Table 4-5
Estimated Sediment Accumulation Rates,
Mattituck Inlet Federal Navigation Channel
Period
Rate (cu/yd year)
1914 August 1923*
7,000
September 1923 September 1927
12,000
October 1927 November 1935
6,500
May 1936 July 1938
8,500
August 1938 September 1946
6,500
November 1950 August 1955
7,000
September 1955 August 1961
7,000
October 1961 September 1965
1,500
October 1965 May 1980
1,500
May 1980 October 1990
1,300
October 1990 March 2004
1,000
* Includes dredging of June - November 1921
The average sediment accumulation rate from 1914 to 1946, the period prior
to the modification of both jetties, is 8,100 cu yd/year and the range is 6,500 to
12,000 cu yd/year. The previous repairs and extensions appear to have greatly
mitigated shoaling along the west jetty, and a reduction in the sediment
accumulation rate is expected after the major rehabilitation of 1946. The
continued high rate indicated from 1946 to 1961 is interpreted as a transitional
period containing adjustments of the morphology of Mattituck Inlet to a new
hydrodynamic equilibrium condition. The decrease in rates for the period 1961
to the present is considerable. The efficiency of the current Federal navigation
project and the mining of sediment directly west of the jetty are identified as the
major factors in the observed decrease.
Seventeen condition survey maps from the years 1969 to 2003 were
digitized, and quantitative analysis of sediment deposition within the Mattituck
Inlet navigation channel was attempted through surface differencing. It was
found that the annual volume difference between successive condition surveys
was too small to detect because the volume error was three to six times the
annual volume change (1,000 to 2,000 cu yd). The surface areas analyzed
measure approximately 60,000 sq yd. A survey error of 0.33 ft, or 0.1 yd, yields
a volume error of +/- 6,000 cu yd. The volumes obtained based on the post-
dredging condition surveys are considered reliable because of the large change
relative to the pre-dredging surveys.
The difference maps generated do, however, provide qualitative information
on the location and persistence of shoaling within the channel. Sediment
accumulation within the channel appears to be constant and evenly distributed,
with the exception of the inlet entrance, where considerable shoaling is observed
along the west jetty. Near the shoreward end, areas of scour and shoaling are
apparent. Figures 4-43a through 4-34e illustrate elevation changes from May
1980 (immediately after a dredging) through October 1990.
157
Chapter 4 Morphology Change, and Channel Shoaling and Migration
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