EVENT SEDIMENTOLOGY AND HYDRODYNAMIC HINDCASTING OF HURRICANE SANDY DEPOSITS, NEW JERSEY

The impact of Hurricane Sandy (October 2012) on the Atlantic Coast of the United States resulted in massive overtopping (aggradation) of coastal barriers, along with localized overwash and breaching. A combination of high-frequency ground-penetrating radar (800 MHz GPR), low-field magnetic susceptibility (MS), and textural and mineralogical analyses was used to reconstruct the sedimentological and hydrodynamic parameters of the storm surge at two New Jersey sites immediately north of hurricane landfall: 1) Mantoloking and 2) Island Beach State Park (IBSP). Mantoloking represents a developed, scour-channelized backdune area with a 30–50 cm thick deposit consisting of 20–23 event horizons. Geophysical images of the overtop deposit reveal gently landward-dipping reflections, with minor surface scour. In contrast, IBSP site is a broad (width: 40 m) vegetated dune swale. GPR transects show landward-dipping tangential-oblique reflections as well as a conformable channel cut-and-fill structure (width: 6 m; depth: 1.5 m), producing a 0.7-1.5-m-thick deposit with 24-30 horizons. Within each sand-dominated event horizon, an upward 5-15% increase in mean grain size and 80-100% decrease in MS highlight the importance of hydrodynamic equivalency in lithological segregation within mixed-density fractions. Basal sub-layers enriched in heavy minerals (magnetite, ilmenite, garnet) yield MS of 100-350 μSI, in contrast to <40 μSI in quartz-rich upper sub-layers and pre-storm deposits. Several peaks in MS values correspond to an up to 65% increase in threshold shear stresses associated with individual unidirectional surge flows, with several most-enriched event horizons likely corresponding to waning-stage storm surge peaks recorded by offshore buoys. A sharp contact with the pre-storm surface produces distinct GPR reflections that allow accurate mapping of the thickness and extent of hurricane deposits. Together with potential correlation between lithological anomalies and high amplitude georadar signal return, the approach used in this study has implications for reconstructing event deposits in Quaternary sedimentary records.