André van der Westhuysen | Nearshore wave modeller

Protecting the low-lying Netherlands against flooding from the sea

André van der Westhuysen has been extensively involved in nearshore wave model development to protect the low-lying Netherlands against flooding from the sea.

A significant part of the Netherlands lies below mean sea level (lowest point is 6.8 m below MSL), and therefore needs to be protected from flooding by means of an extensive system of dikes. By Dutch law, the safety level of these primary flood defenses must be assessed every six years. This assessment is based on a set of so-called hydraulic boundary conditions (HBC), derived by means of numerical hydrodynamic and wave modeling, and statutory safety assessment rules. The Dutch government agency Rijkswaterstaat is sponsoring the program SBW (Strengths and Loads on Flood Defenses) to improve the models and methods for determining these HBC and the methodologies for the flood safety assessments.

One of the focus areas of the SBW program is the Dutch Wadden Sea in the north of the Netherlands. This complex coastal system poses significant challenges to nearshore wave modeling. The region is enclosed by a series of barrier islands and the mainland coasts of the provinces of Friesland and Groningen (see picture of dyke along the coast of Groningen). Tidal inlets are found between the barrier islands,  each featuring a complex system of tidal channels extending into the Wadden Sea interior. The rest of the Wadden Sea interior is shallow and flat, with tidally-modulated depths normally ranging between 0 m (drying, see picture) and 3 m, but going up to 6 m during extreme storms.

Before joining NOAA/NWS/NCEP, Dr. Van der Westhuysen was extensively involved in the improvement of nearshore wave modeling in the complex Wadden Sea. The model development work has been based on a multi-year measurement campaign in this region (see observational mast and wave buoy in picture). The model improvements implemented by Dr. Van der Westhuysen and his colleagues at Deltares and ARCADIS/Alkyon include: depth-induced wave breaking on the shallow flats, wave-current interaction in the tidal channels and the penetration of offshore swell waves to the mainland dikes. The research has been presented at a number of international conferences and in the form of journal publications, like for instance: Van der Westhuysen, A. J. (2010). Modelling of depth-induced wave breaking under finite-depth wave growth conditions. J. Geophys. Res, 115, C01008, doi:10.1029/2009JC005433.