An assessment of the impact of floodplain woodland on flood flows


H. Thomas, Forest Research, Talybont Research Office, Cefn Gethiniog, Talybont on Usk, Brecon, Powys LD3 7YN. Email:


This paper examines the potential role of floodplain woodland in flood alleviation. In theory, the presence of trees and associated woody debris on the floodplain increases the hydraulic roughness, thus slowing down flood flows and enhancing flood storage. One and two-dimensional models were used to simulate a 2.2 km reach of river in south-west England to test this theory for a 1 in 100 year flood using appropriate roughness parameters. Both models predicted a reduction in water velocity within the woodland, increasing water level by up to 270 mm and creating a backwater effect that extended nearly 400 m upstream. Flood storage increased by 15 and 71%, while flood peak travel time was increased by 30 and 140 min for the two scenarios simulated. The results suggest that there is considerable scope for using strategically placed floodplain woodland to alleviate downstream flooding. In particular, it offers a means of tackling the increased flood risk associated with climate change.