Statistical Analysis of Small-Scale Bedforms Formed by Hurricane Sandy Offshore Fire Island, New York

Abstract:

Multibeam bathymetry surveys acquired two months after hurricane Sandy offshore of Fire Island, NY, revealed broad areas of small-scale (<15 m) bedforms on top of large-scale (200–3000 m) sand ridges and sorted bedforms. The small-scale bedforms were absent prior to the hurricane, as evidenced by previous surveys in 2003 and 2011. Here we statistically analyze these bedforms in conjunction with seabed sedimentary properties and storm history to understand the correlation of sedimentary deposition with the storm history. The Fire Island shoreface/inner shelf has been the site of ongoing studies by the USGS. The western half is dominated by 1–3 km-wide sand ridges, with abundant fine-medium sand. The eastern half, on the other hand, is largely starved of modern sand, and the bedform morphology is dominated by 0.2–1.0 km-wide sorted bedforms. We utilize two post-storm surveys, one along the western half of the island and the other to the east, providing an opportunity to compare these different nearshore settings. A Gaussian covariance model is used, with four parameters: rms height, orientation and characteristics length and width. An iterative, least-squares inversion is used to estimate these parameters and their uncertainties from selected sample areas. Bedforms with aspect ratio (length/width) > 1.5 are categorized as two-dimensional, and have rms heights ~3–7 cm and widths 4–6 m. Bedforms with aspect ratio < 1.5 are categorized as three-dimensional, and have smaller rms heights (~1.5–4 cm) and larger widths (~6–13 m). Parameters are correlatable with grain size and large-scale bedform topography. Three-dimensional bedforms tend to form in finer sands and have been interpreted as hummocky bedforms formed by long-period surface gravity waves generated during the storm. The two-dimensional bedforms are interpreted as large ripples that show consistent lineation oblique to the orientation of larger sand ridges. For this study we will compare bedform lineations to storm history in order to infer stage at which these bedforms originated with respect to the passage of the storm. These results should help to improve our understanding of storm processes and their impact on sediment transport and deposition, as well as deduce the magnitude and frequency of ancient storm processes based upon the preserved rock record.