Developments in Operational Shelf Sea Modelling in Danish Waters

A three-dimensional hydrodynamic model is used to investigate the potential for offshore bathymetric deeps in the North Sea to act as fine sediment traps. Simple conditions are established, representative of the observed conditions, and the response of the model investigated. Two deeps are considered here, Inner Silver Pit and Sole Pit. The model study suggests that in Inner Silver Pit deposition of fine sediment is only possible under weak tidal (neap) flows. Under all other conditions the flow is too energetic to allow deposition. A transport path for fine sediment across Inner Silver Pit is established but does not significantly modify the sediment dynamics in the pit. Experiments including wave action show that 50-year return period waves induce bed shear velocities sufficiently large to remove coarse sediment. For Sole Pit a cycle of deposition followed by erosion at spring tides will inhibit deposition in the pit. Long term accretion of fine sediment does not seem possible in either pit.