Modelling the effect of an ebb-tidal delta nourishment of local grain size distribution patterns

The ebb-tidal delta of the Ameland Inlet system is of great ecological importance since it hosts a variety of benthic habitats. In a pilot project, a (mega)nourishment of 5 million cubic meters of sand was carried out on the outer ebb-tidal delta. Since the strong correlation between habitat preference and sediment characteristics on the ebb-tidal delta, it is crucial to gain insight in the mechanisms governing sediment sorting in this area. To study the initial response of grain size distribution patterns, and subsequently potential changes to benthic habitats, process-based models are useful tools. Here, we present a hydro- morphodynamic model of the Ameland Inlet which is able to simulate grain size sorting patterns. Building upon previous modelling efforts, we specifically focus on the inclusion of graded sediment transport and bed stratigraphy. The simulations, here with the nourishment excluded, reveal that the model results on grain size distribution can be related well to the forcing in combination with the local depth. Particularly, it turns out that tidal motion is responsible for the sorting pattern in the deeper parts of the ebb-tidal delta, whereas the grain size distribution in the shallower, offshore parts is particularly shaped by wind and wave processes. Model sensitivity to several numerical parameters (thickness transport layer, MORFAC) is also investigated and seems relatively small, although further investigation is needed here. A preliminary comparison reveals that the model results compare qualitatively well to field data, although the shallower offshore areas seem to be predicted too coarse in the model