Topographic Effects on Lagrangian Residual Flow in Stratified Estuaries

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The goal of this project is to provide a better understanding of the Lagrangian residual flow generated in shallow estuaries. Particles and solutes are carried with the Lagrandain velocity. Therefore, knowledge of the controlling mechanisms of the Lagrandain velocity is esstenial for the prediction of salinity and pollutant distributions. Both stratification and topography have strong effects on the Lagrangian residual flow. Because the two-layer flow in a strongly forced estuary is in a nearly critical (flood) or partially supercritical (ebb) internal hydraulic state, local topographic features (e.g., bends, bars and holes) with wave lengths of less than one tidal excursion have a strong effect on isopycnal displacement and vertical mixing, and therefore on Lagrangian drifts. Thus, the salinity and Lagrangian flow distribution, vertical exchange processes and esturine topography are all closely related. This three-year research project combines accurate, small-scale field measurements of flow over typical topographic features with computer modeling to determine the essential interactions. The models include turbulent mixing effects caused by stratified boundary layers and free shear layers.