National Assessment of Coastal Vulnerability to Sea-Level Rise: Preliminary Results for the U.S. Atlantic Coast

Risk Variables

In order to develop a database for a national-scale assessment of coastal vulnerability, relevant data have been gathered from local, state and federal agencies, as well as academic institutions. The compilation of this data set is integral to accurately mapping potential coastal changes due to sea-level rise. This database is based loosely on an earlier database developed by Gornitz and White (1992). A comparable assessment of the sensitivity of the Canadian coast to sea-level rise is furnished by Shaw et al. (1998).

Table 1. Ranking of coastal vulnerability index variable. Click on figure for larger image.

Table 1 summarizes the six physical variables used here:

• geomorphology,
• shoreline erosion and accretion rates (m/yr),
• coastal slope (percent),
• rate of relative sea-level rise (mm/yr),
• mean tidal range (m),and
• mean wave height (m).

As described below, each variable is assigned a relative risk value based on the potential magnitude of its contribution to physical changes on the coast as sea-level rises.

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from state geologic maps and USGS 1:250,000 scale topographic maps.

Shoreline erosion and accretion rates for the U.S. have been compiled by May and others (1983) and Dolan and others (1985) into the Coastal Erosion Information System (CEIS) (May and others, 1982). CEIS includes shoreline change data for the Atlantic, Gulf of Mexico, Pacific and Great Lakes coasts, as well as major bays and estuaries. The data in CEIS are drawn from a wide variety of sources, including published reports, historical shoreline change maps, field surveys and aerial photo analyses. However, the lack of a standard method among coastal scientists for analyzing shoreline changes has resulted in the inclusion of data utilizing a variety of reference features, measurement techniques, and rate-of-change calculations. Thus, while CEIS represents the best available data for the U.S. as a whole, much work is needed to accurately document regional and local erosion rates. The CEIS data are being augmented by and updated with shoreline change data obtained from states and local agencies, in addition to new analyses being conducted as part of this study.

The regional slope of the coastal zone was calculated from a grid of topographic and bathymetric elevations extending approximately 50 km landward and seaward of the shoreline. The regional slope permits an evaluation of not only the relative risk of inundation, but also the potential rapidity of shoreline retreat, since low-sloping coastal regions should retreat faster than steeper regions (Pilkey and Davis, 1987). In order to compute the slope from the subaerial coastal plain to the submerged continental shelf, the slope for each grid cell was calculated by defining elevation extremes within a 10 km radius for each individual grid cell. In areas where the shelf/slope break was less than 10 km offshore, the slope was recalculated with a more appropriate radius. For the U.S. East Coast, north of Florida, elevation data were obtained from the National Geophysical Data Center (NGDC) as gridded topographic and bathymetric elevations to the nearest 0.1 meter for 3 arc-second (~90 m) grid cells. These data were subsampled to 3-minute (approximately 5 km) resolution. For the Florida coast, the U.S. Navy ETOPO5 digital topographic and bathymetric elevation database was used. This gridded data set has a vertical resolution of one meter, and a horizontal resolution of approximately 8 km, which we resampled to a horizontal resolution of approximately 5 km.

The relative sea-level change variable is derived from the increase (or decrease) in annual mean water elevation over time as measured at tide gauge stations along the coast (e.g., Emery and Aubrey, 1991). Relative sea-level change data were obtained for 28 National Ocean Service (NOS) data stations and contoured along the coastline. This variable inherently includes both the global eustatic sea-level rise as well as local isostatic or tectonic land motion. Relative sea-level change data are a historical record, and thus show change for only recent time scales (past 50-100 yr).

Tide range data were obtained from the NOS. Tide range is linked to both permanent and episodic inundation hazards. Tidal data were obtained for 657 tide stations along the U.S. coast and their values contoured along the coastline.

Wave height is used here as an indicator of wave energy, which drives the coastal sediment budget. Wave energy increases as the square of the wave height; thus the ability to mobilize and transport beach/coastal materials is a function of wave height. In this report we use hindcast nearshore mean wave height data for the period 1976-1995 obtained from the U.S. Army Corps of Engineers Wave Information Study (WIS) (see references in Hubertz et al., 1996). The model wave heights were compared to historical measured wave height data obtained from the NOAA National Data Buoy Center. Wave height data for 151 WIS stations along the U.S. coast were contoured along the coastline.