Initial erosion-deposition patterns in a sheltered coastal environment: the role of wind, waves, tides and graded sediment

Abstract

Nature-based sandy coastal protection measures are becoming increasingly more important as a sustainable alternative for hard engineering protection works (e.g. dikes, sea walls and breakwaters). Currently, most sandy coastal interventions are sited at open-ocean coastal systems and consist of a uniform sediment mixture (e.g. ‘Sand engine’ at the Holland coast). However, there is also a need for sustainable coastal protection measures in more sheltered coastal environments such as estuaries, deltas and bay-side beaches. In addition, natural coastal systems often consist of a wide range of sediment sizes which can substantially alter the morphodynamic processes. Therefore, it is essential to understand the hydrodynamic and morphodynamic processes to predict the evolution of sheltered coastal interventions with non-uniform sediments.

This study applies the widely used morphodynamic Delft3D model to study the sediment transport dynamics of such a system. The ‘Prins Hendrikzanddijk’ (PHZD), situated on the bay side of the Texel barrier island in the Dutch Wadden Sea, is used as a prototype system for a sheltered coastal intervention with non-uniform sediments. A Delft3D model is set up for the PHZD and the hydrodynamics are extensively calibrated with wave height and velocity data from a 6-week field campaign. The design of the model experiments is twofold: 1) the environmental boundary conditions are systematically varied to identify the role of different drivers (waves, tides, winds) on sediment transport, 2) the number of sediment sizes and initial bed composition is varied to determine the role of graded sediments and the bed composition on sediment transport. In order to model the bed composition changes, a multi-layer bed stratigraphy module is applied.

The results show that the sediment transport is a function of the wind velocity, wind direction and the geometry of the basin. During moderate energetic conditions, the orbital wave motion dominates the bed-shear stress. However, for more extreme storm conditions, the wind- and wave-driven currents become increasingly more important. With an increasing number of sediment fractions, the total sediment transport converges towards a larger suspended transport and a smaller bedload transport compared to a single fraction. Furthermore, the process of preferential transport, hiding and exposure effect and bed armoring are modelled by the multi-layer bed stratigraphy. Nevertheless, the hydrodynamic conditions are of greater importance for the bed evolution than the bed composition as there is little difference in bed level changes for an initial fine or a coarse bed composition. In addition, it is shown that the initial vertical bed composition is crucial for bed composition changes. However, the multi-layer bed stratigraphy model threats the sediment transport as diffusive fluxes while the observations show the deposition of discrete layers. Therefore, the model is not able to reproduce the observed vertical sorting.