OBSERVATIONS OF SAND-BAR EVOLUTION ON A NATURAL BEACH

Waves, currents, and the location of the seafloor were measured on a b arred beach for about 2 months at nine locations along a cross-shore t ransect extending 255 m from 1 to 4 m water depth. The seafloor locati on was measured nearly continuously, even in the surf zone during stor ms, with sonar altimeters mounted on fixed frames. The crest of a sand bar initially located about 60 m from the shoreline moved 130 m offsh ore (primarily when the offshore significant wave height exceeded abou t 2 m), with 1.5 m of erosion near the initial location and I m of acc retion at the final location. An energetics-type sediment transport mo del driven by locally measured near-bottom currents predicts the obser ved offshore bar migration, but not the slow onshore migration observe d during low-energy wave conditions. The predicted offshore bar migrat ion is driven primarily by cross-shore gradients in predicted suspende d sediment transport associated with quasi-steady, near-bottom, offsho re flows. These strong (> 50 cm/s) currents, intensified near the bar crest by wave breaking, are predicted to cause erosion on the shorewar d slope of the bar and deposition on the seaward side. The feedback am oung morphology, waves, circulation, and sediment transport thus force s offshore bar migration during storms.