Storm Surge

Storm surge—abnormally high water levels generated by severe storms—can produce sea levels much higher than normal high tide, resulting in extreme coastal and inland flooding.
Hurricane Sandy Causes Flooding in New York City Subway Stations

Storm surge refers to abnormally high water levels generated by severe storms such as hurricanes, cyclones, and nor'easters. A surge forms when strong winds over the ocean combine with low pressure to drive water onshore. Storm surges can produce sea levels much higher than normal high tide, resulting in extreme coastal and inland flooding. Sometimes called "storm tides," storm surges can cause tremendous damage; if they coincide with high tide, they can raise water levels by 20 feet or more above mean sea level. As a result of global sea level rise, storm surges that occur today are eight inches higher than they would have been in 1900. By 2100, storm surges will happen on top of an addtional 8 inches to 6.6 feet of global sea level rise.

Illustration Depicting Storm Surge

 

Water weighs about 1,700 pounds per cubic yard, so extended pounding by wind- and tide-driven waves moving at 10 to 15 mph can damage or destroy any structure not built to withstand such forces. Storm surges driven by strong hurricanes or extra-tropical storms can cause deaths and extensive property loss, including erosion of beaches, damage to coastal habitats, and undermining the foundations of vital infrastructure like roads, railroads, bridges, buildings, and pipelines. Hurricane Katrina (2005) is a prime example of the damage and devastation that is possible. At least 1,500 people lost their lives during Katrina, and many of those deaths occurred either directly or indirectly because of storm surge. Katrina also caused well over $100 billion in damage from its surge and winds.

Hurricane Katrina

Hurricane Katrina was one of the most powerful storms to strike the United States, with winds of 160 miles per hour (257 kilometers per hour) and stronger gusts. The massive storm covers much of the Gulf of Mexico, spanning from the U.S. coast to the Yucatan Peninsula.

The width and slope of the continental shelf where a storm comes ashore are key factors in determining whether and how severely a storm surge will impact the coast. Shallow sloped lands will likely experience a greater storm surge than steep sloped lands (to view animated clips, click here to see storm surge on shallow sloped lands and click here to see storm surge on steep sloped lands).

In order to clearly communicate the potential impacts of flooding from storm surge, National Weather Service predictions for inundation from storm surge are described in terms of height above ground level. For example, a storm surge prediction of 20 feet above ground level for a particular area means that forecasters expect 20 feet of water to cover that area. The depth of the flooding above ground level will gradually decrease as it moves further inland. However, as some coastal regions have vast areas of low-lying land inland from coast, it's important for residents and visitors to be aware of local conditions, and know how far inland local authorities consider lands to be vulnerable from various heights of storm surge.

Wave impacts

Map Showing Possible Future Flood Depths in Mobile, AL, with Rising Sea Level

Many coastal areas in the United States, including the Gulf Coast, are especially vulnerable to sea level rise impacts on transportation systems. This is particularly true when one considers the interaction among sea level rise, wave action, and local geology. This map shows that many parts of Mobile, Alabama—including critical roads, rail lines, and pipelines—would be exposed to storm surge under a scenario of a 30-inch sea level rise combined with a storm similar to Hurricane Katrina. 

Winds from coastal storms generate waves in the ocean. The stronger the wind, the larger the waves. As these waves move toward the shore, wave heights increase due to a process called “shoaling” as they interact with the ocean bottom. As the wave heights increase, the waves eventually break and impart their momentum to the water, causing onshore flow near the surface and offshore flow or “undertow” near the bottom, and an overall increase in elevation in water level at the coast. Two processes add to this elevation increase: “wave setup,” or the rise in the water surface caused by breaking waves, and “wave runup,” which is the rush of wave water up a slope or structure. Wave run-up/setup is often modeled by coastal engineers for project design to make sure structures are built to withstand strong waves, and that overtopping will not occur. Setup and run-up are also major causes of beach erosion and overtopping of man-made and natural coastal structures (like dunes).

Banner Image Credit
Aerial views of the damage caused by Hurricane Sandy to the New Jersey coast taken during a search and rescue mission by 1-150 Assault Helicopter Battalion, New Jersey Army National Guard, Oct. 30, 2012. This appears to be Casino Pier, Seaside Heights, New Jersey. By Master Sgt. Mark C. Olsen/U.S. Air Force/New Jersey National Guard (Flickr). CC BY 2.0, http://creativecommons.org/licenses/by/2.0, via Wikimedia Commons
Last modified
6 October 2017 - 11:45am