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 Climate Change Indicators: Coastal
Flooding
This indicator shows how the frequency of coastal flooding
has changed over time.
Key Points
Flooding is becoming more frequent along the U.S. coastline.
Nearly every site measured has experienced an increase in
coastal flooding since the 1950s (see Figure 1). The rate is
accelerating in many locations along the East and Gulf
Coasts (see Figure 2).
The Mid-Atlantic region suffers the highest number of
coastal flood days and has also experienced the largest
increases in flooding. Since 2010, Wilmington, North
Carolina, has flooded most often—49 days per year—followed
by Annapolis, Maryland, at 46 days per year. Annapolis,
Wilmington, and two locations in New Jersey (Sandy Hook and
Atlantic City) have also seen some of the most dramatic
overall increases in frequency: floods are now at least 10
times more common there than they were in the 1950s. The
Mid-Atlantic’s subsiding land and higher-than-average
relative sea level rise both contribute to this increase in
flooding (see the Sea Level indicator).
Flooding has increased less dramatically in places where the
local flood threshold is higher (for example, the Northeast
and locations on the Gulf of Mexico) or where relative sea
level has not risen as quickly as it has elsewhere in the
United States (for example, Hawaii and the West Coast, as
shown by the Sea Level indicator). |
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Background
Changing sea levels are affecting human activities in
coastal areas. Rising sea level inundates low-lying wetlands
and dry land, erodes shorelines, contributes to coastal
flooding, and increases the flow of salt water into
estuaries and nearby groundwater aquifers. Higher sea level
also makes coastal infrastructure more vulnerable to damage
from storms.
“Relative sea level change” refers to the height of the
ocean relative to the land at a particular location. As
relative sea level rises due to climate change (see the Sea
Level indicator), one of the most noticeable consequences is
an increase in coastal flooding. Flooding typically occurs
during seasonal high tides (“king tides”) and storms that
push water toward the shore. In recent years, however,
coastal cities are increasingly flooding on days with less
extreme tides or little wind, even on sunny days. Floods are
happening more often as rising sea level reduces the gap
between average sea level and the height of the land.
Many coastal cities have defined minor or “nuisance”
flooding thresholds. When water rises above this level,
minor flooding typically occurs in some streets, many storm
drains become ineffective, and a coastal flood advisory may
be issued. Recurrent coastal flooding can cause impacts such
as frequent road closures, reduced stormwater drainage
capacity, and deterioration of infrastructure not designed
to withstand frequent inundation or exposure to salt water.
These impacts are of particular concern because more than
8.6 million Americans live in areas susceptible to coastal
flooding, and more than $1 trillion of property and
structures is within a few feet of current sea level.1
Coastal flooding can also affect human health—for example,
by increasing the risk that drinking water and wastewater
infrastructure will fail, putting people at risk of being
exposed to pathogens and harmful chemicals. |
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About the Indicator
This indicator tracks coastal flooding based on measurements
from tide gauges at locations along U.S. coasts. These
devices measure water levels continuously, which makes them
ideal for measuring the height of the tides, tracking
long-term local relative sea level rise, and detecting
coastal floods. This indicator focuses on 27 tide gauges
where local weather forecasting offices have defined
thresholds for minor, moderate, and major flooding and where
complete data are available from 1950 to present. The
indicator tracks the number of days per year when each tide
gauge measured water that was higher than the minor flooding
level. Figure 1 shows the 27 stations on a map and compares
the first and last decades of data, while Figure 2 provides
a more detailed comparison using all the available years of
data.
Indicator Notes
Flood thresholds are defined locally based on observed
impacts. They vary from one city to another, depending on
land cover, topography, and the presence of flood defenses
such as seawalls. For example, the minor flood level is set
at approximately 10 inches above an average high-water mark
in Wilmington, North Carolina, compared with 31 inches in
Galveston, Texas. A city with flat land and infrastructure
close to sea level will likely have a lower flooding
threshold than a more elevated city, and it will naturally
experience more days of coastal flooding. For this reason,
it is more useful to compare change over time at a single
location than it is to compare patterns across different
locations.
This indicator is limited to tide gauges with defined flood
levels and several decades of data. These tide gauges tend
to be clustered along the East Coast, particularly in the
Mid-Atlantic region. Other parts of the U.S. coast have
fewer tide gauges with defined flood levels and long-term
data.
Data Sources
This indicator is based on hourly water level data that are
publicly available from the National Oceanic and Atmospheric
Administration at:
https://tidesandcurrents.noaa.gov. Flood thresholds are
defined by the National Weather Service at:
https://water.weather.gov/ahps/forecasts.php.
Technical Documentation
Download related technical information PDF |
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 Figure
1. Frequency of Flooding Along U.S. Coasts, 2010–2015
Versus 1950–1959
This map shows the average number of days per year in which
coastal waters rose above the local threshold for minor
flooding at 27 sites along U.S. coasts. Each small bar graph
compares the first decade of widespread measurements (the
1950s in orange) with the most recent decade (the 2010s in
purple).
Data source: NOAA, 20163 |
 Figure
2. Average Number of Coastal Flood Events per Year,
1950–2015
This graph shows the average number of days per year in
which coastal waters rose above the local threshold for
minor flooding at 27 sites along U.S. coasts. The data have
been averaged over multi-year periods for comparison.
Data source: NOAA, 20164 |
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EPA Page |
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This is the
EPA page for this topic. To see if the Trump
administration has changed the EPA page, simply click the
link and compare the information with this page. If you
notice changes were made to the EPA page, please post a
comment. Thanks. |
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Additional Climate Change Information |
Climate Change and Carbon Dioxide
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change
and carbon dioxide.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Carbon Dioxide and Climate Change
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of carbon dioxide
and climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Environmental Group Warns Earth's Health at Risk
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A report by the World Wildlife Fund looked at thousands of animal populations
and found they have dropped significantly in 40 years. |
Sea Levels Rising at Fastest Rate in 3,000 years
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A group of scientists say sea levels are rising at record rates. Another group
found that January temperatures in the Arctic reached a record high. |
Capturing CO2 Gas Is Not Easy
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Most scientists agree that carbon-dioxide gas is partly to blame for climate
change: rising global temperatures. But capturing the CO2 gas released by power
stations is costly and difficult. |
Growth, Climate Change Threaten African Plants and
Animals
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Researchers believe Africa may lose as much as 30 percent of its animal and
plant species by the end of this century. |
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