on sandy coasts (Jarrett 1976). The unusually large minimum cross-sectional
area of the channel at Mattituck Inlet experiences a weak tidal current. The large
area is attributed to a combination of overdredging of the inlet by commercial
mining, low longshore sediment transport rate, and sediment blockage by the
jetties.
Given their significant differences, it is remarkable that these two inlets have
remained open, with the possible exception of intermittent short closings, for
more than two centuries and likely much longer. The stability of inlets on the
north shore derives in part from a relatively steep inner shore face, presence of
geologic controls such as glacial erratics or hard points on shore, origins of ponds
as low-lying areas created after glaciation, and relatively weak longshore
sediment transport that is about an order of magnitude less than that on the south
shore. However, other factors have entered in controlling stability, in particular,
commercial mining of sediment, such as at Mattituck Inlet.
Mattituck Inlet, Conclusions
Prior to the 1938 extension of the west jetty, Mattituck Inlet experienced
substantial sediment intrusion through shoal development on the west side. The
extension was effective in eliminating this shoaling. Prior to the 280-ft landward
extension of the east jetty in 1946, a breach and spit formation occurred on the
east side of the east jetty. The 1938 seaward extension of the west jetty and the
1946 landward extension of the east jetty have effectively protected Mattituck
Inlet from sediment intrusion, indicating successful modification or tuning of
these coastal structures. The improvements decreased the long-term average-
shoaling rate at Mattituck Inlet from approximately 8,000 cu yd/year to 1,500 to
2,000 cu yd/year.
While the breach was open, sediment was transported into the inlet entrance
to the west and into the navigation channel where the channel turns eastward. At
present, large dunes and a berm abut the east jetty, reducing breach potential.
The presence of accretionary features on the Long Island Sound side of the east
barrier indicates that shoreline position adjacent to the east jetty has become
stable under typical wave conditions. Some cutting into the backside of the east
barrier, adjacent to the east jetty, by the tidal current and, possibly, reflected
waves is observed. This cutting should be monitored so that the integrity of the
barrier near the east jetty is not compromised.
Considerable commercial mining of sand and gravel took place at Mattituck
Inlet from the 1920s to the mid-1970s. The exact volumes of mining are
undocumented, and some records are missing. It is estimated that mining
resulted in the removal of 250,000 to 500,000 cu yd of sediment from between
the jetties and that mining on the beach directly west of the west jetty removed
260,000 to 400,000 cu yd of sediment. Commercial mining permanently
removed mined material from the littoral zone and beaches. Such mining
contributed to maintenance of the navigation channel, and it is hypothesized in
this study that the amount of commercial mining of the inlet channel increased
the channel cross-sectional area beyond that supported by tidal flow. The
channel will not readily return to an equilibrium cross section because of the low
rate of longshore sediment transport at the site and protection of the channel by
the jetties.
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Chapter 7 Comparative Analysis and Conclusions
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