Analysis of hummocky bedforms offshore Fire Island, New York, generated by superstorm Sandy

Hummocky bedforms are mainly studied in ancient sedimentary outcrops; they are interpreted to be associated with storms, an inference that is based on paleohydraulic conditions derived from sedimentary deposits. However, the modern hummocky bedform catalog is sparse, with few associations of hydraulic conditions resulting in their deposition and preservation. In this study we investigate small-scale (< 20 m wide) surficial bedforms superimposed on top of large-scale (200-3000 m wide) westward-migrating shoreface-attached sand ridges and sorted bedforms, using multibeam bathymetry surveys acquired two months after “superstorm” Sandy offshore of Fire Island, NY. Geostatistical analysis of two post-storm surveys, one along the western half of the island dominated by 1-3 km-wide sand ridges, and the other to the east, dominated by 0.2-1.0 km-wide sorted bedforms, is utilized in conjunction with robust visualization techniques to classify the surficial bedforms into medium dunes (7.4-13.6 m wavelength, 2.2-4.6 cm high), hummocky bedforms (4.9-9.4 m wavelength, 2.6-6.7 cm high) and 2.5-dimensional dunes (4.7-6.8 m wavelength, 3.9-7.2 cm high). Storm-driven hydrodynamics that drive the migration of the large-scale shoreface-attached bedforms effect the grain size distribution and hence the distribution of small-scale bedforms across both sand ridges and sorted bedforms. The small-scale bedforms are interpreted to have formed under wave-dominated conditions, based on the near-zero asymmetry index. The nature and character of hummocky bedforms examined in this study improves our three-dimensional perspective of this sedimentary structure, providing tools for better identification of the structure from the past geological record, and interpretation of hydraulic processes attributed to its origin.