Surfzone Tracer Transport and Dispersion during the IB09 Field Experiment

Transport and dilution of pollutants discharged into the surfzone are not well understood, limiting water quality forecast accuracy and exposing beachgoers to health risks. Surfzone dispersion in an alongshore current has been characterized using observations of the downstream evolution of Rhodamine WT fluorescent dye tracer. Continuously released near the shoreline, dye was advected by alongshore mean currents while spreading cross-shore, forming shore-parallel plumes. Initial observations (HB06) with a single jetski sampling near-surface dye concentrations in relatively short (400 m) plumes have been used to estimate bulk eddy diffusivities within the surfzone [Clark et al., JGR, in press 2010]. However, questions about the seaward extent of surfzone mixing, exchange with offshore waters, vertical tracer structure, and mixing over longer distances and times were unresolved. Results from more comprehensive observations during the fall 2009 IB09 experiment at Imperial Beach, California will be discussed. The near-surface dye field was mapped with two GPS- and fluorometer-equipped jetskis. Vertical dye concentration profiles were measured with a boat-towed vertical fluorometer array as dye leaked offshore from the surfzone. Additionally, near-shoreline fluorometers were deployed at various downstream distances, and a cross-shore array of six bottom-mounted instrument packages (1-4 m depth) measured dye, waves, and currents. Aerial photographs of the dye field were also acquired. Continuous near-shoreline dye releases spanned a range of wave and current conditions. Dye was measured up to 3 km downstream of the source and over 500 m from shore. Fluorescent Rhodamine WT dye tracer plume during the IB09 experiment at Imperial Beach, California, fall 2009.