Observations of sand bar evolution on a natural beach

Waves, currents, and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross-shore transect extending 255 m from 1 to 4 m water depth. The seafloor location was measured nearly continuously, even in the surf zone during storms, with sonar altimeters mounted on fixed frames. The crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore (primarily when the offshore significant wave height exceeded about 2 m), with 1.5 m of erosion near the initial location and I m of accretion at the final location. An energetics-type sediment transport model driven by locally measured near-bottom currents predicts the observed offshore bar migration, but not the slow onshore migration observed during low-energy wave conditions. The predicted offshore bar migration is driven primarily by cross-shore gradients in predicted suspended sediment transport associated with quasi-steady, near-bottom, offshore flows. These strong (> 50 cm/s) currents, intensified near the bar crest by wave breaking, are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side. The feedback amoung morphology, waves, circulation, and sediment transport thus forces offshore bar migration during storms.