Gravity Currents in Lakes, Reservoirs and Coastal Regions: Two-Layer Stratified Flow Analysis

Abstract

Suspension gravity currents (turbidity currents) occurring in lakes, reservoirs, and the ocean are investigated theoretically. When the turbid water, a mixture of suspended particles and clear water, is introduced in a quiescent body of clear water, the turbid mixture flows downslope due to its larger density. Turbidity currents derive this driving force from the sediment in suspension. Such turbidity currents are essentially non-uniform and in disequilibrium because of water entrainment across the upper boundary of the flowing layer. Therefore, turbidity currents can be erosive or depositive, accelerating or decelerating as they travel over the sloping bed, exchanging sediment between the bed and the flow. The governing equations which describe the movement of gradually varied steady-state turbidity currents occurring in sloping channels with constant as well as variable widths are developed. In order to incorporate the mechanism of sediment exchange into the model, the sediment entrainment function is derived by utilizing data for suspended sediment transport in free-surface open channel flows and is applied to the analysis. The model is also applied to actual field sites. Such turbidity currents can be originated by various processes when large amounts of sediment are put into suspension. One mechanism investigated both theoretically and experimentally is the direct inflow of dense river water into a reservoir. This phenomenon is sometimes referred to as "plunging flow." The results obtained herein are expected to be useful for applications by geologists, oceanographers, and hydraulic engineers, who are concerned with understanding the impact of turbidity currents on natural environments, for example, the redistribution of sands in the ocean, subsurface morphology, and with modeling of water quality and transport of dissolved solids, suspended matter in lakes and reservoirs.