DOI: 10.17603/ds2-qgd5-jk92
title: Experimental investigation of short-crested wave breaking in a laboratory directional wave basin
title: in NHERI RipCurrents - Transient Rip Current Dynamics: Laboratory Measurements and Modeling of Surfzone Vorticity

description: Relative to waves that break with long crests, short-crested wave breaking during directionally spread wave conditions alters sediment transport processes, forces on structures, and wave run-up. Additionally, the energy associated with horizontal eddies (vertical vortices) injected by short-crested breaking waves can be transferred to larger-scale horizontal eddies, altering surf-zone dispersion and transient rip current activity. Laboratory experiments over an alongshore-uniform barred beach in the O.H. Hinsdale Wave Research Laboratory's Directional Wave Basin at Oregon State University were completed to assess the relationships between offshore wave conditions, breaking wave characteristics, and surf-zone eddy evolution and energetics. Experiments spanned a range of wave directional spreads, heights, periods, and water levels. The combined observations from in situ sensors (wire resistance gauges, pressure sensors, and acoustic Doppler velocimeters), a multibeam lidar, and stereo photogrammetry and particle image velocimetry applied to imagery enabled dense spatio-temporal coverage of the water surface elevation and velocities relative to previous studies. The remotely sensed surface elevations and velocities were rigorously compared to in situ observations. We find that the average along-crest breaking length decreases and the number of crest ends increases with increasing wave directional spread from wave-by-wave identified breakers. Eddy evolution for trials with large directional spread waves was consistent with a two-dimensional inverse energy cascade, while trials with unidirectional waves did not exhibit this behavior. Surf-zone dye releases visualize offshore ejections during directionally spread waves. This dataset could be used to investigate nearshore wave and circulation dynamics (e.g., cross-shore transformation of waves, surf-zone eddy evolution and dispersion), test numerical model simulations of directionally spread wave conditions in shallow water environments, or develop machine learning algorithms for coastal applications.
description: The nearshore region, valued as essential intertidal habitat and recreational grounds, is often compromised by pathogens and excess nutrient supply from terrestrial runoff. When an area becomes contaminated, beach closures, ecological disruption, and associated economic losses can result. Understanding the transport of materials throughout the nearshore region can have important implications for ecosystem and human health. Previous studies analyzing cross-shelf exchange suggest that transient rip currents are the dominant mechanism driving exchange between the surf zone and the inner shelf. Evidence also suggests that transient rip currents are generated from the inverse cascade of energy from vorticity injected by short-crested breaking waves to larger scale rotational motions. 

This project utilized laboratory and numerical modeling experiments to study the generation and evolution of surf zone eddies to form transient rip currents. Laboratory experiments were performed in the Directional Wave Basin at the NHERI O.H. Hinsdale Wave Research Laboratory. Wave conditions were run for 45 min for a range of wave heights, periods, water levels, and directional spreads. The experiments included in situ and remote (stereo visible and lidar) measurements of sea-surface-elevations and velocities. Phase-resolved numerical modeling of laboratory experiments were compared with observations to investigate the relationships between wave conditions, vorticity injection, low-frequency eddy activity, and transient rip-current-driven cross-shore exchange. The knowledge of mixing and exchange in the nearshore region will be significantly advanced through this study, which will have important implications for the transport of nutrients, larvae, and pollutants.