Surfzone to inner-shelf exchange estimated from dye tracer balances

TitleSurfzone to inner-shelf exchange estimated from dye tracer balances
Publication TypeJournal Article
Year of Publication2015
AuthorsHally-Rosendahl K., Feddersen F, Clark D.B, Guza RT
JournalJournal of Geophysical Research-Oceans
Date Published2015/09
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000363470300023
Keywordscontinental-shelf; cross-shore transport; currents; driven; inner-shelf; Mixing; pollution; shore; Surfzone; tracer; transport; undertow; waves; zone

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 tenfold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner-shelf dye mass balances close, and in 5 h, roughly half of the surfzone-released dye is transported offshore to the inner-shelf. Observed cross-shore transports are parameterized well ( r2=0.85, best fit slope 0.7) using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference. The best fit cross-shore exchange velocity u*=1.2x10-2ms-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.

Short TitleJ Geophys Res-Oceans
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