Bram de Vries

Project Background

To prevent catastrophes, like the flood of 1953 or the Rhine floods of 1995, the Dutch authorities checks the strength of the water defenses in the Netherlands every five years. The assessment rules for the safety check are provided by the Dutch government. The Rijkswaterstaat programme ''Strength and Loads on Water Defenses (SBW)'' is aimed at establishing new assessment rules for the future assessment of water defenses and is in particular focused on water defenses that can't be accurately assessed with the current rules. The hybrid water defenses, a combination of dunes and hard unerodable structures, fall in this category and still lack reliable assessment rules. In the framework of the SBW programme Rijkswaterstaat had commissioned Deltares to execute a series of pilot experiments on dune erosion near unerodable structures. The main objective was to verify qualitatively the hypothesis that unerodable structures cause an increase in erosion of the dunes next to it. The hypothesis was confirmed by the experiments and the current assessment rules were reviewed.

Existing tools to determine dune erosion under storm conditions assume uniform condition along the coast which works well for relatively undisturbed coastlines. However these tools are inadequate to predict dune erosion in more complex situation, such as a connection between dunes and a seawall. Here the alongshore variability of the coast and hydrodynamic parameters requires a more advanced 2-dimensional tool. XBeach is a 2DH process-based tool that comprises the essential processes responsible for dune erosion under storm conditions.

Objective

The objective of the project is to validate the results of the pilot experiments with the numerical model XBeach. After identifying the dominant processes for dune erosion the capability of XBeach in predicting additional erosion near unerodable structures was investigated. Currently the measurements of the pilot experiments are analyzed in order to extract data suitable for comparison. The focus is on the comparison of erosion quantification parameters, such as the retreat point R and the erosion volume above storm surge level. Hydrodynamic parameter such as the wave spectrum in front of the seawall are also investigated. In a later stage this data is compared with simulation data of the XBeach model.