Modelling of wave overtopping flow over complex dike geometries: case study of the Afsluitdijk

Grass cover erosion by overtopping waves is one of the main failure mechanisms of dikes. Transitions in cover type and geometry can increase the hydraulic load and are therefore identified as vulnerable locations for grass cover erosion. Two models are applied to the inner slope of the Afsluitdijk in the Netherlands to show how transitions can be included in overtopping models. Firstly, the analytical grass-erosion model is used to simulate the erosion depth along the profile for a six-hour storm. The model results show that the erosion depth is maximal at the end of the two slopes in the profile. Secondly, the effect of transitions on the hydraulic load is computed with a detailed hydrodynamic model. The model results show that geometric transitions significantly influence the shear stress, the normal stress and the pressure. Four vulnerable locations for grass cover erosion are identified based on the model results that are related to slope changes along the profile. Furthermore, the model results show that the overtopping flow is mainly affected by geometric transitions, while no effect of roughness transitions on the modelled forces was observed.