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Fire Island Coastal Change

Storm Impact and Recovery: Hurricane Sandy - Overwash Mapping (Print Version)

Overwash processes during Hurricane Sandy carried substantial volumes of sand to the interior and back-bay areas of Fire Island. To estimate the post-storm washover volumes from Hurricane Sandy, a combination of field, remote sensing, and GIS analytical techniques were used. A group of ground-based National Park Service (NPS) volunteers surveyed 50 washover fans following a protocol developed by the USGS field team to create a dataset of storm washover deposit thicknesses. A total of 797 thickness measurements were obtained used to create isopach (thickness) maps. Volumes for each deposit were calculated from the isopach surfaces. The results of this method indicate an estimated 510,000 cubic meters (m3) of sand was transported from the primary dunes and beaches to the island interior during Sandy. More detailed information on the methodology and volumes are found in Hapke and others, 2013.

In addition, calculations of profile-based volume loss from the active beaches and dunes were derived from pre-Sandy (May 2012) and post-Sandy (November 2012) lidar data. The analysis indicates that Sandy moved or removed 3,500,000 cubic meters (m3).

Based on the estimations of erosion and deposition during Sandy, it can be determined that only 14% of the volume of sand removed during Sandy was deposited as washover. The remaining material was transported offshore and downcoast. The USGS is continuing to monitor the beaches and dunes to evaluate how much of the sand removed by Sandy returns to the beach via natural beach-building processes (see Shoreline Change and Beach Profiles)

Overwash Maps

thickness of overwash desposit Area 1

thickness of overwash desposit Area 2

thickness of overwash desposit Area 3

thickness of overwash desposit Area 4

thickness of overwash desposit Area 5

thickness of overwash desposit Area 6

thickness of overwash desposit Area 7

thickness of overwash desposit Area 8

thickness of overwash desposit Area 9

thickness of overwash desposit Area 10

thickness of overwash desposit Area 11

Overwash Characteristics

Examples of overwash styles at Fire Island, New York : (a) sheet wash in front of Fire Island Lighthouse, (b) surge channels through dunes and overwash fan along central-eastern Fire Island, and (c) tongues of overwash in eastern Fire Island.

index map showing location of overwash characteristics Overwash Location A Overwash Location B Overwash Location C

In the western portion of the island, overwash processes resulted in washover sheets
A. In the western portion of the island, overwash processes resulted in washover sheets rather than fans and surge channels, resulting in deposits that are somewhat continuous alongshore. The inland distance of the overwash along western Fire Island was limited in many locations by private homes and other community infrastructure in the developed stretches of coast. [larger version]

In the central portion of Fire Island, the occurrence of overwash is relatively low and is confined to narrow channels that coincide with existing cuts in the dunes
B. In the central portion of Fire Island, the occurrence of overwash is relatively low and is confined to narrow channels that coincide with existing cuts in the dunes (vehicle access points or other lows). Volumes of overwash are also significantly lower in the central portion, likely due to the higher overall pre-Sandy dune and island elevations and (or) lower waves and storm surge along this segment of the island compared to adjacent areas. [larger version]

Eastern Fire Island experienced the greatest amount of overwash and inundation. In areas with high pre-Sandy dunes, surge channels cut through the dunes and carried large volumes of material to the island interior, deposited as overwash fans. In lower-lying portions, continuous overwash fans and fields of overlapping overwash tongues extend across the width of the barrier island.
C. Eastern Fire Island experienced the greatest amount of overwash and inundation. In areas with high pre-Sandy dunes, surge channels cut through the dunes and carried large volumes of material to the island interior, deposited as overwash fans. In lower-lying portions, continuous overwash fans and fields of overlapping overwash tongues extend across the width of the barrier island. [larger version]

Relevant Publications

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