Modelling the effects of floodplain woodland in flood mitigation. A short-term case study.

Abstract

Three floodplains were selected for study in the Mawddach catchment, just north of the town of Dolgellau in Central Wales. Vegetation cover scenarios were modelled for each site using the hydrodynamic model River 2D. The first two sites provided no real potential for using floodplain woodland in flood mitigation, although modelled water depths increased by up to 50 cm due to the presence of dense vegetation on the floodplain. At the third site, dense woodland, with an average basal area of 0.10 m2/m2, reduced modelled peak discharge by 45 m3sec-1 when compared with grassland. This also delayed peak discharge by more than 30 minutes and created a backwater effect that increased water depths by more than 1.2 m for a distance of 1.3 km upstream of the floodplain. Two other vegetation scenarios, sparse and clustered woodland, were also modelled for the third site. Although sparse woodland had just 45% of the basal area of the clustered woodland, it had a similar effect in mitigating modelled flood levels downstream.