As noted by MNE (2002), the recent value of the tidal prism is greater than those reported in
the past (e.g., in Jarrett 1976), whereas the minimum cross-sectional area is less than measured in
the past. However, the nearly annual dredging of the entrance makes conclusions difficult to
reach about trends in channel cross sectional area. The Escoffier closure curve for Fire Island
Inlet as calculated by MNE indicates the inlet is only marginally stable, likely the result of
(1) large amounts of littoral sediment transport entering the inlet, and (2) great length of the inlet,
which makes it hydraulically inefficient.
In summary, Fire Island Inlet in its present state is tending to close, which increases dredging
requirements because the navigation channel through the inlet is only marginally self-scouring.
Also, at least 30 million cubic meters of sediment are available in the ebb shoal. Assuming net
westward longshore transport rate of 300,000 m3/year, the inlet ebb shoal contains approximately
a 100-year supply of sediment.
Circulation Modeling
A regional circulation model established for Long Island (Militello et al. 2000) was run to
examine the circulation for the existing condition. Although the model had been calibrated for
Shinnecock Inlet, for the present work no calibration or grid refinement was done for Fire Island
Inlet and Great South Bay. However, calculated values of water level compared well with recent
measurements in Great South Bay and Fire Island Inlet. Figs. 5 and 6 show contours of current
speed (shading) and direction of the current (arrows) at peak ebb and peak ebb and peak flood
tide for the existing inlet and hypothetical relocated inlet, respectively. The existing inlet was
closed for the simulation with the hypothetical relocated inlet.
For the existing inlet (Fig. 5), the calculated ebb current is strong along Oak Beach and near
the seaward end of the dike, which is known to experience erosion at its tip. There is also a
strong ebb current along the NW side (backside) of Democratic Point, an area that has required
revetting to stop erosion. The flood current, and to a lesser extent, the ebb current is relatively
strong between Sexton Island and Captree Island. The ebb and flood current seaward of the
entrance of the existing inlet spreads widely and weakens because of the wide opening of the
inlet. Therefore, the transport capacity of the current is greatly reduced at the entrance, only
being strong at the narrowest constriction between the dike and backside of Democrat Point.
The calculated ebb and flood currents for the hypothetical relocated inlet are much more
constricted due to the presence of the jetties than for the existing inlet. The ebb shoal that would
form at the relocated inlet would be farther offshore than the finger shoals (Fig. 3) that comprise
much of the ebb shoal of the existing inlet. The flood and ebb currents in the vicinity of Sexton
Island and Fire Islands are much weaker than for the existing condition, so once the island
configurations adjust to the new inlet currents, there would be less potential for sediment to be
removed from them as compared to the existing condition. Based on the horizontal flow pattern
in Fig. 6B, a flood shoal would be created southeast of Sexton Island. The tidal current in the
former inlet and along Oak Beach would be greatly reduced.
REGIONAL SEDIMENT MANAGEMENT CONSIDERATIONS
This section collects and organizes information and concepts, as well as introduces new
considerations, for evaluation of the hypothetical relocation of Fire Island Inlet from a regional
sediment management perspective. Not all issues can be introduced in this short paper, and
those discussed are prominent or obvious ones that serve the purpose of illustrating a "system-
wide approach" to sediment management as discussed in the Introduction.
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