Coastal Flooding

2023-03-21

Table of contents

Coastal flooding
Climate Change Indicators: Coastal Flooding
Key points
Background
About the Indicator
About the Data
- Notes on indicators

Coastal flooding

Coastal floods usually occur when dry, low-lying lands are flooded with seawater. A coastal floodplain results from the uplift of inland penetrating floodwater controlled by the topography of the flooded coastal land.

Flood damage modeling was limited to local, regional, or national scales. However, with climate change and the increase in population rates, flood events have intensified, creating global interest in finding different modalities with both spatial and temporal dynamics.

Coastal flooding

Seawater can flood the land in several ways: direct flooding, breaking a barrier, breaking a barrier.

Coastal flooding is largely a natural phenomenon, but the human impact on the coastal environment can exacerbate coastal flooding. Withdrawal from groundwater reservoirs in the coastal zone can cause land subsidence, increasing the risk of flooding.

Conservation structures designed along the coast, such as seawalls, alter the natural processes of the beach, often leading to erosion of adjacent parts of the coast, and increasing the risk of flooding. In addition, sea level rise and extreme weather conditions caused by climate change will increase the intensity and amount of coastal flooding, affecting hundreds of millions of people.

Climate Change Indicators: Coastal Flooding

This indicator shows how the frequency of coastal flooding has changed over time.

Figure 1. Frequency of flooding along US coasts, 2011–2020 versus 1950–1959

This map shows the average number of days per year coastal waters exceeded the local flood threshold at 33 locations along the US coast. Each small bar graph compares the first decade of extended measurements with the last decade.

Site name	Latitude	Longitude	1950-1959 avg	2011-2020 avg
Bar Harbor, ME	44.3917	-68.205	1.5	10.8
Portland, ME	43.6567	-70.2467	2.555556	8.5
Boston, MA	42.3548	-71.0534	2.9	13.1
Newport, RI	41.505	-71.3267	1.25	4.8
Montauk, NY	41.0483	-71.96	0.888889	4.5
New London, CT	41.36139	-72.09	1.3	4.1
Kings Point, NY	40.8103	-73.7649	3.1	9.2
Sandy Hook, NJ	40.4669	-74.0094	1.4	10.8
The Battery, NY	40.7006	-74.0142	1.555556	9.8
Atlantic City, NJ	39.355	-74.4183	0.4	10.2
Lewes, DE	38.78169	-75.12	0.857143	8.2
Philadelphia, PA	39.93333	-75.1417	1.222222	5.4
Sewells Point, VA	36.94667	-76.33	0.9	10.5
Annapolis, MD	38.98328	-76.4816	1.111111	7.8
Baltimore, MD	39.26667	-76.5783	1.2	6.6
Washington, DC	38.87333	-77.0217	1.2	7.6
Wilmington, NC	34.2267	-77.9533	0.1	3.7
Charleston, SC	32.7817	-79.925	0.111111	6.2
Fort Pulaski, GA	32.0333	-80.9017	0	6.6
Mayport, FL	30.3967	-81.43	0	2.7
Fernandina Beach, FL	30.6717	-81.465	0.75	4.6
St. Petersburg, FL	27.7606	-82.6269	0.111111	2.1
Cedar Key, FL	29.135	-83.0317	0.7	4.9
Pensacola, FL	30.4044	-87.2112	0.222222	3.7
Galveston Pier 21, TX	29.31	-94.7933	0.1	9.6
Port Isabel, TX	26.06	-97.215	0	3.1
San Diego, CA	32.71419	-117.174	0.1	4.9
La Jolla, CA	32.8667	-117.258	0	1.8
Los Angeles, CA	33.72	-118.272	0.4	1.9
Port San Luis, CA	35.1767	-120.76	0.75	0.7
Seattle, WA	47.60264	-122.339	0.9	2.5
San Francisco, CA	37.80669	-122.465	0	0.1
Friday Harbor, WA	48.54667	-123.01	2.666667	3

Figure 1. Frequency of Flooding Along U.S. Coasts, 2011–2020 Versus 1950–1959
Data source: NOAA, 2021
Web update: April 2021
Units: degrees; degrees; the average number of flood days per year; the average number of flood days per year

Figure 2. Average Number of Coastal Flood Events per Year, 1950–2020

This chart shows the average number of days per year that coastal waters exceeded the local flood threshold at 33 locations along the U.S. coast. Data were averaged over multiple years for comparison.

	1950-1969	1970-1989	1990-2009	2010-2020
Bar Harbor, ME	2.052632	5.277778	4.625	11.54545
Portland, ME	2.166667	3.105263	3.4	9.636364
Boston, MA	2.75	2.7	4.15	13.81818
Newport, RI	1.277778	1.444444	1.9	5
New London, CT	1.526316	1.529412	2.25	4.181818
Montauk, NY	1.166667	0.9375	2.277778	4.363636
Kings Point, NY	3.578947	3.111111	5	9.272727
The Battery, NY	1.894737	1.842105	4.294118	9.727273
Sandy Hook, NJ	1.75	2.052632	4.15	10.72727
Atlantic City, NJ	0.684211	1.8125	5.210526	10
Philadelphia, PA	1	1.6	2.55	5.272727
Lewes, DE	1.4375	1.722222	4.2	7.909091
Baltimore, MD	0.75	1.1	1.9	6.727273
Annapolis, MD	0.666667	0.789474	1.421053	7.727273
Washington, DC	0.9	1.9	2.388889	7.636364
Sewells Point, VA	1.45	1.95	5.05	10.09091
Wilmington, NC	0.052632	0.05	0.45	3.363636
Charleston, SC	0.210526	0.631579	1.05	5.727273
Fort Pulaski, GA	0.15	0.75	1.65	6.090909
Fernandina Beach, FL	0.611111	0.894737	1.722222	4.181818
Mayport, FL	0.1	0.05	0.3	2.545455
St. Petersburg, FL	0.333333	0.4	1	1.909091
Cedar Key, FL	0.7	0.65	1.166667	4.636364
Pensacola, FL	0.222222	0.5	1.526316	3.363636
Galveston Pier 21, TX	0.4	0.705882	2.5	9.272727
Port Isabel, TX	0.15	0.4	0.85	3.090909
San Diego, CA	0.157895	1.052632	1.45	4.818182
La Jolla, CA	0.111111	0.941176	1.85	1.727273
Los Angeles, CA	0.3	1.368421	1.05	1.909091
Port San Luis, CA	0.625	1.368421	0.5	0.636364
San Francisco, CA	0.05	0.666667	0.7	0.090909
Seattle, WA	1.3	1.55	1.75	3.090909
Friday Harbor, WA	3.235294	2.9	2.55	4

Figure 2. Average Number of Coastal Flood Events per Year, 1950–2020
Data source: NOAA, 2021
Web update: April 2021
Units: average number of flood events per year

Key points

Flooding is becoming more common along the US coast. At every location measured, there has been an increase in coastal flooding since the 1950s (see Figure 1).

The growth rate is accelerating in most sites along the East and Gulf Coasts (see Figure 2).
The East Coast experiences the most frequent coastal flooding and has generally seen the most significant increase in flood days.

Since 2011, Boston, Massachusetts, has exceeded the flood threshold most frequently—an average of 13 days per year—followed by Bar Harbor, Maine, and Sandy Hook, New Jersey.

In more than half of the locations shown, floods are now at least five times more frequent than in the 1950s.
Flooding increased less dramatically in places where the relative sea levels did not rise as quickly as elsewhere in the United States.

Background

Changing sea levels affect human activities in coastal areas. Rising sea levels inundate low-lying wetlands and dry land, erode coastlines, contribute to coastal flooding, and increase saltwater flow into estuaries and nearby groundwater.

Higher sea levels also make coastal infrastructure more vulnerable to storm damage.

“Relative Sea Level Change” refers to the height of the ocean relative to the land at a particular location. As relative sea levels rise due to climate change, one of the most obvious consequences is increased coastal flooding.

Flooding usually occurs during high seasonal tides and storms pushing water toward the shore. In recent years, however, coastal cities have increasingly experienced flooding on days with less extreme waves or light winds, even on sunny days.

Flooding occurs more frequently because rising sea levels reduce the difference between mean sea level and land elevation. This type of tidal flooding is expected to increase in depth, frequency, and extent in the United States during this century.

Many coastal cities have defined “nuisance” flood thresholds. When the water rises above this level, some streets typically experience minor flooding impacts, many storm drains become ineffective, and coastal flood advisories may be issued.

Repeated coastal flooding can cause frequent road closures, reduced stormwater drainage capacity, and deterioration of infrastructure not designed to withstand periodic flooding or exposure to saltwater.

These impacts are particularly problematic because more than 40 percent of Americans live near the coast, and more than $1 trillion in property and structures are at risk.

Coastal flooding can also affect human health—for example, by increasing the chance that drinking water and wastewater infrastructure will fail and put people at risk of exposure to pathogens and harmful chemicals.

About the Indicator

This indicator tracks coastal flooding based on readings from tide gauges at locations along the US coast. These devices measure water levels continuously, making them ideal for measuring tide heights, monitoring long-term local relative sea level rise, and detecting coastal flooding.

This indicator focuses on 33 tide gauges where data from 1950 to the present are available. The hand tracks the number of days per year that each tide gauge measured water higher than the local flood threshold set by the National Oceanic and Atmospheric Administration.

Figure 1 shows the 33 stations on a map and compares data from the first and last decade, while Figure 2 provides a more detailed comparison using all years of data since 1950.

About the Data

Notes on indicators

Flood thresholds for this indicator are defined based on a statistical approach that considers typical local tidal fluctuations. This method is helpful because it allows scientists to calculate the flood threshold for any location along the coast if enough data has been collected.

However, these flood thresholds do not necessarily correspond to the observed disruption or damage level. Each city is unique in terms of land cover, topography, roads and other infrastructure, and the presence of flood defenses such as seawalls.

As a result of these differences, a flood that reaches the threshold in one city may cause minor inconvenience to only a few people. In contrast, water from the hall in another town may inundate large parts of a low-lying central area.

This indicator is limited to tide gauges with several decades of data. These tide gauges tend to be clustered along the east coast, especially in the mid-Atlantic region. Other parts of the US coast have fewer tide gauges with long-term data.