Estimates of exposure times in the Wadden Sea : A comparison of methods

In this study, we have compared three methods to determine the exposure times of intertidal flats in the DutchWadden Sea. The first method was based on a triangulation (TRIA) of the sea level elevations measured at thetidal gauges surrounding the Dutch Wadden Sea, following Rappoldt et al. (2004); for the second, method numericalsimulations with the General Estuarine Transport Model (GETM) were used, and the third method (HYBRID)is a combination of the previous two. The first two methods showa good agreement for thewestern DutchWadden Sea, an area where the density of intertidal flats is low. However, the results of TRIA and GETM showdifferences of as much as 20% for the much shallower eastern part of the DutchWadden Sea.To explore the influence of the number and distribution of tidal gauge stations on these differences, virtual tidalgauges were added to the existing network of tidal gauge stations, based on model results. An analysis showedthat there is limited added value to an even large (three-fold) increase in the number of tide gauges, largely becauseof the highly non-linear behavior of the tidal wave in themodel compared to the linear approach adoptedin the triangulation method.The third approach (HYBRID) was developed by combining the previous two methods. Tidal prediction was obtainedfrom applying a Least Squares Harmonic Analysis on the Sea Level Height (SLH) in the simulation withGETM at every grid point. Moreover, the unpredictable part, e.g. the set-ups or set-downs induced by windsfrom the North Sea or the European continent, was determined by applying the triangulation method to thewind-induced SLH observed at the tidal gauge stations. This wind-induced SLH was defined as the observedsea level height minus the tidal prediction and its long-term mean value. This combination of methods offers anewapproach to determine exposure times in theWadden Seamore accurately than eithermethod individually.