Assessing the magnitude and direction of storm-related sediment fluxes on barrier islands

Abstract:

Storms drive sediment redistribution on natural barrier islands, including overwash fluxes that maintain barrier heights and widths as they adjust to sea level. However, storms do not only move sediment landward and overwash does not occur uniformly along the coast, so quantifying the proportions and directions of storm-eroded sediment flux is critical for predicting barrier resilience. Using lidar and satellite data, we quantify sediment fluxes associated with Hurricane Sandy at Fire Island, NY, where alongshore-variable geomorphology allows us to explore how the island’s geomorphic history contributes to storm flux partitioning. On the easternmost portion of the island, where stable transgression dominates, landward- and seaward-directed fluxes are balanced, with ~51% of the sediment eroded from beach and dunes deposited as overwash and ~49% delivered offshore. The central portion of the island i) alternates between stable-aggradational and transgressive behavior or ii) is characterized by cross-shore progradation and amalgamation. Characteristic high dunes increase seaward losses drastically, comprising ~95% of beach and dune volume loss. In the westernmost zone, which has historically experienced along- and cross-shore progradation, about 30% of beach and dune volume loss is deposited as overwash with the remaining ~70% transported seaward. The large proportions of seaward-directed flux observed have important implications. First, our results suggest that narrow barrier stretches with high dunes shed significant volumes of sediment seaward, starving the backbarrier and potentially increasing barrier vulnerability to erosion and passive drowning if losses are not replenished from updrift. Second, they show that a characteristic post-storm shoreface response may not occur along barriers with diverse geomorphic histories. The results also suggest observations of beach and dune volume loss that do not account for offshore fluxes dramatically overestimate the magnitude of landward-directed fluxes in natural systems. Comparisons between post-storm beach and dune volume changes calculated in this analysis and corresponding shoreface bathymetric changes will further refine our understanding of sediment exchanges between shoreface and subaerial barrier environments.