Coastal Vulnerability to Sea-Level Rise:
A Preliminary Database for the U.S. Atlantic, Pacific and Gulf of Mexico Coasts
U.S. Geological Survey Digital Data Series - 68

Coastal Vulnerability to Sea-Level Rise:
A Preliminary Database for the 
U.S. Atlantic, Pacific and Gulf of Mexico Coasts

Erika S. Hammar-Klose and E. Robert Thieler 
U.S. Geological Survey  Digital Data Series - 68
2001

 


Coastal Changes Due to Sea-Level Rise 

One of the most important applied problems in coastal geology today is determining the physical response of the coastline to sea-level rise. Predicting shoreline retreat, beach loss, cliff retreat, and land loss rates is critical to planning coastal zone management strategies and assessing biological impacts due to habitat change or destruction. Presently, long-term (>50 years) coastal planning and decision-making has been done piecemeal, if at all, for the nation's shoreline (National Research Council, 1990; 1995). Consequently, facilities are being located and entire communities are being developed without adequate consideration of the potential costs of protecting or relocating them from sea-level rise related erosion, flooding and storm damage.

Recent estimates of future sea-level rise based on climate modeling (Wigley and Raper, 1992) suggest an increase in global eustatic sea-level of between 15 and 95 cm by 2100, with a "best estimate" of 50 cm (IPCC, 1995). This is more than double the rate of eustatic rise for the past century (Douglas, 1997; Peltier and Jiang, 1997). 

The prediction of coastal evolution is not straightforward. There is no standard methodology, and even the kinds of data required to make such predictions are the subject of much scientific debate. A number of predictive approaches have been used (National Research Council, 1990), including: 1. extrapolation of historical data (for example, coastal erosion rates); 2. static inundation modeling; 3. application of a simple geometric model (for example, the Bruun Rule); 4. application of a sediment dynamics/budget model; or 5. Monte Carlo (probabilistic) simulation based on parameterized physical forcing variables. Each of these approaches, however, has its shortcomings or can be shown to be invalid for certain applications (National Research Council, 1990). Similarly, the types of input data required vary widely, and for a given approach (for example, sediment budget), existing data may be indeterminate or may simply not exist (Klein and Nicholls, 1999). Furthermore, human manipulation of the coast in the form of beach nourishment, construction of seawalls, groins, and jetties, as well as coastal development itself, may dictate Federal, State and local priorities for coastal management without proper regard for geologic processes. Thus, the long-term decision to renourish or otherwise engineer a coastline may be the primary determining factor in how that coastal segment evolves.

Variables Affecting Coastal Vulnerability 

We use here a fairly simple classification of the relative vulnerability of different U.S. coastal environments to future rises in sea-level. This approach combines the coastal system's susceptibility to change with its natural ability to adapt to changing environmental conditions, and yields a relative measure of the system's natural vulnerability to the effects of sea-level rise (Klein and Nicholls, 1999).  The vulnerability classification is based upon the relative contributions and interactions of six variables:

1. Tidal range, which contributes to 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 the given section of shoreline has been eroding.
5. Geomorphology, which indicates the relative erodibility of a given 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.

The input data for this database of coastal vulnerability have been assembled using the original, and sometimes variable, horizontal resolution, which then was resampled to a 3-minute grid cell. A data set for each risk variable is then linked to each grid point. For mapping purposes, data stored in the 3-minute grid is transferred to a 1:2,000,000 vector shoreline with each segment of shoreline lying within a single grid cell.

The Coastal Vulnerability Index  

The coastal vulnerability index (CVI) allows the six physical variables to be related in a quantifiable manner that expresses the relative vulnerability of the coast to physical changes due to sea-level rise. This method yields numerical data that cannot be equated directly with particular physical effects. It does, however, highlight those regions where the various effects of sea-level rise might be the greatest.

We used a rating system that classifies the data variables according to risk. Rating system -Atlantic coast, Pacific coast, Gulf of Mexico coast. The CVI values reported here apply specifically to the U.S. Pacific coast and to the U.S. Atlantic and Gulf of Mexico coasts. Thus, absolute CVI values given for the Pacific coast are not directly comparable to the data presented for the U.S. Atlantic and Gulf of Mexico coasts. In addition to the CVI values, the data ranges are also subdivided using values different from other studies so that the values used here reflect only the relative vulnerability along each coast. We feel this approach best describes and highlights the vulnerability for each of the different continental margin types that make up the U.S. coast.

Once each section of coastline is assigned a risk value for each specific data variable, the coastal vulnerability index (CVI) is calculated as the square root of the geometric mean of these values, or the square root of the product of the ranked variables divided by the total number of variables as

CVI = ( ( a*b*c*d*e*f ) / 6 )

where, a = geomorphology, b = coastal slope, c =relative sea-level rise rate, d = shoreline erosion/accretion rate, e = mean tide range, and f = mean wave height.

The coastal vulnerability index (CVI) value yields a relative ranking of the possibility that physical changes will occur along the shoreline as sea-level rises. For example, along the East Coast of the U.S., there are many areas of high vulnerability (Fig. 1). These high-vulnerability areas are typically barrier islands with 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). Rocky cliffed coasts, such as most of the Maine shoreline, with large tidal ranges, steep coastal slopes, and lower historical rates of sea level rise are represented as least vulnerable (Fig. 3). Along the Pacific Coast of the U.S., there are also many areas of high vulnerability. These high-vulnerability areas are typically along the high energy coast, where pocket beaches are sandwiched between rocky headlands. This contrast in geomorphology leads to sharp contrasts in susceptibility (Fig. 4). Finally, the low energy, beach and barrier island, Gulf Coast of the U.S. is particularly vulnerable; this vulnerability is enhanced by high rates of relative sea-level rise, low tide range, and high erosion rates (Fig. 5).

The coastal vulnerability index (CVI) provides insight into the relative potential of coastal change due to future sea-level rise. The maps and data presented here can be viewed in at least two ways: 1. as a base for developing a more complete inventory of variables influencing the coastal vulnerability to future sea-level rise to which other elements can be added as they become available; and 2. as an example of the potential for assessing coastal vulnerability to future sea-level rise using objective criteria.

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.

Klein, R, and Nicholls, R, 1999, Assessment of Coastal Vulnerability to Climate Change. Ambio, 28 (2):182-187

National Research Council, 1990, Managing Coastal Erosion. Washington, D.C.: National Academy Press, 163p.

National Research Council, 1995, Beach Nourishment and Protection. Washington, D.C.: National Academy Press, 334p.

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., 1999. National Assessment of Coastal Vulnerability to Future Sea-Level Rise: Preliminary Results for the U.S. Atlantic Coast. U.S. Geological Survey, Open-File Report 99-593, 1 sheet. [Also published as PDF files on this CD-ROM.]

Thieler, E.R., and Hammar-Klose, E.S., 2000a. National Assessment of Coastal Vulnerability to Future Sea-Level Rise: Preliminary Results for the U.S. Pacific Coast. U.S. Geological Survey, Open-File Report 00-178, 1 sheet. [Also published as PDF files on this CD-ROM.]

Thieler, E.R., and Hammar-Klose, E.S., 2000b. National Assessment of Coastal Vulnerability to Future Sea-Level Rise: Preliminary Results for the U.S. Gulf of Mexico Coast. U.S. Geological Survey, Open-File Report 00-179, 1 sheet. [Also published as PDF files on this CD-ROM.]

Wigley, T. M. L., and Raper, S. C. B. 1992, Implications for climate and sea level of revised IPCC emission scenarios. Nature, v.357, p. 293-300.

 

 

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