Risk reduction by combining nature values with flood protection?

Abstract

In the Netherlands, the concept of a multifunctional dike has already often been implemented, and has been identified as a promising climate adaptation measure. In a multifunctional dike, functions like urban development, transport infrastructure, recreation, agriculture or nature are deliberately combined with its primary flood protection function. This means that the design must be based on the requirements and life span of all different functions, while in a monofunctional dike only the flood protection function is considered. By accommodating other functions, a multifunctional dike may easier fit into, or even contribute to the quality of the landscape. Moreover, these other functions may help in financing the flood protection works, but governance is more complicated. To avoid costly adjustments forthcoming from changed safety standards, incorporation of multiple functions can require a more "robust" flood defence than a monofunctional flood defence. A robust flood defence can withstand more extreme situations than required by the present safety standards, and has a substantially lower flooding probability. Therefore, a multifunctional dike may be attractive in view of the uncertainties regarding the effects of climate change and a changing world. Moreover, it will result in reduced flood risk. As part of the Dutch Delta programme, several explorative studies on multifunctional dikes were initiated. Most studies focused on urban areas, but also in the rural area interest emerged for multifunctional dikes, e.g. for the integration of salt marshes into the flood defences. Marshes provide valuable habitat for vegetation and invertebrate species, and are important for wading birds. Furthermore, under condition of abundant sediment availability they can keep pace with sea level rise. Explorative modelling results indicate that vegetated forelands affect wave heights, even under extreme conditions. However, the inclusion of a vegetated foreland into the dike design does not automatically mean that nature values and flood protection are well integrated. Flood protection imposes rather different requirements on the extent and features of marshes than nature conservation and development. Wave damping is most effective with a high and stable marsh, while nature thrives with dynamic processes and differences in elevation. Therefore, only a design that allows natural marsh dynamics and includes different marsh zones could combine nature values with flood protection. In practice, this means a dike design with an uncertain foreland, that offers space for natural processes. The uncertainty in foreland development reduces the possible flood risk reduction. In our paper we describe the critical points of interest concerning risk reduction in this system.