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

Abstract

In this study, we have compared three methods to determine the exposure times of intertidal flats in the Dutch Wadden Sea. The first method was based on a triangulation (TRIA) of the sea level elevations measured at the tidal gauges surrounding the Dutch Wadden Sea, following Rappoldt et al. (2004); for the second, method numerical simulations 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 show a good agreement for the western Dutch Wadden Sea, an area where the density of intertidal flats is low. However, the results of TRIA and GETM show differences of as much as 20% for the much shallower eastern part of the Dutch Wadden Sea.To explore the influence of the number and distribution of tidal gauge stations on these differences, virtual tidal gauges were added to the existing network of tidal gauge stations, based on model results. An analysis showed that there is limited added value to an even large (three-fold) increase in the number of tide gauges, largely because of the highly non-linear behavior of the tidal wave in the model compared to the linear approach adopted in the triangulation method.The third approach (HYBRID) was developed by combining the previous two methods. Tidal prediction was obtained from applying a Least Squares Harmonic Analysis on the Sea Level Height (SLH) in the simulation with GETM at every grid point. Moreover, the unpredictable part, e.g. the set-ups or set-downs induced by winds from the North Sea or the European continent, was determined by applying the triangulation method to the wind-induced SLH observed at the tidal gauge stations. This wind-induced SLH was defined as the observed sea level height minus the tidal prediction and its long-term mean value. This combination of methods offers a new approach to determine exposure times in the Wadden Sea more accurately than either method individually.