Room for the Wadden Sea

Abstract

A significant part of the Netherlands lies below sea level. The Dutch have protected their land against seawater since the 10th century. Nowadays, the Netherlands is protected by 3800 km of primary flood defences. Since the Flood Defences Act was included in the Dutch law in 1996, all flood defences are tested against the national safety standards every twelve years. In the latest assessment round, a 47 kilometres long stretch of a Wadden Sea dike in the northern part of the Netherlands dike section, from Koehool to Lauwersmeer, did not meet the requirements. It is rejected to multiple foreshore influenced failure mechanisms. The dike area of Koehool-Lauwersmeer consists of cultural heritage in means of historical dikes and historical land reclamation structures. The (embanked) salt marshes that are present at the foreshores of the dike section, are of great value for both ecology and water safety in the area. The length of the dike section and the unique project location, provide an opportunity to explore alternative solutions for the dike strengthening project. Innovative dike strengthening solutions can potentially minimize the anthropogenic influence of the reinforcement projects, increase the systems adaptability potential to sea level rise and contribute to the ecological values of the area. In this research, an explorative study is conducted to find an alternative dike strengthening design and to assess the morphological developments and ecological opportunities of such a design. Based on the criteria to contribute to both flood safety and ecological value, a slufter alternative is chosen to be studied further. The slufter alternative engages an inland area between two dikes into the Wadden Sea by an artificial dike breach. The area is regularly inundated, creating opportunities for ecosystems to evolve. A depth-averaged (2DH) Delft3D model is used to explore the morphological developments of the slufter area and the potential to add to the ecological value of the area. The project area is characterized by a sediment-rich environment and a flood dominant tide. Higher flood than ebb velocities are present, causing an import of sediment into the slufter area. Model results indicate that the net sediment import of the slufter area mainly depends on the suspended sediment concentration, the critical shear stress and the tidal prism of the slufter area. A geometry that causes an increase in the flow velocity shows higher suspended sediment concentrations that lead to higher amounts of sediment import. Under the influence of a sea level rise (SLR) of +1m, the sediment transport increases with a factor of 2 to 3, depending on the initial geometry. SLR enhances the sediment trapping of the area. The slufter area can potentially keep up with SLR. Assuming an equal distribution of sediment over the slufter area, the average growth rate of the bed level is between 0.22 (minimum sediment import, maximum SLR) and 21.12 (maximum sediment import, minimum SLR) times SLR. Applying the slufter intervention adds 60 ha to the intertidal area of the Wadden Sea. Depending on the applied geometry this results in 30-50ha between MLW and MHW (lower marsh) and 10-30ha above MHW (middle and upper marsh). The slufter alternative creates opportunities for new ecosystems and contributes to the Natura2000 goals to create more gradual transition areas between the Wadden Sea and the surrounding land by creating an area with a gradual wet-dry transition. These potentially add to the biodiversity of the area as they create opportunities for specific plant species and waders. It is concluded that creating a slufter area at the Koehool-Lauwersmeer dike, provides opportunities for sediment trapping due to the flood dominance of the system and the high sediment concentrations. The amount of sediment trapping is influenced by the critical shear stress, the area’s tidal prism, and the suspended sediment concentrations. SLR increases the amount of sediment trapping. Depending on the applied geometry, the bed level growth rate is larger than SLR. Creating a slufter area creates an opportunity to improve both the flood safety and the ecological value in one design.