Modeling Sedimentation Dynamics of Sediment-Laden River Intrusions in a Rotationally-Influenced, Stratified Lake

Abstract

River inflows are often the dominant source of sediment into a lacustrine system and the extent of sediment transport can be an important indicator of the impact of introduced constituents on an ecosystem. Inflow intrusions, which intrude into the middle of the water column at a depth of neutral buoyancy, can be common in deep, stratified lakes and reservoirs. However, the dynamics and associated sediment transport of these flows have received less attention in the literature compared to surface or underflow plumes. A three-dimensional hydrodynamic and sediment transport model was used to investigate sediment transport due to inflow intrusions in Pallanza Bay, an embayment within Lake Maggiore, Italy. The model was validated using field observations of inflow intrusions over varying conditions and was then used to simulate a range of realistic scenarios to evaluate sediment transport and deposition from inflow intrusions. The extent of sedimentation across the range of simulated scenarios can be estimated using a simple analytical expression with only a priori known parameters. The expression is a function of the intrusion plume speed and thickness, which are governed by the volume flow rate and the ambient stratification. We utilize the first-mode wave speed to characterize the ambient stratification in the simplified expression, which eliminates the need to characterize near-field mixing and entrainment to predict the intrusion depth. While the simple expression gives a reasonable estimate of the sedimentation extent, it does not capture lateral variability in the plume due to effects like lateral spreading and the earth's rotation.