The vertical structure of turbidity currents and a necessary condition for self‐maintenance

Abstract

A simple numerical model predicts that properties of a turbidity current can vary significantly in the vertical direction, particularly when more than one sediment size is in suspension. Each sediment size decreases in concentration with increasing distance from the bottom boundary at a rate which depends on its settling velocity, the current velocity, the entrainment coefficient, and the bottom slope. However, it is shown that the details of the vertical distribution of sediment load do not necessarily greatly influence either the vertical structure of the hydraulic properties of the flow (velocity, stress) or the bulk properties such as the entrainment coefficient, at least when only one sediment size is in suspension. The ability of a turbidity current to sustain itself is strongly dependent on the sediment load and an autosuspension criterion is obtained by taking into account the vertical structure of the currents. The model turbidity currents often cannot maintain their sediment in suspension even when the bottom stress is greater than the critical stress of deposition for low concentration flows. When this happens, the concentration near the bottom increases without limit as the turbidity current evolves with time. In real turbidity currents, grain‐grain interactions, which become important at high concentrations, may at times cause the current to quickly deposit its near‐bottom sediment load. This is suggested as a possible mechanism for the formation of the poorly sorted sediments frequently found in turbidites.