Coastal Flooding
Identification
1. Indicator Description
This indicator describes how the number of coastal floods exceeding local "nuisance" threshold levels
has changed over time. Sea level rise related to climate change is a key driver of the increasing
frequency of coastal flooding.
Components of this indicator include:
• A map that shows the change in flood days per year along U.S.
decade of record with the most recent decade (Figure 1).
• A more detailed graph that shows average flood days per year
2015 (Figure 2).
2. Revision History
August 2016: Indicator published.
Data Sources
3. Data Sources
Coastal flooding trends are based on measurements from permanent tide gauge stations. The original
tide gauge data come from the National Water Level Observation Network (NWLON), operated by the
Center for Operational Oceanographic Products and Services (CO-OPS) within the National Oceanic and
Atmospheric Administration's (NOAA's) National Ocean Service (NOS). Daily maximum water levels are
derived from the hourly data set maintained by CO-OPS. Mike Kolian of EPA developed this indicator in
collaboration with William Sweet of NOAA. The analysis is adapted from Sweet and Marra (2015), which
is an update to an analysis published in NOAA (2014) and Sweet and Park (2014).
4. Data Availability
Individual tide gauge measurements can be accessed through NOAA's "Tides and Currents" website at:
https://tidesandcurrents.noaa.gov/stations.html?type=Water+Levels. This website also presents an
interactive map that illustrates sea level trends over different timeframes. Station-specific flood
thresholds for 26 of the 27 locations presented in this indicator, as well as other locations, are identified
in NOAA (2014). See also Table TD-1 in Section 6 for a complete list of the NOAA tide gauge stations and
flood levels used in this indicator.
EPA obtained the 1950-2015 analysis from the authors of Sweet and Marra (2015) and NOAA (2014).
Although NOAA has not published a table of the compiled results presented in this indicator, a user
could reproduce these numbers by processing the individual site data available on the website listed
coasts, comparing the first
along U.S. coasts from 1950 to
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above. Updates to this indicator are likely to coincide with NOAA's annual report, State of Nuisance Tidal
Flooding, which is released each spring.
Methodology
5. Data Collection
This indicator presents the change in flood days, which are defined as days in which a tide gauge records
water levels that exceed the National Weather Surface (NWS) "minor flooding" threshold for a given
location. Thus, this indicator captures all flooding events that exceed the minor flooding threshold,
including the much less frequent occurrences of moderate or major floods.
Coastal water levels have traditionally been measured using tide gauges, which are mechanical
measuring devices placed along the shore. These devices measure the change in sea level relative to the
land surface, which means the resulting long-term analysis reflect both changes in flood frequency
occurring from changing absolute sea surface height and local land levels.
Tide gauge data for this indicator come from NWLON, which is composed of 210 long-term, continuously
operating tide gauge stations along the United States coast, including the Great Lakes and islands in the
Atlantic and Pacific Oceans. This indicator shows trends for a subset of stations along the ocean coasts
that had sufficient data from 1950 to 2015 and had established thresholds for what constitutes a minor
or "nuisance" flood (see "Indicator Derivation"). NOAA (2014) describes tide gauge data and how they
were collected. Data collection methods are documented in a series of manuals and standards that can
be accessed at: www.co-ops.nos.noaa.gov/pub.html#sltrends. This indicator uses hourly averages based
on each tide gauge's continuous measurements.
6. Indicator Derivation
This indicator was derived by calculating each day's maximum water level based on hourly water level
data, then comparing these daily maxima with established threshold levels for minor flooding at each
tide gauge. Flood impact levels have been established locally by NWS weather forecasting offices based
on many years of impact monitoring. Each location has a unique flooding threshold, which depends on
the local land cover, topography, the built environment, and human mitigation strategies in place. For
example, an area with flat topography and extensive infrastructure near sea level might have a relatively
low flooding threshold, because even a small increase in water level can cause impacts to humans. All of
these local thresholds have been established as objectively as possible by working with local emergency
managers and reviewing impacts relative to water levels.
Of the 210 NWLON tide gauges, 75 have locally defined minor flooding thresholds. Most of these
stations are located along the Atlantic and Gulf Coasts, as these areas tend to be more vulnerable to
large storm surges. A total of 27 of these stations had complete records for the 1950-2015 period of
study, resulting in 27 sites for this indicator. Table TD-1 identifies the NOAA tide gauges and
corresponding minor ("nuisance") flood levels for the 27 sites in this indicator.
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Table TD-1. Tide Gauges and Nuisance Flood Levels Used in This Indicator
Tide gauge
Nuisance flood level
(meters above mean
higher high water
[MHHW])
Boston, Massachusetts
0.68
Providence, Rhode Island
0.66
New London, Connecticut
0.60
Montauk, New York
0.60
Kings Point, New York
0.52
Battery, New York
0.65
Sandy Hook, New Jersey
0.45
Atlantic City, New Jersey
0.43
Philadelphia, Pennsylvania
0.49
Lewes, Delaware
0.41
Baltimore, Maryland
0.41
Annapolis, Maryland
0.29
Washington, DC
0.31
Sewell Point, Virginia
0.53
Wilmington, North Carolina
0.25
Charleston, South Carolina
0.38
Fort Pulaski, Georgia
0.46
Fernandina Beach, Florida
0.59
Mayport, Florida
0.44
Key West, Florida
0.33
St. Petersburg, Florida
0.84
Galveston Bay, Texas
0.79
Port Isabel, Texas
0.34
La Jolla, California
0.51
San Francisco, California
0.35
Seattle, Washington
0.65
Honolulu, Hawaii
0.22
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The total number of days exceeding the minor flooding threshold was calculated for each tide gauge and
for every calendar year. Annual totals were averaged together over multi-year periods for Figures 1 and
2. Figure 1 provides a simple comparison between the first and last decades of record: the 1950s (1950-
1959) and the 2010s (2010-2015). Figure 2 covers the entire period of record by sorting the data into
bins, most of which are 20 years in length.
Sweet and Marra (2015) provide more information about station selection, data compilation, and
calculation methods.
7. Quality Assurance and Quality Control
Quality assurance and quality control procedures for U.S. tide gauge data are described in various
publications available at: www.co-ops.nos.noaa.gov/pub.html#sltrends.
Analysis
8. Comparability Over Time and Space
All of the tide gauges included in this indicator have used the same methods for determining hourly
water levels. These methods remained constant over time and across gauges, except as documented in
NOAA (2014). Tide gauge measurements at specific locations are not indicative of broader changes over
space, however, and the network is not designed to achieve uniform spatial coverage. Rather, the
gauges tend to be located at major port areas along the coast, and measurements tend to be more
clustered in heavily populated areas like the Mid-Atlantic coast. Nevertheless, in many areas it is
possible to see consistent patterns across numerous gauging locations—for example, increases in the
frequency of flooding along the Atlantic and Gulf Coasts.
Flooding thresholds are established on a local basis, accounting for site-specific characteristics such as
topography. Thus, thresholds are not exactly the same across space. For example, minor flood levels are
0.25 meters above the MHHW datum in Wilmington, North Carolina, compared with 0.79 meters in
Galveston, Texas. This degree of variation is appropriate, as the indicator is designed to characterize an
impact rather than just a physical condition, and thresholds for that impact naturally vary from one
location to another. Thus, this indicator focuses more on comparing change over time at individual
locations rather than comparing different locations over space. Each location's flood threshold has been
applied consistently throughout the period of record, which supports this analysis of trends over time.
9. Data Limitations
Factors that may impact the confidence, application, or conclusions drawn from this indicator are as
follows:
1. Coastal flooding relates to relative sea level change, which is the height of the sea relative to the
height of the land. Changes in coastal flooding frequency cannot be solely attributed to absolute
sea level change, but instead may reflect some degree of local changes in land elevation (e.g.,
subsidence). Tide gauge measurements generally cannot distinguish between these two
influences without an accurate measurement of vertical land motion nearby.
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2. Some changes in coastal flooding may be due to multiyear cycles such as El Nino/La Nina and
the Pacific Decadal Oscillation, which affect coastal ocean temperatures, water density (due to
salt content), winds, atmospheric pressure, and currents.
3. As described in "Comparability Over Time and Space," local flooding thresholds vary according
to land cover, topography, and human mitigation strategies in place. This variation complicates
regional frequency-change comparisons.
4. Not all tide gauges have NWS-defined flooding levels. This indicator only includes tide gauges
with NWS flood levels and with hourly data since 1950, which results in sparse coverage on the
Pacific and Gulf Coasts and no coverage of Alaska and Hawaii.
5. Impacts are localized and not necessarily readily observable. When water levels are expected to
exceed the flooding threshold, coastal flood advisories are typically issued. Minor impacts
typically, but not always, manifest.
6. Local topography may affect the relative influences of various environmental processes on a
specific site's flooding. For example, offshore barriers such as coral reefs or barrier islands may
help to buffer certain contributing effects, such as wind. By contrast, other areas may have
topographical features that amplify the flooding caused by slight changes in the environment.
Although these differences do not negate the site-specific trends observed, they do contribute
to differences between stations.
10. Sources of Uncertainty
Error measurements for each tide gauge station are described in NOAA (2009), but many of the
estimates in that publication pertain to longer-term time series (i.e., the entire period of record at each
station, not the exact period covered by this indicator). Uncertainties in the data do not impact the
overall conclusions. Tide gauges provide precise, reliable water level data for the locations where they
are installed.
11. Sources of Variability
Changes in sea level and corresponding changes in coastal flooding can be influenced by multi-year
cycles such as El Nino/La Nina and the Pacific Decadal Oscillation, which affect coastal ocean
temperatures, salt content, winds, atmospheric pressure, and currents.
12. Statistical/Trend Analysis
This indicator does not report on the slope of the apparent trends in flood frequency, nor does it
calculate the statistical significance of these trends. Separately, NOAA (Sweet and Park, 2014) analyzed
trends in the annual number of flood days from 1950 to 2013 using quadratic and linear fits, and
reported those with statistically significant fits above a 90% level. Of the 27 stations presented in this
indicator, 19 had significant quadratic fits and four had significant linear fits. There were four stations,
whose "nuisance-level" thresholds were quite high, that did not have a significant fit of any sort.
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References
NOAA (National Oceanic and Atmospheric Administration). 2009. Sea level variations of the United
States 1854-2006. NOAA Technical Report NOS CO-OPS 053.
www.tidesandcurrents.noaa.gov/publications/Tech rpt 53.pdf.
NOAA (National Oceanic and Atmospheric Administration). 2014. Sea level rise and nuisance flood
frequency changes around the United States. NOAA Technical Report NOS CO-OPS 073.
https://tidesandcurrents.noaa.gov/publications/NOAA Technical Report NOS COOPS 073.pdf.
Sweet, W.V., and J. Park. 2014. From the extreme to the mean: Acceleration and tipping points of
coastal inundation from sea level rise. Earth's Future 2(12):579-600.
http://onlinelibrarv.wilev.com/doi/10.1002/2014EF00Q272/full.
Sweet, W.V., and J.J. Marra. 2015. 2014 state of nuisance tidal flooding.
www.noaanews.noaa.gov/stories2015/2014%20State%20of%20Nuisance%20Tidal%20Flooding.pdf.
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