Location: 0.2km of the River Allen, Dorset at Witchampton Mill.
Issue: An old mill tail race mechanically cleared of vegetation, resulting in a total loss of in-stream habitat.
Objective: Increase channel sinuosity, flow speed and depth, reduce siltation, use a two stage channel to create marginal habitat and increase resilience to low flows.
Method: flow deflectors and brushwood berms.
WTT role: Lead partner — project design and delivery.
Partners: Local residents, Dorset Wildlife Trust.
Project date: October 2014, 5 days.
A section of the River Allen in Dorset, which had previously functioned as the water mill tail race, was overwide and suffering from sedimentation and excessive marginal encroachment. After some unsympathetic mechanical clearance of the in-stream vegetation, the channel was left barren and devoid of habitat. Local residents and the Environment Agency became concerned about the poor quality habitat and the ongoing maintenance of the channel.
The Wild Trout Trust was approached by the residents’ association and proposed a project to introduce a sinuous two-stage channel that would both control plant encroachment and also provide excellent in-stream and marginal habitat for invertebrates (including white-clawed crayfish), fish, water voles and waterfowl.
Working closely with Dorset Wildlife Trust, a series of brushwood berms, flow deflectors and hinged willow features were installed to accelerate flow through a narrow meandering low-flow channel that could be readily over-topped by flood flows. Brushwood faggots were created from material won from a nearby over-stood hazel copse and also from coppicing work on DWT reserves.
Three years later, the reach is completely transformed with dense and diverse margins and a deep, fast-flowing and sinuous two-stage channel. The channel has increased resilience to both high and low flows, providing better habitat for a range of chalkstream species.