TOWARDS RECONSTRUCTING SHOREFACE SEDIMENT FLUXES AT FIRE ISLAND, NY AND LONG BEACH ISLAND, NJ

Accurate assessments of sediment availability in sandy coastal systems are needed to predict their long-term vulnerability to enhanced rates of sea-level rise, as well as resilience to potential increases in storm frequency/intensity. However, historical records often provide inadequate temporal baselines to evaluate past system sensitivity to changes in sediment supply at decadal to centennial scales. To address this temporal data gap, we reconstruct sediment budgets by applying concepts describing the morphology of the combined beach, foredune, and backbarrier system as a function of sediment input and partitioning at the shoreface. These relationships inform a quantitative, cross-shore morphodynamic model, which is used to infer records of time-variable shoreface sediment fluxes. We evaluate our modeling framework at two field sites along the U.S. Atlantic coast: Fire Island (NY), and the Holgate Peninsula (NJ), at the southern end of Long Beach Island. Despite modern geomorphological similarities, these barrier systems evolved in dissimilar geologic settings and sea-level rise regimes, allowing us to test the model’s capabilities under different temporal scales and sediment input/storage conditions. The Holgate Peninsula formed as a southerly-elongating spit over the last century and experienced significant human intervention. Conversely, Fire Island retrograded to its present position as much as a thousand years ago and historically developed under largely natural conditions. Modern Fire Island is suspected to be an amalgamation of at least two barrier-island cores that formerly experienced episodes of progradation and later became linked alongshore. The now-continuous island is currently undergoing erosion, alongshore extension, and landward transgression. Planned collection of chronometric, sedimentological, and geophysical data at both Fire Island and Holgate will constrain timeframes of morphological evolution, as well as provide insights into volume storage. This will allow us to quantify the threshold magnitudes of sediment fluxes that drive transitions between progradational, stable, and transgressive barrier states.