Estimation of volume change of the offshore shoal for the spatial extents
shown in Figure 4-6 provides insight into the morphology change and patterns
of erosion and deposition for the 75-year period considered. The total study
area gained 343,000 cu yd, yielding a rate of deposition of approximately
4,500 cu yd/year. A considerable portion of this deposition is contributed by
impoundment directly west of the west jetty, as shown in Figure 4-5c. As a
result of seaward migration of the shoal, the seaward area of the shoal (polygon
A) was an area of deposition from 1927 to 2002, and the landward portion
(polygon B) was an area of erosion. Deposition took place in both areas between
1969 and 2002. Polygons A and B are of equal area. Considered collectively,
polygon A+B experienced a net growth of 33,000 cu yd from 1927 to 1969, and
a net growth of 10,000 cu yd for the period 1969-2002.
The rate of deposition for the seaward area was approximately 2,000 cu yd/
year between 1927 and 2002. One thousand cu yd/year of this deposition can be
attributed to migration of the offshore shoal, because the landward area is shown
to be eroding at a rate of 1,000 cu yd/year. Deposition of new material,
therefore, accounts for 1,000 cu yd/year, and can be considered the actual growth
rate of this feature for the period considered. The east bypassing bar is accreting
at a rate of 300 to 400 cu yd/year. The areas of growth are along the seaward
edge of this feature and at the western tip, nearest to the jetty entrance.
Volume change for the total spatial extent of the offshore shoal surveyed on
6-8 October 2002 survey was calculated for the period 1969-2002. The lowest
elevation of either TIN, -21.5 ft NAVD88, was selected as the reference datum.
The calculated volume growth is 13,500 cu yd, and the calculated rate of volume
change is 500 cu yd/year. The volume of the spatial extent of the offshore shoal
shown in Figure 4-7 was calculated to be 460,000 cu yd.
The relative movement of the features offshore of Mattituck Inlet has the
potential to alter the sediment transport pathways. Figure 4-8 shows the center
lines of the offshore shoal in 1927 and 2002. Figure 4-9 shows the relative
location of this feature as well as the relative location of the center lines of the
main longshore bar east of Mattituck Inlet. By 2002, the center line of the main
longshore bar had migrated landward (referenced to its contemporary shoreline),
the shoal has migrated seaward, and the two features no longer directly
connected. The separation of the features resulted in the formation of a secondary
longshore bar adjacent to the east jetty, which may bypass finer sediment to the
downdrift beach.
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Chapter 4 Morphology Change, and Channel Shoaling and Migration
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