Dynamics of Well-Mixed Estuaries

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Mixing between fresh and salt water is the fundamental process that is common to all estuaries. However well-mixed estuaries present an intriguing paradox: how can mixing occur if the estuary is already mixed? This project addresses that paradox and how it relates to the broader problem of estuarine dispersion in the examination of two well-mixed estuaries with similar tidal and freshwater forcing but highly contrasting geometries, one a narrow estuary with a single, uniform channel and the other a broad lagoon-type estuary with a complex, dendritic channel network. The study will address the coupled momentum and salt balances of these two estuaries with a combination of intensive field measurements and high-resolution numerical modeling. The mixing and exchange processes affecting well-mixed estuaries are fundamentally important to the ecology and life histories of nearshore organisms, to the biogeochemistry (including the carbon budget) of coastal environments, and to the morphodynamic stability of coastal embayments. The results of this study will thus inform research and management related to marine resource management, carbon sequestration, and coastal sustainability. The project will focus a large fraction of its resources on the training of a graduate student. It also includes support for a field methods short-course to provide training for students from different graduate programs in advanced estuarine field methods.Whereas the dynamics of partially mixed and highly stratified estuaries have a firm theoretical framework, well-mixed estuaries are subject to a wide range of hypothesized regimes, such as lateral shear dispersion, chaotic advection and tidal-asymmetry-induced circulation, but with no consistent synthesis or scaling of these different processes. In the case of the narrow estuary, mixing is hypothesized to result from brief, episodic restratification events localized in space and time. For the broad estuary mixing may exhibit a much more complex spatial structure, dominated by lateral fronts at the edges of the channels and intensified at channel junctions. This study will examine the roles of these different mechanisms in well mixed estuaries with strongly contrasting geometries, using observations and realistic simulations to identify the processes, and using model sensitivity studies with varying geometries to parameterize the scaling of the different mechanisms and to generalize the results.