Nourishment strategies for the Ameland Inlet

Abstract

Sea level rise poses challenges to the tidal inlet systems of the Wadden Sea, including the barrier islands. If no measures are taken, the shorelines of the islands will retreat and the basins may drown. In the Ameland Inlet, the Boschplaat at the eastern tip of Ameland and Bornrif at the western tip of Terschelling are currently shrinking.
This thesis evaluates different nourishment strategies to counteract the coastal retreat, with the ultimate goal of improving our understanding of how nourishment sand of different grain sizes is shared between the different morphological units of the Ameland Inlet.
The process-based model Delft3D is used as a means of predicting the transport pathways of nourishment sand on a time scale of 10 years. In this thesis an existing model of the Ameland Inlet is improved, in order to make it suitable for medium term nourishment calculations. The Mormerge (parallel online) approach is applied in combination with a phase shift in the tidal signal for the different wave conditions. As a result the computational time is significantly reduced. The bathymetry is updated based on the 2016 Vaklodingen and Bed Composition Generation is used to create a suitable bottom composition. The composition of the nourishments is based on the particle size distribution of the borrow area.
The model results suggest that different objectives, such as feeding the Terschelling or Ameland coast, feeding the basin, or reinforcing the ebb tidal delta, can be met by varying the nourishment location and composition.
The main findings regarding the transport pathways within the inlet system are as follows: Near the Terschelling beach import via Boschgat is dominant. Further offshore, bypassing via the ebb tidal delta is the most important transport pathway. Boschgat feeds the western part of the basin and Borndiep feeds the eastern part, but interaction between the two is limited. The model predicts some transport from Boschgat to Borndiep via Westgat under the influence of the tide, or driven by waves over the shallow area in between the channels. There is no connection in the other direction. Borndiep connects the basin to the ebb tidal delta, resulting in transport in and out of the system. Bornrif is the only location where a net westward transport into the inlet is observed.
Finer fractions are imported further into the basin by the tidal flow. When bypassing the inlet, they are transported further offshore, since they are more influenced by the tidal currents flowing out of the inlet. As a result coarser nourishment fractions end up closer to the Ameland shoreline.
The model has several limitations, mainly since insufficient data is currently available for a sound calibration and validation. However important phenomena observed in the field are reproduced by the model, such as the coastal retreat at Boschplaat and the bar migration at Bornrif. This gives confidence in the model performance regarding the predictions of the sediment transport patterns.