Observed flow patterns near hummocks in intertidal mussel beds

Intertidal mussel beds are a key species in the ecosystem of the Dutch Wadden Sea. Mussels beds are considered eco-system engineers which accumulate and stabilize sediments and therefore create possibilities for coastal protection. While some mussel beds remain uniform and flat others form hummocks. Here, we investigate the influence of hummock type mussel beds on hydrodynamics. Height observations over intertidal mussel beds show that hummock height increases with increasing average water depth. A four week field campaign revealed that hummocks (height 0.4 m) significantly influence the tidal flow. When the flat is covered with little water, i.e. hummocks are not fully covered, the flow velocity between hummocks is large. When the hummocks become covered flow over the hummocks is relatively large while flow between hummocks is small. As the water level rises further the flow area increases resulting in smaller velocities over the hummock. During ebb flow similar flow observations were made. In the presentation we will discuss the physical mechanism behind feedbacks between hummock formation and hydrodynamics.