Hydrodynamics over a patch structure on an intertidal mussel bed

Abstract

The Mosselwad project investigates the stability and restoration opportunities of mussel beds in the Dutch Wadden Sea. The experiments in this study, which were performed in the framework of this project, investigate the key hydrodynamic processes for the survival chances of mussel beds and especially the possible important effects of the mussel bed patch size on hydrodynamical forcing. Previous studies on hydrodynamic processes in relation to mussel beds were performed in flume experiments. This limits the possibilities of expanding these results to field conditions. Therefore a field study was performed on a young intertidal mussel bed near Texel in the Dutch Wadden Sea to obtain key hydrodynamic parameters. The length and width of the investigated mussel patch was ~20m, while the elevation of the patch above its surroundings was ~10cm. Turbulent kinetic energy dissipation rates were estimated from obtained measurements and used to calculate turbulence parameters and bed roughness values. Results showed higher current velocities above the patch than downstream of the patch and further away of the patch. Also higher turbulent kinetic energy values and a higher eddy viscosity on the patch than downstream of the patch were observed. Obtained wave heights on the site were small and effects of the mussel patch on the wave height were not observed, however even orbital velocities of these small waves dominated the total bed shear stress and generated additional turbulent kinetic energy. On the other hand observed current velocities showed weak indications for flow routing, however the measurement setup was insufficient to formulate strong conclusions. The higher turbulent kinetic energy values and eddy viscosity above the mussel patch shows that vertical turbulent mixing was enhanced as flow passed the upstream patch boundary, while flow still experienced a high roughness length behind the downstream patch boundary due to flow recovery effects. Overall, the results of this study confirm the importance of patch boundaries for the turbulent mixing and thus food supply of the mussels. Additionally flow routing might play a major role for patch structures, but additional research is required to support this assumption.