The water authority ”Hunze en Aa’s” initiated a demonstration project adjacent to the Dollard where non traditional dike reinforcement strategies could be developed. For this project Sweco designed a sea dike with an outer slope more gentle than traditional. This dike is called the ”Wide Green Dike” (WGD) and has an outer slope of 1:7. This dike is designed for 10 l/s/m overtopping with hydraulic boundary conditions provided by HYDRA-NL. This report examines whether 10 l/s/m of overtopping leads to failure of the WGD. In this context failure means erosion of the inner dike slope. Whether failure occurs is based upon the cumulative overload method (Steendam et al., 2015).

The WGD is rebuilt in a computational model called SWASH, which is a general-purpose numerical tool for simulating non-hydrostatic, free-surface, rotational flows and transport phenomena in one, two or three dimensions. The flow velocities on the rebuilt dike for the overtopping design condition are analysed. In this study SWASH is used to analyse 2D wave interactions from nearshore along a shallow foreshore towards and over a sea dike.

The simulation using the locally generated waves results in no overtopping. The overtopping criterion is normative for the crest height and no overtopping indicates that it is likely that the design of the dike can be optimized. But before any decisive conclusion can be made more research into the wave cli-mate in the Dollard is needed.

As the scenario using the locally generated waves resulted in no overtopping, a new scenario is created. This scenario has other wave boundary conditions, such that there is overtopping. This simulation results in 57 l/s/m overtopping but without any initiation of damage on the lee side according to the cumulative overload method. This result would indicate that exceeding an average overtopping of 10 l/s/m might not necessary lead to failure for a dike with a more gentle, compared to standard, outer slope. The same simulation shows that the current relation for the flow on the lee side is likely not sufficient to describe the full flow pattern. According to literature the flow velocity on the lee side of a dike increases and stabilizes. However, SWASH simulation results show after the initial increase also a decrease in flow velocity. The current relations for the flow velocity on the lee side only include the first initial increase but do not include the observed decrease in flow velocity. The kinematic shock theory equation, shows good agreement with the decrease in velocity. The results show that the tipping point (change from flow velocity increase to decrease) is located at 97% of the predicted final flow velocity (Van Gent equation). It seems that a combination of the Van Gent equation and kinematic shock equation can represent the flow velocity along the lee side of a dike.