Surfzone to inner-shelf exchange estimated from dye tracer balances

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6289–6308, doi:10.1002/2015JC010844.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 ( inline image, best fit slope inline image) using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference. The best fit cross-shore exchange velocity inline image is similar to a temperature-derived exchange velocity on another day with similar wave conditions. The inline image 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.National Science Foundation Graduate Research Fellowship Grant Number: DGE1144086, California Sea Grant Number: R/CONT-207TR2016-03-1

Nearshore Tracer Fate: Observations and Modeling of Cross-Shore Exchange Between the Surfzone and Inner-Shelf

The nearshore region, consisting of the surfzone (shoreline to seaward boundary of depth-limited wave breaking) and inner-shelf (surfzone boundary to $\approx$20~m water depth), is vitally important to coastal economies, recreation, and human and ecosystem health. Yet, despite the detriments of frequently contaminated coastal water, dynamical complexities of the surfzone/inner-shelf interface have limited our understanding of nearshore transport and dilution.A shoreline-source contaminant's fate is ultimately determined by its exchange with the inner-shelf. Here, cross-shore exchange is examined with coupled surfzone/inner-shelf observations of temperature and dye during two continuous releases at alongshore-uniform Imperial Beach, CA. Dye is mixed and alongshore-transported in the surfzone while being ejected to the inner-shelf by transient rip currents (TRCs). The second release is simulated with a wave-resolving model.In situ 29 September observations reveal a vertically-mixed surfzone that is warmer than the inner-shelf, where elevated temperature and dye co-occur in alongshore-narrow TRC ejections and are depth-uniform in a warm upper layer. Below, stratification limits vertical dye mixing to magnitudes of ocean interiors, despite proximity to the well-mixed surfzone. A temperature-derived bulk cross-shore exchange velocity $u^*_T=0.9\times10^{-2}\;\mathrm{m\,s}^{-1}$ suggests TRCs dominate the exchange.Observations from 13 October include aerial-based multispectral dye images, enabling novel surfzone/inner-shelf tracer mass budget closure. Over 5~h and 3.25~km downstream, 1/2 the surfzone-released dye is transported offshore to the inner-shelf. Near-shoreline dye follows power-law decay (exponent $-0.33$). Observed cross-shore transports are parameterized well using a bulk exchange velocity and mean surfzone/inner-shelf dye difference. The best-fit velocity $u^*=1.2\times10^{-2}\;\mathrm{m\,s}^{-1}$ is similar to temperature-derived $u^*_T$ from 29 September. The $u^*$ magnitude, inner-shelf dye vertical structure, time- and length-scales indicate TRC dominance again during this release.The 13 October release is simulated with the wave-resolving, Boussinesq model funwaveC, which generates TRCs but does not resolve inner-shelf vertical variation. The model largely reproduces observed dye cross-shore profiles, alongshore transport, near-shoreline power-law decay, and surfzone/inner-shelf mass budgets for $1.2\times10^4$~s. Thereafter, inner-shelf (surfzone) dye is somewhat over-predicted (under-predicted), possibly due to funwaveC's lack of tide or vertical variation. The good overall model-data agreement indicates that nearshore tracer dispersion is realistically simulated, and that the funwaveC TRCs accurately induce cross-shore surfzone/inner-shelf exchange