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Costal Inlets Research Program
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> Ebb Shoal/Swash Platform Complex
Change in Area of the Shoals and Adjacent Spits
Estimated Volume Change of the Inlet Shoals and Adjacents Spits
CHL-TR-01-26-1
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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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