eastern half of Long Island. Littoral sediments on eastern Long Island have been
assumed to be supplied either from bluff erosion at Montauk Point, or from offshore
sources (Taney 196l; Kana 1995; Williams 1976; Schwab et al. 1997). Sediment
budget analyses indicate that the inlet removes sediment from the littoral system at a
rate of 70,000 to 115,000 m3/year (USACE 1958, 1988; Kana 1995; Morang 1999;
Rosati et al. 1999). Empirical relations are available to estimate the large volume of
sediment contained in the ebb and flood shoals (Dean and Walton 1975; Walton and
Adams 1976; Carr de Betts 1999). Properties of inlets along the Atlantic, Gulf, and
Pacific coasts of the United States were examined, and the combined control by the
tide and waves were related to the ebb shoal volume as (Walton and Adams 1976),
Vs = CP n
(2)
where Vs is ebb shoal volume, and C and n are empirical coefficients that depend on
wave energy. For Shinnecock Inlet, a moderately exposed inlet, the coefficients C
and n are 10.5 x 10-5 and 1.23 respectively. Using the most recent estimate of tidal
approximately 11,200,000 m3. This empirically-estimated volume is larger than the
amount of material contained in the ebb shoal in 1996 (5,803,000 m3), 1997
(5,988,000 m3), and 1998 (6,463,000 m3) (Morang 1999). Increase in volume
between 1996 and 1998 indicates the ebb shoal complex was trapping sediment prior
to the dredging of the channel and deposition basin in 1998. By 1998, the ebb shoal
had trapped 60 % of the theoretical equilibrium volume. Using the estimated rates of
trapping, the ebb shoal is expected to attain equilibrium in approximately 42 to 70
years. A previous calculation of the theoretical ebb shoal volume determined
equilibrium would be reached in approximately 75 years, although dredging of the
channel and ebb shoal would prolong the evolution (Kraus 2001). As equilibrium is
approached, it is expected that more persistent sediment transport pathways will be
established, and the inlet will bypass sediment more effectively to the neighboring
beaches.
Inlet morphology responds to variations in gross longshore sediment transport
rates, which have been estimated to range from 230,000 m3/year to 305,000 m3/year
in the vicinity of Shinnecock Inlet (Williams et al. 1998). Easterly transport
predominates during the months of May through August, and during December.
West-directed transport dominates the remainder of the year with the largest overall
net transport rates (for either direction) occurring between the months of September
and November. Inlet sediment bypassing controls the rate, location, and composition
(Bruun and Gerritsen 1959, 1960; Liu and Hou 1997) of sand nourished to downdrift
beaches. Sediment bypassing across inlets is achieved through the transport modes of
1) wave-induced transport along the periphery of the ebb shoal (terminal lobe),
2) transport through channels by tidal currents, and 3) migration of tidal channels and
sand bars (Bruun and Gerritsen 1959, 1960). The dominant mode of sediment
bypassing can be determined from the ratio,
r = P/M
(3)
4
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