Alternative 2: Natural morphology with offshore shoal
To examine a possible hydrodynamic, hence sediment-transport, relation
between Mattituck Inlet and the offshore shoal located to the east, an alternative
grid of Mattituck Inlet representative of its natural state was developed. The
results of this alternative are analyzed here and compared to the pre-dredging
configuration of 2002. The morphology representing the natural (pre-jetty) state
is based on that of the inlet in 1891 (Figure 4-10). The east-directed spit shown
in Figure 4-10 was truncated in this grid, because the apparent great spit length
was not considered to be representative of Mattituck Inlet in typical natural
equilibrium. The depth at the center of Mattituck Inlet and Mattituck Creek was
taken to be approximately 1.3-1.5 m NAVD88. This depth was selected because
the spring tide range at Mattituck Inlet is about 2 m, and there is no indication in
the historic record that the inlet and Mattituck Creek became dry.
The tidal prism at Mattituck Inlet is 4.32 107 cu ft. Applying the Jarrett
(1976) relation for Atlantic Coast inlets with no jetties (discussed in
Chapter 6), AC = 5.37 10-6 P0.107 , the channel cross-sectional area of Mattituck
Inlet in its natural state is estimated to be 794 sq ft. If the width of Mattituck
Inlet in a natural state is taken to be 175 ft, the resulting average depth is 4.5 ft
NAVD88, or 1.38 m. Finally, the historic presence of a tidal mill at Mattituck
Inlet suggests presence of water of at least this depth.
The hydrodynamic behavior of Mattituck Inlet in its assumed (synthetic)
natural state differs considerably from that of Mattituck Inlet in its present
modified state. The current at the natural Mattituck Inlet displays similar
properties to the present Goldsmith Inlet. The natural Mattituck Inlet is found to
be strongly flood-dominant, and the phase lag between the offshore and creek
water-level peaks and troughs is pronounced.
Figures 5-14a through 5-14i display calculated tidal velocities offshore of
and through Mattituck Inlet for various times during a full spring tidal cycle.
Figures 5-14a and 5-14b illustrate Mattituck Inlet at 1200 GMT 7 October 2002,
the time of near-maximum offshore current velocity. This time is equivalent to
that of the near-maximum offshore current velocity illustrated in Figure 5-8a.
For comparison to Figure 5-8a, Figure 5-14a is displayed with the same contour
interval range (0-0.6 m). Figure 5-14b illustrates calculation results for this time
with a contour interval range of 03 m, for comparison to subsequent figures.
During this time of near-maximum offshore flood current, the inlet is nearing the
end of ebb tide.
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Chapter 5 Circulation Analysis
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