Remote and In Situ Observations of Surfzone and Inner-Shelf Tracer Dispersion

Abstract:

Surfzone and inner-shelf tracer dispersion was observed at the approximately alongshore-uniform Imperial Beach, California during the IB09 experiment. Rhodamine dye tracer, released continuously near the shoreline for several hours, was advected alongshore by breaking wave- and wind-driven currents, and ejected offshore from the surfzone to the inner-shelf by transient rips. Aerial multispectral imaging of inner-shelf dye concentration complemented in situ surfzone and inner-shelf measurements of dye, temperature, waves, and currents, providing tracer transport and dispersion observations spanning approximately 400 m cross-shore and 3 km alongshore. Combined in situ and aerial measurements approximately close a surfzone and inner-shelf dye budget. Mean alongshore dye dilution follows a power-law relationship, and both spatial and temporal dye variability decrease with distance from the release. Aerial images reveal coherent inner-shelf dye plume structures extending over 300 m offshore with alongshore length scales up to 400 m. Plume tracking among successive images yields inner-shelf alongshore advection rates consistent with in situ observations. Alongshore advection is faster within the surfzone than on the inner-shelf, and the leading alongshore edge of inner-shelf dye is due to local transient rip ejections from the surfzone. A combination of in situ and aerial surfzone and inner-shelf measurements are used to quantify cross- and alongshore dye tracer transports. This work is funded by NSF (including a Graduate Research Fellowship, Grant No. DGE1144086), ONR, and California Sea Grant.

Figure: Aerial multispectral image of surface dye concentration (parts per billion, see colorbar) versus cross-shore coordinate x and alongshore coordinate y, approximately 5 hours after the start of a continuous dye release (green star). The mean shoreline is at x=0 m. Dark gray indicates the beach and a pier, and light gray indicates regions outside the imaged area. Black indicates unresolved regions due to foam from wave breaking. Vertical dashed line delimits the surfzone (SZ) and inner-shelf (IS). Yellow diamonds indicate locations of in situ measurements of dye, temperature, waves, and currents. Yellow circles indicate locations of in situ dye and temperature measurements.