Density- and wind-driven lateral circulation and the associated transport of sediments in idealized partially mixed estuaries

Abstract

Lateral circulation and the associated transport of sediments in idealized partially mixed estuaries are investigated using a three-dimensional, hydrostatic, primitive equation numerical model (ROMS). The model simulates a straight estuarine channel with a triangular cross-section. Attention is focused on lateral density (salinity) gradients, the major driving force for lateral circulation. Lateral salinity gradients can result from boundary mixing on a slope and differential advection of axial salinity gradients.

Without wind forcing, the numerical experiments suggest that boundary mixing on a slope can drive significant lateral circulation when the water column is stratified. Boundary mixing is at least as important as differential advection for the modeled scenarios, when the two mechanisms are evaluated using the salt balance equation. Sediments are eroded in the channel and preferentially deposited on the right slope (looking seaward), mainly due to tidal pumping.

Both stratification and axial salt transport show strong responses to axial wind forcing. While stratification is always reduced by up-estuary winds, stratification shows an increase-to-decrease transition as down-estuary wind stress increases, due to the competition between wind-induced straining of the axial salinity gradient and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition. A regime classification diagram is proposed.

Axial winds also exert important controls on lateral circulation. When the water column mixes vertically, surface Ekman transport is not a significant contributor to lateral circulation. Instead, wind-induced differential advection of the axial salinity gradient establishes lateral salinity gradients that in turn drive lateral circulation. A Hansen-Rattray-like scaling shows good predictive skill for variations in lateral flow. Event-integrated sediment transport is from channel to shoals during down-estuary winds but reversed for up-estuary winds. Accounting for wind-waves results in an order-of-magnitude increase in lateral sediment fluxes.

The effects of wind-waves and seagrass beds on nearshore (< 2m) sediment dynamics are explored separately using a nearshore model (NearCoM). Without seagrass beds, wind-waves greatly enhance sediment resuspension, providing a large sediment source for lateral sediment transport. Seagrass beds attenuate wind-wave energy and trap sediments, thus reducing net sediment loss from the shallow shoal.