The north and south spit areas and Tern Island itself were estimated at a 10-ft
(3-m) thickness based on a topographic survey of Tern Island done by the New
England District in 1994. Average elevation was 10 ft (3 m) above mlw or 8 ft
(2.4 m) above mllw. From stereo pairs it was estimated that the average elevation
on both the north and south spits were around 3 to 5 ft (1 to 1.5 m) above mllw
relative to the elevation of 8 to10 ft (2.4 to 3 m) on Tern Island. Elevations were
highly variable through time as the spits were subject to frequent overwash and
dune erosion. Tern Island's area included the shoals adjacent to the north of the
island. A thickness of 5 ft (1.5 m) above mllw and 5 ft (1.5 m) below mllw were
estimated to cover the volume of these subaerial features. This accounted for the
intertidal volumes adjacent to the shorelines of these features that were outside the
measurements of the wet/dry line indicator of shoreline area on the spits and the
highly variable shoals adjacent to the north end of Tern Island. Shoal and spit
volume estimates are shown in Table 4.
Ebb Shoal/Swash Platform Complex
The largest area feature at Chatham Inlet was the ebb shoal and swash plat-
form complex. This feature did not exist prior to the inlet formation, but quickly
established itself within the first four months after the breach. The areal changes
in the ebb shoal and swash platform complex indicate a persistent growth of the
shoal into the ocean with a southward migration of the swash platform corre-
sponding to the southward migration of the main ebb channel (Figure 26). When
the new north ebb channel established itself in 1995, the ebb shoal again emerged
into the ocean on the north end, as the southern end of the shoal complex con-
tinued to elongate to the south adjacent to South Beach.
Figure 27 shows the rapid growth in area of the swash platform/ebb shoal
within the first 3 years. Figure 28 mirrors the area change with the estimate of
volume change assuming a 5-ft (1.5-m) thickness in the shoal. Peaks in area and
volume were measured in 1989, 1991, 1994 and reached a maximum in 1996.
The area and volume have been constant from 1998 to 1999.
North Spit
The north spit rapidly moved to the north and west within the first 2 years as
the inlet throat expanded. The spit continued to recurve to the north but at a
slower rate and finally stabilized and is now moving to the west into Chatham
Harbor. The area of the spit was measured to a constant point on North Beach
shown by the line on Figure 29. The preinlet area (Figure 30) and volume (Fig-
ure 31) of the barrier island was measured from the 1982 aerials from the 1987
breach position to the northern area limit. This north spit has decreased in area
and estimated volume from the initial breach to a minimum in 1994. Since 1994
the spit has moved south again and to the west and has gained back almost all of
the area and volume it had after the initial breach as the spit has formed several
complex recurves from sand being transported to the south along Nauset Spit.
46
Chapter 4 Ebb and Flood Shoal Evolution
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