David B. Clark

Modeling surf zone tracer plumes: 1. Waves, mean currents, and low‐frequency eddies

[1] A model that accurately simulates surf zone waves, mean currents, and low‐frequency eddies is required to diagnose the mechanisms of surf zone tracer transport and dispersion. In this paper, a wave‐resolving time‐dependent Boussinesq model is compared with waves and currents observed during five surf zone dye release experiments. In a companion paper, Clark et al. (2011) compare a coupled tracer model to the dye plume observations. The Boussinesq model uses observed bathymetry and incident random, directionally spread waves. For all five releases, the model generally reproduces the observed cross‐shore evolution of significant wave height, mean wave angle, bulk directional spread, mean alongshore current, and the frequency‐dependent sea surface elevation spectra and directional moments. The largest errors are near the shoreline where the bathymetry is most uncertain. The model also reproduces the observed cross‐shore structure of rotational velocities in the infragravity (0.004 < f < 0.03 Hz) and very low frequency (VLF) (0.001 < f < 0.004 Hz) bands, although the modeled VLF energy is 2–3 times too large. Similar to the observations, the dominant contributions to the modeled eddy‐induced momentum flux are in the VLF band. These eddies are elliptical near the shoreline and circular in the mid surf zone. The model‐data agreement for sea swell waves, low‐frequency eddies, and mean currents suggests that the model is appropriate for simulating surf zone tracer transport and dispersion.

Surfzone to inner-shelf exchange estimated from dye tracer balances

Surfzone and inner-shelf tracer dispersion are observed at an approximately alongshore-uniform beach. Fluorescent Rhodamine WT dye, released near the shoreline continuously for 6.5 h, is advected alongshore by breaking wave- and wind-driven currents, and ejected offshore from the surfzone to the inner-shelf by transient rip currents. Novel aerial-based multispectral dye concentration images and in situ measurements of dye, waves, and currents provide tracer transport and dilution observations spanning about 350 m cross-shore and 3 km alongshore. Downstream dilution of near-shoreline dye follows power law decay with exponent -0.33, implying that a 10-fold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner-shelf dye mass balances close, and in 5 h roughly 1/2 of the surfzone-released dye is transported offshore to the inner-shelf. Observed cross-shore transports are parameterized well using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference (r2 = 0.85, best fit slope = 0.7). The best fit cross-shore exchange velocity u*=1.2 × 10−2 ms−1 is similar to a temperature-derived exchange velocity on another day with similar wave conditions. The u* magnitude and observed inner-shelf dye length scales, time scales, and vertical structure indicate the dominance of transient rip currents in surfzone to inner-shelf cross-shore exchange during moderate waves at this alongshore-uniform beach. This article is protected by copyright. All rights reserved.

Aerial Imaging of Fluorescent Dye in the Near Shore

Aerial images are used to quantify the concentration of fluorescent Rhodamine water tracing (WT) dye in turbid and optically deep water. Tracer releases near the shoreline of an ocean beach and near a tidal inlet were observed with a two-band multispectral camera and a pushbroom hyperspectral imager, respectively. Theaerialobservationsarecomparedwithnear-surfaceinsitumeasurements.Theratioofupwellingradiance near the Rhodamine WT excitation and emission peaks varies linearly with the in situ dye concentrations for concentrations ,20ppb (r 2 5 0.70 and r 2 5 0.85‐0.88 at the beach and inlet, respectively). The linear relationship allows for relative tracer concentration estimates without in situ calibration. The O(1m) image pixels resolve complex flow structures on the inner shelf that transport and mix tracer.