Hurricane Sandy deposits on Fire Island, NY: Using washover deposit stratigraphy to understand sediment transport during large storms.

Abstract:

By examining coastal deposits from modern inundation events, a number of criteria have been developed to help distinguish between onshore storm and tsunami deposits. The presence of sedimentary structures such as planar lamination and foreset beds in storm deposits suggest a dominance of bedload transport, although some tsunami deposits also contain these structures. The presence of normally graded beds in many tsunami deposits is indicative of the dominance of suspended load transport. Understanding how the stratigraphy in modern event deposits develops during inundation and post-deposition modification improves our ability to differentiate between tsunami and storm deposits in the geologic record. We investigated Hurricane Sandy washover deposits on Fire Island, NY a year after the storm in order to describe deposit stratigraphy on sandy barrier islands. Stratigraphic descriptions from trenches, bulk sediment samples, and push cores were collected. So far, grain size and microtextural analyses have been performed on bulk sediment samples. Deposits ranged from 1-100 cm thick and consist of very well-sorted, fine to coarse sand. Elevation profiles of washover features were measured using Differential GPS. Comparisons of GPS elevations to Lidar data from pre- and post-Sandy surveys show that at some locations, the upper 10-40 cm of the deposit had been reworked in the year following Hurricane Sandy. However, there were still areas where the deposit was almost entirely intact, consisting of massive sand layers near the base, overlain by laminated layers. We hypothesize that the lower sand layers represent deposition from overwash carrying sand in suspension that may have occurred during dune breaching. The upper laminated layers are indicative of deposition from bedload transport by successive storm waves. Detailed analyses of vertical grainsize trends will help determine if the lower unstratified sand layers were deposited by one or multiple flows.