Change of offshore shoal morphology. To assess long-term change
offshore of Mattituck Inlet, a comparative analysis of the 6-8 October 2002
survey, a 1969 survey by the National Oceanic and Atmospheric Administration
(NOAA) (survey #H09087), and an 8 December 1927 New York District survey
was conducted. Because the 8 December 1927 covered the smallest area, the
spatial extent of this survey defined the area of examination of elevation change
for the periods considered. Elevation change was obtained through surface
differencing, calculated between triangulated irregular network (TIN) grids
generated in ArcView 3.3.
The 8 December 1927 condition survey was conducted approximately
2 months after the dredging of September October 1927. The New York
District condition survey of 8 December 1927 was referenced to a mlw datum
that lies 2.2 ft below a New York District msl datum as determined in April-May
1906. To adjust the 8 December 1927 survey to the NAVD88 datum, 3.63 ft was
subtracted from the elevation values of this condition survey. The difference
between the datums was derived after Batten and Kraus (2005), and can be
accounted for in the following manner. The mlw datum of reference for the 1927
survey was determined at the NOS tide station at Ft. Schuyler, NY. At Mattituck
Inlet, mlw is 0.3 ft below mlw at Ft Schuyler, NY. According to the NOS, the
conversion factor between Mattituck Inlet and the NOS Bridgeport, CT,
reference station is 0.76, and the difference between mlw and NAVD88 at
Bridgeport, CT, is 3.59 ft. The difference between the present day mlw and
NAVD88 at Mattituck Inlet is therefore 2.73 ft. Sea level has risen
approximately 0.6 ft at Port Jefferson for the 75-year period considered. The
difference between mlw at Mattituck Inlet and Ft. Schuyler (0.3 ft), sea level rise
(0.6 ft), and the calculated elevation of mlw datum in the present tidal epoch
(2.73 ft), yield the adjustment of 3.63 ft applied here. The datum relations are
illustrated in Figure 3-1a and 3-1b.
The 1969 NOAA survey was referenced to mean lower low water (mllw).
The NOS published conversion factor for the 1983-2001 epoch at Mattituck Inlet
defines NAVD88 at 2.96 ft above mllw. According to the NOS tide station at
Port Jefferson, sea level has risen approximately 0.26 ft between 1969 and 2002.
To adjust the mllw datum of this survey to NAVD88, 3.22 ft was subtracted
from the elevation values of this survey. The datum relations discussed here are
illustrated in Figure 3-1a.
The condition survey of 8 October 1927 is referenced to a local coordinate
system. The background aerial photograph shown in Figures 4-3 through 4-5c is
circa 1930. Horizontal conversion to the New York State Plane Grid, Long
Island Lambert, NAD83, was accomplished by georeferencing the data to this
aerial photograph of Mattituck Inlet, and the subsequent digitization of the
adjusted survey points. The root mean square (rms) error in the georeferencing
was 60 ft. The circa 1930 aerial photograph is large scale (1:24,000), and covers
more than 10 miles of shoreline. The rms error is considered reasonable because
Mattituck Inlet is near the center of the aerial photograph, where the estimated
error is less. Overall, mapped high-water line (hwl) lines and jetty and channel
location matched well, and visual inspection of the alignment of the jetties
between the aerial photograph and the 8 December 1927 bathymetry survey
indicated error between 10 to 30 ft.
Chapter 4 Morphology Change, and Channel Shoaling and Migration