Background Information
This project was in Carshalton, focusing on a stretch of the River Wandle at Butterhill Bridge, which had been impounded by a weir from a former mill. This had resulted in a large build-up of sediment on the riverbed, which had completely degraded the riverine habitat. On top of this, the weir itself is impassable to fish.
Scope of the Project
To address the impoundment the existing weir was lowered by permanently opening the sluice gates at the mill to drop the river level. A new fish ladder would then be installed once the upstream habitat had been improved to allow fish access upstream of the weir.
Dredging the sediment from the channel and using it to construct berms and lower the bank profile through the heavily modified stretch, reinstated the natural chalk stream characteristics of the river.
Additional habitat features such as imported gravels, coarse and large woody debris and planting would be introduced to further enhance the scheme and improve biodiversity.
Our Solution
The channel was dredged using an excavator positioned on bog mats inside the channel. The accumulated sediment was removed down to the original bed level and repositioned behind Hazel Faggots installed as berms.
This introduced natural meanders to the stretch. The right-hand bank was re-profiled and landscaped using the excavator to form a gentler sloping bank profile more suitable for the colonisation of wetland plant species.
A new gravel substrate was introduced to the riverbed to provide gradient to the stretch, increasing water velocity and creating vital pool-riffle habitat. Habitat was further enhanced by the introduction of large woody debris, root wads and marginal aquatic planting.
Since completion, this project has been awarded the 2016 UK river prize award and Nigel Holmes trophy in the urban category, by the River Restoration Centre.
