MORPHOLOGIC AND HYDRODYNAMIC PROCESSES
Westward Growth of Democrat Point and Relation to Neighboring Beaches
Fire Island Inlet has migrated to the west since at least 1824, when a lighthouse was built
close to its eastern bank. Its rapid migration is a textbook example of spit growth and has been
well documented (Gofseyeff 1952; Saville 1960; Panuzio 1968; Kumar and Sanders 1974;
Leatherman and Allen 1985; Morang et al. 1999; Smith et al. 1999). Shoaling in the inlet tends
to drive the navigation channel to the north. Erosion on Oak Beach was attributed to the close
proximity of the natural ebb channel to shore, and a training dike was constructed in 1959 to
move the channel southward (Figs. 2&3) and limit erosion. Property owners along Oak Beach
are concerned about opening Fire Island Inlet wider or aligning the channel more in a north-
south orientation that might expose the beaches to greater wave action. Cedar Beach has
accumulated sediment, whereas Gilgo Beach further to the west of the inlet experiences chronic
erosion and is a Demonstration Site in the National Shoreline Erosion Control Development and
Demonstration Program (http://limpet.wes.army.mil/sec227/Demosites/gilgo.htm). Beaches to
the east of Fire Island Inlet are also experiencing erosion, and back passing of dredged material
is performed. Most dredged material is placed off Gilgo Beach. Historically, westward growth
of Democrat Point modified the islands to the north and west, as well as abandoned flood shoals
to the east. Examination of historic U.S. Coast and Geodetic Topographic Sheets (T-Sheets)
suggests that Sexton Island and the Fire Islands of Great South Bay may be the remnants of the
distal end of the Oak Beach barrier that has disintegrated by erosion during the barrier overlap
process.
To investigate the westward movement of Democrat Point, T-Sheets were obtained in digital
format from the National Oceanic and Atmospheric Administration's Coastal Services Center.
The T-sheets included survey dates between 1834 and 1924. Historical aerial photographs
covering 1941 through 2000 were obtained from the U.S. Army Engineer District, New York
archives and from the Beach Erosion Board archive at the U.S. Army Engineer Research and
Development Center, Coastal and Hydraulics Laboratory, and digitized at high resolution. The
digital T-sheets, which included benchmark information referenced to NAD 1927, were imported
into ArcView and referenced to NAD 1983 with the ArcView Projection Utility. The aerial
images were imported into ArcView as image analysis themes and referenced to the local
plane coordinate systems (NY State Plane NAD 83) using USGS Digital Ortho Quarter
Quadrangle images (DOQQ's). The approximate position of the mean high water line was then
mapped with the Beachtools ArcView Extension (Hoeke et al. 2001).
The analysis is summarized in Fig. 4. The approximate 8-km extension of Fire Island
mapped from the sequence of T-Sheets and aerial photographs shows the response of the Oak
Beach barrier to the overlap process. A narrow tidal inlet (Oak Island Inlet) is noted in the 1851
configuration. By 1873, the east end of Oak Beach was fragmented. Shortening of Oak Beach
continued to at least 1924, and by 1941 the east end of Oak beach was nearly in its present
location. The inlet entrance was narrow by the early 1950's (Fig. 3) due to shoal building, and
thinning of the Oak Beach barrier became severe from erosion by tidal currents within the inlet
channel. At this point in time it seems likely that a major storm would have to cut a new inlet
across a narrow portion of Oak Beach just to the west of Oak Island and Captree Island. The
impoundment of sand and deflection of tidal currents by the sand dike constructed in 1959 may
have prevented breaching of a new inlet. The impacts of sand impoundment, sand bypassing,
and shoal growth are evident in the 1960 to 2000 shoreline sequence.
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