Nearshore current pattern off south texas: an interpretation from aerial photographs

Abstract

Current patterns in a 4-km-wide zone along the south Texas coast were interpreted from patterns of water turbidity visible in aerial photographs taken during a winter day of moderate northerly winds. Features of the turbidity pattern remained recognizable on photographs taken 25 min apart. Currents measured from the movements of these features were southward and nearly parallel to shore, increasing from about 17 cm/sec in an offshore zone to about 40 cm/sec at the line of breaking waves. The shoreward increase in velocity was probably a manifestation of wave-driven longshore currents. Rip-current plumes were drifting with this longshore current and during their drift were deformed by the horizontal shear of the current. As a result of the shear, the plume axes trended seaward in an obliquely updrift direction. Much of the turbidity visible in the photographs was caused by suspended sediment apparently supplied to the Gulf of Mexico through Aransas Pass, a tidal inlet from which a large ebb-tidal plume of turbid water was issuing at the time of the photography. Another discrete mass of turbid water along the coast to the south was probably the preceding ebb-tidal plume, which had become separated from the inlet and was drifting with the shore-parallel shelf current. Still farther south, a linear pattern of turbid and less turbid bands in the offshore zone suggested the development of Langmoir circulation having cell axes parallel to the shelf current. The turbid bands probably marked the zones of surface divergence and upwelling of the Langmuir circulation. The spacing between the turbid bands averaged about ten times the water depth, and thus the cells were much flatter than Langmuir cells that have been observed in deeper water.