Moulton, MelissaNuss, Emma Shie2025-01-232025-01-232025-01-232024Nuss_washington_0250E_27659.pdfhttps://hdl.handle.net/1773/52753Thesis (Ph.D.)--University of Washington, 2024In the surf zone, waves steepen and break, driving nearshore currents and impacting transport of nutrients, larvae, sediment, and other particulate matter. One type of nearshore current that is important to the cross-shore transport of surf-zone tracers is a transient rip current. Transient rip currents are strong offshore directed flows driven by short-crested wave breaking. Short-crested waves form in the surf zone when the wave field is directionally spread (i.e. wave energy is spread across many directions), forming an irregular sea surface that leads to wave breaking over finite regions of the wave field. As these short-crested waves break, they generate small-scale vertical vorticity (i.e. horizontal rotational motion). Energy from this small-scale vorticity can be transferred to larger-scale coherent rotational motions, or eddies, that interact and enhance cross-shore exchange of surf-zone tracers. Open questions remain about vorticity generation, evolution, and resulting cross-shore transport associated with surf-zone eddies. We use phase-resolved numerical simulations to quantify surf-zone vorticity evolution and exchange for varying wave conditions. Additionally, we couple this Eulerian perspective of surf-zone vorticity with a semi-Lagranian eddy-focused perspective by tracking individual eddies, similar to tracking of mesoscale ocean eddies. The combination of these two approaches provides a new perspective of how surf-zone vorticity evolves and implications for cross-shore transport of surf-zone tracers. Part 1 quantifies how wave field characteristics (i.e. crest length, number of crest ends) vary for a range of wave directional spreads and peak periods using a suite of phase-resolved numerical simulations. Additionally, we relate wave field characteristics to the generation of small-scale vorticity, low frequency rotational power associated with large-scale vorticity, and cross-shore exchange velocities. We find that while crest lengths decrease and the number of crest ends increases for increasing directional spread, small-scale vorticity generation and cross-shore exchange velocities exhibit a peak at intermediate directional spread. A weaker relationship is observed for peak period, with shorter crest lengths and a greater number of crest ends observed for decreasing peak period, while vorticity generation and exchange velocities are similar for intermediate and longer peak period, but decrease for shorter peak period. Part 2 investigates how large-scale coherent vorticity, or eddies, vary across the surf zone and offshore for varying wave directional spreads through application of a modified mesoscale ocean eddy identification algorithm. This modified algorithm identified individual eddies in the surf zone and offshore for low, intermediate, and high directional spread simulations. Eddy characteristics, such as size, shape, and circulation strength were quantified for all identified eddies. We find that while eddy characteristics vary minimally between directional spreads in the surf zone, the total number of eddies in the surf zone increases with increasing directional spread. However, we also find that a greater number of eddies, with a higher median nonlinearity, are found offshore at intermediate directional spread, consistent with a peak in cross-shore exchange velocities at intermediate directional spread (finding from Part 1). Part 3 applies a tracking algorithm to explore how large-scale eddies (identified in Part 2) vary over their life cycle. Individual eddies are tracked spatially recording their characteristics (i.e. size, circulation, etc.) and translation information (i.e. translation speed, direction, etc.). Additionally, a clustering algorithm is applied to classify types of eddy trajectories observed throughout the model simulation. We find that eddy trajectories fall into three categories, eddy tracks that tend to: 1) start and stay within the surf zone, 2) start and stay offshore, and 3) start offshore and persist for a long duration. While these trajectory types exhibit distinct behavior, eddy characteristics exhibit similar responses to bathymetric changes within the surf zone and generally conserve potential vorticity. Findings also highlight several tracer transport pathways, including eddy pairs, nonlinear eddies, and offshore jet features, but suggest that further work is needed to understand when and why specific transport mechanisms may dominate.application/pdfen-USCC BY-NC-NDEddy trackingNumerical modelingPhase-resolved numerical modelingRip currentsSurf zoneWave breakingCivil engineeringOcean engineeringCivil engineeringThesis