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