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CEE 269C EnvEng Seminar - Stephen Monismith: "Salinity Intrusion in Northern San Francisco Bay: Observations and Models"

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Abstract: The problem of predicting how the salinity field in estuaries responds to freshwater inflows is one that draws attention from both physical oceanographers and hydraulic engineers since it has both scientific and practical dimensions. In Northern San Francisco Bay, examination of 20+ years of data spanning the estuary shows that the overall structure of the salt field can be described using a single parameter, X2, the distance in km measured from the Golden Gate Bridge to where the salinity on the bottom is 2. In this talk I will discuss the observed behavior of X2 and how it responds to flow, Q. In general, the tendency of freshwater flows to carry salt out of the estuary is balanced by the tendency of dispersion to move salt upstream. A surprising aspect of the X2-Q relation in Northern San Francisco Bay is that it is much weaker than would be inferred from classical estuarine circulation theory, behavior that we attribute to the effects of stratification on the turbulent flows that support upstream salt flux. I will present a rigorously derived albeit simplified integral model of salinity dynamics that can be used to understand this behavior and that can be used to create a dynamically based (rather than purely empirical) model of unsteady salinity intrusion. Finally, examination of the relevant data also suggests that inability to accurately measure freshwater flows during relatively dry periods may be a bigger limitation on accurate predictions of low-flow behavior than is choice of model structure.

Bio: Bay Area native Stephen Monismith, the Obayashi Professor in the School of Engineering at Stanford University, received all his degrees (in Civil Engineering ) from UC Berkeley. Following completion of his thesis, he did a postdoc in Western Australia focusing on the fluid mechanics of stratified flows in lakes. He has been at Stanford in the Dept of Civil and Environmental Engineering since 1987 and was department chair between 2009 and 2016. His research is focused on estuarine and lake physics, as well as nearshore flows with waves and stratification, focusing on mixing and transport processes that are central to ecology, biogeochemistry and environmental management. Through his work on estuarine dynamics, he has been active in San Francisco Bay-Delta issues, including helping to develop the scientific underpinnings of freshwater flow regulations. In recent years, much of his efforts (and travel) have focused on the physics of coral reef flows, with fieldwork and modeling carried out on reefs in the Red Sea, and in nearshore waters of Hawaii, Moorea, Palmyra Atoll, American Samoa, and Palau. He has parallel interests studying the inner shelf flows found near and inside the kelp forests of California (Alta y Baja). He is a fellow of the Fluid Dynamics division of the American Physical Society and was elected to the National Academy of Engineering in 2022.

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