Predicting Coastal Flooding and Wetland Loss

These models augment field and

laboratory studies by integrating our

knowledge and understanding of

natural and physical systems into a

single framework. We examined the

impacts of sea-level rise scenarios on

wetland habitats of federal parks and

refuges along the Gulf of Mexico. A

geographic information system (GIS)-

based simulation model was developed

to predict ecosystem response to

changing environmental conditions for

wetland areas of the Big Bend region in

northwest Florida. The model describes plant community sensitivity to

hydrologic conditions linked with a

GIS database of site characteristics,

including habitat type, elevation, soils,

and land use.

St. Marks Refuge

This regional model application

includes the aquatic and terrestrial

habitats of St. Marks National Wildlife

Refuge (Fig. 1), a 65,000-acre refuge 20

mi south of Tallahassee covering parts

of Wakulla, Jefferson, and Taylor

counties. The refuge borders

Apalachee Bay on the south,

Ochlockonee Bay on the west, and

Aucilla River on the east. The landscape is characterized by a relatively

low elevational gradient intersected by

several rivers and a number of freshwater springs and intertidal creeks.

Upland pine sandhills drain into wet

pine flatwoods and hardwood swamps

within the freshwater zone and into

tidal salt marsh and mudflats at bays

edge. Seagrass beds are abundant

throughout Apalachee Bay, a shallow,

low energy system open to the Gulf of

Mexico. Elevations of these major

habitat types range from below sea

level for seagrass; 0-2 ft above mean

sea level for salt and fresh marsh (Fig.

2); 1-4 ft above mean sea level for

lowland pine, palm, hardwood hammocks; 4-6 ft above mean sea level for

bottomland hardwood and pine

flatwoods; and 6-40 ft above mean sea

level for pine sandhill and oak associations in the higher elevations.

Field Surveys

Land elevation and tidal inundation

are key factors controlling habitat type

and distribution in this coastal

environment. We conducted plot and

transect surveys to derive a conceptual model of community types and

succession under different environmental conditions and disturbance

regimes. Vegetation descriptions and

elevation data were collected across

the forest-marsh transitional zone.

The significance of our field study

was that we could plot elevation

Fig. 1. Predicted shoreline changes by 2100 showing coastal inundation of the

St. Marks NWR area, Florida, for low, mid (best), and high projections of sea-level

rise, adopted by the Intergovernmental Panel on Climate Change.

The southeastern coastal region encompasses vast areas of wetland habitat important to wildlife and other economically

valuable natural resources. Located on the interface between sea and land, these wetland habitats are affected by both sealevel rise and hurricanes, and possibly by hydroperiod associated with regional climatic shifts. Increased sea level is expected

to accompany global warming because of higher sea temperatures and ice melt. To help determine the effects of sea-level rise

on these wetlands, USGS scientists created computer models of coastal flooding and wetland loss.

95 cm - high estimate

50 cm - mid (best) estimate

15 cm - low estimate

Based on IPCC sea-level rise model (1995)

Red line approximates present shoreline
15 cm 50 cm 95 cm

0

5

10

15

-5

-10

-15

Water Marsh Forest

Sea-level rise

Low projection Mid projection High projection

Thousands (ha)

Wetland gain

Wetland loss

increase of 0.95 m (high) over the next

century. The model predicts an

effective migration of emergent marsh

into forested zones, but it also indicates an overall net loss of terrestrial

habitat to an open water environment.

For more information, contact

Thomas W. Doyle

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

318-266-8647

tom_doyle@usgs.gov

http://www.nwrc.gov

inundation over space and time for

projections of sea-level rise. Model

simulations were generated to predict a

likelihood index of habitat change and

conversion under flooded conditions

for different scenarios of sea-level rise.

Sea-level rise was simulated based

on a series of case projections (low,

mid, and high) for the years 2000 to

2100 adopted by the Intergovernmental

Panel on Climate Change (1995; Fig. 3).

At these levels, major portions of the

coastal zone in this region would be

permanently inundated over the next

century, bringing about a migration

upslope of marsh habitat and a loss in

the total area and proportion of forest

habitats.

Results

Model results show that there is a

large landbase that will be converted

from marsh to open water and forest to

marsh. However, due to the slope of

the landform, coastal marsh is predicted to increase slightly in land cover

as it migrates upslope and replaces

existing forest habitat. A significant

portion of coastal pinelands that stand

at or below the 1-m contour would be

lost given a projected sea-level

Fig. 3. Predicted changes of net loss and/or gain of open water, emergent marsh,

and forest for low, mid, and high sea-level rise by the year 2100.

U.S. Department of the Interior

U.S. Geological Survey

USGS FS 094-97

June 1997

Fig. 2. USGS researcher showing the

height of marsh surface above

surrounding water.

contours within the needlerush marsh

landscape by identifying the vegetation on aerial photography and even

satellite imagery. Sand flats occupied

by turtleweed and glasswort are highly

visible on remote imagery and occur at

the same elevation as mean high water

(about 36-40 cm above mean sea level)

from predicted tide tables for the area.

Also highly visible is the distinct

marsh-forest boundary occurring near

the elevation of the highest predicted

tides from the same tide tables (60-70

cm above mean sea level). These

vegetation boundaries provided proxy

elevation contours that were used to

estimate the slope and produce an

elevation gradient model of the study

area. The probability of species and

plant communities tolerating coastal

flooding was also determined from

these field surveys and used to predict

changes in habitats.

Model Development

A digital elevation model of the

study area was constructed to track

the process and pattern of coastal