In-Situ Observations of Cross-Shore Sediment Transport in Response to Waves Along the Inner-Continental Shelf Offshore from Fire Island, NY
In-Situ Observations of Cross-Shore Sediment Transport in Response to Waves Along the Inner-Continental Shelf Offshore from Fire Island, NY
Abstract:
Fire Island is a 50 km long barrier island along the southern shore of Long Island, NY (USA) that protects a populated mainland from large waves and storms. Previous sediment budget analyses have identified cross-shore sediment flux from the inner-continental shelf as the likely sand source [O(200,000 m3/yr)] required to account for accretion along the central segment of the island. To investigate the mechanisms driving cross-shore sediment flux, quad-pod instrument frames were deployed from January to April 2014 along the inner-shelf offshore of Fire Island at 12-15 m water depth equipped with acoustic sensors to concurrently measure surface waves, hydrodynamic flows, near-bed wave orbital velocity and turbulence, vertical profiles of near-bed suspended sediment, and bedform morphology and migration. The measurements were made at this location to quantify cross-shore sediment flux in response to wave forcing at depths where both wave skewness and wave-current interactions should significantly contribute to cross-shore sediment transport.
Bedform migration and geometry were computed from rotary sonar images and consisted of orbital-scale ripples with wavelengths between 0.3 to 1.2 m, and oriented ±40° from directly onshore. Ripple heights ranged from 0.04 to 0.19 m with a slight onshore-directed asymmetry. Ripple migration occurred in response to wave events and was used as a proxy to estimate transport at the bed. Migration rates were related to the cube of near-bed wave-orbital velocity, 〈ubr3〉 in the peak direction of wave propagation. The ripple geometry and migration enabled estimation of the net cross-shore bedload sediment volume flux, which was directed onshore O(1 m3/yr per m alongshore) at the quad-pods during the deployment. These rates will be compared to migration at other locations to determine relative migration scales.
Cross-shore suspended sediment flux was computed from profiles of velocity and acoustic-backscatter estimates of suspended-sediment concentration. These estimates were more sensitive to wave-current interactions than was bedload transport, and will be examined as another possible mechanism for onshore transport.
Bedform migration and geometry were computed from rotary sonar images and consisted of orbital-scale ripples with wavelengths between 0.3 to 1.2 m, and oriented ±40° from directly onshore. Ripple heights ranged from 0.04 to 0.19 m with a slight onshore-directed asymmetry. Ripple migration occurred in response to wave events and was used as a proxy to estimate transport at the bed. Migration rates were related to the cube of near-bed wave-orbital velocity, 〈ubr3〉 in the peak direction of wave propagation. The ripple geometry and migration enabled estimation of the net cross-shore bedload sediment volume flux, which was directed onshore O(1 m3/yr per m alongshore) at the quad-pods during the deployment. These rates will be compared to migration at other locations to determine relative migration scales.
Cross-shore suspended sediment flux was computed from profiles of velocity and acoustic-backscatter estimates of suspended-sediment concentration. These estimates were more sensitive to wave-current interactions than was bedload transport, and will be examined as another possible mechanism for onshore transport.