Due to the inevitable effects of climate change, more frequent and more extreme storms will take place. In addition, Sea Level Rise (SLR) will be present such that the pressure on coastal flood protection increases. Moreover, since the magnitude of the effects of climate change is still uncertain, there is a great need for primary flood defences to be adaptive. Due to the self-organising behaviour of ecosystems, there is more often looked for solutions that work together with nature resulting in Nature-based flood defences. In this research, the Wide Green Dike (WGD) pilot study in the Ems-Dollard estuary in the Netherlands is used as an example of a Nature-based flood defence. For the WGD pilot study, 1 km of dike is being reinforced with a thick clay layer on the seaward side while also decreasing the outer dike slope to fulfil the safety standards against erosion. Besides the adaptive capacity of this reinforcement, a wide vegetated foreshore is present in front of the dike. Due to the self-sustaining behaviour of this foreshore, caused by the enhancement of sedimentation by the presence of vegetation, the foreshore should be able to grow along with SLR. Moreover, due to the capacity of the vegetation to mitigate wave conditions by increased bottom friction, especially effective during daily conditions, the vegetated foreshore lowers the hydraulic boundary conditions. As a result, the vegetated foreshore could limit the required dike dimensions under extreme storm conditions but could also strongly influence the needed dike maintenance if dike erosion is prone to arise for example. Doing so, a more sustainable dike reinforcement arises. The above-described processes are visualised in Figure 1.
Figure 1: Visualisation WGD pilot study
With the use of a Simulating Waves Nearshore (SWAN) model, a sensitivity as well as scenario analysis was conducted. For the sensitivity analysis, there was looked into the model input parameters as well as the foreshore dimensions and the method of the SWAN model to implement the effect of vegetation. Within the scenario analysis, changes to foreshore dimensions were combined with an indirect method to include vegetation. Finally, the potential influence of the foreshore was linked to their influence on the required dike design.
The sensitivity analysis showed that the water depth on the foreshore has a greater influence on the significant wave height than the width of the foreshore (for the WGD transect). Moreover, by including vegetation in the method of the model, the significant wave height showed a significant decrease. Together with the conducted scenarios analysis it could be concluded that the foreshore potential depends on the ability of the foreshore to pace with SLR and the presence of vegetation which causes increased bottom friction.
When comparing the required dike dimensions if foreshore height increase (with SLR) and the influence of vegetation are not taken into account (current approach) with using an approach where those influences are taken into account, a required height decrease of 22 cm and clay layer thickness decrease of 31 cm arises. This concludes that the (changing) foreshore dimensions and vegetation are of great importance to lower the hydraulic boundary conditions and thus required dike dimensions of the WGD.