Nonhydrostatic simulation of hyperpycnal river plumes on sloping continental shelves: Flow structures and nonhydrostatic effect

We use the SUNTANS model to simulate plunging hyperpycnal plumes on different shelf slopes in idealized domains. Our simulations reveal detailed three-dimensional flow structures in both transient and steady states. An important flow feature is the formation of the triangular suspended sediment concentration pattern at the surface, which results in a flow convergence upon plunging and closely relates to the plume’s three-dimensional flow structure. As the slope is steeper, the triangular pattern becomes shorter in the cross-shore distance, resulting in stronger flow convergence. We show that the nonhydrostatic effect is particularly important before plunging, where the plume is subject to a significant change in the vertical direction. Moreover, the great fluctuations of the velocity field from the hydrostatic calculation result in a much higher eddy diffusivity compared with the nonhydrostatic case, leading to slower sink of the plume while plunging during the transient state. We also conduct depth-integration analysis of both the cross- and along-shore momentum components to examine the bulk effect of the shelf slope, the three-dimensional flow structure, and the nonhydrostatic pressure. We find that the flow convergence due to plunging of the plume provides strong lateral transport of the cross-shore velocity component toward the central line, which leads to the difference between three- and two-dimensional plumes. The flow becomes divergent when the plume forms an undercurrent, in which the lateral transport becomes an important momentum sink.