ANTHROPOGENIC INFLUENCES ON THE DUNE/BEACH MORPHOLOGY OF A MODERATELY DEVELOPED BARRIER ISLAND; FIRE ISLAND, NEW YORK

Technical Report NPS/NER/NRTR--2008/131

Meredith G. Kratzmann¹, Cheryl J. Hapke^2
1Department of Geosciences
University of Rhode Island
Kingston, RI 02881

²U.S. Geological Survey,
Woods Hole Science Center
384 Woods Hole Rd.
Woods Hole, MA 02543

October 2008

U.S. Department of the Interior
National Park Service
Northeast Region
Boston, Massachusetts
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Executive Summary

This study assesses the influence of anthropogenic alterations to the dune/beach morphology of Fire Island, a moderately developed barrier island located along the south shore of Long Island, New York. Alterations include beach replenishment, beach scraping, and the presence of developed communities. Beach replenishment is an engineering method that adds material to beaches and dunes from an upland or offshore source. Beach scraping involves the transfer of sand from the berm to the foredune zone creating an anthropogenic foredune intended to protect property from overwash processes and erosion. Analyzed datasets include volume (dune and subaerial beach) and shoreline change calculated from lidar and RTK GPS grids to quantify spatial and temporal changes to the beach, and investigate whether development and/or human alterations affect volume and shoreline position at Fire Island. Beach profile characteristics were used to study the effects of beach scraping on dune/beach morphology in scraped versus non-scraped areas, and to determine if beach scraping has morphological effects on undeveloped areas downcoast.

Two study sites were selected for analysis that contain both developed and undeveloped areas. The study sites are located within Fire Island National Seashore (FIIS) which contains seventeen communities that are managed by the National Park Service (NPS). Understanding the effects of anthropogenic alterations to the dune/beach system within the National Seashore is needed for effective preservation of the beaches and other natural resources for future generations.

Results indicate that anthropogenic alterations in developed areas are detectable via changes in volume, shoreline position, and several beach profile characteristics. The western site increased in volume between 2002 and 2005 as a result of beach replenishment in the developed area which subsequently moved downcoast into the undeveloped area. On a decadal timescale (1998-2007), volume increased within 77% of the western site, but in contrast, 75% of the eastern site decreased in volume. Shoreline position retreated at the eastern site over the same time period and prograded at the western site which are results consistent with the volume change analysis. In the long-term (28 years, 1979-2007), both sites are characterized by an accretional trend. An erosional trend is present in one section of the eastern site from 1979 to 2007, and is located in the developed area.

The results of the beach profile analysis reveal morphological differences in scraped areas compared to non-scraped areas of the beach. Dunes constructed via beach scraping contain a greater volume of material than the natural foredunes of Fire Island. Within the western site, scraped material moved downcoast as shown by higher beach and dune volumes and wider beaches in undeveloped areas that have never been scraped. At the eastern site, the scraped profile location is the most erosional and dune elevations are lower than non-scraped locations.

These results indicate that the most potential for overwash and flooding is at the scraped location. Beach scraping appears to accelerate downcoast transport in accreting locations and is ineffective protection in eroding areas. This study quantified alterations to the dunes and subaerial beach of a moderately developed barrier island by comparing specific morphological characteristics in developed versus undeveloped areas. In addition, a new dataset directly isolated the effects of beach scraping which is a method of property protection on sandy coasts that has not been well studied. Methods established in this work are applicable at other barrier islands to determine the influence anthropogenic modifications have on the dune/beach system.

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