SPACE AND TIME SCALES OF SHORELINE CHANGE AT FIRE ISLAND NATIONAL SEASHORE, NY

Fire Island is a south-facing 50 km long barrier island bounded by stabilized inlets in a wave-dominated microtidal regime. There is a net westward longshore export of at least 360,000 m³yr^-1, split about equally between an alongshore input past Moriches Inlet and from offshore sources to the western half of the island. A dataset of 31 files describing shoreline positions and changes analyzed in GIS is used to provide detailed alongshore change at 50 m intervals between 1870 and 1999 for the island. Half-century-scale, system-wide means of shoreline change were 0.5m yr^-1 prior to inlet stabilization in 1953 and 0.3m yr^-1since. However, the means were dominated by spatial variance due to inlet processes and apparent “waves” of sand advection alongshore. When analyzed at more frequent intervals, the data contain decadal variability associated with offshore bathymetric interaction, interannual variability is due to nearshore bar morphologic states controlled by recent summer storms (normally offshore hurricanes decaying into the north Atlantic) defining end-point comparisons, and seasonal variability is prominent but with a dimension correlated to winter storm frequency/magnitude. One event scale measurement of the effect of Hurricane Edouard passing offshore in 1996 displays a a mean erosion of 13 m with less spatial structural variability than any other time scale but still contains high frequency spatial variance.

The dataset also points to the development of effective sediment bypassing between 1986 and 1994 of the stabilized Moriches Inlet, which had built an ebb-tidal delta of over 2.3 million m³ in the 40+ years. Although the sediment deficit resulted in erosion over 10 km downdrift, the beach/dune system has accreted over 100 m in a few km reach 2 km downdrift of the inlet and this accretion is expanding westward. Principal Components Analysis of the very large data matrices points to decadal scale variation as dominant but that inlet dynamics and offshore bathymetry controls are substantial over several time scales. The higher frequency changes are small “hotspots” of erosion, largely attributed to storm wave interactions with nearshore bar/trough and beach morphology evolution from the a priori state.