Eva Kwoll

Processes controlling cohesive sediment transport in complex coastal environments

My current research focuses on cohesive sediment transport in tidal rivers in the form of distinct clouds of sediment. These structures are observed (e.g. in the Elbe Estuary) for a short period (up to 1 hour) after slack water during accelerating tidal currents until there is a homogenous sediment mixture occupying the water column.
The intermittent nature of suspended transport is commonly associated with the occurrence of coherent flow structures generated in the bottom boundary layer. Coherent Flow Structures are irregular, but repetitive turbulent events believed to be caused by the deformation and evolution of vortices in the seabed region. They are characterised by rapid in- and outrush of fluid from the near-bed region, interspersed with quiescent inter-event periods.
In a field campaign, high frequency hydro-acoustic and optical sensors have been deployed to derive time series of velocity fluctuations and suspended sediment concentration throughout the water column over several tidal cycles. This will enable the determination of inter-event period of the flow structures and suspension events close to the seabed and possibly scaling to dune or flow variables. Furthermore, it is aimed at a better understanding of the processes causing the confined upward transport of material and sustainment of the clouds at the water surface.