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Surfzone tracer dispersion : methods, observations, and modeling


Decisions about recreational beach closures would be enhanced with better estimates of surfzone contaminant transport and dilution. New in situ methods, developed here, for measuring fluorescent Rhodamine WT dye tracer in the surfzone increase the temporal and spatial resolution over previous techniques. Bubbles and sand suspended by breaking waves in the surfzone interfere with in situ optical fluorometer dye measurements, but turbidity correction reduces root-mean-square dye concentration errors to < 5% of dye concentration magnitude. Alongshore tracer plumes, formed by continuously releasing dye in a wave driven alongshore current are used to examine cross- shore surfzone tracer dispersion at Huntington Beach, California. Ensemble averaged cross-shore tracer concentration profiles are generally shoreline attached (maximum at or near the shoreline), with increasing cross- shore widths and decreasing peak concentrations with downstream distance. For each release, cross-shore surfzone absolute diffusivities [kappa]xx, estimated using a simple Fickian diffusion solution with a no-flux boundary at the shoreline, range from [kappa]xx = 0.5-2.5 m²s⁻¹. The [kappa]xx scale best with a mixing-length scaling (correlation r² = 0.59 and the expected scaling versus [kappa]xx best-fit slope < 1), indicating that horizontal rotational motions are important for cross- shore tracer dispersion in the surfzone. The five tracer plumes used for [kappa]xx estimates are simulated with a time-dependent wave-resolving Boussinesq surfzone model (funwaveC) initialized with the observed bathymetry and incident wave spectra. The modeled and observed cross- shore structure of significant wave heights and mean alongshore currents have good qualitative agreement. Modeled and observed low frequency (< 0.03 Hz) horizontal rotational velocities, possibly important for cross-shore dispersion, have similar cross-shore structure, although magnitudes are slightly over predicted. Modeled tracer is spread by model currents and eddies, a breaking wave eddy diffusivity, and a small (0.01m̃²s⁻¹) background diffusivity. Mean model tracer concentration skill (compared to a zero prediction) is highly variable (from negative to 0.73), however cross-shore integrated moments (normalized by the cross-shore tracer integral) have consistently high skills (0̃.9). Modeled [kappa]xx estimates are similar to the observations, but the skill (0.4) is only moderate. The model breaking wave eddy diffusivity does not effect dispersion significantly

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