Hydrodynamic processes and the stability of intertidal mussel beds in the Dutch Wadden Sea

The mussel population in the Dutch Wadden Sea, especially in the Western parts, is currently under pressure. Hydrodynamic process influence intertidal mussel beds in two ways: on the one hand currents are responsible for the transfer of food and sediments towards the beds, on the other hand, large shear stresses from waves and currents are capable of eroding mussels from intertidal mussel beds. A previous study suggested that wave exposure is an important factor determining the suitability of an intertidal area to sustain a stable intertidal mussel bed. However, the actual shear stresses associated with wave forcing have not been studied in soft-sediment environments; most research has focused on the role of currents.

In this thesis the wave induced, and current induced bed shear stresses were quantified. Wave induced bed shear are larger than those resulting from currents. They are caused by physical interaction of the wave orbital motion with the bed; wave breaking does not occur because of the small slope of the tidal flat. Current induced shear-stresses were small. However, they have a substantial effect on the total bed shear stress due to wave-current interaction. Waves lose energy as they propagate over an intertidal mussel bed as a result of increased bottom roughness of the mussel bed. Wave exposure of the seaward edge of the mussel bed is largest. Areas deeper inside the bed are protected from high wave induced bed shear stresses by this seaward edge. The seaward edge is vulnerable erosion by wave-action but it can, as observations show, also be damaged by ice-action. Both processes not only erode parts of the bed they also elevate parts of the mussel bed surrounding the eroded area. This increases the wave exposure of these areas, making these areas more susceptible to erosion. The break-up of broad band structures, commonly observed in intertidal mussel beds, by storms might also influence food availability and thereby the persistence of an intertidal mussel bed. Results reveal the occurrence of flow routing around and acceleration around the elevated mussel covered areas. Flow routing reduces food transport towards the mussel covered area but this can be compensated by increased vertical mixing resulting from the flow acceleration. The net effect on food availability depends on geometry; broader elevated mussel covered areas tend to increase the hummock acceleration type flow; while narrower mussel covered areas tend to promote flow routing.

Wave exposure is thus an important factor influencing mussel bed persistence. Therefore, the relation between mussel bed occurrence and wave exposure in the Dutch Wadden Sea was investigated. Results show that wave exposure is largest in the Western basins of the Dutch Wadden Sea. Wave exposure may not only be a factor limiting persistence but the high wave exposure during normal conditions might also limit settlement chances for mussel beds in the Western Wadden Sea. A comparison between wave exposure and mussel bed occurrence per tidal basins shows that mussel coverage is smaller in more wave exposed tidal basins.