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Observations of Seafloor Evolution on a Natural Barred Beach

  • Author(s): Gallagher, Edith L.
  • et al.
Abstract

Waves, currents, and morphology were measured nearly continuously for more than 2 months at 9 locations along a cross-shore transect extending from the shoreline to 4-m water depth. Changes in seafloor location were measured with newly de­veloped sonar altimeters which can locate the seafloor even in the surf zone during storms. During the deployment the sand bar moved 130 m offshore (primarily when the offshore significant wave height exceeded about 2m), with 1.5 m of erosion near the initial location of the bar crest and 1 m of accretion offshore. Onshore bar migration was observed for low-energy conditions. An energetics-type sediment transport model driven by locally measured near-bottom currents predicts the ob­served offshore bar migration accurately, but the slight onshore migration observed during low-energy wave conditions is not well modeled. The predicted offshore bar migration is driven primarily by gradients in suspended sediment transport asso­ciated with quasi-steady, near-bottom, offshore flows. These strong (>50 cm/sec) currents, possibly enhanced by intensified wave breaking near the bar crest, are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side. The feedback between the morphology, waves, circulation, and sediment transport thus forces offshore bar migration when waves are large enough to break on the bar. During lower energy events and onshore migration the skill of the model is reduced. This may be the result of phase lags between sediment suspension in the lee of bedforms. Megaripples (not accounted for in the transport model) with heights of O(20 cm) migrating at speeds as high as 100 cm/hr were often observed with a coherent array of altimeters located in the trough shoreward of the sand bar.

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