Observed finite depth wave growth limit in the Wadden Sea

The spectral wind wave model SWAN (Booij et al. 1999) plays a key role in the estimation of the Hydraulic Boundary Conditions (HBC) for the primary sea defences of the Netherlands. Since some uncertainty remains with respect to the reliability of the wind wave model SWAN for application to the geographically complex area of the Wadden Sea, a number of activities have been initiated under the project \u91SBW Waddenzee\u92 to devise a strategy for the improvement of the model. This activity is carried out in parallel with a measurement campaign that is being undertaken in the Dutch Wadden Sea to provide observational data for the calibration and validation of SWAN (\u91SBW-Veldmetingen\u92). The present study aims to compile a reliable set of observations (waves, water depth and wind) from three westerly storms recently observed in shallow regions in the Dutch Wadden Sea (Royal Haskoning 2007), and from three field experiments in shallow lakes, with which to calibrate SWAN for finite depth wave growth in the Wadden Sea. Firstly, the reliability of the observations taken during the mentioned westerly storms in the Wadden Sea interior was assessed, and improved in terms of the interpretation of the wave buoy observations (which were taken in small water depth, namely 1.5-2.0 m) and the modelling of water levels. Whereas Royal Haskoning (2007) reports maximum Hm0/d ratios in excess of 0.6 for the Wadden Sea (0.55 if a plotting error is removed), the present study has shown maximum Hm0/d ratios to rather be in the order of 0.43. It is recommended to reduce the uncertainty of this result further by verifying finite depth wave buoy observations with alternative instrumentation, such as pressure sensors or capacitance probes, and by measuring water depths at the wave buoy locations. Secondly, this data set was compared with the above-mentioned data sets from shallow lakes representing depth-limited wave growth, from which a reasonable agreement was found. In this regard, the empirical relationships for the growth limit proposed by Young and Babanin (2006) appear to be applicable for Lake George and Lake Sloten, but not for the overall data set. Thirdly, a selection of 30 calibration cases of depth-limited wave growth for SWAN has been made from the data sets of the four geographical areas considered. It is recommended to investigate the performance of SWAN for these field cases by hindcasting, and to subsequently apply them in the calibration and validation of the model.SB