Transient and morphologic rip current dynamics on an alongshore variable beach

Annika O'Dea, US Army Engineer Research and Development Center, Duck, United States, Nirnimesh Kumar, University of Washington, Department of Civil & Environmental Engineering, Seattle, WA, United States and Merrick C Haller, Oregon State University, School of Civil and Construction Engineering, Corvallis, OR, United States
Abstract:
Rip currents, offshore-oriented currents generated in the surf zone, can be classified as either bathymetrically-controlled (morphologic) or transient, depending on their forcing mechanism. X-band radar observations from the 2017 Inner Shelf Dynamics DRI (ISDRI) field experiment show both transient and morphologic rip current events on a relatively straight section of coastline with alongshore varying bathymetry. Although several past studies have assessed transient rip dynamics on alongshore uniform beaches, the conditions leading to transient rip current activity on alongshore variable beaches are still poorly understood.

Here, a phase-resolving Boussinesq wave model (funwaveC) is used to characterize transient and morphologic rip dynamics on an alongshore variable beach. A range of wave conditions (wave height and directional spread) and an alongshore variable bathymetry that are representative of the wave and bathymetric conditions observed during the ISDRI are used in model simulations. In order to isolate the role of bathymetry, simulations with an alongshore uniform bathymetry are also conducted for each set of wave conditions. The relative contributions of directional spread versus bathymetry are determined by comparing dominant eddy length scales identified from the alongshore vorticity wavenumber spectra. The drivers for these eddies are explored through analyses of the radiation stress gradients and the curl of the wave breaking force. Analyses of cross-shore exchange due to rotational motions in different frequency bands further delineate the role of both the underlying bathymetry as well as the incident wave conditions in controlling rip dynamics. These rotational motions have implications for tracer dispersion both within the surf zone and from the surf zone to the inner-shelf. Model results are qualitatively compared to ISDRI observations to determine if similar trends and variability are simulated. Funded by the Office of Naval Research.

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