Observations of sand bar evolution on a natural beach

TLDR

Offshore sand‑bar migration is driven by cross‑shore gradients in suspended‑sediment transport from strong near‑bottom offshore currents, which intensify near the crest and cause shoreward erosion and seaward deposition. The study measured waves, currents, and near‑continuous seafloor location over two months at nine sites along a 255‑m transect using sonar altimeters mounted on fixed frames. The sand bar crest migrated 130 m offshore during storms with significant wave heights >2 m, eroding 1.5 m near its original location and accreting 1 m at the new position; a sediment‑transport model driven by measured near‑bottom currents reproduces this offshore migration but not the slow onshore movement seen in low‑energy conditions, indicating that morphology, waves, circulation, and sediment transport feedback drive offshore bar migration during storms.

Abstract

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 1 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.