Erosion mechanism of the landward Oostgat tidal channel slope, The Netherlands

Abstract

The Oostgat tidal channel, located in the mouth of the Scheldt estuary in The Netherlands, migrates in the landward direction by about 0.2 m/year as a result of erosion of the landward channel slope. This erosion threatens beach stability and the effectiveness of beach nourishments. The physical mechanisms controlling this channel slope erosion are yet unknown. We have studied this erosion problem with a high-resolution Delft3D coupled wave-flow-sand transport model, which we have validated against available hydrodynamic data. Initial sedimentation and erosion patterns were inferred from the computed net sand transport rates. The model results show that channel slope erosion is controlled by the tide; effects of waves and wind are relatively small. The net sand transport is mainly controlled by the interaction between the residual current and the tidal velocity variation. Previous studies attributed a prominent role to the sand ridge Bankje van Zoutelande offshore from the Oostgat channel. This has not been confirmed by our model results: erosion of the landward channel slope was still present if the ridge was removed, albeit smaller. Instead, the erosion seems to be related to a sharp channel bend just north of the erosional area. The bend drives cross-channel differences in tide-dominance resulting in an increasing ebb-directed/northward net sand transport away from the erosional area.