Wind Effects on Exchange Flow, Lateral Circulation and Salt Dynamics in Microtidal Estuaries

Wind affects estuarine circulation and stratification both by increasing vertical mixing and by modifying the along-channel velocity profile. The effects of axial (down-estuary) and nonaxial wind on the exchange flow, stratification and lateral circulation in microtidal estuaries were investigated in this dissertation using a 3D hydrodynamic model of an idealized straight estuary. For the axial wind, a theoretical model based on the depth-integrated momentum budget predicts the transverse structure of exchange flow. An expression of surface mixed layer thickness (hs) is derived by equating along-channel and vertical buoyancy fluxes. The effects of wind forcing on the estuarine circulation can be characterized by the ratio of hs to the maximum water depth (hs/hmax), which is shown to be a function of Wedderburn number. Nonaxial wind causes cross-sectional asymmetries in the exchange flow, lateral circulation and salt field. Lateral Wedderburn number (Wy) is derived to capture the competition between wind-driven and density gradient driven lateral circulation. Previous theories of the effects of wind on the estuarine circulation and salt dynamics are mostly based on either real or idealized straight channel estuaries. However, natural estuarine channels often exhibit meandering planforms which influence the flow and salinity structures. In this dissertation, a 3D hydrodynamic model was used to study the effects of channel curvature on the wind-driven hydrodynamics and salt dynamics in microtidal estuaries. An idealized channel with a 90-degree bend in the middle was used and simulations with three wind directions were conducted. In each leg the wind tends to drive distinct circulation patterns that are consistent with the local wind angle relative to the channel but the dynamics are also affected by remote effects from the other leg and bend. Within the bend, the wind conditions change consistently along the bend and the circulation and salinity structure respond to it but these dynamics are modified locally by flow separation and the centrifugal force. A strong salt front formed around the upstream end and along the sides of the salt wedge and the inner bank. An opposite salt front with negative salinity gradient formed along the outer bank and the top of the salt wedge in the bend. This dissertation advances understanding of circulation and salinity dynamics in wind-dominated estuaries with both straight and curved channels and reveals mechanisms driven by wind that are often difficult to isolate in estuaries with tides.