darker, deeper water is visible on all of the photos. Based on the limited bathy-
metry it was estimated that the digitized shoal boundaries represented the 5 to
10 ft (1.5 to 3.0 m) mean lower low water (mllw) depth contours.
Prior to inlet formation a remnant north and south shoal, and south sand flat
were present in Chatham Harbor. Tern Island and its associated south shoal were
also present in the vicinity of the Fish Pier. These shoals were probably remnant
flood shoal deposits of the last inlet breach in the 1846 to 1926 time period (Giese
1988) and were visible on the preinlet 1982 photographs. An ebb shoal and
northern swash platform formed within the first month and was well established
by the first set of photography in May 1987. The main ebb channel migrated to
the south and the main body of the ebb shoal became asymmetric to the south as it
followed the migration of this single channel. As the second ebb channel formed
to the north the ebb shoal began to advance seaward on its northern side, begin-
ning in 1995. The bed forms and shapes associated with the ebb shoal and swash
platform are easily seen in the photographs.
Because of the long narrow Chatham Harbor shape and close proximity of the
mainland shoreline to the inlet, two flood shoal deposits formed. One developed
around the north remnant shoal and one around the south remnant shoal, as flood
flow entered the estuary and flowed both to the north and south. The south flood
shoal eventually overtook the south sand flat. Later, the south shoal/sand flat area
was incorporated in the south and westward migrating south beach spit until it
finally welded to the mainland in early 1992. The entire flood flow was then
forced to the north, resulting in the north flood shoal expanding and moving north
toward Tern Island. Tern Island and an associated shoal to its south also migrated
to the north as the north flood shoal encroached on the channel to the Fish Pier.
This is the area of most concern to maintaining a navigation channel through the
shifting shoal deposits. Each shoal outline was stored on a separate layer in the
GIS.
Bathymetry
Only one set of bathymetry was available for the area after the inlet was
formed. A SHOALS survey was conducted in November 1998. The SHOALS
system is described in Irish, Parson, and Lillycrop (1995). Due to limitations in
the lidar system, the survey was limited to the ebb channels. Figure 4a shows a
bathymetric map constructed from the SHOALS survey. The predominant south
ebb channel and the incipient north channel are visible extending seaward through
the ebb shoal as seen on a three-dimensional Digital Terrain Model (DTM) of the
SHOALS data in Figure 4b. A large scour hole is prominent just to the bayside of
the northern ebb channel. All depths are referenced to mean lower low water
(mllw) on the SHOALS survey. Depths reaching to 29 ft (8.8 m) mllw in this
scour area. The western side of the flood shoal is also visible showing the location
of the west flood channel and the Tern Island channel. An ebb lobe is also present
on the edge of the flood shoal where the west channel is trying to re-establish a
continuous channel through the ebb shield on the western side of the flood shoal.
A maximum depth of 54 ft (16.5 m) was recorded at the seaward end of the ebb
shoal at the south ebb channel. Landward elevations reached a maximum height
12
Chapter 2 Methods
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