National Assessment of Coastal Vulnerability to Sea-Level Rise: Preliminary Results for the U.S. Atlantic Coast
Introduction
Recent estimates of future sea-level rise based on climate
model output (Wigley and Raper, 1992) suggest an increase in global
eustatic sea-level of between 15-95 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). Thus, sea-level rise will have the largest sustained
impact on coastal evolution at the societally-important decadal
time scale. For example, Zhang et al. (1997) showed that sea-level
rise over the past 80 years at two locations on the U.S. East Coast
contributed directly to significant increases in the amount of time
the coast is subjected to extreme storm surges. From 1910-1920, the
coast near Atlantic City, New Jersey was exposed to anomalously
high water levels from extreme storms less than 200 hours per year,
whereas during the early 1990's the coast was exposed to high water
from storms of the same magnitude 700 to 1200 hours per year.
Interestingly, the authors found that although storm surge varied a
great deal on annual to decadal scales, there was no long-term
trend showing increases in storm intensity or frequency that might
account for the increasing anomalously high water levels. Zhang et
al. (1997) concluded that the increase in storm surge exposure of
the coast was due to sea-level rise of about 30 cm over the 80-year
period. This finding suggests that the historical record of
sea-level change can be combined with other variables (e.g.,
elevation, geomorphology, wave characteristics) to assess the
relative coastal vulnerability to future sea-level change.
The prediction of future 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:
- extrapolation of historical data (e.g., coastal erosion rates),
- static inundation modeling,
- application of a simple geometric
model (e.g., the Bruun Rule),
- application of a sediment
dynamics/budget model, or
- 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 (e.g. sediment budget), existing
data may be indeterminate or simply not exist. Furthermore, human
manipulation of the coastal environment in the form of beach
nourishment, construction of seawalls, groins, and jetties, as well
as coastal development itself, may drive federal, state and local
priorities for coastal management without regard for geologic
processes. Thus, the long-term decision to renourish or otherwise
engineer a coastline may be the sole determining factor in how that
coastal segment evolves.
Although a viable, quantitative predictive approach is not
available, the relative vulnerability of different coastal
environments to sea-level rise may be quantified at a regional to
national scale using basic information on coastal geomorphology,
rate of sea-level rise, past shoreline evolution, and other
factors. The overall goal of this study is to develop and utilize a
relatively simple, objective method to identify those portions of
the U.S. coastal regions at risk and the nature of that risk (e.g.,
inundation, erosion, etc.). The long-term goal of this study is to
predict future coastal changes with a degree of certainty useful
for coastal management, following an approach similar to that used
to map national seismic and volcanic hazards (e.g., Miller, 1989;
Frankel et al., 1996; Hoblitt et al. 1998). This information has
immediate application to many of the decisions our society will be
making regarding coastal development in both the short- and
long-term.
This study involves two phases. The first phase, presented in
this report for the U.S. East Coast, involves updating and refining
existing databases of geologic and environmental variables, such as
that compiled by Gornitz and White (1992). For all of the variables
in this data set, updated or new data exist and are presented here.
The second phase of the project has two components. The first
component entails integrating model output such as eustatic,
isostatic, and short-term climatic sea-level change estimates in
order to assess the potential impacts on the shoreline due to these
changes. The second component involves developing other databases
of environmental information, such as relative coastal sediment
supply, as well as including episodic events (hurricane intensity,
track, and landfall location, Nor'easter storm intensity data, and
El Niño-related climate data such as short-term sea-level
rise).
In this preliminary report, the relative vulnerability of
different coastal environments to sea-level rise is quantified for
the U.S. East Coast. This initial classification is based upon
variables such as coastal geomorphology, regional coastal slope,
and shoreline erosion and accretion rates. The combination of these
variables and the association of these variables to each other
furnishes a broad overview of regions where physical changes will
occur due to sea-level rise.
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