S. Xu1, M. van der Vegt1, N. Geleynse2 and K.J.H. Lenstra1
Ameland Inlet has recently been studied intensively because of the mega nourishments (~20106 m3) of its ebb-tidal deltas that are being considered. Therefore, to understand how such nourishment will behave, a good understanding of the sediment transport patterns and its spatial and temporal variability is needed. In a recent study (Lenstra et al., 2019) we showed that sediment transport patterns in Ameland Inlet not only depend on wave and tide conditions, but also strongly depend on the different phases of its cyclic evolution. However, we neglected the effect of wind and assumed the Ameland Inlet to be an isolated system where the tidal watersheds could not be flooded. The latter is realistic during calm weather conditions, but during storms the exchange of water over the tidal watersheds can be quite large. In this study we present results on the effect of wind and basin connectivity on estimated sediment transport patterns in Ameland Inlet based on the results from a high resolution model including the effects of winds, waves and flooding of tidal watersheds.
We set up a model in Delft3D/SWAN with two-way coupling (domain decomposition) between a high-resolution Ameland Inlet grid and a medium coarse grid Wadden Sea grid. Boundary conditions for the tides and waves were based on a North Sea continental shelf model and wave buoy data, respectively. We included effects of wind growth because waves in the Wadden Sea are to a large extent determined by local wind growth. Wind and pressure input came from WRF and HIRLAM data. Wind forcing was schematized by binning the wind events into 5 main directions and studying for each direction an extreme scenario and a quiet scenario in the year 2007, 2011 and 2012.
The model shows that wind force and connectivity with adjacent basin can alter the flow and the sediment transport significantly. For example, as shown in Figure , during the strong NW storms, roughly 80% of water is imported into the basin via its western watersheds and the rest 20% flow through the Westgat. Besides, the eastern watershed was the main exit of water during the storm while Borndiep become dominant in exporting water after the storm. In terms of sediment transport, Westgat contribute to almost 80% of the sediment transport during the storm while only 10% of the sediment is transported through the western watersheds. The export of a small volume of sediment can be observed via eastern watersheds. To sum up, the system accumulates the sediment during a strong NW storm. During the presentation we will also discuss the results of other wind directions and magnitudes and present the weighted mean sediment balance of Ameland Inlet and compare it with previous estimates.
Figure 1 The time-series of residual flow and sediment transport through 5 cross-sections of Ameland Inlet.