Surfzone vorticity dynamics in a directional wave basin

Abstract

The surf zone is an energetic and evolving region where waves break near the coast. This region, the transition zone from the shoreline to the deeper waters of the shelf, is critical for supporting ecosystems and valuable recreational and economic resources, and at the same time, is increasingly vulnerable to anthropogenic impacts and the effects of climate change. Breaking waves in this region drive complex circulation patterns, including eddies and rip currents, that disperse material such as pathogens, contaminants, larvae, and excess nutrients from terrestrial runoff. Rip currents, an important material transport mechanism, are fast, narrow, offshore-flowing jets driven by breaking waves in the surf zone. They occur on beaches under a wide range of conditions and are a dominant contributor to cross-shore exchange of material between the surf zone and the shelf. Transient rip currents are ephemeral ejections associated with surfzone eddies that can occur even on alongshore-uniform beaches. These rip currents are less understood in comparison to bathymetric rip currents that persist at fixed locations determined by bathymetry, but are ubiquitous on many beaches. Under directionally spread wave conditions, waves break in finite-length -- short-crested -- regions, leading to spatial variations in the breaking force and a corresponding input of rotational motion -- vorticity -- to the water column. Energy associated with injected vorticity is hypothesized to nonlinearly transfer to larger scale rotational currents -- eddies -- that enhance dispersion within the surf zone, interact with other eddies, and episodically eject offshore as a transient rip current. This hypothesis is widely discussed, however, we do not have strong observational evidence for, or a good understanding of, the processes connecting directionally spread wave fields to the formation of large-scale eddies. To overcome the challenges of isolating and studying surfzone vorticity dynamics in the field, large-scale wave basin experiments with an alongshore-uniform barred beach were conducted. In the first chapter of the thesis, I detail the ecosystem and human health implications and scientific knowledge gaps motivating the laboratory study of surfzone eddies. I also review the dynamics of surface gravity waves, breaking-wave driven mean surfzone currents, including longshore currents and bathymetric rip currents, surfzone eddy forcing and evolution, and transient rip current formation. The second chapter describes my work investigating short-crested wave breaking that results from directionally spread wave conditions. I quantified wave transformation and directional properties with a 3-d scanning lidar, stereo cameras, and in situ pressure and velocity sensors, which yielded similar estimates. Highly resolved spatiotemporal patterns of wave breaking were characterized by developing a remote breaker identification scheme (RBIS) using a combination of thresholded brightness imagery and stereo camera reconstruction of the water surface elevation. The RBIS estimated average along-crest-length of breaking waves decreases while the number of crest ends increases with increasing directional spread. Parameterized relationships between directional spread and crest properties exhibit similar trends to observed breaking crest length and the number of crest ends within the surf zone. In the third chapter, I investigate how the wave breaking patterns characterized in the previous chapter relate to the forces driving surfzone eddies and the subsequent eddy evolution. This is hypothesized to include nonlinear energy transfers to larger scales through an inverse cascade, similar to two-dimensional turbulence. Using stereo reconstructions of the laboratory water surface, I applied a bore model to estimate the along-crest profile of wave dissipation. Next, I computed the breaking force and its curl, which drives a time-rate of change in surfzone vorticity. The estimated curl of the breaking force is highly irregular along individual wave crests and varied from crest to crest. Averaging over many crests, the curl of the breaking force is positively and negatively signed near opposite crest ends. The shape of the crest-averaged curl of the breaking force varies by surfzone region, and the shape near crest ends strongly depends on assumptions about the decay of dissipation outside of an identified crest region. The spatial characteristics of low-frequency currents, estimated from in situ sensors and remote particle image velocimetry, varies with wave directional spread and is similar to expected relationships for a two-dimensional turbulence inverse energy cascade. I synthesize the relationships between the short-crested wave field, estimates of the curl of the breaking force, and low-frequency currents, and describe these relationships in the context of a conceptual model of mechanisms leading to transient rip currents. In the final chapter, I describe the overall findings and implications of this research, including observational evidence for a conceptual model linking directionally spread wave fields to large-scale, low-frequency motions in the surf zone. I also discuss future research directions needed to improve understanding of these links, including field-based studies, by quantifying individual eddy evolution within and beyond the surf-zone, and exploring implications for swimmer hazard and transport of pollutants, larvae, and sediment.