Coastal Changes due to

Sea-Level Rise

One of the most important problems

in applied coastal geology today is determining the physical response of the

coastline to sea-level rise. Prediction of

shoreline retreat and land loss rates is

critical to planning future coastal management strategies. To date, long-term

planning for the Nation’s shoreline has

been done piecemeal, if at all (National

Research Council, 1990, 1995). Consequently, entire communities are being

developed without adequate consideration of the potential costs of erosion,

flooding, and storm damage related to

sea-level rise.

Recent estimates based on global

climate models (Wigley and Raper, 1992)

suggest an increase in sea level of

between 15 and 95 centimeters (cm)

by 2100, with a “best guess” of 50 cm

(IPCC, 1995). This is more than double

the rate of sea-level rise for the past century (Douglas, 1997; Peltier and Jiang,

1997). Thus, sea-level rise will have a

large sustained impact on coastal evolution in the future and will occur as both

population and infrastructure in coastal

regions are projected to increase.

Variables Affecting Coastal

Vulnerability

Predicting future coastal evolution

and vulnerability to change is difficult

because many factors are involved. No

standard method is used by scientists to

predict coastal change. In order to

address these problems, the U.S. Geological Survey (USGS) is implementing a

fairly simple classification of the relative

vulnerability of different U.S. coastal

environments to future rises in sea level

(fig. 1). The vulnerability classification is

based on the relative contributions and

interactions of six variables:

National Assessment of Coastal Vulnerability to

Future Sea-Level Rise

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–076–00

June 2000

77° 30' 75° 72° 30'

40°

35°

30°

25°

45°

COASTAL VULNERABILITY INDEX

Low

Moderate

High

Very High

RISK RANKING

EXPLANATION

ATLANTIC OCEAN

200 100 0 200 km

Cape Cod

Cape Hatteras

Cape Canaveral

Maine

Massachusetts

Connecticut RI

New

York

New

Jersey

Virginia

North Carolina

South Carolina

Florida

DE

Maryland

New

Hampshire

Georgia

Ontario

Quebec

Vermont

Pennsylvania

West Virginia

Ohio

Canada

Figure 1. Map of the Coastal Vulnerability Index (CVI) for the U.S. East Coast showing the relative vulnerability of the coast to changes due to future rises in sea level.

Areas along the coast are assigned a ranking from low to very high risk, based on

the analysis of physical variables that contribute to coastal change. From Thieler

and Hammar-Klose (2000).
g, 

inundation hazards.

2. Wave height, which is linked to inundation hazards.

3. Coastal slope (steepness or flatness of

the coastal region), which is linked to

the susceptibility of a coast to inundation by flooding and to the rapidity of

shoreline retreat.

4. Shoreline erosion rates, which indicate how fast a section of shoreline

has been eroding.

5. Geomorphology, which indicates the

relative erodibility of a section of

shoreline.

6. Historical rates of relative sea-level

rise, which correspond to how the

global (eustatic) sea-level rise and

local tectonic processes (land motion

such as uplift or subsidence) have

affected a section of shoreline.

Coastal Vulnerability Index

The USGS rating system classifies

the data variables according to risk. A

mathematical formula allows scientists

to relate the different types of data to

each other and to calculate an index

value. This coastal vulnerability index

(CVI) value yields a relative ranking of

the possibility that physical change will

occur along the shoreline as sea level

rises. For example, along the U.S. East

Coast, high-vulnerability areas are typically barrier islands having small tidal

ranges, large waves, a low coastal slope,

and high historical rates of sea-level

rise, such as the Mid-Atlantic Coast

(fig. 2). Coasts having rocky cliffs

(steep coastal slopes), large tidal ranges,

and low historical rates of sea-level rise,

such as most of the Maine shoreline

(fig. 3), are represented as least vulnerable and have a low CVI value. The distribution of vulnerability ranges for the

U.S. East Coast is shown in figure 4.

National Assessment

The USGS is conducting a national

assessment of coastal vulnerability to

future sea-level rise by using simple,

objective criteria; the assessment for the

U.S. East Coast has been released (Thieler and Hammar-Klose, 2000). More

detailed investigations of the impact of

sea-level rise are needed in order to forecast coastal change with a degree of certainty that is useful for the management

of coastal resources.

References

Douglas, B.C., 1997, Global sea rise; A redetermination: Surveys in Geophysics, v.

18, p. 279–292.

IPCC, 1995, IPCC second assessment—Climate change 1995; A report of the Intergovernmental Panel on Climate Change,

IPCC: Geneva, Switzerland, 64 p.

National Research Council, 1990, Managing

coastal erosion: Washington, D.C.,

National Academy Press, 163 p.

——1995, Beach nourishment and protection: Washington, D.C., National Academy Press, 334 p.

Peltier, W.R., and Jiang, X., 1997, Mantle

viscosity, glacial isostatic adjustment

and the eustatic level of the sea: Surveys

in Geophysics, v. 18, p. 239–277.

Thieler, E.R., and Hammar-Klose, E.S.,

2000, National assessment of coastal

vulnerability to sea-level rise; Preliminary results for the U.S. Atlantic Coast:

U.S. Geological Survey Open-File

Report 99-593, 1 sheet. (Also available

on the web at

http://woodshole.er.usgs.gov/epubs/open

files/ofr99-593/.)

Wigley, T.M.L., and Raper, S.C.B., 1992,

Implications for climate and sea level of

revised IPCC emissions scenarios:

Nature, v. 357, p. 293–300.

For additional information, please

contact:

E. Robert Thieler

U.S. Geological Survey

Woods Hole Field Center

384 Woods Hole Road

Woods Hole, MA 02543–1598

E-mail: rthieler@usgs.gov

Telephone: 508–457–2350

Figure 2. Rodanthe, North Carolina,

during the “Halloween Storm” of

1991. The low coastal slope, barrier

island morphology, small tidal

range, and large waves make this

high-vulnerability area particularly

susceptible to flooding and overwash. Photograph courtesy of

H.C. Miller, U.S. Army Corps of

Engineers.

Figure 3. West Quoddy Head,

Washington County, Maine. The

rocky cliffs (steep slopes) of the

Maine coast and the large tidal

range lead to an index designation

of very low risk. Photograph courtesy of the Maine Department of

Conservation.

Percentage of the U.S. East Coast

shoreline in each risk category

0 5 10 15 20 25 30 35

Very High

High

Moderate

Low 6,630 km

5,332 km

5,094 km

6,328 km

Figure 4. A total of 23,384 kilometers of shoreline is ranked for the

U.S. East Coast. Of this total, 27

percent of the mapped shoreline

is classified as being at very high

risk due to future sea-level rise,

22 percent is classified as high

risk, 23 percent as moderate risk,

and 28 percent as low risk.

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