Cat Island Evolution, Morphology, and
Hurricane
Response
– 1995 to 2000
By Keil
Schmid
Mississippi
Department of Environmental Quality
Office
of Geology
Open
File Report 132
S.
Cragin Knox
State
Geologist
Coastal
Section
Energy
and Coastal Division
February
2001
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Cat Island Evolution, Morphology, and
Hurricane
Response
– 1995 to 2000
By Keil
Schmid
Mississippi
Department of Environmental Quality
Office
of Geology
Open
File Report 132
S.
Cragin Knox
State
Geologist
Coastal
Section
Energy
and Coastal Division
February
2001
Cover:
Chart and sailing directions for Cat Island, 1847. Chart courtesy of Office of
Coast
Survey, National Ocean Service, NOAA
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Table of
Contents
1997
–1998 Post Georges GPS....................................................................
10
1998
Post Georges – 2000
GPS................................................................... 12
Middle
Spit to West Point
Shoreline........................................................ 19
Mud
Dominated
Shorelines......................................................................
25
Sand
Dominated Shorelines.....................................................................
27
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1
Introduction
Changes in
barrier island position, elevations, and
morphology typically happen at
irregular
and exaggerated rates (Byrnes et al., 1989). The Mississippi barrier islands
are
no
exception, with average shoreline position changes in some areas as high as 90
meters/year
(McBride et al., 1995). These high rates make simple Global Positioning
System
(GPS) shoreline surveys with accuracies of better than five meters a viable way
to
document island evolution at yearly to semi-yearly scales.
This is an important milestone
in the
study of hurricane change along the Mississippi Barrier Islands; at no time
before
could
shoreline evolution be as densely and completely quantified prior to and
following
a
hurricane’s passage without incurring high costs. Previous studies have shown
short
temporal
changes; however, they have been mainly qualitative (Byrnes et al., 1989).
Temporally
dense data help highlight small trends and their causes that together drive
island morphology and evolution. Moreover, the availability of
Light Detection and
Ranging
(LIDAR) data to researchers has brought highly accurate elevation data sets to
the greater
research community. LIDAR has been successfully used in documenting
seasonal
change in coastal California as well as the Atlantic Coast (Morgan et al.,
1999;
Sallenger
et al., 1998). This report focuses on Cat Island and is part of a series covering
recent
change on the Mississippi Barrier Islands.
Study
Site
Coastal
Mississippi stretches from Louisiana in the west to Alabama in the east
(Figure 1)
and contains five nearly shore-parallel barrier islands. The Mississippi
Barrier
Islands are
an elongate east – west chain, located 15 to 20 kilometers from the mainland
coast. From
east to west the islands are Petit Bois, Horn, East Ship, West Ship and Cat
(Figure 2).
Petit Bois, Horn, East Ship, and West Ship are presently part of the Gulf
Islands
National Seashore and Cat is in the stages of
being acquired. The eastern islands
appear to
exhibit a shoal to island geology (Otvos,
1970a, b) with the main source of
sediment
from the Alabama mainland coast (Otvos, 1985). Their early formation into
islands has
been placed in the Mid-Holocene (about 3-4 thousand years ago).
Historically,
within the last 300 years, the eastern islands (Petit Bois and Horn) have had
a
dominantly translational – longshore drift movement, such that they are not
moving
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2
landward
but rather along the coast. Cat Island, the westernmost island,
has had very
little
translational movement and is instead eroding in place. The two Ship Islands
are in
the middle
of the spectrum.
Figure 1.
General Study site in Southeastern United States.
The eye of
Hurricane Georges entered and passed through the Mississippi Sound
between
East Ship and Horn Islands on September 28, 1998 (Figure 1 and 2). It was a
category 2
storm before making landfall near Biloxi, Mississippi. Although Hurricane
Georges was
only a category 2 storm when impacting the Mississippi Gulf Coast, its slow
forward
motion of about 5 mph (Otvos, 1999) caused significant damage to barrier
islands in
the area. Island changes in Louisiana from
Georges have been compared to
those
caused by Hurricane Camille, a category 5 storm (Penland et al., 1999). The
only
other
tropical storm of note to pass fairly close to Mississippi during the study
period was
Hurricane
Danny in 1997 (Figure 1); however, it only glanced the area and was
substantially
weaker.
r
r
r
r
r
r
r
r
r
r
r
r
rrr
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
Mississippi
Louisiana
Alabama
Florida
N
E
W
S
r
Danny
Georges
100
0
100
200
Miles
Study
Area
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3
Figure 2.
Study area with Hurricane Georges track.
Methods
Several
different methods were used to document island
evolution prior to, in response
to, and
following Hurricane Georges. Kinematic GPS surveying techniques were used to
highlight
shoreline changes in time periods prior to Hurricane Georges (1995-1997),
encompassed
by Hurricane Georges (1997-1998), and following Hurricane Georges
(1998-2000).
To further document island changes,
morphology from LIDAR elevation
data sets
was analyzed in relation to hurricane-driven shoreline change. At
representative
locations,
cross shore profiles generated from LIDAR elevations taken following
Hurricane
Georges were compared with profiles measured with conventional survey
procedures
in 2000/2001 to highlight morphology changes during the two-year period.
Yearly GPS
shoreline surveys of the Mississippi Barrier Islands have been performed
since 1993
by the Mississippi Office of Geology and semi-annually by the National Park
Service
since 1998. In each survey the high tide shoreline was mapped using kinematic
GPS
techniques. All data were post processed, yielding accuracies on the order of ± 2-5
meters
(Hutchins and Oivanki, 1994). The high tide line, denoted by either a
wet-to-dry
line or
debris wrack line, has been chosen as a repeatable datum and represents the
state-
owned
boundary. Errors in interpreting the high-tide line and from differences in
tide
range
exist, although effort has been taken to insure a level of consistency between
mapping
parties.
Cat Island
E.
Ship
W.
Ship
Horn
Island
Petit
Bois
Mississippi
Sound
G
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lfp
o
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Biloxi
P
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4
Locations
of shoreline retreat and advance beyond chosen levels were computed using
buffers on
GPS shorelines. The buffer width was chosen to highlight areas of significant
change
within the confines of the survey accuracy. Any portions of the compared
(later)
shoreline
landward of the negative buffers of the base (earlier) shoreline were
highlighted
as retreat;
portions seaward of the positive buffers were highlighted as advance.
Buffer
widths varied from island to island based on historic levels of shoreline change
reported in
Byrnes et al. (1991). Cat Island has an average shoreline change of roughly
2.5 m/yr
and a high of 3.5 m/yr, which was rounded up to 4.0 m/yr for the purpose of
analysis.
These values were used to construct buffers of average change (years x 2.5) and
high change
(years x 4.0) during the ambient and recovery periods. The high change
buffer was
doubled for the hurricane period to highlight major changes; the average value
was not
used for the hurricane period analysis. Buffers help highlight areas of
targeted
change
levels; they do not represent all areas of change, which would, in most cases,
highlight
the entire shoreline.
Total island changes in acres were also computed using
GPS shorelines. A buffer
method was
used to approximate the acreage change because the GPS’ed shorelines did
not include
the entire island. Only surveyed shoreline
segments common in each
compared
year could be used; thus the acreage change is not representative of the entire
island, but does provide comparison between the time periods
because each had similar
GPS
shoreline segments.
Shoreline
configurations prior to 1993 were taken from National Ocean Service (NOS)
T-sheets
and aerial photography (Byrnes et al., 1991). These data are less accurate than
GPS surveys;
they are used only to document broad historic trends spanning several tens
of years.
General
analysis of morphology was preformed using LIDAR data flown in November
1998 (U.S.
Geological Survey et al., 1998). Horizontal accuracies are on the order of 1
meter;
vertical accuracies are ±15 to 20 centimeters. For general analysis of morphology,
a 10 x 10
meter grid was used; the minimum value within each grid was used. The
minimum
value in the grid was specifically chosen to minimize the effects of
vegetation.
Elevation
values were imported into AUTOCAD MAP (AutoDesk, 1998) and 10 x 10
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5
meter
gridded surface was generated using QUICKSURF (Schreiber Instruments, 1998).
A
triangulated grid method was chosen based on the normal spacing of data points
(Schreiber
Instruments, 1998).
Areas with
representative morphology and shoreline change were further analyzed
with a
higher density of LIDAR elevation points; for these areas 2 x 2 meter grids and
surfaces
were chosen. The high-density data were used to produce cross-shore profiles.
These LIDAR
profiles were then compared to conventional profile surveys taken with a
total
station in 2000/2001. Survey benchmark locations were GPS’ed and elevations
taken from
the 2 x 2 meter LIDAR grid. In some locations the benchmark elevation
taken from
the 1998 LIDAR survey was not completely accurate for 2000/2001. In these
cases, the
benchmark elevations were adjusted slightly so that measured beach face
morphology
was consistent with respect to its elevation. LIDAR elevations over the
subaqueous
portions of the profiles (below sea level) are suspect in some cases so
interpretation
of the bathymetry changes is tenuous.
Data
Cat Island
Cat Island is the westernmost barrier island in Mississippi. It has a unique shape
(Figure 3)
with the main island dominated by beach/dune
ridges (Rucker and Snowden,
1989),
middle spit dominated by marsh, and the eastern shoreline from South Spit to
North Point
dominated by active wave and wind features. Cat Island has recently
received
considerable attention as it is set to be purchased and incorporated into the
Gulf
Islands
National Seashore. Cat Island
also has a unique geologic evolution among the
present
islands in the National Seashore (Rucker and Snowden, 1990). Its complex
history was
influenced by the extension of the Saint Bernard subdelta just to the south of
the island. The Saint Bernard subdelta complex was a
major delta lobe of the Mississippi
River from
4000 to 2000 years ago (Roberts, 1997). During this period, Cat Island was
surrounded
by marsh that shielded it from the typical northwest longshore drift. The
eventual
destruction of the Saint Bernard subdelta coupled with the southeastern wind
regime has
given Cat Island
a distinctive T-bone shape. Several studies have attributed
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6
the N-S
spit on the eastern end as a result of modified sediment transport patterns
following
decline of the Saint Bernard subdelta (Otvos, 1993; Rucker and Snowden,
1990). Yet,
the curious shape and trend from 1850 to present (Figure 4), where the
southern
portion of the spit has changed drastically as compared to the rest of the island,
leaves room
for future analysis. Based on historical shoreline configurations the central
point of Cat Island has
moved only meters, while the other islands in the chain have
moved
kilometers (Schmid, 2000). These differences in Cat Island’s evolution certainly
make it
unique among the five barrier islands. In addition, it is also the only barrier
island in the chain with a significant fringing coastal marsh
habitat.
Yearly wind
patterns from 1995 to July 2000 show little change (Figure 5), although
the period
between August 1997 and November 1998 does have a slightly higher
southwest
component than the other periods. Average wind speeds during the 1995-2000
period are
highest from the northeast to northwest (about 14 knots), and lowest from the
south
(about 10 knots).
Figure 5.
Wind data taken from National Data Buoy Center – buoy #42007, approximately 10
miles south
of Cat Island.
Shaded area is the percent of time the wind was from the specific
direction.
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7
Figure 3
1996 photo
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8
figure 4
historical shorelines for cat
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9
Shoreline
Change
Historically,
Cat Island
is associated primarily with cross-shore changes (Figure 4) as
opposed to
translation (Byrnes et al., 1991; McBride et al., 1995). Cat
Island has an
average
shoreline change of roughly 2.5 m/yr and a high of 3.5 m/yr (Byrnes et al.,
1991),
which was rounded up to 4.0 m/yr for the purpose of analysis. These values were
used to
construct buffers of average change (years x 2.5) and high change (years x 4.0)
during the
ambient and recovery periods. The high change buffer was doubled for the
hurricane
period to highlight major changes; the average value was not used for the
hurricane
period analysis. Shoreline surveys on the Mississippi Sound shoreline of Cat
Island are difficult to perform because of overhanging trees and
large root mats over the
water’s
edge. Results from these areas, while surveyed on several occasions, are not
used
here
because errors in locating the shoreline are high.
Table 1. Cat Island
shoreline inventory
Shoreline
Buffers
(high, ave)
July, 1995
Baseline
July, 1997
8 m, 5 m
November,
1998
8 m
June, 2000
7.5 m, 4 m
1995-1997
GPS
Comparison
of the two GPS shorelines shows only a moderate degree of change
during this
period (Figure 6). Middle Spit, however, shows high shoreline retreat. The
eastern
shoreline of the island shows only localized
high retreat on the southern half.
Unfortunately,
the southern spit was not surveyed in 1997 and could not be compared to
earlier
data. The northern half of the eastern shoreline is associated with accretion
or
little
change during this period. Similarly, western Cat Island shows about equal
shoreline
retreat and advance.
Over the
region GPS’ed in both 1995 and 1997, a total of 18.5 km, the island lost 7.2
acres
during this period. This value is conservative as it does not include the
Mississippi
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10
Sound
shoreline of the main island, Southern Spit,
and most of the marsh dominated
shoreline.
1997
–1998 Post Georges GPS
During this
roughly one-year period only high change areas are highlighted. Once
again
Middle Spit is dominated by shoreline retreat (Figure 7). Also notable is the
erosion on
the western end of Cat Island,
which was not dominated by erosion in the
preceding
period. The small area of shoreline advance is near the very tip where
washover
features are present (Picture 1). On the eastern portion of the island, which saw
the highest
wave energy from the storm, the southern half, including South Spit, is
uniformly
dominated by high shoreline retreat. In contrast, the northern half of the
eastern
shoreline has a large area of advance (Figure 7). Note that the 1995 shoreline
was
used to
construct the buffer on the southern spit because of the lack of data for 1997.
An aerial
photo taken after the hurricane (Picture 2) also depicts a series of ebb
overwash
fans at the intersection of the main island with
the eastern shoreline. The
distinct
change in hurricane-driven change on the eastern shoreline is associated with
the
intersection
of Cat Island
proper. It is evident that the beach ridges making up the spine
of the island are not only a sediment source, but also
create a natural funnel for ebbing
storm
surge.
Over the
region GPS’ed in both 1997 and 1998, a total of 17 km, the island
lost 21
acres
during this period. This value is also conservative as it does not include the
Mississippi
Sound shoreline of the main island, Southern
Spit, and most of the marsh
dominated
shoreline. It does represent, however, about a 300% increase in erosion from
the
previous period.
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11
Figure 6,
1995 –1997 data
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12
1998
Post Georges – 2000 GPS
As in the
preceding periods, Middle Spit is largely dominated by shoreline retreat
(Figure 8).
Shoreline retreat also continues on western Cat Island, but to a lesser degree.
The major
difference between this period (recovery) and the hurricane period (1997-
1998) is on
the eastern shoreline where patterns have switched. The northern half, which
saw either
subtle retreat or shoreline advance during Hurricane Georges, now shows a
dominant
retreat trend. In addition, the southern half shows only localized areas of
retreat.
This is a temporal change in pattern from both ambient and hurricane periods.
Over the
common region GPS’ed in both 1998 and 2000, a total of 20 km, the island
lost 4.6
acres during this period. Again, this is a conservative value; however, it is
directly
comparable to the other values and shows a major decrease in the area of
erosion
from the
hurricane period (1997 – 1998) and is slightly lower than the initial period
(1995
– 1997).
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13
(Picture
1 and 2)
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14
(Figure
7. 1997-1998)
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15
Elevations
and Profiles
Eastern
Shoreline
LIDAR
elevation data taken after Hurricane Georges passage show a nearly
continuous
dune ridge running parallel to the shoreline on the southern half of the
eastern
shoreline
(Figure 9). Washover morphology exists immediately north of the intersection
between
relict dune ridges on Cat Island proper and the active eastern shoreline. It
further
suggests that this area was fully breached by the ebbing storm surge. The right
angle configuration
of the northern (Sound-side) shoreline appears to facilitate piling up
of the ebb
tide surge, creating a situation that causes ebbing flow over this section. In
response to
the seaward-directed washover, the shoreline in this area moved seaward
following
Hurricane Georges, and probably contributed to the general accretion north of
this
location following storm passage (prior to GPS shoreline survey). To the north
of the
breached
section, there is a semi-continuous dune ridge with higher forested dunes at
its
most
northern extent. This area has a higher elevation than most of the eastern
shoreline,
and is
consistent with long-term stability (Figure 4).
Five
profiles were surveyed on the eastern shoreline in October 2000 and compared to
LIDAR
generated profiles following Hurricane Georges (Figure 8). These locations were
chosen
based on morphology and shoreline change. The two northern profiles (Figures 8
and 10; NE
1 and NE 2) show little change in shoreline location as is indicated by GPS
surveys
1998-2000, but a positive volume change above zero elevation (Table 2). The
northernmost
profile (NE 1) displays vertical growth of two dune sets (Picture 3) and the
highest
positive normalized cross-shore volume change of all profile locations.
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Table 2. Cat Island
profile attributes. Volume change is in cubic yards per linear foot
of the
shoreline, normalized volume change is volume change/profile length. Zero
Elevation
change represents shoreward (-) or seaward (+) change of the 0 elevation
contour.
Profile
Length (ft)
Volume
Change
(cyd/ft)
Normalized
Volume
Change
(cyd/ft/ft)
Zero
Elevation
Change (ft)
NE 1
482
11.8
0.025
7.1
NE 2
381
2.5
0.006
0.5
Cntrl N
431
8.0
0.019
-50.7
Cntrl S
364
-11.8
-0.030
-5.1
SW
519
2.5
0.005
-1.0
Midwest
186
-2.5
-0.014
-5.57
West
107
-2.2
-0.021
-23.7
In the
central section, the two profiles show very different topography in spite of
their
close
proximity. The northern one, Cntrl N, (Figure 10) has modest vertical relief,
high
positive
volume change, and a substantial shoreline retreat since 1998 (Table 3). There
has been
net vertical accretion on the shoreface and establishment of one dune set.
Increased
elevation and high shoreline retreat are indicative of recovery in a region
generally
typified by ebb overwash during Hurricane Georges. Scour marks are still
present
after nearly two years (Picture 4).
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17
Figure 8
1998 to 2000
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18
(Figure 9.
LIDAR elevation grid on eastern shoreline)
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19
The
morphology of the southern profile, Cntrl S (Figure 10), located at the
intersection
of the older ridge sets with the eastern shoreline, has changed little since
Georges’
passage. This profile is typified by a large dune, reflecting the abundant
source
of sediment
in the exposed relict beach ridges. There is, however, overall deflation along
the profile
with the highest negative normalized volume change. A lengthy deflation
history is
evident in the many dead trees and elevated stumps on the beach and nearshore
platform
(Picture 5). The differences in these two closely spaced profiles highlight the
varied
influence of older ridges on present shoreline dynamics.
One of the
more interesting profiles is on the southern spit, not because there is
marked
change, but because there has been very little. On a section of the island that
would seem
to be the most responsive to changing conditions and has been in the past
(Figure 4),
there has been little volume or shoreline change since 1998. A general
increase in
dune height is the only notable change.
Middle Spit
to West Point Shoreline
The western
shoreline morphology is dominated by subdued beach berms (Figure 11)
perched on
an old marsh surface and backed by an active marsh. Non-exposed shorelines
(between
Middle Spit and Cat Island)
are typically composed of marsh banks, with small
sections of
pocket beaches where older beach ridges are exposed. On Middle Spit,
elevations
scarcely reach one meter. Near West Point where slightly greater wave
energies
are present, berm elevations are somewhat higher, but here again are mainly
less
than 1
meter. Two profiles were surveyed on the western shoreline (Figure 8); both are
in areas
experiencing recent (1998 to 2000) retreat.
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20
Figure 10 -
profiles
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21
The profile
on Middle Spit (Figure 12 - Midwest) is a good example of the
morphology
found along this marsh dominated shoreline and highlights some shortfalls
of the
LIDAR data. The 2000 profile has a broad gently sloping (1/100) mud platform
that
abruptly drops off at zero elevation (Picture 6). On this mud platform there
are small
remnant
clumps of marsh grass (S. alternaflora, J. roemerianus). The slight
perturbations
in the LIDAR profile along the mud platform represent berm crests and
possibly
some wrack and debris. Sand berms are perched on top of the mud platform.
Note the
differences in the offshore portions of this profile. Here there is a change of
–
4.5
c.yds/ft on only a small segment (100 ft) of the profile. This may represent
limitations
of LIDAR to map below the water surface where water turbidity is high, as is
the case in
this region from constant erosion of the mud platform. Likewise, there is
dense marsh
behind the most landward beach berm that the LIDAR appears to have
sensed as
ground elevation.
The most
westerly profile (Figure 12 - West) highlights, like Cat
Cntrl N on the
eastern
shoreline, the process of ebb flooding process and subsequent recovery. Aerial
pictures
following hurricane passage (Picture 1) show the large ebb overwash
morphology
in this area, and especially to the west of the profile. Likewise, the LIDAR
profile
depicts a wider beach than at present (2000). At this location, sandy beach
deposits
are perched on top of a mud unit that is exposed below sea level (Picture 7).
Comparison
of LIDAR and total station surveys would suggest that the mud step has
been eroded
about 20 ft in two years.
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22
(Pictures 3
to 5)
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23
(Pictures
6 and 7)
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24
(Figure
11. LIDAR elevation grid for the western portion of Cat Island)
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25
Figure 12.
Profiles on western portion of island.
Discussion
During the
5 year period of investigation Cat Island lost more than 32 acres of land,
most of
which occurred during the Hurricane Georges period (1997-1998) (Table 3).
There are
three major areas on Cat Island
associated with prevalent shoreline change at a
level
distinguishable in the short term: Middle Spit, the western end of Cat Island, and
the
entire
eastern shoreline. More generally, there are two separate shoreline types: mud-
platform
dominated shorelines and sand dominated shorelines.
Table 3. Island area change along surveyed shorelines.
Year period
Measured shoreline distance (km)
Acres lost
1995-1997
18.5
7.2
1997-1998
17.0
21
1998-2000
20.0
4
Mud
Dominated Shorelines
Middle Spit
shows the most dominant retreat trend with little shoreline advance (Table
4).
Shoreline retreat did not change significantly between any period; shoreline advance
was minimal
in all three periods. The only area of shoreline advance was the
progradation
of a small cuspate spit on the western end. High levels of retreat may
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26
partially
be the result of a decrease in size of South Spit since 1850. In 1850 Southern
Spit
extended 1.8 kilometers (Figure 4) to the south of its present tip and
protected much
of Middle
Spit from the dominant southeastern wind direction (Figure 5).
Table 4. Cat Island
shoreline change
Middle Spit
Years
Shoreline
retreat %
Shoreline
advance
%
High
Ave
High
Ave
95-97
45
58
6
6
97-98
37
-
0
-
98-00
24
38
0
1
Cat Island – West Point
Years
Shoreline
retreat %
Shoreline
advance
%
High
Ave
High
Ave
95-97
4
12
7
19
97-98
60
-
1
-
98-00
15
30
2
7
East
Shoreline
Years
Shoreline
retreat %
Shoreline
advance
%
High
Ave
High
Ave
95-97
7
15
7
18
97-98
38
-
28
-
98-00
21
34
18
26
Middle
Spit’s shoreline has little elevation; sand that is present forms a perched
beach
on top of
relict marsh mud so there is a limited amount of local coarse sediment
available.
Nearly all of the sand-sized sediment is from longshore drift; onshore sand
movement
and fair weather beach aggradation is precluded by the nearly vertical mud
step.
Cat Island is characterized by several sets of
relict beach/dune ridges (Figure 3) and,
like Middle
Spit, is open to southerly winds and waves on the western end. The amount
of high
retreat areas during the hurricane (1997-1998) period was significantly greater
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27
than during
ambient (1995-1997) and recovery (1998-2000) periods (Table 3) even
though
there were large ebb washover areas. During the month and a half period between
the
hurricane and shoreline survey it is possible that the shoreline retreated
landward.
Moreover,
exposure of the mud platform below mean water elevation suggests that
transport
dominates over deposition along this shoreline. Higher shoreline retreat during
the
recovery period as compared to the ambient may be a sign that significant
sediment
was made
available (moved seaward) by the storm and subsequently lost to western
longshore
transport, which may help explain why the western tip of Cat
Island has only
lost 70 m
since 1986. It is also possible that the storm caused higher erosion of the mud
platform
and thus left a smaller shelf for sediments to rest on. The percent of
shoreline
advance
area is lower than retreat during the recovery period, whereas during the
ambient
period it
was higher than retreat. This suggests that the shoreline had reached a semi-
equilibrium,
such that beach/dune ridges supply enough sand to stabilize the shoreline in
ambient
conditions. However, intense storms overpower the system causing increased
levels of
dune ridge erosion and, thus, a high short-term sediment supply followed by a
deficit
until equilibrium between dune erosion and longshore drift is re-established.
Sand
Dominated Shorelines
Although
change on the eastern shoreline since 1850 is quite dramatic, this shoreline
has changed
little since 1986, especially on the northern portion. It now appears to be
very nearly
balanced in terms of shoreline position. Retreat and advance are almost equal
for all
three periods (Table 4), despite the higher energy setting. In addition, the
southern
spit has
actually grown since 1950 (Figure 4). An interesting pattern has developed such
that the
northern and southern portions of this shoreline retreat and advance out of
phase.
During
ambient and hurricane periods, the northern portion was dominated by shoreline
advance,
the southern portion by retreat. Conversely, during the recovery period erosion
was highest
on the northern portion and the southern portion had higher levels of
accretion
than erosion (1200 m vs. 670 m). It is important to note that the ‘hinge point’
corresponds
to the location of the intersection between the shoreline and Cat Island’s
ridges.
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28
It is clear
that ebb overwash during the hurricane (Figure 13) and subsequent transport
is one of
the main causes of the advance-retreat cycle on the northern half; and that the
islands
antecedent morphology is controlling the overwash locations. The temporal
regularity
of ebb overwash, whether it occurs only in large storms (hurricanes) or as a
more normal
process, is uncertain. Subsequent growth of a dune ridge (Figure 10-Cntrl
N) to the
north of the hinge point in the years following Hurricane Georges suggests that
ebb
overwash is a limited process and does not account for shoreline advance during
the
ambient
period (1995-1997). This is likely a result of a long-term northerly sediment
transport.
Shoreline
retreat on South Spit is to be expected given the dominant southerly winds
(Figure 5).
Shoreline advance during the recovery period is less intuitive. It is possible
that the
ebb overwash built-up the nearshore platform (Figure 10; Cntrl N, Cntrl S) and
facilitated
some onshore sediment transport in the following seasons; however, there are
limited
data to substantiate this. It remains an interesting point and may suggest that
the
present
shoreline configuration is now aligned such that the rapid retreat from 1850 to
1986
(Figure 4), when the shoreline was more north-south oriented, is retarded.
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29
(Figure 13.
Cat Island
in 1998- aerial photo)
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30
Conclusions
Using high
accuracy, simple, and cost effective GPS surveys and publicly available
LIDAR
elevation data combined with ground-based survey data has helped detail the
differences
and similarities of shoreline and morphology changes during the period from
1995 to
2000. During this time one major storm, Hurricane Georges, passed very near
Cat Island. Shoreline and morphology changes
were analyzed for the periods before the
hurricane,
in response to the hurricane, and during the recovery period. Several trends
were
recognized.
1) Mud
platform shorelines show higher retreat/advance ratios for all periods.
2) Sand
dominated shorelines are more dynamic and appear to be operating in semi-
equilibrium,
such that retreat and advance are nearly balanced.
3)
Retreat/advance on the eastern shoreline is associated with its intersection
with
Cat Island’s beach/dune ridges. The shorelines
north and south of the intersection
appear to
function as opposing systems, such that general shoreline change is
opposite
from one to the other. The ridges themselves provide sediment to the
system and
funnel ebb flow during large storms.
4) Profiles
done with traditional survey equipment can be compared with profiles
generated
from LIDAR data to analyze morphology changes.
5) Dramatic
changes seen on the eastern shoreline of Cat Island from 1850 to 1986
appear to
be subdued from 1986 to 2000, despite the passage of Hurricane
Georges.
Changes on the mud-dominated shoreline (western shoreline) appear to
be higher
during the 1995-2000 period than during the 1850-1986 period. The
cause for
the deviations from the historical norm may be the slight re-alignment
of the
eastern shoreline.
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31
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