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Intratidal Transport and Mixing Processes at the Shoal-Channel Interface in a Partially-Stratified Estuary


This work addresses transport and mixing processes at work during each tide across the shoal-channel interface. These are fundamental to long-term, large-scale, salt, sediment and biomass budgets yet remain poorly characterized and understood. Most of the analyses presented here are based on field observations collected in South San Francisco Bay, CA, during winter 2009.

Horizontal transport across the interface is governed by the transverse flow, which is predominantly characterized by the presence of one or two lateral circulation cells. This study reveals that these circulation cells can evolve more rapidly than previously thought, reversing multiple times during a single ebb tide. The formation of convergence fronts is found to be sensitive to the direction of the lateral circulation and is therefore marked by a similar intratidal variability. It is hypothesized that these intratidal variations are the result of competing lateral density gradients which are the main forcing mechanism for transverse flows. The outcome of this competition depends strongly on vertical mixing conditions on the slope.

Vertical mixing on the slope is driven for the most part by turbulence generated in the bottom boundary layer, except for occasional late-ebb bursts generated by internal vertical shears. These internal shears result from the destabilizing straining of lateral shear by the lateral circulation which overcomes in this case the stabilizing straining of lateral density gradients.

Horizontal mixing across the interface is quantified indirectly and is found to be driven mostly by lateral shear instabilities, but is also affected by lateral dynamics. As a result, transverse mixing also displays significant intratidal variability. This result poses a challenge to existing parametrization developed for steady shear layers.

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