Process-response analysis for Hörnum tidal inlet in the German sector of the Wadden Sea

Abstract

The Hörnum tidal inlet is located in the German sector of the Wadden Sea between the barrier islands Sylt and Amrum. On the basis of seven bathymetric surveys of the tidal inlet covering a 55-yr period from 1939 to 1994 and long-term records from two tide gauges, a process-response analysis for the inlet was carried out. Following the method described by Dean and Walton (1975), the volume of sediment stored in the ebb–tidal delta (Vebd) was calculated for each survey. Furthermore, the cross-sectional area of the tidal inlet throat (Ac), and the area-height distribution (hypsographic curve) of the study area were established. Between 1939 and 1994, Ac increased by about 32%, whereas Vebd diminished by about 18%. Earlier investigations by, e.g., O'Brien (1931), Walton and Adams (1976), Dean (1988), and Eysink and Biegel (1992) documented a strong positive correlation between these two morphological parameters and tidal currents in the inlet. In conformity with the literature, regression of the values of Ac with the corresponding fall velocities of the water level in the inlet (a direct indication of ebb–tidal currents) produced a correlation coefficient (r) of 0.87. However, Vebd shows a weak negative correlation (r=−0.74) with fall velocities of the water level. Apparently, other factors than changes in ebb–tidal currents must have been responsible for the decrease in Vebd. It is suggested that one factor might be the observed considerable increase in storminess, i.e. wave action, in the region since approximately 1960. Consequently, storm wave action on the ebb–tidal delta intensified and erosion was here initiated. As a result, the swash bars of the ebb–tidal delta, in particular, suffered from severe erosion between 1959 and 1994. As the ebb–tidal currents increased simultaneously, sand supply to the terminal lobe of the ebb–tidal delta increased as well, thereby balancing the loss of sediment from the terminal lobe caused by the increasing storminess. The results of this case study indicate that the ratio of ebb–tidal currents to (storm) wave action determines, in large part, the value of Vebd.