Tidally- and Wind-Driven Horizontal Flow Patterns on Intertidal Flats in the Wadden Sea

Abstract

The Wadden Sea is of great ecological importance. Key elements in the Wadden Sea are the extensive intertidal flats. These contain vast amounts of life, serve as feeding grounds for millions of birds, and provide ecosystem services such as coastal protection. It follows that it is important to be able to understand and predict how these vital elements of the highly dynamic Wadden Sea change over time.

The existence and evolution of intertidal flats rely on the intricate balance between erosion and sedimentation caused by the forcing of wind, waves, and the tide. Exact horizontal flow patterns on intertidal flats are not fully understood, especially under the influence of wind. This study aims to systematically investigate the tidally- and wind-driven horizontal flow patterns on the scale of an intertidal flat in the western Dutch Wadden Sea. To do so, existing literature is assessed and a subgrid-based hydrodynamic model of the Dutch Wadden Sea is created. Most hydrodynamic models require a wetting-and-drying algorithm to resolve the flow patterns near the wet/dry interface. Several types of wetting-and-drying algorithms exist, each with unique advantages and limitations. Subgrid modeling technique does not require a wetting-and-drying algorithm as wetting and drying is automatic. The adequacy of using a subgrid-based hydrodynamic model to achieve the aim of the research is investigated through a comparison of simulations with different settings. Computational grid resolutions are manipulated on the scale of the intertidal flat, along with whether subgrid modeling technique is applied. The tidally and wind-driven horizontal flow patterns are assessed using simulations in which the phase of the spring-neap tidal cycle is manipulated, along with whether wind forcing is applied. These scenarios are applied to two non-fringing intertidal flats with different geometries.

Simulations show that subgrid modeling technique is beneficial for representing the impact of small-scale bathymetric features on the flow pattern while using a coarser computational grid. Subgrid modeling technique offers an increase in how well the flow pattern is represented compared to simulations using interpolated bathymetry and the same computational grid resolution. The automatic wetting and drying is crucial for representing shallow flows, especially at depths where measurements and other hydrodynamic models face difficulties.

Results indicate that the horizontal flow patterns on intertidal flats follow the water level gradient between surrounding channels and are influenced by the intertidal flat geometry (i.e. size, bed elevation, and degree to which the intertidal flat is intersected by tidal channels), hydraulic boundary condition (i.e. phase of the spring-neap tidal cycle, asymmetry of the tidal wave), and wind forcing (which can completely alter hydrodynamics at several scales).

This report contributes to the knowledge on horizontal flow patterns on intertidal flats, benefiting studies into ecology, navigational maintenance, flood protection, and the ability of intertidal flats to keep up with sea level rise. However, the flow pattern is not solely responsible for morphological change. Moving forward, continued research into the drivers of the morphological change of intertidal flats will be crucial for informed management of ecologically valuable estuarine areas like the Wadden Sea.