ASSESSING A DECADE OF MORPHOLOGIC CHANGE IN A DYNAMIC SYSTEM: A SPATIAL AND TEMPORAL ANALYSIS OF DUNE/BEACH BEHAVIOR AT FIRE ISLAND NATIONAL SEASHORE, NEW YORK, 1998 - 2008

A spatial and temporal analysis has been completed for the dune/beach system at two study sites within Fire Island National Seashore, New York. The study sites are located in segments of Fire Island that behave differently from one another: the eastern site is in an area that is undergoing landward roll-over migration, while the western site is in an area that is narrowing but remaining relatively fixed-in-place. A substantial replenishment project supplied sediment to beaches and dunes at four communities within the western site during the winter of 2003-2004. Differences in the morphologic behavior of the two sites were characterized using a dense temporal dataset of high-resolution 3D surfaces generated from RTK GPS and existing lidar surveys. Changes were assessed from December 1998 to October 2008. The 3D surfaces were used to calculate total volume change and positional change of the dune crest, dune toe, and shoreline, as well as to measure cross shore changes in characteristics such as dune elevation, beach width, and profile slope. Results show that statistically significant differences exist between the two study sites. Prior to the replenishment, dune crest elevation and profile slope were lower at the western site (by 20% and 10% respectively) and highlight differences in morphology that may predispose the two sites to behavioral variations. Between 2002 and 2008, observed feature differences are largely attributable to replenishment: site-wide volume increased by 18%, shoreline erosion rates (-2.7 m/yr) essentially reversed to become accretional (2.3 m/yr), and average beach width increased by 20% at the replenished western site. The lower gradient and dune elevations at the more dissipative western site may make it less vulnerable to storm impacts when compared to a more reflective eastern site. In addition, results suggest that the replenishment may have exacerbated differences already inherent in the system. Because beach replenishment impacts are temporary in affecting beach behavior and mitigating erosion, these costly efforts should encompass a thorough understanding of factors that contribute to their durability. Given the results of this analysis, an understanding of the antecedent geology may be a critical component in predictions of future replenishment project longevity at Fire Island.