Publication Date:
2015-07-18
Description:
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 ( r 2 = 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.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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