A Model of Mixing in a Stratified Tidal Flow

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by Stephen Monismith, Stanford Univ, Stanford, United States,
Derek Fong, Stanford Univ, Stanford, United States,
Mark Stacey, Stanford Univ, Stanford, United States,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering:

Abstract: We will describe a simple model of vertical mixing in a stratified tidal flow that might be observed in shallow estuary like San Francisco Bay. The model flow is driven by a specified surface pressure gradient due to barotropic tidal motions. The model incorporates two means for accomplishing vertical mixing: (1) shear instability; (2) bottom mixed-layer deepening by turbulence produced at the estuary bottom and in the shear layer at the top of the mixed layer. We will discuss model results that show how the combined action of shear instability and tidal `stirring' accomplishes vertical mixing. The interaction between the instabilities that develop because of the substantial tidal time-scale velocity shear caused by stratification is the key feature of mixing. Shear instability weakens the stability of the mixed-layer, allowing stirring to efficiently entrain lighter fluid into the mixed layer. As it is traditionally described, i.e., without shear, tidal stirring appears ineffectual and not likely to lead to significant changes in bottom mixed layer depth or buoyancy.

Subject Headings: Tides | Water stratification | Stratified flow | Water flow | Estuaries | Bays | Mathematical models | Hydrologic models |

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