Abstract

This paper presents a theoretical model that considers near bottom flow and sediment transport resulting from moderate to strong wind-driven, mid-depth currents. It includes the extremely important effects of wind waves interacting nonlinearly with the local currents to produce substantially enhanced boundary shear stresses and sediment transport rates. The model incorporates essential sediment transport related processes such as: 1. turbulence damping by stable density stratification caused by suspended sediment; 2. boundary roughness enhancement as a consequence of bed load and suspended load transport; and 3. bed armouring resulting from more rapid removal of particles with low settling velocities from the bed. This paper describes results of the model for the type of extreme winter storms that rework the surface sediments on the central part of the Washington continental shelf. The predicted stratigraphic signature is a 50 mm thick layer grading from very fine sand at the base to fine silt at the top, produced by the coinciding of a large amplitude, long wavelength swell and severe storm driven currents. Available wave and current data suggest that such conditions occur about once every four years. However, the net deposition rate is only 1 mm/year. Consequently, if there were no bioturbation on this segment of the Washington continental shelf, the geological record there would consist of a sequence of layers about 4 mm thick, each of which would be graded from fine sand to coarse silt. The medium and fine silts would not be preserved at this location but rather would exist only as a transient surface deposit. In reality, the bioturbation rate is sufficient to incorporate much of the silt into deeper layers and obliterate most of the physical record. Although the quantitative predictions of our model are specific to the Washington continental shelf, the processes characterized by it are applicable to many ancient and modern geological settings, as are the qualitative aspects of the results.