BART have implemented measures to reduce the impact of suspended sediment and open up the dense tree canopy to encourage habitat diversity within an over wide section of the Congresbury Yeo below Blagdon Lake. In November 2019, BART carried out a programme of strategic coppicing along the river banks to open up sections of dense canopy to enable light to reach the river to encourage habitat diversity. With a doppled light now reaching the channel there is greater potential for in-stream vegetation to colonise which will provide multiple benefits to riparian wildlife.
The second phase of the project was to address the issue of fine sediment from upstream sources which were making the river silty and smothering river bed substrate and habitat for invertebrates and fish. The over-wide, straightened channel below Blagdon Lake was silty and had little habitat or flow diversity.
The sediment accumulating on the river bed was being flushed downstream during high flows, smothering river bed gravels and impacting on water quality. To address this issue BART designed three structures using wood harvested from the river banks during the first phase of works.
We produced brushwood bundles, called ‘faggots’, tightly bound and secured to the river bed to create the shape of the first and largest structure. We filled this in with brushwood and secured it down with arms made of branches and a small amount of wire to secure these to the chestnut posts.
The structure will capture fine sediment – as water flows through the structure, sediment will drop out in the brushwood, thus reducing the fine sediment load transported downstream. During spate flows the brushwood will fill up with silt and, over time, will formalise and grow up with vegetation such as branched bur reed.
A second structure was installed which is designed to capture fine sediments whilst pinching the channel to generate flow diversity necessary for flushing river bed gravels thus reducing the quantity of fine sediments being deposited.
Where the channel is narrowed between the two structures the increased flow velocity will help to clean the bed of silt and improve habitat for aquatic invertebrates and fish.
A third structure was installed at a location downstream where a another channel joins the main river, bringing with it more fine sediment. BART used large woody material and finer brushwood to pinch the channel at the confluence to generate flow diversity, encourage bed scour to encourage river bed diversity and creating a pool.
BART are taking catchment based approach to the Congresbury Yeo and have recently delivered catchment wide surveys for the Congresbury Yeo and its tributaries and drainage channels. BART will be exploring opportunities with stakeholders and partners to develop plans to deliver benefits to water quality, riparian habitat and natural flood management (NFM) across the Congresbury Yeo catchment.
For more information about this project please get in touch with Simon Hunter: simon@bristolavonriverstrust.org
