Collaborative Research: Three-Dimensional Surfzone Eddies

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Surfzone eddies mix pollutants, erode coast- lines, endanger swimmers, and transport marine organisms. Eddies may be directly forced by groups of breaking surface gravity waves, or may develop as shear instabilities of mean alongshore currents ('shear production'). The relative importance of direct forcing and shear production has not yet been determined, but recent numerical simulations suggest that wave group forcing may be an important, yet mostly neglected, factor. Surfzone eddies have often been viewed as two-dimensional and depth-independent, but recent observations indicate that eddies can in fact be strongly three-dimensional. Indeed, observed three-dimensionality can be so strong that total shear production cannot be calculated unless depth-dependence is resolved. This study will examine the dynamics of three-dimensional surfzone eddies using depth-resolving field observations and numerical modeling.A 15-element array of Acoustic Doppler Current Profilers (ADCPs) will be deployed to gather the first synoptic, vertically-resolved observations of surfzone mean currents and eddies. ADCP observations will be used in conjunction with a numerical model that will take advantage of recently developed 3D wave forcing formulations, while also simulating eddy generation by wave groups. Shear production and direct forcing terms in the eddy energy equation will be evaluated in a range of numerical simulations, and in observations, to determine their relative importance under a variety of incident-wave conditions. The project will provide the first horizontally-resolved in-situ observations of near-surface eddy motions. In addition to quantifying magnitudes, phasing, and de-correlation across-shore, the team will test whether alongshore propagation speeds and de-correlation distances vary in the vertical (both vary across-shore). These observations will be compared with simulations. Numerical simulations and observations, especially vertical phasing and coherence with wave groups, will be examined to distinguish vertical shear generated by breaking from shear generated by bottom friction or horizontal advection. Simulations will be used to study the effects of three- dimensionality on mixing, including possible rapid lateral mixing of passive tracers and momentum.This experiment will build on substantial existing resources, including Washington State University experience working with ADCPs, and Oregon State University expertise in three-dimensional circulation modeling. Deployment and recovery will carried out by one of the world's best-equipped field crews at the US Army Corps of Engineers' Field Research Facility. Broader Impacts: This project will improve our understanding of the nearshore eddies that mix pollutants and organisms, transport sediments, and endanger swimmers. A community model for predicting nearshore currents and eddies will be tested and refined and made available to the community, constituting an important step towards the goal of a predictive model for surfzone pollution. Such a model would be valuable to managers, who must currently rely on bacterial cultures that are slow to return actionable results. Field data, constituting the first depth-resolving synoptic measurements of surfzone mean currents, and the first profiling array to extend 1 km from the shore to the inner shelf, will be made available to the oceanographic community. Three graduate students will be funded. To disseminate knowledge beyond universities, students will likely partner with high school teachers (through Washington State University's GK-12 program) and PIs will partner with a community college teacher (facilitated by the COSEE program).