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Modelling the hydraulic impact of reintroducing large woody debris into watercourses



Huw Thomas, Centre for Forestry and Climate Change, Forest Research, Forestry Commission Office, Rheola, Resolven, Neath SA11 4DR, UK



Very little large woody debris (LWD) is present in UK river systems due to its removal from watercourses mainly for flood defence and angling purposes. Some researchers now believe that restoring LWD dams in rivers can aid in flood retention, improve habitat and biodiversity, and help improve water quality and reduce sediment transport. This study was carried out to model the potential impact of restoring five LWD dams into a small Welsh tributary within an existing area of flood plain woodland on flood flows. The model results suggest that the dams could increase water levels sufficiently during the design 1-in-100 year flow to reconnect the channel with its flood plain. The model predicts as much as a 2.1 m/s reduction in flow velocities behind the dams. Both factors contribute to the delaying of the flood peak by up to 15 min over a 0.5-km reach. The results support the use of LWD dams as a viable soft engineering technique for complementing existing flood defences and aiding in downstream flood mitigation, although to be effective at a larger scale would require an extensive series of dams across the upper and middle reaches of a catchment.