UC San Diego

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.