A numerical investigation of the dynamics and structure of hyperpycnal river plumes on sloping continental shelves

A 3‐D hydrodynamic model (Regional Ocean Modeling System) is used to investigate the dynamics and structure of hyperpycnal river plumes over sloping continental shelves. The focus is on the plume's response to varying slopes and settling velocities ( w s ). The idealized model is configured to represent small mountainous river systems during a flood event. A hyperpycnal sediment concentration of 60 g/L is specified at the river mouth such that the sediment–freshwater mixture is denser than the seawater, causing the plumes to traverse the shelves as undercurrents. A realistic range of shelf slope of 0.001–0.03 is chosen. The settling velocity is varied based on river's carrying capacity. The model‐derived velocity profiles and the entrainment rate compare favorably against prior laboratory experiments. Both cross‐shore and alongshore momentum balances are primarily between gravitational forcing and bottom friction. But, the Coriolis deflection is significant at the plume core in the alongshore momentum budget (i.e., Ekman balance). As the slope increases and settling velocity decreases, hyperpycnal plumes transition from depositional to autosuspending regime. An estimate of critical slope governed by a dimensionless parameterw s3 / q b( q b is buoyancy input) reasonably captures the regime transition. In the depositional regime, the plume's runout (cross‐shore penetration) scales with advective distance: increasing slopes and discharge enhance the gravitational forcing and plume velocity, leading to an increase in runout. In contrast, increasing settling velocity shortens the vertical settling time, thereby reducing the plume's horizontal footprint. For the range of parameters considered, the runout of depositional plumes is confined within 15 km from the mouth, whereas the penetration of autosuspending plumes is essentially unlimited.