Sorted bedforms off Western Long Island, New York, USA: Asymmetrical morphology and twelve-year migration record

Sorted bedforms are widely present in sediment-starved littoral and inner shelf settings; they are indicators for hydrodynamic conditions and a primary contributor for the subsurface structure. This study investigated the morphology and migration of sorted bedforms on the inner shelf of Long Beach Barrier Island, New York, USA, by repeat geophysical and geological surveys in 2001, 2005 and 2013 (following superstorm Sandy) involving swath bathymetry, backscatter, chirp seismic reflection data and grab sampling. Swath data revealed that the western sector, comprising the western 75% of the survey region, is dominated by NNE–SSW-oriented, 0·5 to 1·0 km wide sorted bedforms with highly asymmetrical cross-sections, with steeper slopes and coarser sands on the eastern (stoss) flanks. Many secondary bedforms were also observed (north–south to north-east/south-west oriented lineation structures) at the western edges of coarse sand zones. The eastern sector displays an unusual sorted bedform pattern that is dominated by coarse-grained substrate, with isolated patches of fine-grained sands oriented north-east/south-west which are 0·15 to 1·0 km in length and ca 30 to 200 m in width, similar in scale and orientation to the secondary bedforms in the western sector. Comparison analysis of the swath data sets indicates that the primary transverse sorted bedform morphology within the western sector was largely stable over this time frame, although the swales were deepened following the storms. The coarse/fine sand boundaries did migrate, however, moving ca 1 to 5 m eastward between 2001 and 2005, and ca 5 to 20 m westward between 2005 and 2013; the higher migration rates (up to 2·5 m year⁻¹) in the latter time period may be attributable to large storm forcing (for example, hurricanes Irene and Sandy). Significant north-westward migration of the secondary bedforms and coarse sand patches in the western sector, as well as fine sand patches in the eastern sector were also observed; these features are far more mobile than the primary sorted bedforms, possibly because they are fine sand drifts that do not erode into the coarse substrate. Seismic reflection data revealed a transgressive ravinement beneath sorted bedforms, merging with the sea floor at the bottom of swales. The authors hypothesize that long-term topographic migration of transverse sorted bedforms contributes to the formation and evolution of the ravinement.