Influence of suspended sediment concentration on hyperpycnal delta progradation

Abstract

When rivers with high suspended sediment load plunge into lakes and reservoirs, the resulting density currents often cause the formation and progradation of hyperpycnal deltas. Suspended load can contribute to delta progradation through two different mechanisms: (1) indirectly, by increasing the excess density of the underflows, thus enhancing the basal shear stresses that drive along-bed transport; (2) directly, by settling out of suspension onto the evolving bed. In this work, we conducted laboratory experiments designed to investigate the relative importance of these two mechanisms, aided by a conceptual model that includes both processes. The experiments are conducted in a narrow tank of constant slope, supplied with prescribed water, sediment, and/or saline influxes. Both suspended sediment load and salinity can therefore contribute independently to the excess density of the inflow. Simultaneous measurements of delta profile evolution and suspended sediment concentration are then acquired using imaging methods. To interpret the results, we construct a simplified one-dimensional model of delta progradation in which along-bed transport is modelled as a diffusion process, and suspended sediment settling as an advection-deposition process. We then examine the influence of process coefficients on the morphology and rate of evolution of the delta fronts and compare simulations with the experiments. It is found that the evolution of the bed profile alone is not sufficient to distinguish between the two mechanisms, hence the importance of simultaneously measuring suspended sediment concentration. Although obtained in a simplified setting and at reduced scale, the results should provide useful guidance for the modeling and monitoring of reservoir sedimentation at field scales.