Hurricane Sandy’s Impact on Coastal Sedimentation on Long Island’s South Shore: Results from a 2013 Rapid Response Study

Abstract:

To understand the impact of Hurricane Sandy on the NY coast, we conducted subsurface and multi-beam analyses, ground-truthed by grab samples, in 3 areas: the western end of Fire Island (FIW), eastern Fire Island (FIE) where a new inlet formed during the storm, and Long Beach (LB). Grab samples yielded sands and muds, a surprise given the shallow (10-25m) water depths. Muds rested on top of sands and were removed for additional analyses. Since percent mud could not be determined absolutely, sediments were washed through a 63 mm sieve, RoTapped for 10 minutes at ¼ Φ, and weight percent calculated for the coarse fraction.

At FIW and FIE, fine sands dominate the shallowest depths studied, consistent with previous studies. At FIE, the sedimentary wedge extends to ~15m, with finest sands (peak 3-3.5 Φ) in shallowest waters surveyed (~10m). Slightly coarser (2.5Φ) sediments plus relict gravels are present in swales where the wedge shoals. This supports mapping results indicating sand ridges migrated to the SW. Medium to fine sand is present at the deepest extent of the wedge; the grain size distribution matches a sample taken in the swash zone on the eastern flank of the new breach. Sediments may have been transported shoreward and then reworked post-Sandy. Samples seaward of the new breach were capped by a mud layer, which in turn had a layer of fine sand resting on it, evidence of a nascent ebb tidal deposit. At FIW, sediments in the shallow NE swale are finer (3.5Φ) and better sorted. As the region is underlain by relict sediments, these fine sands may be relicts exposed by storm-driven bedform migration. Deeper water (~22m w.d.) samples at FIW are coarser and contain shell hash. Sand on the lee side of the sand ridge, which CHIRP profiles show did not migrate significantly and accumulated sands, are medium (1.5 Φ), and match the grain sizes found on Fire Island beach. Muds contain heavy metals in concentrations consistent with transport from adjacent estuaries.