Pink Water: Surfzone Dye Measurements at Huntington Beach, CA.

Mixing in the surfzone controls the dispersal and transport of pollution, bacteria, and other tracers near the shoreline. However, the difficulty of measuring tracer concentrations in breaking waves has limited studies of surfzone mixing. Surfzone turbulent length scales range from sub-meter in the bores of broken waves to 100's of meters in shear waves, and the mechanisms primarily responsible for tracer dispersion are unclear. Dye tracer experiments will be part of HB06, a multi-institutional study at Huntington Beach, California in Fall 2006. Waves, currents and temperature will be measured simultaneously at seven cross-shore and four alongshore locations in the surfzone. Fluorescent Rhodamine-WT dye will be measured using in-situ fluorometers, bottle samples, and a novel GPS tracked jetski platform. Onboard, flow-through fluorescence and turbidity measurements will be made 20 cm below the surface at an effective sample rate of 0.5 Hz, as the jetski crosses the surfzone at about 4 m/s. Five fixed fluorometers on the cross-shore transect will record dye fluorescence and turbidity approximately 50 cm from the bottom, and shoreline bottle samples will determine dye concentration in water too shallow for the jetski. Measured in-situ Rhodamine-WT fluorescence is reduced by the presence of sand and bubbles. Laboratory tests simulating average surfzone conditions have found that this corresponds to a 10-20% reduction in measured dye concentration. While this creates noise in the signal, dye concentrations usually vary over 1-2 orders of magnitude and observations are still useable. Both patch and continuous releases of dye will be used to infer dispersion statistics, and preliminary results will be reported. This research was supported by the California Coastal Conservancy, Sea Grant, and ONR.