The authors of the present study estimate that these mining activities
removed 260,000 to 380,000 cu yd from the system. The consequence of this
mining on sediment accumulation within Mattituck Inlet can be seen in the
dredging records (Table 2-5), where a substantial decrease in dredged volume
within the channel is observed after 1965. Cessation of these mining activities
can also, in turn, be expected to eventually increase sediment accumulation rates
within Mattituck Inlet. Impoundment west of the west jetty is observed to have
increased in recent years, and the shoreline there has advanced (see Figures 4-
29h through 4-29l; Batten and Kraus 2005). Sediment accumulation rates can be
expected to increase significantly when the west jetty has reached impoundment
capacity.
Elevation change and dredging volume. In its present condition, Mattituck
Inlet is a hydraulically efficient channel that experiences small rates of sediment
accumulation. Gradual shoaling within the Federal navigation channel
eventually leads to constriction, and this shoaling is the main requirement for
periodic dredging, jetty repair, and jetty extension. The good performance of the
Mattituck Inlet navigation project in its present condition is largely the result of
proper maintenance and jetties of adequate length. Sand and gravel mining that
occurred directly west of the inlet which served to keep the volume of sediment
located there below the impoundment capacity of the west jetty, is also a
significant factor. Dredging records (Table 2-5) indicate that the Federal
navigation channel at Mattituck Inlet accumulated sediment at a rate of 1,000-
2,000 cu yd/year from 1965 to 1990. The repair of the west jetty in 1975 may
partially explain the decrease in accumulation rate. (The west jetty was last
repaired in 1996.) Improvements in dredging technology may also account for
the smaller volumes that have been dredged in recent years, where the channel is
more accurately dredged to the project depth of 7 ft mlw with 2-ft overdraft.
The volume dredged in August to October 1961 was 43,550 cu yd. This
volume resulted in average channel depths of about 10 to 11 ft mlw, 1 to 2 ft
greater than the authorized depth of 7 ft mlw with 2 ft advance dredging. The
volume change for the September to October 1965 dredging was not calculated
because a full pre-dredging condition survey was not available. The channel
depth observed after dredging in May 1980 and October 1990 for this same
location is 9 ft mlw, equal to the authorized project depth of 7 ft mlw with 2-ft
advance dredging.
Maintenance dredging of Mattituck Inlet was performed in May 1980 and
October 1990 (Table 2-5). The most recent maintenance dredging of the channel
took place on 17-24 March 2004. Dredging volume changes for years 1961,
1980, and 1990 are analyzed here. The dredging of 1965 included section B as
shown in Figure 2-7 as well as section A. The dredgings of 1980 and 1990
covered only section A, the area of typical dredging. Figure 4-31a plots channel
elevations for June 1961, prior to dredging, and Figure 4-31b shows channel
elevations for September 1961, immediately after dredging. Figure 4-31c plots
the net change in elevation as a result of the maintenance dredging of September
1961. Figures 4-32a through 3-32c show channel elevation and elevation change
for the maintenance dredging of 1980. Figures 3-33a through 3-33c show
channel elevation and elevation change for the maintenance dredging of 1990.
The shoaling patterns observed in these figures are largely the same as those
that followed the 1946 landward extension of the east jetty, though the rate of
150
Chapter 4 Morphology Change, and Channel Shoaling and Migration
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