M
V=
(6-7)
ρs(1 - a)
where
V
= sediment volume
M
= sediment mass
ρs
= specific density of the sediment
= sediment porosity
a
For this calculation, the porosity was taken to be 0.4 and the specific density 2,650 kg/cu
m, corresponding to quartz sand. The resultant value of 57,000 cu m/year converts to
73,000 cu yd/year and must be considered doubtful because it is inconsistent with
estimates of Leatherman et al. (1997) and Fields et al. (1999), as well as with channel
maintenance volumes, which are much less at Mattituck Inlet. Such a large net rate
implies an even larger gross transport that is not considered feasible for the north shore of
Long Island and the waves in the Long Island Sound.
To estimate the gross longshore sediment transport rate at Goldsmith Inlet, the
present study considers the rate of accumulation at the fillet west of the Goldsmith Inlet
jetty, the annual rate of sediment accumulation in Goldsmith Inlet , the rate of sediment
transported offshore, and the rate of west-directed longshore sediment transport. The
annual rate of accumulation at the fillet is taken to be 8,000 cu yd/year, as inferred from
the analysis by Fields et al. (1999). The annual sediment accumulation rate in Goldsmith
Inlet is at least 5,000 cu yd/year, based on dredging records (1977 to 1990) and the fact
that sediment is observed to accumulate beyond the point of typical dredging. The rate of
transport offshore is not known, but may be significant given the pronounced depression
located directly offshore of Goldsmith Inlet. The rate of west-directed transport is not
known. Given the previous information and unknowns, the authors conclude that a gross
longshore sediment transport rate of 25,000 cu yd/year at Goldsmith Inlet is a reasonable
upper limit estimate.
The coast from Duck Pond Point to Horton Point (Figure 2-3) can be considered as a
littoral cell, with the bluffs serving as the sediment source. A littoral cell is a
semienclosed reach of the coast that is relatively isolated sedimentologically from
adjacent coastal reaches and that may contain its own sources and sinks of sediment. The
shoreline from Duck Pond Point to Goldsmith Inlet increases in orientation toward the
east. This change in orientation increases the angle between the crests of the
predominant waves and the shoreline, thereby increasing potential longshore sediment
transport directed to the east. In contrast, the section of shore that includes Mattituck
Inlet is oriented more parallel to the crests of the predominant waves and would,
therefore, be expected to have a smaller east-directed longshore sediment transport rate
for some wave conditions.
As described in Chapter 4, the present study estimates an average-annual sediment
accumulation rate of 8,000 cu yd/year for Mattituck Inlet prior to jetty modifications in
1938 and 1946. Based on this range of rates, a gross sediment transport rate of
25,000 cu yd/year as found for Goldsmith Inlet is judged to be an overestimate for
Mattituck Inlet. Recently, Batten and Kraus (2005) analyzed shoreline change, beach
profile surveys, and dredging records at Mattituck Inlet, and developed a local sediment
budget within the context of a regional budget. The local sediment budget was balanced
19
Chapter 2 Study Area and Physical Setting
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