Why Storm Surge Is Dangerous—And Becoming More Frequent

How hurricanes push water onto shore in deadly storm surges, and why storm surges are getting worse

By Clara Moskowitz, Jen Christiansen & Daniel P. Huffman edited by Jen Christiansen & Clara Moskowitz

Schematic shows a hurricane over the ocean, with a large wave on the leading edge flooding the coast. — Jen Christiansen

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The beginning of June marks the start of the Atlantic hurricane season, the six-month period when strong storms can brew in the ocean and then wreak havoc on land. Among the hazardous consequences of hurricanes are storm surges, in which water rapidly rises above the normal tide level on shore. These dangerous events can cause flooding and pick up and displace homes and other structures. “Water is very powerful,” says Heather Nepaul, a meteorologist at the National Oceanic and Atmospheric Administration’s National Hurricane Center in Miami. “It can be a deadly situation.”

Surges occur when the strong winds of a hurricane interact with ocean waters, piling up water ahead of the storm. As the hurricane heads toward shore, it travels over shallower ocean, and the water it carries has nowhere to go but upward onto land.

How severe a surge will be depends on many factors, including the characteristics of the coastline and the intensity, size and angle of approach of the storm. In general, though not always, stronger and larger storms produce higher storm surges.

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As the climate warms, hurricanes are becoming more intense, and sea levels are rising. Both of these effects are likely to worsen storm surges. Coastal areas that are already vulnerable to storm surge could experience worse impacts, and places that aren’t quite vulnerable now may become increasingly at risk.

HOW IT WORKS

The bulk of a storm surge is caused by wind pushing water ashore. A small part of the effect, however, results from the low atmospheric pressure inside a storm, which decreases the amount of downward force on the ocean, triggering a rise in water level.

Schematic shows a hurricane approaching the coast, with a large wave on the leading edge. A small bulge in water level is immediately under the eye of the storm.

As the storm advances, its spiral of air pulls ocean water up into its center. When it nears land, the excess water surges over the shore above and beyond the normal tide level.

The same scene as above, but the hurricane is closer to shore. The wave breaks up and over the coast, flooding land and destroying property. Storm surge is sea-level rise beyond the predicted astronomical tide at that time. — Jen Christiansen

VARIABLES THAT AFFECT STORM SURGE HEIGHT

The severity of storm surges is hard to predict because it depends on so many variables: the speed and radius of the wind associated with the storm, the hurricane’s size, the speed and angle at which the storm approaches land, and the specific shape of the shoreline where it hits.

A table of variables indicate that faster wind speed, larger wind radii and gently-sloping wide shelves generally lead to higher storm surge heights. — Jen Christiansen

STORM SURGE RISK

These charts, based on the National Hurricane Center’s National Storm Surge Risk Maps, show the potential amount of surge along the U.S. Atlantic coast for hurricanes of different severity. Storms are categorized along a scale from 1 to 5 based on the strength of their winds, with 5 being the most intense. Surges can affect not just the coastline but locations many miles inland.

A series of three maps show possible storm surge heights along the Atlantic coast for category 1, 3, and 5 hurricanes. — Daniel P. Huffman; Source: “A National View of Storm Surge Risk and Inundation,” by Brian C. Zachry, William J. Booth, Jamie R. Rhome and Tarah M. Sharon, in Weather, Climate, and Society, Vol. 7, No. 2; April 2015; via www.nhc.noaa.gov/nationalsurge (*map data*)

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