Suspended sediment storm modeling

Abstract

A variety of sediment-transporting storms was observed in the HEBBLE long deployment indicating several distinct forcing mechanisms; in particular, some storms appear to originate locally (Gross and Williams, this issue). In this paper, a simple model of a locally forced storm is presented to elucidate the time-dependent dynamics of erosion and deposition events in the deep sea. Based on these modeling efforts, we make the following inferences. Storms exhibit peak sediment concentration before peak velocity. This behavior is due to a limited amount of erodible bed sediment. After near-instantaneous erosion of the bed is complete, fine-sediment concentration in the water column drops due to dilution throughout a thickening turbulent mixed layer in the accelerating flow. During and immediately after a storm event, turbulence intensity above the 40-m-thick momentum mixed layer can be sufficient to suspend fine particles in low concentration to elevations of 100 m above bottom or more. Following the storm, the rate of sediment clearing depends on gravitational settling and the potential importance of additional turbulence-driven deposition and aggregation mechanisms. Syn- and post-storm "overthickening" of the suspended sediment mixed layer relative to the Ekman momentum mixed-layer thickness, as well as sensitivity of the post-storm concentration profile to turbulence-driven deposition, can explain some complex features of near-bed nepheloid layers in the HEBBLE region observed by other workers. Validation of suspended sediment transport models is based on field observations obtained with light transmissometers. Transmissometer measurements of suspended sediment concentration depend, in turn, on calibration of instrument response to both mass concentration and particle size distribution. By analyzing the temporal and spatial changes in mass concentrations with the model presented herein, we calculate an operational calibration function. During HEBBLE storms, this calibration function does not change by more than 30% due to particle sorting by turbulent suspension. © 1991.