Evaluation of floodplain management strategies: The added value of wetlands

The objective of this project was to evaluate the beneficial effects of wetlands on flood risk reduction and water quality improvement in the Rhine. The analysis of wetland effects on flood risk reduction aimed at evaluating whether the position of a wetland in the basin – upstream or downstream – will influence the added value of the wetlands in terms of reduced flooding damage. It was found that storing one cubic metre of water in an upstream wetland is generally more beneficial than storing a cubic metre of water in a downstream wetland, because the beneficial effects of upstream wetlands are felt throughout the basin and the effects of downstream wetlands are only felt downstream. However, upstream wetlands can not be used as a means for flood mitigation during extreme discharge conditions. Under such conditions, upstream wetlands will be completely saturated and directly discharge additional water. Extreme flood events in the Netherlands can thus only be mitigated by downstream detention areas. The analysis of wetland effects on water quality improvement aimed with evaluating (i) whether increased areas of active downstream floodplains may increase nutrient retention in rivers, and (ii) whether rehabilitation of natural floodplain wetlands from agricultural grasslands may increase nutrient retention. This study showed that both an increase in the area of active floodplains in the Rhine delta, as well as rehabilitation of natural wetlands within floodplains will likely significantly improve water quality with respect to phosphorus, but will hardly affect water quality with respect to nitrogen. Moreover, nature restoration from agricultural grasslands towards reedbeds and ponds will be more beneficial for river water quality, than restoration towards woodlands or semi-natural grasslands.