Commercial mining by local permit on the beach directly west of Mattituck
Inlet served to prevent the west jetty from reaching impoundment capacity. The
volume of the attachment fillet located there has grown considerably in recent
years (Batten and Kraus 2005). Because mining of the fillet west of the jetty no
longer takes place, the west jetty will eventually reach impoundment capacity.
This occurrence can in turn be expected to substantially increase bypassing and
sediment accumulation within the inlet.
Most dredging conducted today is performed to remove the flood shoal that
forms at the base of the east jetty and along the west bank opposite this flood
shoal. Sediment accumulating on the flood shoal encroaches into the channel as
a steep shoal growing southward. Wave action is inferred to be a significant
contributing mechanism in mobilizing finer sediments for transport by the flood
tidal current. Waves have been observed to enter directly into the inlet to break
on the opposing gravel beach located at the eastward turn in the channel.
Sediment in this area encroaches into the channel as a steep shoal growing
northward. The sediment arriving to this beach is sorted, with gravel and pebbles
left behind and finer sand transported into the channel by tidal currents and
wave-induced currents.
For the past approximately 50 years, Mattituck Inlet has provided a reliable
channel for the shallow-draft navigation it supports, requiring maintenance
dredging approximately every 10 years. Shoaling patterns occurring today are
largely the same as those observed prior to the improvements of 1938 and 1946,
although removed volumes have diminished.
The existence of the modern-day flood shoal can be traced to the behavior of
the spit that formed in 1891 from the east bank, directed to the west. This spit
migrated and reached an equilibrium position around 1980. Because the spit was
subject to littoral forces over this time and is now mostly below water, it can be
considered a flood shoal. The main area of shoaling occurs along the east bank
where Mattituck Inlet empties into Mattituck Creek and the west bank opposite
this location experiences significant shoaling. Shoaling is caused primarily by
sediment brought into the inlet by storms from the northwest quadrant, to which
the inlet is directly exposed. The major form of shoaling at Mattituck Inlet is in
the form of bank encroachment, particularly at the location of the flood shoal.
Wave action, and the production of waves by boats passing through the inlet are
judged to be significant mechanisms for the redistribution of sediment onto the
banks of the Federal navigation channel.
It is concluded that the morphologic feature at Mattituck Inlet that has the
appearance of a relict ebb-tidal shoal is not an ebb shoal. Reasons for this
conclusion are:
a. The shoal experienced minimal growth from 1927 to 2002.
b. The feature is not hydraulically connected to Mattituck Inlet. Numerical
simulations of the tidal circulation for the present condition and
representative natural inlet condition demonstrate that the ebb-tidal
current is too weak to transport sediment to the shoal.
c. The shoal has a large gravel content, making transport to it by a weak
tidal current infeasible.
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Chapter 7 Comparative Analysis and Conclusions
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