Modeling Nourishment in the Ameland Inlet - a long-term and short-term study with Delft3D

Abstract

Sea level rise and human impact pose threats to the ebb-tidal deltas of the Wadden Sea by severely reducing its ebb-tidal delta volume. Ebb-tidal deltas are important features because they protect the barrier islands and backbarrier basins against waves and because they can be a source of sand. Sand nourishment is an environmentally friendly solution to make the Wadden Sea and its coastline more resilient to sea level rise.

This study used the process-based numerical model Delft3D to predict the hydrodynamics and morphodynamics in the Ameland Inlet and its response to nourishment. Ameland inlet was chosen because its morphodynamic behavior is very typical of Wadden Sea systems and because management authorities are planning sand nourishments in this system. I studied the effect of different nourishment locations and sizes on the short-term (about 25 hours) and long-term (years) hydro- and morphodynamics. To study the short-term effect, this paper has used the Ameland model with the bathymetry of the measured data in the study area. The idealized model with a simplified bathymetry, which is able to reproduce the cyclic evolution of the ebb-tidal delta, has been applied to simulate the long-term morphology change with nourishment.

The short-term results show that mega nourishment (20Mm3) affects the hydrodynamics more efficiently than the pilot nourishment (5Mm3) by causing more hydrodynamic changes and extending the influenced area. Nourishment at different location leads to various hydrodynamic changes. For the outer nourishment location around the Kofmansbult (A) and the location offshore of Terschelling (B), the residual transport is dampened in the nourished area and strengthened in other areas of the ebb-tidal delta. Nourishment in the inlet (C) cuts the transport in landward of the main channel and weakened the seaward transport flux at the end of Akkepollegat, but displays an augmentation to the other inlets and the eastern side of the ebb-tidal delta. With similar hydrodynamic changes in short-term, the result of the idealized model could give insights in the evolution of the Ameland inlet. The long-term simulation results show that nourishment accelerates the cyclic evolution by shortening 15 – 30% of the period and changes the morphological features in evolution. It is expected that the nourishment near Kofmansbult might shorten 30% of the period, and nourishment near Terschelling and in the Borndiep might shorten approximate 15% of the period.

Nourishment affects the long-term development by inducing different sediment transport pattern, which results from the change in tides and waves. Processes influenced by nourishment are discussed including friction, wave refraction and tidal asymmetry. Nourishment A has the best wave shelter effect for the barriers and basin due to its wave guarding location. Nourishment B shelters waves toward Terschelling and thereby leads to a weakened littoral transport. Wave refraction in the channel and tidal asymmetry are relative important for nourishment C, which results in more sediment import into the basin and its poor performance in wave shielding effect.