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CEE 269B Environmental Engineering Seminar - Stephen Monismith, PhD "Observations of (and Questions About) Drag in Wavy Flows; 5 Stories".

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Time-averaged (mean) flows in the shallow waters of estuaries and the inner continental shelf can be significantly affected by friction associated with a combination of mean flows and surface waves. I will present field observations drawn from a variety of studies of these turbulent, shallow water flows in which drag, either in the form of bed stresses acting on a variety of bottom surfaces, e.g. corals, or in the form of depth-integrated forces acting on Macrocystis pyrifera, the giant kelp found throughout the world’s oceans, are affected by surface waves. I will examine cases where bed stresses can be computed using the now classical and widely accepted Grant-Madsen (GM) theory, as well as the more complicated case of kelp. In all cases mean frictional resistance can be computed by assuming that the mean drag experienced by the flow is the simple time average of the instantaneous drag including the waves and a constant drag coefficient that is a function of the physical geometry of the drag elements. In cases where GM theory can be applied, it appears that the simple theory gives values of CD, the drag coefficient, of comparable accuracy. This also holds in the case of corals, although stresses inferred from log-fits to mean profiles can be somewhat larger than those inferred by direct measurement of Reynolds stresses computed from acoustic Doppler velocimeter data by removing waves. Finally, using recent observations from a large field experiment carried out near San Diego with colleagues from Scripps Institute of Oceanography, I will show that drag on kelp is more complex than either muddy estuary bottoms or picturesque coral reefs in that the drag elements themselves are moving, likely in ways that recent calculations suggest are more complex than what I have assumed in order to apply the simple model. Moreover, what makes predicting these flows especially challenging is the drag appears to be strongly associated with internal waves of unknown origin



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 (he went there voluntarily rather than being transported). He has been at Stanford in the Dept of Civil and Environmental Engineering (CEE) since 1987 and the new Oceans Department in the Stanford Doerr School of Sustainability since 2022. He has been the director of the Environmental Fluid Mechanics Laboratory at Stanford since 1996, and was CEE department chair between 2009 and 2016. He is interested in 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. 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, American Samoa, and Palau. He has parallel interests studying the inner shelf flows found near and inside the kelp forests of California. While he is a native Californian, he appreciates both cricket and rugby and loves pub lunches, all of which he was able to enjoy during a recent extended stay in Oxford.