Removal of a stepped weir to allow fish passage and habitat diversity
The River Wandle at Revensbury Park splits into a main and side channel, both of which are impounded. A stepped concrete weir on a side channel of the River Wandle created a complete barrier to fish passage. The weir also led to sluggish upstream river speeds and the silting up of the channel limiting its habitat diversity.
The Wandle Trust engaged Cain Bio-Engineering to provide easement of the side channel weir, opening the side channel as a migration route for Salmonids and allow by-pass of the tilting weir impoundment on the main channel. It was imperative that the solution employed would need to maintain the existing head drop of 75cm over 5m.
Cain Bio-Engineering was able to remove the stepped weir and attenuate flows to match the existing gradient by installing a stepped rock ramp. Once the weir had been removed using a hydraulic breaker, sweet chestnut hardwood posts were driven into to the bed in order to retain a combination of rocks and gravels to create a series of ‘stepdown weirs’ or rock ramps. Concrete blocks won from the weir cutting were used as the base material for these new rock ramps and gravel riffles.
The existing banks were then stabilised with ‘coirnet’ revetment, the bed dressed with imported gravels, to enhance Salmonid habitat, and marginals plants were planted. Cain Bio-Engineering also installed a small area of LWD and planted with live willow whips, in order to encourage sinuosity and create an area of low flow, further diversifying the habitat. The dilapidated oak boarding was also replaced with a geotextile revetment which is designed to fully vegetate.
The project team were great; professional, knowledgeable, skilful and ultimately provided us with the solution we wanted! The rock ramp is novel, durable and will allow natural processes and diversity to develop. As well as providing fish passage a variety of habitats for spawning and juvenile fish have been created in what was essentially a dead stretch of river. I cant wait to see the site mature over the next few years! Tim Longstaff, Catchment Project Officer, Wandle Trus