Morphologic projections of the Boschplaat overwash

Abstract

This study investigates the morphological development of a planned artificial washover on the Boschplaat, Terschelling. The washover is part of a plan to reintroduce dynamics to the island tail. This study uses a Delft3D FLOW-SWAN model coupled to a MATLAB model to simulate six different storm classes, of which the effects are added linearly to investigate whether this method can be used to model mid-term timescales. The influence of waves, washover height and storm direction is examined on a storm and yearly time scale.

It is shown that wave set-up is an important factor in sediment transport as it results in larger onshore-directed water level gradients during overwash. An absence of waves leads to reduced onshore or even offshore transport. Northwesterly storms are expected to be most important for sediment input onto the Boschplaat as these occur more frequently and move more sediment per storm. Sediment input into the washover fan increases for lower gap heights as the washover is affected by more storms and more sediment is moved per storm. The inclusion of a dune model for longer periods is advised as it impacts the erosion-sedimentation balance significantly: there is 13-22% less erosion in the washover throat of a 1.7 m high washover when dunes are included.

Sediment transport is largely governed by the water level gradient over the island tail. Washover height and storm orientation are important factors for sediment transport. To investigate the development of the washover on decadal timescales, a better way to upscale needs to be found, and aeolian processes should be included.