A DIFFUSIVE TRANSPORT MECHANISM FOR FINE SEDIMENT

Abstract

A simple conceptual model (SCM) for the net transport of suspended sediments associated with a simple harmonic tidal current superimposed on a spatially varying, wind-wave-induced near-bottom concentration is developed. Because the SCM accounts for the response of the sediment concentration in the water column to the varying near-bottom concentration, it predicts a net transport rate which is proportional to and in the downward direction of the imposed bottom concentration gradient, i.e. expressible as a diffusive process. In view of its extreme simplicity, which allows for a closed form analytical solution for the apparent diffusivity, the predictions of the SCM are shown to be (surprisingly) accurate by comparison with predicted net transport rates afforded by the numerical solution of the Princeton Ocean Model (POM). Furthermore, the transport rate predicted by the SCM is found to be of comparable magnitude to the long-term average transport rate predicted by conventional equilibrium-based modeling of suspended transport of very fine sediment for a site on the Northern California shelf. Since the diffusive transport mechanism identified and quantified in this paper is entirely missed by equilibrium-based models for suspended sediment transport, the results obtained here strongly suggest the necessity to properly account for non-equilibrium effects when modeling suspended sediment transport in coastal waters.