Effects of river floodplain lowering and vegetation cover

The current river management policy in the Netherlands is to give rivers more space, mainly by main channel widening and floodplain lowering. The aim is to reduce flood water levels and to create more favourable conditions for river ecology. However, the effect on water levels gradually disappears due to sedimentation and vegetation growth on floodplains. The key questions are how effective these measures are and how long the desired effect remains. A two-dimensional, depth-averaged, morphological model was applied to the Meers pilot project on the Common Meuse River. The project consisted of channel widening by floodplain excavation, carried out over a short reach. Spatially varying floodplain vegetation was included. The results showed that the pilot project would be effective in lowering local water levels during a period of 20 years for floods having a return period of 1-4 years but not for floods having a return period of 100 years. In the latter case, water levels would be dominated by a restriction in floodplain width, causing backwater effects. Floodplain vegetation was found to shorten the lifetime of the project by enhancing sedimentation.