Figure 4-18a shows the approximate hwl position in 1927, indicating
considerable accretion on the west side of Mattituck Inlet. Dredging from
August to September 1923 removed 49,168 cu yd of sediment. Of this dredging
and the dredging of September-October 1927, Ralston (1928) states that the
removed sediment was "principally from the entrance channel between the
jetties." Newly dredged channels will often be subject to sediment slumping, in
morphology. Given the large volumes dredged in the period 1921 1927, it is
hypothesized that sediment slumping is responsible for a portion of the sediment
dredged, and that the volumes dredged during this early period cannot be totally
attributable to longshore sediment transport rates for this area.
Condition surveys indicates that the Federal navigation channel had
narrowed to less than 50 ft at the base of the east jetty by May 1925, and that
controlling depths of the channel had decreased to 5 ft mlw by May 1925, to 3 ft
by June 1926, and to 2.5 ft by August 1927 (Ralston 1928). Preliminary
examination (submitted 29 April 1927) and subsequent survey (submitted
22 June 1928), concluded that "shoaling is caused mainly by heavy material
(sand and gravel) being driven into the inlet by the action of storms from the
northwest quadrant, to which the inlet is directly exposed" (Ralston 1928). The
report notes that sand fences had been erected with no appreciable effect, which
precludes wind-blown sand as a significant sediment source (or that the sand
fencing was ineffective). These surveys produced a recommendation that the
west jetty be extended seaward 350 ft, or to the 12-to-15 ft mlw depth contour.
The survey also recommends a possible further seaward extension of 150 ft for
the west jetty and a seaward extension of 300 ft for the east jetty "if required and
conditions indicate" (Ralston 1928). The Rivers and Harbor Act of 30 August
1935 authorized a 250-ft seaward extension of the west jetty.
The dredging from September to October 1927 restored Mattituck Inlet to the
specified project depth. Figure 4-18a shows channel elevations, and Figure 4-
18b shows the areal extent of the flood shoal, for 23 September 1935, 8 years
after this dredging. The east and west lobes of the flood shoal have joined, and
the controlling depth is 2 ft mlw. At this time, the Federal navigation channel
was presumably navigable only during times of high water. Infilling along the
insides of both jetties created significant tracts of dry shoals. The condition
survey of 23 September 1935 did not fully record the shoreline position. The
landward breach at the base of the east jetty can be discerned. Evidence of this
breach becomes clearer in successive surveys. The shoreline fragments for this
period that were recorded are indicated in black, referenced against the hwl
shoreline of 8 December 1927. A portion of this area, within the channel at the
base of the east jetty, appears to be below mlw, further indicating that the breach
had begun to form at this point.
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Chapter 4 Morphology Change, and Channel Shoaling and Migration