ELEVATION (FT)-MSL

TRANSI-

HIGH MARSH 5%]

FIGURE 2

COASTAL WETLAND HABITATS

LOW MARSH 12%

VERTICAL EXAG 1:100

| TIDAL FLAT 6% |

WATER 27%

— 10 YR STORM

— PEAK YEARLY TIDE

— SPRING HIGH WATER
— MEAN HIGH WATER
— NEAP HIGH WATER

va

— MEAN LOW WATER
— SPRING LOW WATER

Sse

-IT-


TABLE 6
ACREAGE OF PRINCIPAL HABITAT IN 1980 AND 2075

Pegs HES pe re AREERSIMEO EELS cpg Sgtlgs EOE
Habitat Acres (%) Acres (%) Acres (%) Acres (%) Acres (%) Acres (%)
Highland 21,200 (46.6) 21,700 (47.7) 20,485 (44.9) 21,195 (46.6) 18,990 (41.7) 21,195 (46.6)
Transition 1,500 (3.3) 2,820 (6.2) 1,355 (3.0) 605 (1.3) 1,420 (3.1) 0 (0)
High Marsh 2,300 (5.1) 3,320 (7.3) 690 (1.5) 690 (1.5) 675 (1.5) 0 (0)
Low Marsh 5,400 (11.9) 3,910 (8.6) 3,235 (7.1) 3,235 (7.1) 860 (1.9) 750 =((1.7)
Tidal Flat 2,600 (5.7) 2,600 (5.7) 5,020 (11.0) 5,020 (11.0) 1,425 (3.1) 1,425 (3.1)
Water 12,500 (27.4) jit (24.5) 48,755 (32.5) 18,755 (32.4) 22,130 (48.7) 22,130 (48.6)
TOTALS 45,500 (100.0) 45,500 (100.0) 45,500 (100.0) 45,500 (100.0) 45,500 (100.0) 45,500 (100.0)
TABLE 7

NET CHANGE IN ACRES FOR PRINCIPAL WETLAND HABITATS: 1980-2075

w Scenario ~ 207: High Scenario - 2
Baseline Without Protection With Protection Without Protection With Protection

Habitat Acres (%) Acres (%) Acres (%) Acres (%) Acres (%)
Highland 500 (+2.4) -7T4y (4) 0 (0) "2,210 (10) 0 (0)
Transition 1,320 (+88) “144 (10) 895 (60) -80 (5) -1,500 (100)
High Marsh 1,020 (+44) -1,610 (70) -1,610 (70) 71,625 (71) 72,300 (100)
Low Marsh -1,490 (-28) 72,165 (40) 72,165 (40) “4,540 (84) -4,650 (86)
Tidal Flats O (0) +2,420 (+93) 42,420 (+93) “1,175 (45) 1,175 (45)

Water -1,350 (-10.8) 42,255 (+18) +2,255 (+18) +9,630 (+77) +9,630 (+77)

-@e-


-28-

Council critical area line). Figure 10 illustrates the various options. If
all property above today's wetlands are protected with a wall, for example,
the wetlands will be squeezed between the wall and the sea. Table 5
illustrates the intermediate protection options, whose economic implications
were estimated by Gibbs (1984).

For our modeling, we used the composite habitat elevations we derived from
the twelve transects (see Figure 9). The cutoff elevation for highland around
Charleston was assumed to be 200 cm (6.5 ft) above mean sea level. In
general, land above this elevation around Charleston is free of yearly
flooding dominated by terrestrial (freshwater) vegetation. Although
terrestrial vegetation occurs at lower elevations that are impounded between
dikes or ridges, this information is less relevant for sea level rise
modeling. The zone of concern is the area bordering tidal waterways, where
slopes are assumed to rise continuously without intermediate depressions.

The transition zone is defined as a salt-tolerant area between
predominant, high-marsh species and terrestrial vegetation. This area is
above the limit of fortnightly (spring) tides but is generally subject to
tidal and minor-storm flooding several times each year. If storm frequency
remains constant, it is reasonable to assume that storm tides will shift
upward by the amount of sea level rise (Titus et al. 1984). However, most
climatologists expect the greenhouse warming to alter storm patterns
significantly. Nevertheless, because no predictions are available, we assumed

that storm patterns will remain the same.


-37-,

CHARLESTON TIDES

4.00:

+2.00:

TIDAL ELEVATION (FT-MSL)

3,00
PROBABILITY (%)

—Fus

wow
usuw

4,00

Figure 12.

784

ELEVATION/TIDAL RANGE
°

-78:

1.00:

NORMALIZED TIDE RANGE VS.
WETLANDS SPECIES

‘Sp pateng (tranertion)

FEES fone mare
‘Rebcorme

ue

aw

0.00 20.00 40.00 60,00 80.00 100.00

CUMULATIVE PROBABILITY (%)

Tide-probability curves based on statistics for Charleston given
in Ebersole (1982).

(A) Probability distribution for the range of astronomic tides.

(B) "Normalized" cumulative probability distribution, indicating

the preferential elevation for various wetland species.

Abbreviations:

MSHW (mean spring high water); MHW (mean high
water); MSL (mean sea level); MLW (mean low water); MSLW (mean
spring low water).


types), and climate. Their elevation in relation to mean sea level is
critical to determining the boundaries of a habitat and the plants within it,
because elevation affects the frequency, depth, and duration of flooding and
soil salinity. For example, some marsh plants require frequent (daily)
flooding, while others adapt to irregular or infrequent flooding (Teal 1958).
Along the U.S. East Coast, the terms "low marsh" and "high marsh" are often
used to distinguish between zones (Teal 1958; Odum and Fanning 1973) that are
flooded at least daily and zones flooded less than daily but at least every 15
days. Areas flooded monthly or less are known as transition wetlands.

Regularly flooded marsh in the southeast United States is dominated by
stands of smooth cordgrass (Spartina alterniflora), which may at first
appear to lack zoning. However, work by Teal (1958), Valiela, Teal, and
Deuser (1978), and others indicates total biomass varies considerably within
the low marsh, ranging from zones of tall S. alterniflora along active creek
banks to stunted or short S. alterniflora stands away from creeks and
drainage channels. The tall S. alterniflora may be caused by a combination
of factors, including more nutrients, a higher tolerance for the reductions in
oxygen that result from subtle increases in elevation along levees (DeLaune,
Smith, and Patrick, 1983); and differences in drainage created by variations
in the porosity of sediment. The zone where S. alterniflora grows is thought
by many to be limited in elevation to mean high water. This is probably too
broad a simplification according to Redfield (1972), who emphasized that the
upper boundary of the low marsh is, at best, indistinct.

High marsh, in contrast, consists of a variety of species. These include
Salicornia spp. (glassworts), Distichlis spicata (spikegrass), Juncus spp.

(black needlerush), Spartina patens (salt-marsh hay), and Borrichia


Batis maritima
Borrichia frutescens
Distichlis spicata
Juncus roemerianus
Limonium carolinianum
Polygonum setaceum
Salicornia virginica
Spartina alterniflora
Spartina patens
Spartina cynosuroides
Suaeda linearis

RANGE # EPA-3 LOT 246, END OF SALTGRASS CT.

KEY: 15 JUNE 1984

—— ——__MHW
——_MLW

250 300 350 400 450 650

DISTANCE (FT.)

a 3.13(4)
4.98(3)
1 ~(<1)
i 307(1)
reser 3.06(9)
ee tt‘(‘(SCOSO 2.45(99)
5.35(1)

=8h-


RANGE # EPA-1 EAST SIDE OF MARSH ISLAND

KEY: *& Stsv 1964

PARK BOARDWALK

207
a
a
= rot
e
a ___ MHW
re or ——MLW
u
-10 + + + + + + + + + +t + + 4
0 50 100 «6150 = 200 250 300 48=—350 400 450 500 550 ©6600 650
DISTANCE (FT.)
Batis maritima
Borrichia frutescens (LE 3.90(1)
Distichlis spicata
Juncus roemerianus {| _#f 1 3.40(1)
Limonium carolinianum = |_S 3.27(80)
Polygonum setaceum
Salicornia virginica |_H i 3.42(1)
Spartina alterniflora A 3.27(<1)
Spartina patens
Spartina cynosuroides
Suaeda linearis

-9%-


RANGE # EPA-7 LOT 57, WATERWAY ISLAND DRIVE
KEY: 15 JUNE 1984

207
a
n
= s0+
Kk
= __MHW
= ot MLW
a
w
10 t +——+—+ t +— —— —

Batis maritima
Borrichia frutescens
Distichlis spicata
Juncus roemerianus
Limonium carolinianum
Polygonum setaceum
Salicornia virginica
Spartina alterniflora
Spartina patens
Spartina cynosuroides
Suaeda linearis

0 50 100 §=6150 =. 200 250 300 350 400 450 500 550 600 650

DISTANCE (FT.)

_ i 3.17(6)

|| 3.20(4)

aI 3.01(4)

*T I 3.10(4)
VAR <1 2.60 3.62 1,95(62)

VAR 3.11(4)

-@S-


RANGE # EPA-11 790 WOODWARD ST., RUNNING SOUTH
KEY: 16 JUNE 1984

207
= ]
on
2 sot
E ~~
x —~__ —MHW
2 OT ___MLW
al
w
-10 t + + t t + t + t t + t i
0 50 100 150 200 250 300 350 400 450 500 550 600 650

DISTANCE (FT.)

Batis maritima

Borrichia frutescens _— JE 4.10(9)
Lt
‘=

Distichlis spicata 3.95(31)
Juncus roemerianus 5.45(<1)
Limonium carolinianum
Polygonum setaceum
Salicornia virginica
Spartina alterniflora
Spartina patens
Spartina cynosuroides
Suaeda linearis

5.45(<1)

3.40(97)

-9S-


-43-

South Carolina Coastal Council, 1985. Performance Report of the South
Carolina Coastal Management Program. South Carolina Coastal Council,
Columbia, South Carolina.

Teal, J.M., 1958. "Energy flow in the salt marsh ecosystem." In Proc. Salt
Marsh Conf., Mar. Inst., Univ. Georgia, pp. 101-107.

Titus, J.G., T.R. Henderson, and J.M. Teal, 1984. "Sea level rise and wetlands
loss in the United States. National Wetlands Newsletter, Environmental
Law Inst., Washington, D.C., Vol. 6(5).

Titus, J.G., "Sea Level Rise and Wetlands Loss." In 0.T. Magoon (ed.) Coastal
Zone '85. American Society of Civil Engineers, New York, New York, pp.
1979-1990.

Titus, J.G., M.C. Barth, M.J. Gibbs, J.S. Hoffman, and M. Kenney, 1984. "An
overview of the causes and effects of sea level rise." In M.C. Barth and
J.G. Titus (Eds.), Greenhouse Effect and Sea Level Rise. Van Nostrand
Reinhold Co., New York, N.Y., Chap. 1, pp. 1-56.

Turner, R.E., 1976. "Geographic variations in salt marsh macrophyle
production: a review." Contributions in Marine Science, Vol. 10, pp.
47-48.

U.S. Department of Commerce, 1979. State of South Carolina Coastal Zone
Management Program and Final Environmental Impact Statement. Office of
Coastal Zone Management, National Oceanic and Atmospheric Administration,
Washington, D.C.

U.S. Department of Commerce, 1981. "Tide tables, east coast of North and
South America." NOAA, National Ocean Survey, Rockville, MD., 288 pp.

Valiela, I., J.M. Teal, and W.G. Deuser, 1978. "The nature of growth forms in
the salt marsh grass Spartina alterniflora." American Naturalist, Vol.
112(985), pp. 461-470.

Ward, L.G., and D.D. Domeracki, 1978. "The stratigraphic significance of
back-barrier tidal channel migration." Geol. Soc. Amer., Abs. with
Programs, Vol. 10(4), p. 201.

Wilson, K.A., 1962. North Carolina Wetlands: Their Distribution and
Management. North Carolina Wildlife Resources Commission, Raleigh, N.C.



-15-

marsh/tidal-flat system behind Isle of Palms (upper right) and Dewees Island,
just outside of the Charleston study area. The inlet shown is Dewees Inlet.
Note the mud flat and circular oyster mounds in the foreground near marsh and
tidal channels.

The photo in Figure 6 was taken at low tide. It shows the vertical

transition from marsh to oyster flats along a tidal creek in the Charleston

FIGURE 6

TRANSITION FROM MARSH TO OYSTER FLAT

Photo by L.C. Thebeau, April 1981.

case study area. Where the waterfront is developed (Figure 7), the transition
from marsh or tidal creeks to highland can be very distinct because of the
presence of shore-protection structures, such as vertical bulkheads and

riprap. This aerial view was taken 50 km (31 mi) north of the case study area.


SCALE -(FT)

TIDAL FLAT WATER
4 2075 2075
12% 33%

+6:

+4

«

° ses 2 2078.8
gee

2 a 1980 MSL

4 ee EXISTING

peas
RANSITION | HIGH MARSH

LOW MARSH

TIDAL FLAT

1980 1980 1980 . 1980
3% 5% 12% 6% 27%
Figure 11. Conceptual model of the shift in wetlands zonation along a shoreline profile if sea level
rise exceeds sedimentation rates. In general, the response will be a landward shift and

altered areal distribution of each habitat because of variable slopes at each elevation
interval.

-¢e-


iii

CONCLUSIONS

Sea level rise could become a significant cause of wetland loss in the
Charleston area. If current trends continue for the next century,
economic development will destroy less than 0.5 percent of the area's
wetlands, and those losses would be more than offset by natural creation
of wetlands. Projected sea level rise, however, could result in the loss
of between 50 and 90 percent of the area's marsh in the next century.

The National Academy of Sciences’ estimate of sea level rise (2.9 feet)
implies that Charleston could lose 50 percent of its marsh by 2075. The
area of high marsh would decline from 2300 to 700 acres, while the area of
low marsh would decline from 5400 to 3200 acres.

The high scenario (5.2 feet) implies that Charleston could lose 80
percent of its marsh by 2075 if human activities do not interfere. As
with the NAS estimate, the high marsh would decline from 2300 to 700
acres; low marsh, however, would decline from 5400 to 900 acres.

The impact of sea level rise on coastal wetlands will ultimately depend
on whether developed areas immediately inland of the marsh are protected
from rising sea level by levees and bulkheads. The above estimates are
based on the assumption that new wetlands will be created as inland areas
are flooded. However, new wetlands can only be created if areas just
inland of the marsh are undeveloped. If these areas are developed and
protected with levees and bulkheads as the sea rises, no additional
wetlands will be created; thus, the net loss will be greater. The high
scenario (5.2 feet) would imply a loss of all high marsh and all but 750
acres of low marsh. Because development in the Charleston area is
generally at least three feet above the high marsh, constructing these
barriers would not increase the loss of marsh by 2075 for the NAS scenario
(2.9 feet), although some transition wetlands would be lost.

Factors not considered in this report could increase or decrease the
vulnerability of wetlands to a rise in sea level. The estimates in this
report are based on the assumptions that the rate of vertical marsh growth
and the shape and position of marsh profiles remain unchanged. More
extensive study could improve upon these assumptions and take into account
such factors as impacts of global warming on peat formation, reworking of
sediment, and oxidation of peat due to marsh drowning.

Other communities with similar types of marsh can obtain inexpensive
first-order estimates of future wetland loss. The data in this report,
along with information on tidal ranges, can be used by those who need only
a rough indication of the vulnerability of wetlands to rising sea level.

Assessments should be undertaken of how to mitigate loss of wetlands from
sea level rise. Although the most substantial losses of wetlands are at
least 50 years away, today's coastal development may largely determine the
success with which wetlands adjust to rising sea level in the future.



-26-

under an accelerated rise in sea level and to apply the model to the case

study area.

Scenario Modeling
Based on an earlier study sponsored by EPA (Barth and Titus 1984), we
chose three scenarios of future sea level rise:
° a baseline scenario, with an average annual rise of
2.5 mm (.1 in), based on current sea-level trends in the
Charleston area (Hicks, DeBaugh, and Hickman 1983);
* a low scenario with an average annual rise of 9.2 mm
(.36 in), based on the National Academy of Sciences
estimate of a worldwide rise of 70 cm (2.3 ft) in the
next century and the EPA mid-low scenario; because sea
level is rising faster in Charleston than globally, this
scenario implies a rise in the Charleston area of 87 cm
(2.9 ft) by 2075;
Ld a high scenario, with an average annual rise of 17.0
mm (.67 in), based on EPA's mid-range high scenario;
because of local subsidence trends, this scenario
implies a rise of 159 cm (5.2 ft) by 2075 in the
Charleston area.
To be consistent with the previous study, we projected the scenarios to the
year 2075--95 years after the baseline date of 1980 used to determine
"present" conditions.

The model for future wetland zonation also accounted for sedimentation and
peat formation, which partially offset the impact of sea level rise by raising
the land surface. Sedimentation rates are highly variable within East Coast
marsh/tidal-flat systems, with published values ranging from 2 to 18 mm (.08
to .71 in) per year (Redfield 1972; Hatton, DeLaune, and Patrick 1983). Ward
and Domeracki (1978) established markers in an intertidal marsh 20 km (12 mi)

south of the Charleston case study area and measured sedimentation rates of

4-6 mm (.16-.24 in) per year. Hatton, DeLaune, and Patrick (1983) reported


-12-

conjunction with the new and full moons. The statistical average of these,
referred to as mean spring high water, has an elevation of 1.0 m (3.1 ft)
above mean sea level in Charleston (U.S. Department of Commerce 1981).

Less frequent tidal flooding occurs annually at even higher elevations
ranging upwards of 1.5 m (5.0 ft) above mean sea level. In a South Carolina
marsh near the case study area, the flooding of marginal highland occurred at
elevations of 4-6 ft above mean sea level (approximately 2.0-2.5 ft above
normal). The peak astronomic tide that was responsible for the flooding
included an estimated wind setup of 0.5-1.0 ft under 7-9 m/s (13-17 mph)
northeast winds.

The Charleston area has a complex morphology. Besides the three tidal
rivers that converge in the area, numerous channels dissect it, exhibiting
dendritic drainage patterns typical of drowned coastal plain shorelines.

To help provide an understanding of these ecosystems, Figures 3-6
illustrate various marshes in South Carolina. Figure 3 is an oblique aerial

FIGURE 3
BACK BARRIER WETLANDS AND TIDAL CREEK NEAR KIAWAH ISLAND

BT OR 0 BEAN
Photo by M.O. Hayes.


-18-

FIGURE 8
LOCATIONS OF STUDY AREA AND TWELVE TRANSECTS

For each transect, we measured elevation and distance from a benchmark

using a rod and level. Data points were surveyed wherever there was a

noticeable break in slope or change in species. The average distance between

points was about 7.5 m (25 ft). Along each transect we collected and tagged
samples of species for laboratory typing and verification, noting such

information as the elevation of the boundaries between different species. By


-14-

such an abrupt transition along the back side of Kiawah Island, just south of
the case study area. Highland terrestrial vegetation appears on the left side
of the photo and the marsh is on the right. The detritus seen in the
foreground has been washed up by extreme high tide.

The photos in Figures 5 and 6 illustrate typical horizontal and vertical
division of these intertidal environments. The most extensive intertidal mud
flats around Charleston generally occur in the sheltered zone directly behind
the barrier islands. They are thought to represent areas with lower
sedimentation rates (Hayes and Kana 1976) away from major tidal channels or

sediment sources. Looking seaward, Figure 5 is an oblique aerial photo of the

FIGURE 5
MARSH TIDAL FLAT SYSTEM BEHIND ISLE OF PALMS

Photo by T.W. Kana, February 1981


-27-

comparable values (3-5 mm per year) for Georgia marshes. Although the rate of
marsh accretion will depend on proximity to tidal channels (sediment sources)
and density of plants (baffling effect and detritus), we believe the published
rate of 4-6 mm per year is reasonably representative for the case study area
(Ward and Domeracki 1978). Thus, for purposes of modeling, we assumed a
sedimentation rate of 5 mm per year. Obviously, the actual rate will vary
across any wetland transect, so this assumed value represents an average.
Lacking sufficient quantitative data and considering the broad application of
our model, we found it was more feasible to apply a constant rate for the
entire study area.

As shown in Table 4, the combined sea level rise scenarios and
sedimentation rates yield a positive change in substrate elevation for the
baseline and a negative change for the low and high scenarios. The positive

change for baseline conditicns follows the recent trend of marsh accretion in

Charleston.
TABLE 4
SEA LEVEL RISE SCENARIOS TO THE YEAR 2075
Annual Annual Net
Average Sedimen- Substrate
Scenario Sea Level Rise by 2075 Annual Rise tation Rate Change
Baseline +23.8 cm (0.78 ft) 2.5 mm 5 mm +2.5 mm
Low +87.0 cm (2.85 ft) 9.2 mm 5 mm -4.2 mm
High +159.2 cm (5.22 ft) 17.0 mm 5 mm 712.0 mm

For each of these three scenarios, we considered four alternatives for
protecting against the rising sea: no protection, complete protection, and
two intermediate protection options. Protective options consist of bulkheads,

dikes, or seawalls constructed at the upper limit of wetlands (S.C. Coastal


-9-

FIGURE 1
CHARLESTON STUDY AREA

Island



RANGE # EPA-9 DEWEES INLET, NEAR RPI MON. STATION 13, RUNNING NORTH
KEY: 16 JUNE 1984

207
>
a
= 10+
i
_— [ie WY —MHW
= ot LEVEE __iw
a
w
-10 + + + + + + + + + t + + 4
0 50 100 «150 =. 200 250 =300 = 3350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima
Borrichia frutescens —"y 3.57 EX] 3.54(29)
Distichlis spicata 3.4 3.29(15)
Juncus roemerianus
Limonium carolinianum a 4.35(<1)
Polygonum setaceum
Salicornia virginica | EE EE En 3.14(31)
Spartina alterniflora | XT TGR 2.71(70)
Spartina patens
Spartina cynosuroides
Suaeda linearis Ei 3.22(5)

-9S-


-1-

INTRODUCTION

Increasing atmospheric concentrations of carbon dioxide, methane, and
other "greenhouse" gases are expected to raise the earth's average temperature
a few degrees centigrade in the next century. Such a warming could cause sea
level to rise a few feet by expanding ocean waters, melting mountain glaciers,
and perhaps causing polar glaciers to melt or slide into the oceans. Because
most coastal wetlands are within a few feet of mean sea level, a rise in water
levels could cause a major loss of these ecosystems.

This study examines the potential impact of future sea level rise on
coastal wetlands in the area of Charleston, South Carolina, for the year
2075. The report builds upon previous EPA studies that had assessed the
potential physical and economic impacts of sea level rise on the Charleston
area. We surveyed twelve wetland transects to determine elevations of
particular parts of the marsh, frequency of flooding, and vegetation at
various elevations. From these transects, we developed a composite transect
representing an average profile of the area. Using this information and
estimates of the sediment provided by nearby rivers, we then estimated the
shifts in wetland communities and net loss of marsh acreage associated with
three possible scenarios of sea level rise for the year 2075: (1) the current
trend, which implies a rise of 24 cm (0.8 ft); (2) the National Academy of
Sciences estimate, which implies a rise of 87 cm (2.9 ft); and (3) a high
scenario rise of 159 cm (5.2 ft).

This report presents background information concerning global warming and
future sea level rise, the ecological balance of coastal wetlands; and the
potential transformation of these ecosystems as sea level rises. Next, it

examines the wetlands in the Charleston study area and describes a field study


-24-

as measured from the 1929 MSL datum. Thus, it would be reasonable to assume
that today's low-marsh S. alterniflora prefers elevations close to the limit
of the neap tide. For Charleston, this is about 15 cm (0.5 ft) below mean
high water. Figure 9 shows that S. alterniflora extends beyond the limits of
low marsh into both high marsh and tidal flat; however, this species occurs
primarily at low-marsh elevations.

The other indicator species are generally considered to be high-marsh

species. These include Distichlis spicata, Borrichia frutescens, Limonium

carolinianum and Salicornia virginica. Spartina patens, while having been

found to coexist with Distichlis spicata in Maryland and North Carolina
marshes (E.C. Pendleton, personal communication, December 1984), is uncommon
at elevations less than 122 cm (4 ft) above mean sea level in Charleston
(Scott, Thebeau, and Kana 1981). The variance in these observations may he
related to the significant difference in tidal range between central South
Carolina and North Carolina.

As illustrated in Figure 5 in the previous section, large portions of the
back-barrier environments of Charleston consist of tidal flats at lower
elevations than the surrounding marsh. Oyster mounds were found at a wide
range of elevations along tidal creek bands, but over tidal flats most were

common at 30-46 cm (1.0-1.5 ft) above mean sea level.

Area Estimates

Two sources of information were available for land area estimates: United
States Geological Survey (USGS) 7.5-minute quadrangles and digitized computer
maps prepared in an earlier EPA-sponsored case study (Kana et al. 1984).

Using topographic and contour maps, we estimated the number of acres of each


-63-

APPENDIX C

AREA DISTRIBUTION BY ELEVATION ZONE FOR EACH OF THE
FIVE PRINCIPAL LAND DIVISIONS IN THE
CHARLESTON STUDY AREA


APPENDIX C. Continued.

NO_PROTECTION LOW SCENARIO HIGH SCENARIO.
with Without With Without,
Low High Anticipation Anticipation Anticipation Anticipation
Existing Baseline Scenario scenario Protection Protection Protection Protection Protection Protection
ZONE (1980) (2075) (2075) (2075) @ 1980 @2030 @2050 @1980 @2010 @2020

SULLIVANS ISLAND: ‘TOTAL ACRES = 1,750

Highland 37.1 36.2 35.6 32.8 37-1 NA NA 37.1 NA NA
Transition 2.9 2.9 2.8 3.4 1.3 - - 0.0 - -
High Marsh ToL 2.3 1.2 0.7 1.2 - - 0.0 - -
Low Marsh 15.7 13.0 9.6 1.7 9.6 - - 1.5 - -
Tidal Flat 8.6 12.4 14.7 3.5 4.7 - - 3.5 - -
Water 28.6 33.2 36.1 57.9 6 ok - - 57.9 - -
‘TOTALS 100.0 100.0 100.0 100.0 100.0 - - 100.0 - -
DANIEL ISLAND: TOTAL ACRES = 4,500
Highland 41.6 40.7 40.2 37.5 41.6 NA NA 41.6 NA NA
Transition 2.8 2.8 2.6 2.8 1.2 - - 0.0 - -
High Marsh 5.0 1-9 1.2 Leb 1.2 - - 0.0 - -
Low Marsh 11.7 9.5 7.0 L.7 7.0 - - 165 - -
Tidal Flat 5.7 8.9 10.9 2.8 10.9 - - 2.8 - -
water 33.2 36.2 38.1 54-1 38-1 - - 54.1 - -
TOTALS 100.0 100.0 100.0 100.0 100.0 - - 100.0 - -

-S9-

‘WrU.S. GOVERNMENT PRINTING OFFIC, 19 8676217734—-00190


-21-

Composite Transect

To model the scenarios of future sea level rise, we had to develop a
composite transect from the data in Table 2. Thus, for each species, one
modal elevation was estimated from the various elevations in Table 2.
Similarly, the percent of each transect covered by an individual species was
used to estimate an average percent coverage for all transects (Table 3).

This information allowed us to choose for our composite the five species
that dominated the high and low marshes in all the transects: Spartina

alterniflora, Salicornia virginica, Limonium carolinium, Distichlis

spicata, and Borrichia frutescens. We call these the indicator species.

Figure 9 shows the modal elevations for these five species, for two other

salt-tolerant plants found in the transects (Juncus roemerianus and Spartina

patens), and for a species found in tidal flats and under water (Crassotrea
virginica). The primary zone where each species occurs is indicated by the
shaded area; occasional species occurrence outside the primary zone is
indicated by the unshaded, dashed-line boxes. Figure 9 also outlines the
boundaries for the six habitats and indicates the estimated percentage of the
study area that each covers.

While this profile is by no means precise, it gives some insight into the
expected habitat for a given elevation and the tolerances various species have
for flooding. For example, it establishes the general lower limit of marsh
for Charleston, where it is presumed that too frequent flooding kills
low-marsh species and transforms the marsh to unvegetated mud flats.

The low-marsh plant Spartina alterniflora was the most dominant species,
making up 69 percent of the composite transect. Its modal elevation was 75 cm

(2.45 ft) above mean sea level (MSL), or 5 cm (0.15 ft) below mean high water


TABLE OF CONTENTS

INTRODUCTION ...

FUTURE SEA LEVEL RISE AND ITS IMPLICATIONS .........-. cece eee crete teens

Global Warming and a Rising Sea
Ecological Balance of Wetlands
Potential Transformation of Wetlands

COASTAL HABITATS OF THE CHARLESTON STUDY AREA .......-....eeeeeeeee eee
WETLANDS TRANSECTS: METHOD AND RESULTS ........: ces eeeeee cece ere eeneee

Data Collection and Analysis ..
Results of Individual Transects ..
Composite Transect ..

Area Estimates ...

WETLAND SCENARIOS FOR THE CHARLESTON AREA: MODELING AND RESULTS ......

Scenario Modeling ...
Scenario Results ....

RECOMMENDATIONS FOR FURTHER STUDY ......-.. cece cece eee e eee e eee eee eeeee

Normalized Elevations ...... sce e cece cence teen eee eee eee e ene
Conclusion 2... eee eee eee eee eee teen t eee teen eteee

REFERENCES 2... eee e ccc e cere eee e eee eee e ee een etree nee eee tent ee ene eene
APPENDIX A: WETLANDS TRANSECTS AND DISTRIBUTIONS OF SPECIES ..........

APPENDIX B: TIDE ELEVATION PROBABILITY DISTRIBUTION FOR CHARLESTON,
SOUTH CAROLINA ..... 0. . cect cece e eee eee teen ene t eens

APPENDIX C: AREA DISTRIBUTION BY ELEVATION ZONE FOR EACH OF THE
FIVE PRINCIPAL LAND DIVISIONS IN THE CHARLESTON STUDY
2 N50 7

35

36
39

41

45

59

63


iv

ACKNOWLEDGMENTS

The authors wish to express their appreciation to several people who
provided assistance to this study.

The research was completed with the assistance of several people at
Coastal Science and Engineering, Inc., including S. Jonathan Siah, who
prepared the tide probability analysis; Bill Eiser, Mark Jordana, and Mike
Bise, who assisted in the field; and Tom Ballouy, who helped identify the
marsh species. Graphics for the report were prepared under the supervision of
Starnell Perez with the assistance of Jerry Cole, Steve Loy, Harriet
Gilkerson, and Cindi Fehrs.

Report production services were provided by staff at EPA, RPI, Inc., and
ICF Incorporated: Joan O'Callaghan of EPA and Susan MacMillan of ICF provided
editing contributions; and finally, the manuscript was prepared by Diana
Sangster of RPI and Margo Brown of ICF Incorporated.

Discussions with Miles 0. Hayes, President of RPI, Inc., were beneficial
to the authors in preparing this report. David Flemer, Alan Hirsch, Howard
Marshall, and Gregory Peck of the Environmental Protection Agency reviewed the
draft and provided substantive comments. Carroll Cordes and Edward Pendleton
of the U.S. Fish and Wildlife Service's National Coastal Ecosystems Team and
Joy Zedler of San Diego State University also made useful suggestions.
Finally, James G. Titus of EPA provided overall guidance for the study and
wrote portions of the final report.


-10-

Six discrete habitats are found in the Charleston area, distinguished by
their elevation in relation to sea level and, thus, by how often they are
flooded (Figure 2):

highland - flooded rarely (47 percent of study
area)

transition wetlands - flooding may range from biweekly to
annually (3 percent)

high marshes - flooding may range from daily to
biweekly (5 percent)

low marshes - flooded once or twice daily (12
percent)

tidal flats - flooded up to half of the day (16
percent)

water - flooded more than half of the day

(27 percent)

This flooding, in turn, controls the kinds of plant species that can
survive in an area. In Charleston, the present upper limit of salt-tolerant
plants is approximately 1.8-2.0 m (6.0-6.5 ft) above mean sea level (Scott,
Thebeau, and Kana 1981). This elevation also represents the effective lower
limit of human development, except in areas where wetlands have been
destroyed. The zone below this elevation (delineated on the basis of
vegetation types) is referred to as a critical area under South Carolina
Coastal Zone Management laws and is strictly regulated (U.S. Department of
Commerce 1979).

The distribution of coastal environments around Charleston is balanced for
tides occurring twice each day. However, the actual upper limit of
salt-tolerant species is considerably above mean high water. Because of the
lunar cycle and other astronomic or climatic events, higher tides than average

occur periodically. Spring tides occur approximately fortnightly in


Table

Table

Table
Table
Table
Table

Table

vii

LIST OF TABLES

Scenarios of Future Sea Level Rise: 1980-2100 ........... .

Modal Elevations and Percentage of Transect Covered
by Principal Species ..

Summary Statistics for Elevations of Marsh Plant Species ..

Sea Level Rise Scenarios to the Year 2075 ........seseeeuee

Shore-Protection Scenarios ......... fade nea seneeee seeeee see

Acreage of Principal Habitat in 1980 and 2075 ........ bees

Net Change in Acres for Principal Wetland Habitats:

1980-2075

20

22

27

30

32

32


-16-

FIGURE 7
A DEVELOPED SOUTH CAROLINA COASTAL BARRIER

Photo by T.W. Kana, February 1981.

Much of the Charleston shoreline has accreted (advanced seaward and
upward) during the past 40 years (Kana et al. 1984). Marshes accrete through
the settling of fine-grained sediment on the marsh surface, as cordgrass
(Spartina alterniflora) and other species baffle the flow adjacent to tidal
creeks. Marsh sedimentation has generally been able to keep up with or exceed
recent sea level rises along this area of the eastern U.S. shoreline (Ward and
Domeracki 1978). Much of the sediment into the Charleston area has derived
from suspended sediment originating primarily from the Cooper River, which
carries the diverted flow of the Santee River (U.S. Army Corps of Engineers,
unpublished general design memorandum). However, the recent rediversion will
reduce sediment input, which could slow the rate of marsh accretion in the

future.


EPA

United States Office of Policy May 1986
Environmental Protection Planning and Evaluation EPA 230-10-85-014
Agency Washington, DC 20460

Potential Impacts of

Sea Level Rise On

Wetlands Around Charleston,
South Carolina



-39-

Conclusion

Wetlands in the Charleston area have been able to keep pace with the
recent historical rise in sea level of one foot per century. However, a
three- to five-foot rise in the next century resulting from the greenhouse
effect would almost certainly upset these ecosystems in a fashion similar to
that occurring in Louisiana, which every year loses over one hundred square
kilometers (fifty square miles) to the sea.

The success with which coastal wetlands adjust to rising sea in the future
will depend upon whether human activities prevent new marsh from forming as
inland areas are flooded. If human activities do not interfere, a three-foot
rise in sea level would result in a net loss of about 50 percent of the marsh
in the Charleston area. A five-foot rise would result in an 80 percent loss.

To the extent that levees, seawalls, and bulkheads are built to prevent
areas from being flooded as the sea rises, the formation of new marsh will be
prevented. We estimate that 90 percent of the marsh in Charleston--including
all of the high marsh--would be destroyed if sea level rises five feet and
walls are built to protect existing development.

This study represents only a first investigation into an area that
requires substantial additional research. The methods developed here can be
applied to estimate marsh loss in similar areas with different tidal ranges
without major additional field work. Nevertheless, more field surveys and
analysis will be necessary to estimate probable impacts of future sea level
rise on other types of wetlands.

In spite of the preliminary nature of this investigation, it appears
reasonable to conclude that a three- to five-foot rise in sea level could

seriously threaten coastal wetlands in the United States, if human activities


warm and expand. Mountain glaciers, which have retreated in the last century,
could melt more rapidly. Glaciers in Antarctica and Greenland could melt
along the fringes, and portions of them could slide into the oceans. Although
a complete disintegration of the West Antarctic Ice Sheet would raise sea
level six meters, it is unlikely to occur in the next century (Meier et al.
1985).

In 1983, two independent reports estimated sea level rise in the next
century. The National Academy of Sciences report Changing Climate estimates
that worldwide sea level will rise 70 cm (2-1/3 ft) in the next century,
ignoring the impact of the global warming on Antarctica (Revelle 1983). The

Environmental Protection Agency report Projecting Future Sea Level Rise

states that the uncertainties regarding the factors that could influence sea
level rise are so numerous that a single estimate of sea level rise is
impossible (Hoffman, Keyes, and Titus, 1983). Instead, it specified high and
low estimates for all the factors that could influence sea level and estimates
resulting high, medium, and low scenarios. As Table 1 shows, the EPA report
estimates that sea level will rise between 38 and 212 cm by 2075, with the
likely range falling between 91 and 136 cm (3 and 5 ft), compared with a
global rise of 10 to 15 cm (4 to 6 in) in the last century. Because most of
the coast is subsiding, sea level rise along the Atlantic coast has been 15 to
20 cm per century higher than the worldwide average; this subsidence trend is

expected to continue into the future.

Ecological Balance of Wetlands
Recent attention concerning rising sea level has been focused on the fate

of economic development in coastal areas. However, the area facing the most


in which we developed wetland transects. Finally, it discusses the potential
impact of future sea level rise on Charleston's wetlands, and suggests ways to
improve our ability to predict the impact of sea level rise on other coastal

wetlands.

FUTURE SEA LEVEL RISE AND ITS IMPLICATIONS

Global Warming and a Rising Sea

A planet's temperature is determined by the amount of sunlight it
receives, the amount of sunlight it reflects, and the extent to which its
atmosphere retains heat. When sunlight strikes the earth, it warms the
surface, which then radiates the heat as infrared radiation. However, water
vapor, carbon dioxide, and other gases in the atmosphere absorb some of the
energy, rather than allowing it to pass undeterred through the atmosphere to
space. Because the atmosphere traps heat and warms the earth in a manner
somewhat analogous to the glass panels of a greenhouse, this phenomenon is
generally known as the "greenhouse effect."

Since the industrial revolution, the combustion of fossil fuels,
deforestation, and cement manufacture have released enough co, into the
atmosphere to raise co, concentrations by 20 percent (Keeling, Bacastow, and
Whorf 1982). Review panels of the National Academy of Sciences have concluded
that a doubling of atmospheric co, expected in the next century, would warm
the earth 1.5°-4.5°C (3°-8°F). Increasing concentrations of methane,
chlorofluorocarbons, nitrous oxide, and other trace gases could roughly double
the warming from co, alone (Lacis et al. 1981; Ramanathan et al. 1985).

A global warming of a few degrees could be expected to raise sea level for

several reasons. Increasing atmospheric temperatures would cause seawater to


=~ N
o o °o

ELEV. (FT.-MSL)

'
nm
o

RANGE # EPA-8 DEWEES INLET, NEAR RPI MON. STATION 13, RUNNING WEST

KEY: 16 JUNE 1984

TIDAL
CREEK

——MHW

Ny ye —_MLW

DYSTE!I

Batis maritima

t t + +
50 100 150 200 250 300 350
DISTANCE (FT.)

Borrichia frutescens
Distichlis spicata
Juncus roemerianus

Limonium carolinianum a
Polygonum setaceum

Salicornia virginica
Spartina alterniflora

Spartina patens

4.01 24 3.82(27)

a17 3.70(23)
3.89(28)

0 a 1323 2531 3.30(18)
3.23 2.79 2.79(57)

Spartina cynosuroides
Suaeda linearis

4.00(7)

-€S-


-35-

city was built, destruction of coastal wetlands came to a virtual halt with
the creation of the South Carolina Coastal Council. Since 1977, the entire
state has only lost 35 of its 500,000 acres of wetlands to dry lands (South
Carolina Coastal Council 1985). Approximately 100 acres have been flooded by
artificial impoundments (U.S. Fish and Wildlife Services, Charleston Office;
personal communication). Thus, without the impact of sea level rise, one
would expect the Charleston area to lose less than 0.5 percent of its wetlands

in the next century.

RECOMMENDATIONS FOR FURTHER STUDY

This study is a first attempt at determining the potential impact of
accelerated sea level rise on wetlands. The experience gained in the
Charleston area should be built upon with case studies in other estuaries.
Louisiana provides a present-day analog for the effect of rapid sea level rise
on wetlands because of high subsidence rates along the Mississippi Delta (see
Gagliano 1984). Additional studies in that part of the coast should attempt
to document the temporal rate of transformation from marsh to submerged
wetlands.

Accurate wetland transects with controlled elevations are required to
determine the preferred substrate elevations for predominant wetland species.
With better criteria for elevation and vegetation, we can use remote-sensing
techniques and aerial photography to delineate wetland contours on the basis
of vegetation. Scenario modeling can then proceed using computer-enhanced
images of wetlands and surrounding areas, for more accurate delineation of
marsh habitats. Using historical aerial photos, it may also be possible to

infer sedimentation rates by changes in plant coverage or species type, which


ELEVATION (FT)-MSL

HIGHLAND 47%

TRANSI-

HIGH MARSH 5%|

LOW MARSH 12%

COMPOSITE TRANSECT-

CHARLESTON, S.C.

| TIDAL FLAT 6%

WATER 27%

+104 + +10
8 z
* —10 YR STORM +8
6a
— PEAK YEARLY TIDE =
__ sprinG HiGH waTeR [~*4 A
= MEAN HIGH WATER t
— NEAP HIGH WATER +2
z
¥ roo
Fol
wean tow water [2 S
VERTICAL EXAG 1:100 = SPRING LOW WATER rt]
4 Lg
~s i
a TSpaiine enecntiers 2 ~~ 6
“6 T T T 8
° 1000 2000 5000
TYPICAL DISTANCE (FT)
Figure 9. Composite wetlands transect for Charleston illustrating the approximate percent occurrence

and modal elevation for key indicator species or ha’
Minor species have been omitted.

transects.

bitats based on results of 12 surveyed

-€Z-


-40-

prevent natural adaptation to that rise. Because of the long-range nature of
the issue, it may be possible to avoid adverse environmental impacts through
low-cost nonregulatory measures and market incentives. Future studies should
not only focus on the loss of wetlands caused by sea level rise, but on
possible measures to avoid those losses.

The assumptions used to predict future sea level rise and the resulting
impacts on wetland loss must be refined considerably so that we can have more
confidence in any policy responses that are based on these predictions. The
substantial environmental and economic resources that can be saved if better
predictions become available soon will easily justify the cost (though
substantial) of developing them (Titus et al. 1984). However, deferring
policy planning until all remaining uncertainties are resolved is unwise.

The knowledge that has accumulated in the last twenty-five years has
provided a solid foundation for expecting sea level to rise in the future.
Nevertheless, most environmental policies assume that wetland ecosystems are
static. Incorporating into our environmental research the notion that
ecosystems are dynamic need not wait until the day when we can accurately

predict the magnitude of the future changes.


-22-

TABLE 3

SUMMARY STATISTICS FOR ELEVATIONS
OF MARSH PLANT SPECIES

Weighted Mean Standard Percent
(feet above Deviation Modal Occurrence
SPECIES mean sea level) (+ ft) Elevation* Composite
Batis maritima - - 3.17 7
Borrichia frutescens 3.76 53 3.16** 14%*
Distichlis spicata 3.71 -27 3.7 1]%* gree
Juncus roemerianus - - 4.17 1
Limonium carolinianum 3.38 +46 3.38% 16**
Polygonum setaceum - - 3.32 1
Salicornia virginica 3.18 20 3.16%* 21%
Spartina alterniflora 2.59 59 2.45% 69%%
Spartina patens - - 5.35 21
Spartina cynosuroides - - 2.51 6
Sueada linearis - : 3.59 4

* Excludes anoma

lous values in some cases and observations

than 2 percent of transect.

wk Recommended indicator species.

covering less


-61-

APPENDIX B. Continued.
Common Elevation Noretonh Probability Propebiity
Reference* (ft) Tidal Range) %) (%)
-0.8 0.154 3.13 62.29
-1.0 -0,192 3.17 65.46
71,2 -0.231 3.47 68.93
-1.4 0.269 3.64 72.57
-1.6 0.308 3.78 76.35
-1.8 0.346 3.72 80.07
-2.0 -0,385 3.77 83.84
2.2 0.423 3.39 87.23
2.4 0.462 3.14 90.37
MLW 72.6 -0.500 2,54 92.91
-2.8 0.538 2.13 95.04
-3.0 70.577 1.67 96.71
MSLW -3.2 -0.615 1.16 97.87
3.4 0.654 0.86 98.73
-3.6 0.692 0.53 99.26
-3.8 0.731 0.35 99.61
4.0 -0.769 0.21 99.82
7 4,2 0.808 0.12 99,94
4,4 0.846 0.03 99.97
4.6 0.885 0.02 99.99
4.8 -0.923 0.01 100.00
-5.0 -0.962 0.00 100.00
-5.2 -1.00 0.00 100.00
*MHW- - mean high water
MLW - mean low water
MSL - mean sea level
MSHW - mean spring high water
MSLW - mean spring low water


vi

LIST OF FIGURES

Figure 1. Charleston Study Area ........ 0. cece ccc cece eee ee eens
Figure 2. Coastal Wetland Habitats 1... .... cece cece cece teens

Figure 3. Back Barrier Wetlands and Tidal Creek Near Kiawah
BSD Ce

Figure 4. Transition from Highland to Marsh: Kiawah Island .........

Figure 5. Marsh Tidal Flat System Behind Isle of Palms ..............

Figure 6. Transition from Marsh to Oyster Flat ............e ee eee eens
Figure 7. A Developed South Carolina Coastal Barrier .............005
Figure 8. Locations of Study Area and Twelve Transects ...........05-
Figure 9. Composite Wetlands Transect for Charleston

Illustrating Modal Elevations and Key Species .............

Figure 10. Marsh Loss Due to Bulkheads ..

Figure 11. Conceptual Model of the Shift in Wetlands Zonation
along a Shoreline Profile ..

Figure 12. Tide-Probability Curves ........ cee cece eet e eee tee cere neces


-13-

photograph of a back-barrier, tidal creek/marsh/mud-flat system near Kiawah
Island, approximately 20 km south of Charleston. It shows a typical drainage
pattern and a recently abandoned, horseshoe-shaped infilling with mud.

Throughout the area, highlands are typically less than 5 m (16 ft) above
mean sea level. With a mean tidal range of 1.6 m (5.2 ft), a broad area along
the coastal edge is flooded twice each day. The natural portions of
Charleston Harbor are dominated by fringing salt marshes from several meters
to over one kilometer wide.

The upper limit of the marsh can usually be distinguished by an abrupt
transition from upland vegetation to marsh species tolerant of occasional
salt-water flooding. On topographic maps of Charleston, this break is often
about 1.5 m (+5 ft) above mean sea level. The ground photo in Figure 4 shows

FIGURE 4
TRANSITION FROM HIGHLAND TO MARSH: KIAWAH ISLAND

Photo by M.L. Williams, May 16, 1984.


RANGE # EPA-6 LOT 54, WATERWAY ISLAND. DRIVE
KEY: 15 JUNE 1984

207
=
a
= 0+
t
= TIDAL CHANNEL —MHW
Ff of F108 — MLW
a
irr)
-10 t + + + t + + + + +t + ti——1
0 50 100 150 200 250 300 350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima | i 3.04(14)
Borrichia frutescens | SEX SEXE) 3.86(40)
Distichlis spicata
Juncus roemerianus | Ef = 3.48(7)
Limonium carolinianum i 3.04(14)
Polygonum setaceum
Salicornia virginica |ER SEET) 3.30(34)
Spartina alterniflora  |_§ VAR --(11)
Spartina patens
Spartina cynosuroides
Suaeda linearis EXT (EXT) 3.61(34)


-59-

APPENDIX B

TIDE ELEVATION PROBABILITY DISTRIBUTION
FOR CHARLESTON, SOUTH CAROLINA


TABLE 5
SHORE-PROTECTION SCENARIOS

Without Anticipating With Anticipating
Area Sea Level Rise Sea Level Rise
Low Scenario
Peninsula Protection after 2050 Protection after 2030
West Ashley/James Island Protection after 2050 Protect half of area
after 2050
Mt. Pleasant None Protection after 1990
Sullivans Island None None
High Scenario
Peninsula Protection after 2020 Protection after 2010
West Ashley/James Island Protection after 2020 Protect half of area
after 2030
Mt. Pleasant Protection after 2050 Protection after 1990
Sullivans Island None None

Note: In West Ashley/James Island, less protection is necessary if sea level
rise is anticipated, because more of the low-lying areas are subject to
an orderly abandonment. Our high scenario was called "medium" by Gibbs.

Source: Gibbs 1984.

High marsh is defined here by a narrow elevation range of 90 to 120 cm (3
to 4 ft) above mean sea level, and low marsh ranges from 45 to 90 cm (1.5 to
3.0 ft) above mean sea level. This delineation follows the results of
surveyed transects and species zonation described earlier. The lower limit of
the marsh was estimated from the typical transition to mud flats. Sheltered
tidal flats actually occur between mean low water and mean high water but were
found to be more common in Charleston in the elevation range of 0-46 cm (0-1.5
ft) above mean sea level. This somewhat arbitrary division was also based on
the contours available on USGS maps, which enabled estimates of zone areas

within the case study region.


-31-

Scenario Results

Based on the shore-protection alternatives for the five suburbs around
Charleston, we computed area distributions under the baseline, low, and high
scenarios. Figure 10 illustrates shore-protection scenarios and their effects
on the wetland transect. Our basic assumption was that the wetland habitats’
advance toward land ends at 200 cm (6.5 ft) above mean sea level. Dikes or
bulkheads would be constructed under certain protection scenarios at that
elevation on the date in question to prevent further inundation.

Because the results are fairly detailed for the five separate subareas and
four protection scenarios within the Charleston case study area, we have only
listed the overall changes in Tables 6 and 7 (complete protection and no
protection). Results by subarea for all four protection scenarios, given in
Appendix C, illustrate the variability of land, water, and wetland acreage
from one subarea to another. For example, the peninsula currently has a much
lower percentage of low marsh than all other areas. Tidal flat distribution
was also variable, ranging from 3.2 percent of the Mt. Pleasant zone to 8.6
percent of the Sullivans Island zone. The summary percentages given in Table
6 are appropriately weighted for the five subareas within the study area.

Table 6 lists the number of acres for each elevation zone in 1980
(existing) and for the baseline, low, and high scenarios with and without
structural protection by the year 2075. The percentage of the total study
area that a habitat covers is given in parentheses in Table 6 and graphically
presented in Figure 11. Table 6 indicates losses under all scenarios with no
protection for the four upper habitats and gains in area for tidal flats and
water areas. For example, without protection, highland would decrease from

46.6 percent of the total area in 1980 to 41.7 percent in 2075 under the


RANGE # EPA-2 BOAT RAMP RD., WEST OF MARSH ISLAND PARK
KEY: 16 MAY 1984

207
“7
a
= rot
&
ad ——-MHW
Zz OT ——_MLW
a
w
-10 + + +t + + + + + + + + + 4
0 50 100 150 8 ©200 250 300 350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima
Borrichia frutescens |RZYD 4,34(33)
Distichlis spicata
Juncus roemerianus
Limonium carolinianum
Polygonum setaceum
Salicornia virginica LEZ 3.38(31)
Spartina alterniflora | SRZGM 2.12(75)

Spartina patens
Spartina cynosuroides
Suaeda linearis

“Ly-


-19-

measuring the length of the transect that a species covered and dividing it by
the transect's total length, we computed percentages for the distribution of
each species along a transect.

National Geodetic Vertical Datum (NGVD) is the reference level we used for
all transects. This datum, derived from the so called 1929 measure of mean
sea level, is about 13 cm (0.4 ft) lower than the mean sea level of Charleston
today (Ebersole 1982) and therefore does not include sea level rise during the
past 50 years. We used the 1929 mean sea level because it is the most widely
applied measure of sea level and is used on most published maps. In addition,
there is no agreement on an absolute measure of today's sea level since it has

to be computed after a period of tidal records become available for analysis.

Results of Individual Transects

Table 2 summarizes the results of the twelve transects. It presents the
principal species observed along each transect, their "modal"--or most
common--elevations, the percentage of each transect they covered, and the
length of each transect. For example, in transect number 6, Borrichia
frutescens was found at a modal elevation of 118 cm (3.86 ft) above mean sea
level and covered 40 percent of the transect, or about 37 m (120 ft). ;

Because species often overlapped, the sums of the percentages exceed 100.
In addition, to omit any marginal plants that exist at transition zones, @
modal elevation differs slightly from the arithmetic or weighted mean.

Appendix A contains plots of the profiles of each transect, showing the

modal elevations of the substrate and zonation of plant species found there.


RANGE # EPA-5 BETWEEN B&RC TRACT "D" BLOCK “P" AND TRACT “D” BLOCK "Q”

KEY: 15 JUN 1984

207
a
a
2 tot
kK
> KR. —_MHW
. ——,
2 0+ ——MLW
aA
w
-10 + + + + t + + + + t + + 4
0 50 100 =150 8=©200 250 300 86350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima | _[E Ys | 2.59 3.20(61)
Borrichia frutescens - 3.60(14)
Distichlis spicata | SE 3.52(10)
Juncus roemerianus |i _&f 3.63(2)
Limonium carolinianum = |__ 3.14(68)
Polygonum setaceum
Salicornia virginica | [i 3.12(77)
Spartina alterniflora [9 __ EEN 2.55(78)

Spartina patens
Spartina cynosuroides
Suaeda linearis

-0S-


-60-

APPENDIX B._ Tide elevation probability distribution for Charleston (based
on data given in Ebersole, 1982).

aconmen,, Elevation NOevation/ Probability Grabapility
Tidal Range) (%)

5.2 1,000 0.00 0.00
5.0 0.962 0.01 0.01
4.8 0.923 0.02 0.03
4.6 0.885 0.03 0.06
44 0.846 0.08 0.14
4.2 0.808 0.13 0.27
4.0 0.769 0.26 0.53
3.8 0.731 0.44 0.97
3.6 0.692 0.72 1.69
3.4 0.654 1.01 2.70
MSHW 3.2 0.615 1.54 4.24
3.0 0.577 2.02 6.26
2.8 0.538 2.55 8.81
MHW 2.6 0.500 2.97 11.78
2.4 0.462 3.20 14,98
2.2 0.423 3.40 18.38
2.0 0.385 3.47 21.85
1.8 0.346 3.48 25.33
1.6 0.308 3.22 28.55
1.4 0.269 3.18 31.73
1.2 0.231 2.89 34,62
1.0 0.192 2.76 37.38
0.8 0.154 2.71 40.09
0.6 0.115 2.69 42.78
O.4 0.077 2.66 45.44
0.2 0.038 2.65 48 09
0.0 0.000 2.66 50.75
0,2 -0,038 2.67 53.42
-0.4 -0.077 2.80 56.22

-0.6 -0.115 2.94 59.16


di

SUMMARY

Recent reports by the National Academy of Sciences (NAS) and others have
concluded that increasing concentrations of carbon dioxide and other gases
released by human activities are likely to warm the earth a few degrees
Celsius in the next century. Such a warming could raise sea level by
expanding ocean water, melting mountain glaciers, and eventually causing polar
glaciers to slide into the oceans. Unfortunately, it is not yet possible to
accurately predict future sea level. Estimates for the year 2025 range from
five to twenty-one inches above current sea level, while estimates of the rise
by 2100 range from two to eleven feet. While the timing and magnitude of the
future rise in sea level is uncertain, there is an emerging scientific
consensus that a significant rise is likely.

Several issues must be resolved for society to rationally address the
possibility of a significant rise in sea level. Officials in coastal areas
making decisions about near-term projects with long lifetimes must determine
whether the risk of sea level rise justifies a shift to strategies that can
more successfully accommodate a rise in sea level. The research community
needs to decide whether to accelerate studies to more accurately project
future sea level. These decisions require assessments of the adequacy of
existing projections and of the value of developing better estimates and the
prospects for doing so.

These decisions also require an understanding of the consequences of sea
level rise and of the potential costs and benefits of adopting measures that
can forestall those consequences. To further this understanding, EPA has
initiated studies of the impacts of sea level rise on economic development in
Charleston, South Carolina, and Galveston, Texas; on municipal drainage
facilities; on salinity of surface and ground water; and on beach erosion on
coastal barrier islands.

This study examines the potential impact of alternative sea level rise
scenarios on wetlands in the area of Charleston, South Carolina. Because
economic development in coastal areas can have long-term impacts on the
viability of wetland ecosystems, and because sea level rise would cause these
ecosystems to migrate inland, wetland protection strategies constitute a class
of decisions that may depend on sea level rise. The purpose of this report is
to enhance our knowledge of the possible impacts of the rise in sea level
projected by previous reports. Because of the uncertainty associated with
these forecasts, additional analysis will be needed to determine what changes
in existing wetland protection strategies, if any, may be appropriate.

In this report, a team of wetland scientists describes surveys of twelve
wetland transects in the Charleston area and presents estimates of the ability
of these wetlands to keep pace with rising sea level. Also presented are
estimates of the shifts in wetland communities and of the net loss of marsh
acreage associated with three possible scenarios of sea level rise for the
year 2075, all of which are well within the range of estimates reported in
previous studies: (1) current trends of 1 foot per century along the Atlantic
Coast; (2) the NAS estimate of a 2-1/3 foot global rise in sea level; and (3)
a high scenario of a 4-1/2 foot global rise. Because sea level is rising
about 8 inches per century more rapidly along the Atlantic coast than
worldwide, these scenarios imply rises of 2.9 and 5.2 feet, respectively, by
2075 in the Charleston area.


-=38-

probability" curves, based on detailed statistics of Atlantic Coast water
levels given in Ebersole (1982) and summarized in Appendix B. The graph of
Figure 12A gives the probability of various water levels for Charleston. In
Figure 12B, the data have been normalized for the mean tide range of 5.2 ft in
Charleston and given as a cumulative probability distribution. These graphs
are applicable to much of the southeastern U.S. coast by substituting
different tide ranges. Each graph provides a measure of the duration of time
over the year that various wetland elevations are under water.

In the case of Salicornia virginica (+3.16 ft MSL for Charleston), the
cumulative frequency of flooding is approximately 4 percent (Figure 12B and
Appendix B). If one wanted to apply these results for an area with a
different tide range but similar species occurrence, such as Sapelo Island
(Georgia), the flooding frequency for S. virginica could be used to estimate
its modal elevation at the locality. With a mean tide range of 8.5 ft at
Sapelo, S. virginica is likely to occur around +5.3 ft MSL (based on
substitution of the tide range in Figure 12B). This procedure can be applied
for other southeastern U.S. marshes as a preliminary estimate of local modal
elevations.

We do not consider elevation results for the transects to be definitive
because of the relatively small sample size. However, the results are
sufficiently indicative of actual trends to allow scenario modeling. With the
tide-probability curves presented, it should be possible to check these
results against other areas with similar climatic patterns, but different tide

ranges.


-36-

could be related to elevation using some of the criteria provided in this
report.

Another problem that remains with this type of study is the frame of
reference for mean sea level. For practical reasons, mean sea level for a
standard period (20 years generally) cannot be computed until after the period
ends. Therefore, earlier mean sea levels, such as the NGVD of 1929, are
used. But sea level has risen about 15 cm since then. If everyone uses the
same reference plane for present and future conditions, the problem may be
minor. But it does not allow us to determine modal elevations with respect to
today's sea level. The transects surveyed for the present study suggest that
S. alterniflora (low marsh) grows optimally at an elevation of 2.45 ft above
mean sea level, close to mean high water (U.S. Department of Commerce 1981).
Compared with today's mean sea level in Charleston, S. alterniflora probably
prefers to grow as much as 0.5 ft below actual mean high water, which may
confuse the reader who forgets that the report uses 1929 sea level.

The basic criteria for delineating elevations of various wetland habitats
in this study can be easily tested in other areas. By applying normalized
flood probabilities (similar to those depicted in Figure 12), it will be
possible to measure marsh transects in other tide-range areas and relate them

to the results for Charleston.

Normalized Elevations

The absolute modal elevation for each species is site-specific for
Charleston. Presuming that the zonation is controlled primarily by tidal
inundation, it is possible to normalize the data for other tide ranges based

on frequency curves for each water level. Figure 12 contains two such "tide


APPENDIX C. Area distribution (%) by elevation zone for each of the five principal communities or land divisions in the Charleston case study area
under various sea level rise and mitigation scenarios.

NO PROTECTION LOW SCENARIO. HIGH SCENARIO
with Without With Without
Low Haigh Anticipation Anticipation Anticipation anticipation
Existing Basel ine Scenario Scenario Protection Protection Protection Protection Protection Protection
ZONE (1980) (2075) (2075) (2075) @ 1980 2030 @2050 @1980 @2010 @2020

CHARLESTON PENINSULA -- 14,000 ACRES -- % AREA @ 2075

Highland 53-4 52.3 51.5 47.9 53.4 52.4 51.9 53.4 51.6 51.0
Transition 3.7 3.6 3eL 3eL 1.2 2.2 2.7 0.0 0.0 0.0
Hagh Marsh Leb 1.4 2.0 2k 2.0 2.0 2.0 0.0 1.5 2-1
Low Marsh 2.5 21 1.9 2.2 1.9 1.9 1.9 1.9 2.2 2.2
Tidal Flat Ta 4.6 3.2 2k 3.2 3.2 3.2 2a. 261 2eL
water 32.0 36.0 38.3 42.6 38.3 38.3 38.3 42.6 42.6 42.6
TOTALS 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
WEST ASHLEY -~ 9,500 ACRES -- % AREA @ 2075
Haghland 29.5 29.0 28.6 26.4 29.5 28.55 28.60 29.5 27.15 28.2
Transition 21 1.9 1.8 21 0.9 1-85 1-80 0.0 1.35 0.3
High Marsh 7.4 2.2 0.9 0.9 0.9 0.9 0.9 0.0 0.9 0.9
Low Marsh 16.8 14.3 9.9 0.9 9.9 9.9 9.9 0.8 0.9 0.9
Tidal Flat 7.4 12.2 15.6 3.5 15.6 15.6 15.6 3.5 3.5 3.5
water 36.8 40.4 43.2 66.2 43.2 43.2 43.2 66.2 66.2 66.2
TOTALS 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
MT. PLEASANT: ‘TOTAL ACRES = 15,750 -- % AREA @ 2075
Highland 53.3 52-1 51-3 47.7 53.3 53.1 51.3 53.3 52.7 49.2
Transition 3.8 3.8 3.7 3.8 1.7 1.9 3.7 0.0 0.0 2.3
High Marsh ToL 2.6 1.6 1.5 1.6 1.6 1.6 0.0 0.3 1.5
Low Marsh 16.7 13.5 9.8 2.3 2.8 9.8 9.8 2.0 2.3 2.3
Tidal Flat 3.2 10.4 14.9 3.9 14.9 14.9 14.9 3.9 3.9 3.9
water 15.9 17.6 18.7 40.8 18.7 18.7 18.7 40.8 40.8 40.8

TOTALS 100.0 100.0 100.0 100.0 100-0 100.0 100.0 100.0 100.0 100.0

-49-


frutescens (sea ox-eye). Teal (1958) reports that Juncus marsh tends to be
found at a slightly higher elevation than the Salicornia/Distichlis marsh.
The high marsh can also be distinguished from low marsh on the basis of
sediment type, compaction, and water content. High-marsh substrate tends to
be firmer and dryer and to have a higher sand content. Low-marsh substrate
seldom has more than 10 percent sand (except where barrier-island washover

deposits introduce an "

"artificial" supply) and is often composed of very soft
mud. Infrequent flooding, prolonged drying conditions, and irregular rainfall
within the high marsh also produce wide variations in salinity. In some
cases, salt pannes form, creating barren zones. But at the other extreme,
frequent freshwater runoff may allow less salt-tolerant species, such as
cattails, to flourish close to the salt-tolerant vegetation. These factors
contribute to species diversity in the transition zone that lies between S.
alterniflora and terrestrial vegetation.

By most reports, low marsh dominates the intertidal areas along the
southeast (Turner 1976), but the exact breakdown can vary considerably from
place to place. Wilson (1962) reported S. alterniflora composes up to 28
percent of the marsh in North Carolina, whereas Gallagher, Reimold, and
Thompson (1972) report for one estuary in Georgia that the same species covers
94 percent of the "marsh" area. Low marsh is thought by many to have a
substantially higher rate of primary productivity than high marsh (Turner
1976). Data presented in Odum and Fanning (1973) for Georgia marshes support
this notion. However, Nixon (1982) presents data for New England marshes that
indicate above-ground biomass production in high marshes comparable to that of
low marshes. Some data from Gulf Coast marshes also support this view

(Pendleton 1984).


RANGE # EPA-4 LOT 177, MARSH HAWK LANE
KEY: 16 MAY 1984

207
>
a
= ot
t
zy mew
w
-10 + + — + t + + + + t + + 1
Q 50 100 150 200 250 300 350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima
Borrichia frutescens _ i 3.48(7)
Distichlis spicata
Juncus roemerianus |i 5,34(5)
Limonium carolinianum i 3.76(<1)
Polygonum setaceum
Salicornia virginica | <—_—__s 3.49(37)
Spartina alterniflora |___ ES CE SU 2.05(85)
Spartina patens
Spartina cynosuroides 3.35 0.96 2.51(72)
Suaeda linearis

-6>-


RANGE # EPA-10 790 WOODWARD RD., RUNNING WEST
KEY: 16 JUNE 1984

207

cary

o”

= i0+

E

ee MHW
zB ot MLW
a

wa

“10 , , ' , , , , , ,

Batis maritima

n n n 4
+ t t t T t T T T iJ

0 50 100 «150 6 =. 200 250 300 350 400 450 500 550 600 650

DISTANCE (FT.)

Borrichia frutescens
Distichlis spicata
Juncus roemerianus

Limonium carolinianum
Polygonum setaceum
Salicornia virginica

| 4.94(1)
| 3.80(9)
5.72(<1)

Spartina alterniflora
Spartina patens

|  3.50(99)

Spartina cynosuroides

Suaeda linearis

-SS-


elevation than at wetland elevation. Therefore, significant changes in the
habitats and a reduction in the area they cover will generally occur with
accelerated sea level rise. Moreover, increasing development along the coast
is likely to block much of the natural adjustment in some areas.

Louisiana is an extreme example. Human interference with natural sediment
processes and relative sea level rise are resulting in the drowning of 100 sq
km of wetlands every year (Gagliano, Meyer Arendt, and Wicker 1981; Nummedal
1982). There is virtually no ground to which the wetlands can migrate. Thus,
wetlands are converting to open water; high-marsh zones are being replaced by
low marsh, or tidal flats; and saltwater intrusion is converting freshwater

swamps and marsh to brackish marsh and open water.

COASTAL HABITATS OF THE CHARLESTON STUDY AREA

As shown in Figure 1, the case study area, stretching across 45,500 acres,
is separated by the three major tidal rivers that converge at the port city:
the Ashley, Cooper, and Wando Rivers. In addition, it covers five land areas:

e West Ashley, which is primarily a low-density
residential area with expansive boundary marsh;

. Charleston Peninsula, which contains the bulkheaded
historic district built partly on landfill;

¢ Daniel Island, which is an artificially embanked
dredge spoil island;

e Mount Pleasant, which derives geologically from
ancient barrier island deposits oriented parallel to the
coast; and

* Sullivans Island, which is an accreting barrier
island at the harbor entrance.


POTENTIAL IMPACTS OF SEA LEVEL RISE ON WETLANDS
AROUND CHARLESTON, SOUTH CAROLINA

Prepared by
Timothy W. Kana, Bart J. Baca
and Mark L. Williams

Coastal Science & Engineering, Inc.
P.O. Box 8056
Columbia, South Carolina 29202

This document has been reviewed in accordance with the U.S.
Environmental Protection Agency's peer and administrative
review policies and approved for publication. Mention of
trade names or commercial products does not constitute
endorsement or recommendation for use. Please send comments
to James G. Titus (PM-220), Strategic Studies Staff, U.S.
Environmental Protection Agency, Washington, D.C. 20460.


-29-

2075 WETLANDS ——>

2075 +6.5
NO 198
<= 0 +6.5,
PROTECTION “SEs
Epjr----... 2075 SUBSTRATE
CUENTA
HOW S85 Zo7Ey-~--------
WAL 2075 WETLANDS —>

\ 2076 +6.5

2020 +6.5

= 1980 +6.5

PROTECTION =
AT 2020 Tate 2078 SUBSTRATE
10 ao
i
1980 WETLANDS —» \
2075 WETLANDS —»
\ WALL 2075 +6.5
—
= 1980 +6.5
ao
PROTECTION ee 2075 SUBSTRATE
SEDIMENTATION 7iggemmscecce

NO WETLANDS
_ /ETLANDS-+¥«—1980 WETLANDS —> \

FIGURE 10. If people build walls to protect property from rising sea level,
the marsh will be squeezed between the wall and the sea. Sketches
show only the upper part of the wetlands which would be affected
by shore-protection structures. Mean sea level is off the diagram
to the right.


-17-

WETLANDS TRANSECTS: METHOD AND RESULTS

To determine how an accelerated rise in sea level would affect the
wetlands of Charleston, one needs to know the portions of land at particular
elevations and the plant species found at those elevations. To characterize
the study area, we randomly selected and analyzed twelve transects (sample
cross sections, each running along a line extending from the upland to the
water). This section explains how the data from each transect was collected
and analyzed, presents the results from each transect, and shows how we

created a composite transect based on those results.

Data Collection and Analysis

For budgetary and logistical reasons, we had to use representative
transects near, but not necessarily within, the study area. For example, a
limiting criterion was nearness to convenient places where reliable
elevations, or benchmarks, had already been established. The marshes behind
Kiawah Island and Isle of Palms are similar to the marshes behind Sullivans
Island, but are more accessible. As Figure 8 shows, all the transects were
within 20 km (12 mi) of the study area.

Each transect began at a benchmark located on high ground near a marsh's
boundary, and ended at a tidal creek or mud flat, or after covering 300 m
(1,000 f£t)--whichever came first. The length of the transects was limited
because of the difficulty of wading through very soft muds. Although this
procedure may have biased the sample somewhat, logistics prevented a more

rigorous survey.


Library of Congress Cataloging-in-Publication Data
Kana, Timothy W., 1949-

Potential impacts of sea level rise on wetlands
around Charleston, South Carolina.

Bibliography: p.
1, Wetland conservation-~South Carolina--Charleston.
2. Wetlands--South Carolina--Charleston. 3. Sea Level--
South Carolina--Charleston. 4, Greenhouse effect, Atmospheric.
|, Baca, Bart J. Il. Williams, Mark L. Ill. Title.
QH76.5.S67K36 1986 333,91'815'09757915  86-29309


-41-

REFERENCES

Barth, M.C., and J.G. Titus (Eds.), 1984. Greenhouse Effect and Sea Level
Rise. Van Nostrand Reinhold Co., New York, N.Y., 325 pp.

Boesch, D.F., D. Levin, D. Nummedal, and K. Bowles, 1983. Subsidence in
Coastal Louisiana: Cases, Rates and Effects on Wetlands. U.S. Fish and
Wildlife Serv., Washington, D.C., FWS/OBS-83/26, 30 pp.

DeLaune, R.D., C.J. Smith, and W.H. Patrick, Jr., 1983. "Relationship of
marsh elevation, redox potential, and sulfide to Spartina alterniflora
productivity." Soil Science Amer. Jour., Vol. 47, pp. 930-935.

Duc, A.W., 1981. "Back barrier stratigraphy of Kiawah Island, South Carolina."
Ph.D. Dissertation, Geol. Dept., University of South Carolina, Columbia,
253 pp.

Ebersole, B.A., 1982. Atlantic Coast Water-level Climate. WES Rept. 7, U.S.
Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss.,
498 pp.

Gagliano, S.M., 1984. Independent reviews (comments of Sherwood Gagliano).
In M.C. Barth and J.G. Titus (Eds.), Greenhouse Effect and Sea Level
Rise. Van Nostrand Reinhold Co., New York, N.Y., Chap. 10, pp. 296-300.

Gagliano, S.M., K.J. Meyer Arendt, and K.M. Wicker, 1981. "Land loss in the
Mississippi deltaic plain." In Trans. 3lst Ann. Mtg., Gulf Coast Assoc.
Geol. Soc. (GCAGS), Corpus Christi, Texas, pp. 293-300.

Gallagher, J.L., R.J. Reimold, and D.E. Thompson, 1972. "Remote sensing and
salt marsh productivity." In Proc. 38th Ann. Mtg. Amer. Soc.
Photogrammetry. Washington, D.C., pp. 477-488.

Gibbs, M.J., 1984. "Economic analysis of sea level rise: methods and
results." In M.C. Barth and J.G. Titus (Eds.), Greenhouse Effect and Sea
Level Rise. Van Nostrand Reinhold Co., New York, N.Y., Chap. 7, pp.
215-251.

Hatton, R.S., R.D. DeLaune, and W.H. Patrick, 1983. "Sedimentation, accretion
and subsidence in marshes of Barataria Basin, Louisiana." Limnol. and
Oceanogr., Vol. 28, pp. 494-502.

Hayes, M.O., and T.W. Kana (Eds.), 1976. Terrigenous Clastic Depositional
Environments, Tech. Rept. NO. 11-CRD. Coastal Research Division, Dept.
Geol., Univ. South Carolina, 306 pp.

Hicks, S.D., H.A. DeBaugh, and L.E. Hickman, 1983. Sea Level Variations for
the United States 1855-1980. National Ocean Service, U.S. Department of
Commerce, Rockville, Maryland.



-25-

habitat in each of the divisions of the Charleston area (see Figure 1). For
budgetary reasons, we could not rigorously calculate areas using a
computerized planimeter. This level of precision would be questionable
anyway, in light of the imprecision of USGS topographic maps in delineating
marshes and tidal flats near mean water levels.

Our results were graphically determined and spot-checked by a second
investigator to ensure they were consistent to within +15 percent for each
measurement. Thus, the error limits for the overall study area are estimated
to be a maximum of +15 percent by subenvironment. (Because the standard
error of a sum is less than the sum of individual standard errors, the errors
are likely to be less. Unfortunately, we had no way of rigorously testing
these results within the time and budget constraints of the project.)

Tidal-flat areas were estimated using aerial photos and shaded patterns
shown on USGS topographic sheets. The marsh was initially lumped together
(high and low marsh) to determine representative areas for each Charleston
community. The total number of acres for this zone was divided into high- and
low-marsh areas by applying the typical percentage of each along the composite
transect (70 percent low marsh and 30 percent high marsh). The transition

zone areas were estimated from the digitized computer maps.

WETLAND SCENARIOS FOR THE CHARLESTON AREA:
MODELING AND RESULTS

After establishing the basic relationships among elevation, wetland
habitats, and occurrence of species for Charleston, the next steps in our

analysis were to develop a conceptual model for changes in saltwater wetlands


-34-

high scenario. This represents a loss of over 2,200 acres or 10 percent of
the present highland area. Land that is now terrestrial would be transformed
into transition-zone or high-marsh habitats a century from now. Under the
2075 high scenario with no protection, high and low marsh, combined, would
decrease from 7,700 acres to 1,535 acres--a reduction of almost 80 percent.
While highland and marsh areas would decrease under the no-protection
scenarios, water areas would increase dramatically--from 27.4 percent to as
much as 48.7 percent--under the high scenario of 2075.

With structural protection implemented at different times for each
community (see Table 5), highland areas would be maintained at a constant
acreage, but transition and high-marsh habitats would be completely eliminated
by 2075 under the high scenario (because of the lack of area to accommodate a
landward shift). Total marsh acreage would decrease from 7,700 acres to 3,925
acres (2075 low scenario), or 750 acres (2075 high scenario), under the
assumed mitigation in Table 5.

The net change in areas under the various scenarios listed in Table 7
indicates that all habitats would undergo significant alteration. Even under
the baseline scenario, which assumes historical rates of sea level rise, 20-35
percent losses of representative marsh areas are expected by 2075. Protection
under the low scenario (as outlined by Gibbs 1984) would have virtually no
effect on high or low marsh coverage; but it would cause a substantially
increased loss of transition wetlands. Under the high scenario with
protection, highland would be saved at the expense of all transition and high
marsh areas and almost 90 percent of the low marsh. Even under the low
scenario, sea level rise would become the dominant cause of wetland loss in

the Charleston area. Although a substantial amount of marsh was filled as the


TABLE 2

MODAL ELEVATIONS AND PERCENTAGE OF TRANSECT COVERED BY PRINCIPAL SPECIES
(in feet at mean sea level)

Modal Elevations (percent of transect covered)

SPECIES 1 2 3 4 5 6 7 8 9 10 u 12
Batis maritima - - 3.13¢ 4) - 3.20(61)  3,04(14) : - - - - -
Borrichia frutescens 3.90(1) 4,983) 3.487) 3.6014) 3.86(40) 3.176) 3.82(27)3.54(29) 4.941) 4.209) -
Distichlis spicata - - - - 3.52(10) - 3.20(4) 3.70(23) 3.2915) 3.809) 3, 95( 35) 3.54(7)
Juncus roemerianus 3..40(1) - - 5.345) 3.63(2)3..48(7) - - - - 5.45(1) -
Limonium carol inianum 3..27( 80) - 3.07(1) 3.762) 3.1468) 3.04(14) 3,014) 3.8928) 4. 35(1) - - -
Polygonum setaceum - - - - - - - - - 5.720.) 5.4501) 3.3207)
Salicornia virginica 3.42(1) -3,38(31)-3.06(9) 3. 4937) -3,.12(77) 3, 3034) 3.109) 3.30018) 3.1431) - - -
Spartina alterniflora 3.271) -2.12(75) 2,.45(99) 2,.05(85) 2.55 78) -(11)—-1.95(62) 2.79(57) 2.7270) -3.50(99) 3.40097) 2.65( 97)
Spartina patens - - 5.35(1) - - - - - - - - -
Spartina cynosuroides - - - 2.51( 72) - - - - - - - -
Suaeda linearis - - - - - 3.61(34) 3.1104) 4,00(7) 3.2205) - - -
Transect Length (in 189 51 440 353 933 300 421 387 232 700 588 402

feet)

-07-


TABLE 1
SCENARIOS OF FUTURE SEA LEVEL RISE: 1980-2100
(centimeters)
2000 2025 2050 2075 2080 2100
Current Trends in
Sea Level Rise
Global 2.0-3.0 4.5°6.8 7.0-10.5 9.5-14.3 10-15 12.0-18.0
East Coast 6 13.5 21 28.5 30 36
EPA Scenarios -
(Global Sea
Level Rise)
High 17.1 54.9 116.7 211.5 - 345.0
Mid-range high 13.2 39.3 78.9 136.8 - 216.6
Mid-range low 8.8 26.2 52.6 91.2 - 144.4
Low 4.8 13.0 23.0 38.0 - 56.2
NAS Estimate
(Global Sea
Level Rise--
excluding
Antarctic
contribution) - - - - 70.0 -
SOURCES: (1) John S. Hoffman, D. Keyes, and J.G. Titus, Projecting Future Sea

immediate consequences would be intertidal wetlands.

Level Rise, U.S. EPA, 1983; (2) R. Revelle, "Probable Future Change

in Sea Level Resulting from Increased Atmospheric Carbon Dioxide,"
in Changing Climate, Washington, D.C.: National Academy Press,
1983; (3) Hicks, S.D., H.A. DeBaugh, and L.E. Hickman, Sea Level
Variations for the United States 1855-1980, Rockville, Maryland:
National Ocean Service, 1983.

Lying between the sea

and the land, this zone will experience the direct effects of changing sea

levels, tidal inundation, and storm surges.

The intertidal wetlands contain productive habitats, including marshes,

tidal flats, and beaches, which are essential to estuarine food webs.

The

distribution of the wetlands is sensitively balanced for existing tidal

conditions, wave energy, daily flooding duration, sedimentation rates (and


RANGE # EPA-12 END OF OAKDALE ST.

KEY: 16 JUNE 1984

207
=
a
= 10+
as
£& oN MHW
= ot > MLW
Pl
w
-10 +t +t t + + + + + + t + + 4
0 50 100 150 200 250 300 350 400 450 500 550 600 650
DISTANCE (FT.)
Batis maritima
Borrichia frutescens
Distichlis spicata | Mil 3.32(7)
Juncus roemerianus
Limonium carolinianum
Polygonum setaceum | Sl 3.32(7)
Salicornia virginica
Spartina alterniflora | __ S 2.65(97)

Spartina patens
Spartina cynosuroides
Suaeda linearis

“15+


-42-

Hoffman, J.S., D. Keyes, and J.G. Titus, 1983. Projecting Future Sea Level
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PP-


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APPENDIX A
WETLANDS TRANSECTS AND DISTRIBUTION OF SPECIES

Surveyed wetlands transects and distribution of species for 12 profiles in
the general vicinity of Charleston, South Carolina (field notes are available
at RPI Coastal Science & Engineering).


Potential Transformation of Wetlands

The late Holocene (last several thousand years) trend has been one of
gradual infilling and loss of water areas (Schubel 1972). During the past
century, however, sedimentation and peat formation have kept pace with rising
sea level over much of the East Coast (e.g., Ward and Domeracki 1978; Duc
1981; Boesch et al. 1983). Thus, apart from the filling necessary to build
the city of Charleston, the zonation of wetland habitats has remained fairly
constant. Changes in the rate of sea level rise or sedimentation, however,
would disrupt the present ecological balance.

If sediment is deposited more rapidly, low marsh will flood less
frequently and become high marsh or upper transition wetlands, which seems to
be occurring at the mouths of some estuaries where sediment is plentiful. The
subtropical climate of the southeastern United States produces high weathering
rates, which provide large fluxes of sediment to the coastal area. Excess
supplies of sediment trapped in estuaries have virtually buried wetlands
around portions of the Chesapeake, such as the Gunpowder River, where a
colonial port is now landlocked.

If sea level rises more rapidly in the future, increased flooding may
cause marginal zones close to present low tide to be under water too long each
day to allow marshes to flourish. To maintain the distribution of their
habitats, wetlands must shift along the coastal profile--moving upward, to
keep pace with rising sea levels (unless sedimentation rates are high). Total
marsh acreage can only remain constant if slopes and substrate are uniform
above and below the wetlands, and inundation is unimpeded by human activities
such as the construction of bulkheads. Titus, Henderson, and Teal (1984),

however, point out that there is generally less land immediately above wetland