Cross-shore Tracer Exchange Between the Surfzone and Inner-shelf

Cross-shore exchange between the surfzone and inner-shelf was observed using rhodamine WT dye tracer at the approximately alongshore-uniform Imperial Beach, California in fall 2009. A case study, with dye continuously released near the shoreline for 4.5 hours, is presented. Obliquely incident waves (H_s about 1 m) drove a mean alongshore current, creating a several km long dye plume in the approximately alongshore-uniform surfzone. Dye, turbidity, and temperature were measured near the seabed with bottom-mounted instruments, near the surface with two jetskis, and over the vertical with CTD+F casts and an alongshore-towed vertical array. A few 100 m downstream from the dye release location, the plume extended across the surfzone (shoreline to ~2 m depth) and was depth-uniform. Offshore, in 4-6 m depth, tracer was alongshore-patchy in 50-100 m-wide bands, and elevated dye concentrations co-occurred with elevated turbidity and temperature, consistent with ejection of surfzone water by transient rip currents. Offshore dye concentrations were approximately depth-uniform in the upper 3 m, where thermal stratification was weak, but near-zero at the seabed in 6 m depth, below the strong thermocline. Within the surfzone, dye was vertically well-mixed with no temperature stratification. Just seaward of the surfzone, the vertical dye and temperature gradients were linearly related, indicating that thermal stratification was limiting vertical tracer mixing just offshore of the outer wave-breaking boundary. These observations highlight the important role that transient rip ejections can play in surfzone/inner-shelf exchange, and indicate the importance of stratification to vertical tracer dispersion immediately seaward of the surfzone.