(5) North flood shoal - has grown in area and migrated north expanding
from a circular shoal into a more typical flood shoal shape, with a
flood ramp, east and west flood channels, ebb shield, ebb spits, and
spillover lobes.
Analysis Methods
All of these areas have undergone major morphologic change from the
preinlet conditions to their present form. Table 3 gives the area of the shoals and
adjacent spits measured from the areal photography. The visible edge of the shoal
was digitized in ArcView from the rectified and georeferenced aerial photogra-
phy. The shoal edge was identified by the change in gray tones from the light gray
of the shoal to the dark gray of the deeper water. In most cases the annual aerial
photography was quite clear and the shoal boundary was distinct. The digitized
outline of the shoal for each year was entered onto a separate layer and the area
was calculated by ArcView. From the one and only bathymetric survey of the inlet
collected by the SHOALS lidar system in November 1998, the approximate depth
range of the visible shoal edge on the July 1998 aerial photography was deter-
mined to be between 5 and 10 ft (1.5 and 3 m) mllw. The water clarity and tone
quality appeared to be consistent on all yearly photo sets, so it is assumed that this
depth range was digitized on each of the shoal edges. This depth range was
assumed for the ebb shoal/swash platform, north and south flood shoal, south
remnant shoal, Tern Island and south Tern Island shoal boundaries.
The shoreline position was determined from all of the aerial photography as a
line digitized along the consistently visible gray tone line separating the wet/dry
line or approximate water table line on the inlet adjacent spit beaches and the
mainland shoreline. Based on a few photographic sets, where a high tide debris
line was visible, this water table line is just below the high tide line.
The volume measurements were computed based on the area of each feature
on a given data multiplied by a constant depth based on the 1987 bathymetry. The
depths of the shoals were estimated to be conservatively 5 ft (1.5 m) thick. The
seaward edge of the ebb shoal was measured at the end of the north and south ebb
channel as around 54 ft (16.5 m) mllw and the deepest scour hole in Chatham
Harbor was around 29 ft (8.8 m) adjacent to the rock revetment along the main-
land shoreline. The actual thickness of the ebb shoal mass can be estimated at
much more than 5 ft (1.5 m) but with only one bathymetry survey and the highly
variable morphology over the study period, a conservative measure of 5-ft (1.5-m)
thickness was used. The flood shoals (both north and south), Tern Island south
shoal, and south remnant shoal were also estimated at a 5-ft (1.5-m) thickness
given that the depth of the channels around the north flood shoal in 1998 were
around 10 to 15 ft (3 to 4.5 m) mllw.
Again, lack of historic yearly bathymetry and high variability in harbor
morphology over the study makes it difficult to be more accurate in shoal
thickness.
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Chapter 4 Ebb and Flood Shoal Evolution
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