase of delivery in October last year. 

Our good friend Luke Kozak waded into the river with a pneumatic drill, and we chipped out a notch from the limestone and concrete weir, creating a nice plume of flow for fish to run up and through. (Even before we’d finished smoothing off the edges with quick-setting mortar, we could see little bullheads darting up the current to investigate this new route upstream).

The second stage of the project, building the flow-deflecting berms above the bridge, proved slightly more complicated. Mike’s initial plans suggested low-level brushwood berms, constructed from locally-won tree thinnings, but on closer inspection it became apparent that the river bed was solid rock under a thin layer of silt and cobbles: not ideal for driving chestnut posts!

The clear alternative was to build berms from suitable rock – a much more expensive option which might even have killed the whole project on financial grounds – but Ewan came up trumps with an offer of 30 tonnes of limestone, from less than 5 miles down the road, very generously donated free of charge by the local Emily Estate (now The Newt In Somerset). 

More challenges arose when it came to getting the rocks into the river: one date had to be cancelled due to high water, another because the stone had been diverted to a different project, and a third when our new pile of rocks got boxed into the yard by an unexpected delivery of a whole lorry-load of palletised breeze blocks! 

But persistence paid off in the end, and on the morning of 24 May, after I’d staked out the dimensions of the berms in the river, Mike and I supervised proceedings as a grab lorry carefully dropped two 15-tonne piles of Hadspen limestone boulders and cobbles into the channel. The following day, a team of 18 local volunteers (including those rock-berm experts Gary Hunt and Matt Bishop from CATCH) made short work of human-chaining the rocks into position.

Over the next few months, Ewan and his Brue CREW colleagues will keep an eye on the berms as they silt up and vegetate, maybe adding more sediment and plants to help them on their way. We’ll also be keeping a close eye on how the river’s new thalweg develops, with reinvigorated flows hopefully keeping the channel much clearer of reeds, and even providing new spawning habitat for wild trout which can now move around more freely – all as a result of this exciting new Trout in the Town community project.

Huge thanks to all our project partners: Brue CREW, Somerset Rivers Authority, Hills to Levels, The Newt in Somerset, the Environment Agency, Luke Kozak, local landowners, and all the volunteers who helped us build the rock berms on the day!