complex. This shoal existed for only two years after the breach until the south
flood shoal migrated south and incorporated this remnant sand flat into it. Fig-
ure 39 shows the measured area and Figure 40 shows the estimated volume of the
remnant south sand flat. While short lived after the inlet opened, this shoal gained
sand as tidal currents flowed into south Chatham Harbor in 1987 and 1988. With
the complicated multichannel configuration and shoaling associated with the loss
of tidal current velocity, the south remnant sand flat became the distal end of the
south flood shoal and finally the landward side of the land bridge.
Tern Island and Tern Island South Shoal
Another preexisting sand body in Chatham Harbor was Tern Island. Tern
Island was the outer boundary of Aunt Lydia's Cove and the anchorage around
the Chatham Fish Pier commercial harbor. Tern Island has historically been used
as a disposal site for sporadic prebreach maintenance dredging. Prior to the 1987
inlet, the channel to the Fish Pier was oriented southeast-northwest and was
between Tern Island and a shoal called south Tern Island shoal (Figure 41). After
the inlet opened, there was little change in Tern Island, its south shoal and Aunt
Lydia's Cove entrance channel, until 1990. The measured area and estimated vol-
ume of Tern Island remained relatively constant gaining slightly from before the
inlet opened (Figures 42a and 43a). At that time, the north flood shoal began to
migrate north, causing the channel to reorient and caused Tern Island to migrate to
the north. Changes in the orientation of the west flood channel around the north
flood shoal also caused the Tern Island south shoal to lose area and volume (Fig-
ures 42b and 43b).
The change in the west flood channel and reorientation of the Aunt Lydia's
Cove entrance channel slowly eroded and finally removed the entire Tern Island
south shoal by 1997. Tern Island remained relatively constant in area and esti-
mated volume over the study period, with a minimum area/volume in 1994 and
1996. The island has been used as a disposal site for the initial dredging of the
entrance channel into Aunt Lydia's Cove by the New England District, with a
placement of about 100,000 cu yd (76,460 m3) in 1995. This additional fill vol-
ume has not been included in the estimated volume calculation for 1995. Sand has
eroded on the south end and is migrating to the sand flats on the north. The
present area and estimated volume is close to the preinlet conditions.
North Flood Shoal
A large shoal was also present in the northern part of Chatham Harbor in mid-
lagoon in 1982 that caused the main channel to split into an east and west channel
on its way to Pleasant Bay. This shoal was oblong and after the breach, this rem-
nant shoal became the north flood shoal (Figure 44a). This remnant shoal was
located just to the north of the expanding inlet throat and quickly took on the
characteristics of the northern side of the flood shoal. Just four months after for-
mation, this shoal almost doubled in measured area (Figure 45) and estimated
volume (Figure 46). The shape became more teardrop as sediment deposited on
the shoal as the tidal flow slowed as it made the turn to the north. Briefly in 1988,
58
Chapter 4 Ebb and Flood Shoal Evolution
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