National Assessment of Coastal Vulnerability to Sea-Level Rise:

Preliminary Results for the U.S. Gulf of Mexico Coast

By

E. Robert Thieler and Erika S. Hammar-Klose

U.S. Geological Survey

Woods Hole, Massachusetts

2000

Open-File Report 00-179

INTRODUCTION

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 and land loss rates is critical to planning
future coastal zone management strategies and assessing biological
impacts due to habitat changes 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 model output
(Wigley and Raper, 1992) suggest an increase in global sea-level of
between 15-95 cm by 2100, with a \"best estimate\" of 50 cm (IPCC,
1995). This rate 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 a large, 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, and
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: 1) extrapolation of historical data (e.g., coastal erosion
rates), 2) static inundation modeling, 3) application of a simple
geometric model (e.g., 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
(e.g. sediment budget); existing data may be indeterminate or may simply
not exist (Klein and Nicholls, 1999). 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. 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 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. Gulf of Mexico coast and previous reports for the
U.S. Atlantic and Pacific coasts (Thieler and Hammar-Klose, 1999; 2000),
involves updating and refining existing databases of geologic and
environmental variables, such as that compiled by Gornitz and White
(1992). The variables included in this database are geomorphology,
regional coastal slope, rate of relative sea-level rise, shoreline
erosion and accretion rates, tide range and mean wave height. 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 Nino-related
climate data such as short-term sea-level rise) and human influences
(e.g. coastal engineering).

In this preliminary report, the relative vulnerability of different
coastal environments to sea-level rise is quantified for the U.S. Gulf
of Mexico Coast. This initial classification is based upon variables
such as coastal geomorphology, regional coastal slope, rate of sea-level
rise, wave and tide characteristics, and historical shoreline change
rates. The combination of these variables and the association of these
variables to each other furnishes a broad overview of regions where
physical changes are likely to occur due to sea-level rise.

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 mapping potential coastal
changes due to sea-level rise. This database loosely follows an earlier
database developed by Gornitz and White (1992). A comparable assessment
of the sensitivity of the Canadian coast to sea-level rise is presented
by Shaw et al. (1998).

The input data for this database of coastal vulnerability have been
assembled using their original, sometimes variable horizontal resolution
and resampled to a 3-minute grid cell resolution. A data set for each
risk variable is then stored within the 3-minute grid. 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.

Table 1 summarizes the six physical variables used here: 1)
geomorphology, 2) coastal slope (percent), 3) rate of relative sea-level
rise (mm/yr), 4) shoreline erosion and accretion rates (m/yr), 5) mean
tidal range (m) and 6) 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, as well as
correlated with descriptive information found in the Living with the
Shore book series (Morton et al., 1983; Kelley et al., 1984; Canis et
al., 1985 and Doyle et al., 1985).

The regional coastal slope permits an evaluation not only of the
relative risk of inundation, but also the potential rapidity of
shoreline retreat, because low-sloping coastal regions should retreat
faster than steeper regions (Pilkey and Davis, 1987). The regional slope
of the coastal zone was calculated from a grid of topographic and
bathymetric elevations extending landward and seaward of the shoreline.
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. Gulf of Mexico coast, elevation data were obtained from the
U.S. Navy ETOPO5 digital topographic and bathymetric elevation database
with elevations to the nearest 1 meter for 5-minute grid cells. These
data were subsampled to 3-minute (approximately 5 km) resolution to be
consistent with our other coastal databases (Thieler and Hammar-Klose,
1999; 2000).

The relative sea-level change variable is derived from the increase (or
decrease) in 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 seven National Ocean Service
(NOS) data stations and contoured along the coastline. This variable
inherently includes 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).

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.

Where higher-quality data are available, we replace and augment the CEIS
data with shoreline change data obtained from states and local agencies.
In this report, for example, the updated erosion rates for the Gulf of
Mexico are from a regional study in the Northern Gulf of Mexico
(Westphal et al., 1991), as well as an Alabama coastal hazards
assessment study (NOAA Coastal Services Center, 1997). The long-term
erosion rates for Alabama were calculated using a linear regression
approach (Dolan et al., 1991) using data derived from aerial photographs
spanning the years 1970-1997. These data were correlated with beach
profile survey data from 92 points covering the Alabama coastline (NOAA
Coastal Services Center, 1997).

Tide range is linked to both permanent and episodic inundation hazards.
Tide range data were obtained from the NOS for 117 tide stations along
the U.S. Gulf of Mexico coast; the values were 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 122 WIS stations along
the U.S. Gulf of Mexico coast were contoured along the coastline.

DATA RANKING

Table 1 shows the six physical variables described above, ranked on a
linear scale from 1-5 in order of increasing vulnerability due to
sea-level rise. In other words, a value of 1 represents the lowest risk
and 5 represents the highest risk. The database includes both
quantitative and qualitative information. Thus, numerical variables are
assigned a risk ranking based on data value ranges, whereas the
non-numerical geomorphology variable is ranked according to the relative
resistance of a given landform to erosion. For the U.S. Gulf of Mexico
coast, regional coastal slopes are considered to be very low risk at
values \>0.115 percent; very high risk consists of regional slopes
\<0.022 percent. The rate of relative sea-level rise is ranked to
reflect the regional to local isostatic or tectonic effects, taking into
account that these data also reflect the modern rate of eustatic rise
(1.8 mm/yr). Shorelines with erosion/accretion rates between -1.0 and
+1.0 m/yr are ranked as moderate. Increasingly higher erosion or
accretion rates are ranked as correspondingly higher or lower risk.
Tidal range is ranked such that microtidal coasts are high risk and
macrotidal coasts are low risk. Mean wave height rankings range from
very low (\<0.55 m) to very high (\>1.25 m).

In previous and related studies (Gornitz, 1990; Shaw et al., 1998),
large tidal range (macrotidal; tide range \> 4m) coastlines were
assigned a high risk classification, and microtidal coasts (tide range
\<2.0 m) received a low risk rating. This decision was based on the
concept that a large tide range is associated with strong tidal currents
that influence coastal behavior. We have chosen to invert this ranking
such that a macrotidal coastline is classified as low risk. Our
reasoning is based primarily on the potential influence of storms on
coastal evolution, and their impact relative to the tidal range. For
example, on a tidal coastline, there is only a 50 percent chance of a
storm occurring at high tide. Thus, for a region with a 4 m tide range,
a storm having a 3 m surge height is still up to 1 m below the elevation
of high tide for half a tidal cycle. A microtidal coastline, on the
other hand, is essentially always \"near\" high tide and therefore
always at the greatest risk of significant storm impact.

COASTAL VULNERABILITY INDEX

The coastal vulnerability index (CVI) presented here is similar to that
used by Gornitz et al. (1994), as well as to the sensitivity index
employed by Shaw et al. (1998). The index 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 may be the greatest.

Once each section of coastline is assigned a risk value based on each
specific data variable, the coastal vulnerability index is calculated as
the square root of the geometric mean, 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 CVI values reported here apply specifically to the U.S. Gulf of
Mexico coast, but are also comparable to the values for the U.S.
Atlantic coast since the data ranges for the Gulf of Mexico are
categorized using overall values for both coasts. Absolute CVI values
given for the Pacific coast, however, (e.g., Thieler and Hammar-Klose,
2000) are not directly comparable to the data presented here. 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.

RESULTS

The calculated CVI values range from 1.2 to 39.5. The mean CVI value is
15.25; the mode is 7.3; and the median is 15.5. The standard deviation
is 7.89. The 25th, 50th, and 75th percentiles are 8.7, 15.6, and 20.0,
respectively.

Figure 1 shows a map of the coastal vulnerability index for the U.S.
Gulf of Mexico coast The CVI scores are divided into low, moderate,
high, and very high-risk categories based on the quartile ranges and
visual inspection of the data. CVI values below 8.7 are assigned to the
low risk category. Values from 8.7-15.6 are considered moderate risk.
High-risk values lie between 15.6 and 20.0. CVI values above 20.0 are
classified as very high risk. Histograms of the CVI values are shown in
Figure 2.

Figure 3 shows a bar graph of the percentage of shoreline in each risk
category. A total of 8058 km of shoreline is evaluated in the study
area. Of this total, 42 percent of the mapped shoreline is classified as
being at very high risk due to future sea-level. Thirteen percent is
classified as high risk, 37 percent as moderate risk, and 8 percent as
low risk.

In the calculation of the Coastal Vulnerability Index, certain variables
add more weight to the index than others. For example, in a region where
most variables score low in the risk ranking (1-3), but one variable
scores high (4 or 5), the high variable adds the most weight to the
index. This variable is said to dominate the index. In most cases along
the U.S. Gulf of Mexico coast, two or three variables dominate the
index, while the other, lower-ranking variables have little impact on
the index value.

The mapped CVI values show large areas of very high vulnerability,
particularly along the Louisiana - Texas coast. The
highest-vulnerability areas are typically lower-lying beach and marsh
areas; their susceptibility is primarily a function of geomorphology,
coastal slope and rate of relative sea-level rise. On the Gulf of Mexico
coast, much of the vulnerability is due to geomorphology and tide range;
two variables which are ranked as generally high for the entire Gulf of
Mexico region. The western Gulf of Mexico is ranked as more vulnerable
than the eastern Gulf of Mexico when described in terms of relative
sea-level rise. Wave energy is highest along sections of the Texas coast
and on the southern tip of the Mississippi delta. The slope variable has
the highest risk ranking along the Louisiana coast, the Texas coast
north of Corpus Christi and the southwest Florida coast. The erosion
rates within the study area range from low risk to very high risk. In
contrast to the Pacific and Atlantic coasts (see Thieler and
Hammar-Klose, 1999; 2000), the erosion rates in the Gulf of Mexico do
not vary consistently on very short spatial scales. Instead, there are
reaches of coastline as long as 150 km with the same risk ranking.

DISCUSSION

The data variables underlying the CVI show variability at several
spatial scales. The geomorphology and tide range vary over a spatial
scale of \>500 km. For geomorphology, this lack of variability
represents the large-scale, rather uniform patterns of landform type
along the Gulf coast (Figure 4). Barrier islands, lagoons, marshes and
deltas dominate the coast, which are landforms that have a very high
risk ranking (see Table 1). In addition, the entire coast is microtidal;
thus, this variable yields a very high risk ranking (Figure 5).

A second group of variables, consisting of relative sea-level rise and
wave height, vary on a \~200 km spatial scale. For example, the
low-energy Gulf of Mexico coast has mean wave heights that on average
are \~0.5 m (Figure 6), and vary between only 0.07 m and 1.04 m.

Changes in relative sea-level rise are greatest around New Orleans,
Louisiana, where the rates can be as much as 10 mm/yr (Figure 7). East
of Louisiana, the rate of relative sea-level rise is \~ 2 mm/yr. This
lower value, however, is still higher than the modern rate of eustatic
rise (1.8 mm/yr), which reflects the ongoing recent subsidence of this
region. To the west of Louisiana, rates of relative sea-level rise are
also lower, decreasing to the 3-5 mm/yr range, which again is well above
the modern eustatic rate and still within the very high risk range of
our rating system. These high rates of relative sea-level rise within
and surrounding Louisiana are primarily due to the natural compaction of
the Holocene deltaic sediments in the Gulf of Mexico (Penland and
Ramsey, 1990; Turner, 1991).

The coastal slope variable changes on a \~ 50 km spatial scale (Figure
8). The areas with the lowest slope are those surrounding the
Mississippi delta. These data show slopes of less than 0.02%. The
highest slopes along the Gulf of Mexico coast are found south of Corpus
Christi, Texas, along the Florida panhandle, and in the greater
Tampa-St. Petersburg, Florida region. While these values of slope yield
a moderate to low susceptibility ranking, they are only in the 0.5%
range, and thus are not steep slopes in an absolute sense.

In some cases, the data describing erosion and accretion rates vary on a
small spatial scales of about 5 km, but most of the variation in erosion
rates varies on the \~20 km scale. There are long sections of coastline
(\~150 km), however, that show little to no variation in erosion rates
(Figure 9), both because of an actual lack of change as well as an
absence of comprehensive erosion rate data.

The CVI rankings for the Gulf of Mexico coast are governed by the
large-scale variations of its variables (Figure 1). The CVI shows a
regional distinction centered on the New Orleans region. From the very
high vulnerability New Orleans region to the west, the CVI rankings
remain as high vulnerability along the coast with lower vulnerability in
the inland bays. To the east of New Orleans, the CVI values decrease to
moderate. The one exception to this trend is Apalachicola Bay, Florida,
which due to its high erosion rates, low slope, and moderate rate of
sea-level rise receives a very high susceptibility ranking. The regional
variation of higher vulnerability to the west and lower vulnerability to
the east is controlled by the mean wave height, the relative sea-level
rise and to some extent the coastal slope.

The CVI shows that the region around New Orleans is the most vulnerable
of all areas along the Gulf of Mexico coast. The Florida panhandle, as
well as the West Florida coast, are considered to be at low to moderate
risk, primarily because of the lower rates of relative sea-level rise,
lower mean wave heights, and a relatively higher coastal slope in this
region. The Texas coast is considered to be at a high to very high risk
because of the relatively high mean wave height and relative sea-level
rise vulnerabilities.

SUMMARY

The coastal vulnerability index (CVI) for the Gulf of Mexico coast
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.

As ranked in this study, coastal geomorphology and tide range are the
most important variables in determining the CVI for the Gulf of Mexico
coast since both variables reflect very high vulnerabilities along
nearly the entire shoreline. Wave height, relative sea-level rise, and
coastal slope provide large-scale (50-200 km alongshore) variability to
the coastal vulnerability index. Erosion and accretion rates, where
complete, contribute the greatest variability to the CVI at short
spatial scales. The rates of shoreline change, however, are the most
complex and poorly documented variable in this data set. The rates used
here are based on a dated, low-resolution data set and thus far
corrections have been made only on a preliminary level. To best
understand where physical changes may occur, large-scale variables must
be clearly and accurately mapped and small-scale variables must be
understood on a scale that takes into account their geologic and
environmental influences.

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FIGURE CAPTIONS

Figure 1. Map of the Coastal Vulnerability Index (CVI) for the U.S. Gulf
coast. The CVI shows the relative vulnerability of the coast to changes
due to future rise 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.

Table 1. Ranking of coastal vulnerability index variables for the U.S.
Gulf of Mexico.

Figure 2. Histograms showing the frequency of occurrence and cumulative
frequency of CVI values for the U.S. Gulf of Mexico coast. The vertical
red lines delineate the chosen ranges for low, moderate, high, and very
high risk areas.

Figure 3. Bar graph showing the percentage of shoreline along the U.S.
Gulf of Mexico coast in each risk category. The graph also shows the
total length of shoreline (in kilometers) in each risk category. The
total length of mapped shoreline in this study is 8058 km.

Figure 4. Map of the geomorphology variable for the U.S. Gulf of Mexico
coast. The shoreline is composed predominantly of very high-risk barrier
island complexes, lagoons, marshes and deltas.

Figure 5. Map of the tide range variable for the U.S. Gulf of Mexico
coast. The tide ranges are less than 1.0 m over the open-ocean coast,
thus, the entire coast has a very high risk ranking.

Figure 6. Map of the wave height variable for the U.S. Gulf of Mexico
coast. The wave height variable displays a regional trend of high wave
heights ( \>.8 m) to the west and lower wave heights (\< .4 m) to the
east.

Figure 7. Map of the relative sea level rise variable for the U.S. Gulf
of Mexico coast. The highest rates of sea-level rise in the Gulf of
Mexico (and in the United States) are in the Mississippi delta region
(\>10 mm/yr).

Figure 8. Map of the coastal slope of the U.S. Gulf of Mexico coast. The
slope is the least, \<0.02%, in the region of the Mississippi delta. The
slope is the highest (\>.0435 %) south of Corpus Christi, along the
western panhandle of Florida, and in the greater Tampa - St.Petersburg
region.

Figure 9. Map of the erosion rate variable for the U.S. Gulf of Mexico
coast. Most of the Gulf of Mexico coast receives a moderate to very high
risk ranking, meaning the coastline is either stable or is eroding.
There are few accreting areas.

DISCLAIMER

This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards (or with the North American
Stratigraphic Code). Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by the U.S.
Government.