Storm sediment transport: observations from the British North Sea shelf

Abstract

Measurements of wave heights, near-bed currents, bed shear stresses and suspended-sediment concentrations and fluxes from a severe storm are described. The data are from 25-m water depth on the British North Sea shelf. The current above the wave boundary layer was retarded by bed roughness that was dominated by wakes shed from saltating grains in the bedload layer and by wave-current interaction in the wave boundary layer. The bed roughness increased by two orders of magnitude immediately prior to overturning of the instrument installation by large waves; the cause for the increase is not clear but is consistent with the formation of large-scale bedforms at high transport stage. Backscatter data revealed wave resuspension of bed sediment, modulation of sediment concentration by wave groups, and advection of dense sediment clouds by the current. The vertical profiles of mean suspended-sediment concentration were, nevertheless, consistent with a simple turbulent-diffusion model that incorporates different diffusion scales within and above the wave boundary layer. A single measurement of the sediment reference concentration at the height of the storm yielded an estimate of 0.00025 for the entrainment parameter, γ o, but at other times γ o was O(10 −3). The change in bed roughness might have caused the decrease in entrainment rate at the height of the storm. The erosion depth required to support the suspended-sediment load was a maximum of ∼1 cm, but was typically an order of magnitude smaller than that; suspension was the dominant transport mode. A wind-driven mean flow distorted the tidal ellipse and caused a net transport of sediment along- and off-shore; depth-integrated sediment transport during the storm was up to two orders of magnitude greater than prior to the storm 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.