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Hydrodynamic Exchange in Estuarine Perimeter Habitats


Hydrodynamic exchange in estuaries is forced by tides, freshwater input, density forcing, and winds, and controls transport of important quantities such as salinity, sediment, nutrients, and pollutants. Previous work has characterized many aspects of estuarine transport and contributed to our understanding of transport mechanisms such as gravitational exchange, tidal dispersion processes, and residual flows due to tidal asymmetries. In general, studies of estuarine transport have focused on large-scale transport processes in the along-channel direction of the estuary, which determine the overall salinity and flow structure in estuarine environments. However, study of hydrodynamic exchange at the perimeter of estuaries has also been recognized to be important, as exchange at the perimeter is relevant for understanding questions related to environmental restoration and management and ecological habitat quality.

In this work, hydrodynamic exchange in estuaries and perimeter habitats is studied using numerical modeling and field observations of South San Francisco Bay. First, the exchange between the estuary and a small perimeter slough is measured using salinity and temperature as tracers to calculate hydrodynamic flushing of the slough through tidal exchange, using a modified tidal prism method. This method applies quasi-Lagrangian analysis to Eulerian measurements of exchange, and the results are compared to previous results from larger-scale estuarine systems, where tidal flushing is found to be significantly affected by the scale of mixing volumes in the system. Next, Lagrangian methods of particle-tracking and Lagrangian coherent structure (LCS) analysis, developed from dynamical systems theory in order to analyze complex, chaotic flows, are applied to analyze tidal transport. The results reveal the significant effects of tidal interactions with perimeter estuarine features on Lagrangian tidal transport over the tidal cycle, where perimeter interactions are found to significantly contribute to longitudinal estuarine dispersion. Finally, the effect of wind forcing on estuarine transport is examined, using Lagrangian analysis methods applied to cases of constant wind forcing with varying wind direction relative to the main axis of the estuary. Wind forcing is found to have a significant effect on hydrodynamic exchange and connectivity between the estuary and perimeter habitats, where wind in all directions increases perimeter exchange and connectivity, with the greatest effect for winds aligned with the along-axis direction of the estuary. The results of these studies are relevant to a wide range of applications requiring analysis of connectivity near the estuarine perimeter, including sediment exchange and transport and seagrass population colonization in the context of wetland habitat restoration.

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