Volumes dredged from the navigation channel were calculated by
differencing bathymetry surfaces. A TIN was generated for each condition
survey, and the volume for each TIN was then calculated. The volumes
calculated represent the volumes of sediment in the study area found above a
datum located beneath the lowest elevation found. Because ArcView GIS
introduces surface area variation in generating grids, the surface areas compared
were normalized to represent equal surface areas. The volume of the post-
dredging grid was divided by its planimetric surface area, yielding an average
height above the reference datum. This height was then multiplied by the
planimetric surface area of the pre-dredging grid. The total volume of a post-
dredging TIN was then subtracted from its corresponding pre-dredging TIN.
Results of these calculations are listed in Table 4-4 and compared to the
reported volumes dredged. The data indicate an accumulation rate of 1,000 to
2,000 cu yd/year for the period following the jetty repairs of 1975 for the area
considered. Good agreement is found between calculated and reported volume
for January 1980 to May 1980. Agreement between June 1961 and September
1961 is reasonable as well. The dredging of September 1961 included a section
of the channel not normally dredged. The volumes dredged from this extra
portion are not, however, expected to be large. Given this fact, the calculated
volume, which is less than the reported volume, is considered to be reasonably
accurate. The calculated volume for the September 1980 to October 1990 is only
50 percent of the reported volume. The cause of this discrepancy is unknown.
Table 4-4
Calculated Versus Measured Dredging Volumes, Mattituck Inlet
Channel
Calculated Volume
Reported Volume
Pre-Dredging Date
Post-Dredging Date
(cu yd)
(cu yd)
June 1961
September 1961
40, 473
43,550
January 1980
May 1980
26,459
24,137
September 1990
October 1990
26,595
13,241
Table 4-5 provides estimated sediment accumulation rates at Mattituck Inlet
for selected periods. Certain periods are not included, in instances when the
extent or purpose of the dredging is beyond normal maintenance (as with the
OctoberNovember 1950, which served to reposition the Federal navigation
channel).
156
Chapter 4 Morphology Change, and Channel Shoaling and Migration
Previous Page
