Graduation of Menno Rekveld

Flow velocity analysis of lee side “Wide Green Dike” using Swash

• Professor of graduation: Dr.  ir. Bas Hofland 

• Supervisors: Ir. Alessandro Antonini (Tu Delft), Henrike Maris (Sweco)

   

The water authority ”Hunze en Aa’s” initiated a demonstration project adjacent to the Dollard were 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 bound- ary conditions provided HYDRA-NL. This report examines whether the current overtopping criteria(10 l/s/m) leads to failure for a dike with such a gentle outer slope. In this context failure means erosion of the inner dike slope. Whether failure or non failure occurs is based upon the cumulative overtopping method.

The dike is rebuild in a computational model called Swash and the flow velocities on the dike that occur for design conditions are analysed. Swash is a general-purpose numerical tool for simulating non- hydrostatic, free-surface, rotational flows and transport phenomena in one, two or three dimensions. The governing equations are the nonlinear shallow water equation including non-hydrostatic pressure and some transport equations(TU-Delft, 2021). The wave boundary condition is provided by HYDRA- NL. However, a closer look to the provided wave climate showed that the predicted wave climate is physically impossible. Consequently, modelling the exact conditions the dike was designed for is not possible.

This resulted in the formulation of new hydraulic boundary conditions. No wave records exist for the Dollard that could give an indication of the wave climate. Several options for the wave boundary con- ditions are discussed, most likely is however that the waves are generated inside the Dollard and are therefore calculated using an empirical fetch based wave generation formula.