Green adaptation by innovative dike concepts along the Dutch Wadden Sea coast

This paper describes the development and application of an approach to adapt the existing flood defences along the Dutch Wadden Sea coast to the effects of climate change and sea level rise in the context of other uncertainties and developments. It starts with the development of a dike-portfolio with traditional as well as new flood protection concepts. Next these concepts are evaluated by means of a multi-criteria analysis by local experts. The objective is to identify realistic adaptation options that use or enable natural processes to strengthen ecological resilience and facilitate sustainable human use in the Wadden region. In our analyses Eco-engineering concepts (in rural areas) as well as a Multifunctional dike (in built-up areas) received the highest scores. Multifunctional dikes are robust and offer space for other functions and values. However, their performance in an integral assessment strongly depends on the applied functions and the weight per evaluation criterion. Eco-engineering concepts can potentially contribute to nature and landscape values, but implementation may lead to tension with nature legislation. For the Wadden Sea landscape, which is characterised by the presence of semi-natural salt marshes, the application of vegetated foreshores for flood protection comes forward as being particularly attractive. For every specific stretch of the 227 km long coastline the study has identified the most promising dike concept. The result of this study has been adopted by the national government and the regional water boards as a basis for more detailed analysis and design per stretch of coast. Additional modelling studies and cost benefit analyses will be required at this stage to verify the outcome of our studies and to optimise the design.