STORM SEDIMENT TRANSPORT - OBSERVATIONS FROM THE BRITISH NORTH-SEA SHELF

Measurements of wave heights, near-bed currents, bed shear stresses an d suspended-sediment concentrations and fluxes from a severe storm are described. The data are from 25-m water depth on the British North Se a shelf. The current above the wave boundary layer was retarded by bed roughness that was dominated by wakes shed from saltating grains in t he bedload layer and by wave-current interaction in the wave boundary Layer. The bed roughness increased by two orders of magnitude immediat ely prior to overturning of the instrument installation by large waves ; the cause for the increase is not clear but is consistent with the f ormation of large-scale bedforms at high transport stage. Backscatter data revealed wave resuspension of bed sediment, modulation of sedimen t concentration by wave groups, and advection of dense sediment clouds by the current. The vertical profiles of mean suspended-sediment conc entration were, nevertheless, consistent with a simple turbulent-diffu sion model that incorporates different diffusion scales within and abo ve the wave boundary layer. A single measurement of the sediment refer ence concentration at the height of the storm yielded an estimate of 0 .00025 for the entrainment parameter, gamma(o), but at other times gam ma(o) was O (10(-3)). The change in bed roughness might have caused th e decrease in entrainment rate at the height of the storm. The erosion depth required to support the suspended-sediment load was a maximum o f similar to 1 cm, but was typically an order of magnitude smaller tha n that; suspension was the dominant transport mode. A wind-driven mean flow distorted the tidal ellipse and caused a net transport of sedime nt along- and off-shore; depth integrated sediment transport during th e storm was up to two orders of magnitude greater than prior to the st orm under tidal currents. Wave-orbital currents directly transported a small, but not negligible, fraction of the total sediment load at 34 cm above the bed.