Complex patterns of wave- and tide-driven flow and sediment transport on the cyclic ebb-tidal delta of the Ameland inlet : A short-term model study with a coupled Delft3D-model

Abstract

Ebb-tidal deltas are shallow accumulations of sand located seaward of tidal inlets. These deltas feature a complex morphology of flood- and ebb-dominated channels, shoals and swash bar complexes. This research focused on the ebb- tidal delta of the Ameland inlet, located along the Dutch Wadden Sea. At present, the ebb-tidal deltas of the Dutch Wadden Sea already show a clear eroding trend. Moreover, many of these deltas display (natural) cyclic dynamics, causing channels and shoals to migrate and change position relatively fast. The aim of this research was to gain insight in the complex patterns of wave- and tide- driven flow, sediment transport and initial morphological change at the ebb-tidal delta of the Ameland inlet on short time scales. The response of these patterns to varying hydrodynamic forcings (one can think of changes in the wave height and direction) and for the (cyclic) bathymetries in the years 1999 and 1971 of the Ameland inlet system was examined. Results of this research show that wave- driven currents and sediment transport is most pronounced at the shoals, while tide-driven currents and sediment transport are more confined to the ebb-and flood-dominated channels of the delta. During quiet (storm) conditions, which occur for about 50% (1%) of the time, the delta is dominated by tides (waves) in terms of residual flow patterns and by tides (waves) and interaction in terms of residual sediment transport. Therefore, it is suggested that waves indirectly govern the patterns of sediment transport and morphological change by means of interaction (one can think of stirring of sediments by waves on the shoals of the delta). Furthermore, it is shown that varying wave directions (especially during storm conditions) and the cyclic morphological evolution of the ebb-tidal delta affects the magnitude and direction of the residual patterns in flow and sediment transport (including sediment bypassing) largely.