Figure 4-22a shows channel elevations for July 1938 (exact date not
available), and the areal extent of the flood shoal is illustrated in Figure 4-22b.
Channel elevation changes between 4 May 1936 and July 1938 are plotted in
Figure 4-22c. For the 26-month period, the net volume change for the study area
was calculated to be 13,882 cu yd, yielding an annual sediment accumulation rate
of 6,500 cu yd/year. The portion of the Federal navigation channel considered
was calculated to have 6,600 cu yd of infilling for the 26-month period, for an
average channel infilling rate of 3,000 cu yd/year for the period.
The average depth of the Federal navigation channel for July 1938 was
calculated to be 7.5 ft. The flood shoal was calculated to have grown by only
400 cu yd during this period. This small volume change can be attributed to the
fact that the condition survey of 1938 did not fully record elevations along the
west side of the inlet. The area of elevations not recorded can be discerned by
comparing the survey extent shown in Figure 4-40 to that of prior condition
surveys. The extent of this area is approximately 26,000 sq ft. Because the area
not surveyed is dry shoal, the depth was taken to be at least 7.5 ft, based on the
morphology of 3 July 1937 and the ambient depth at this location, resulting in a
volume of shoaling that is estimated to be at least 7,000 cu yd. This figure, in
addition to the 400 cu yd calculated, yields an estimated growth of the total flood
shoal of 7,400 cu yd.
Figure 4-22a. Mattituck Inlet channel elevation, 1938
Chapter 4 Morphology Change, and Channel Shoaling and Migration