Evidence type: Research
Organisation: Forest Research
Author(s): Thomas, H. & Nisbet, T.
A research paper which 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 2.2km 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 270mm and creating a backwater effect that extended nearly 400m 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.
A range of options are currently being considered including the creation of washlands, river corridor widening and river restoration.
Floodplain woodland is thought to have naturally carried out this role in the past, and its historic removal may have contributed to an increase in flooding severity.
The River Parrett is one of a number of major river systems in the country facing a serious and recurrent flooding problem.
The roughness associated with the presence of a complete cover of woodland on the north side of the floodplain increased the flood level by around 190mm.
Both floodplain woodland scenarios significantly increased the cumulative flood volume stored within the modelled reach.
As expected, the presence of trees, undergrowth and woody debris decreased the water velocity over the floodplain, both within and upstream of the wooded area.
The presence of a 50 ha central block of woodland would increase the downstream progression of the flood peak by 30 min.
The modelling work demonstrated that water levels were raised by up to 190mm immediately above the forest. The implications of this factor would need to be carefully considered on a site-by-site basis when assessing the suitability of individual sites for the restoration of floodplain woodland.