A storm surge is a rise in sea level that occurs during tropical cyclones, intense storms also known as typhoons or hurricanes. The storms produce strong winds that push the water into shore, which can lead to flooding. This makes storm surges very dangerous for coastal regions.

Tropical cyclones are circular storms characterized by high winds and heavy rainfall. They form over warm, tropical oceans. The center of a cyclone is called the eye. The eye is surrounded by a ring of clouds called the eye wall, where the winds are strongest. Surrounding the eye wall are clouds that spiral outward, called spiraling rain bands.

A storm surge is primarily caused by the relationship between the winds and the ocean’s surface. The water level rises where the winds are strongest. In addition, water is pushed in the direction the winds are blowing. The rotation of the Earth causes winds to move toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere—a phenomenon known as the Coriolis effect. If a cyclone develops in the Northern Hemisphere, the surge will be largest in the right-forward part of the storm. In the Southern Hemisphere, the surge will be largest in the left-forward part of the cyclone.

Another factor contributing to storm surge is atmospheric pressure. Atmospheric pressure is the force exerted by the weight of air in the Earth’s atmosphere. The pressure is higher at the edges of a cyclone than it is at the center. This pushes down the water in the outer parts of the storm, causing the water to bulge at the eye and eye wall—where the winds have helped add to the rise in sea level.

More factors contribute to the strength of a storm surge as the dome of water comes ashore. The water level can reach as high as 10 meters (33 feet) if the storm surge happens at the same time as high tide. The slope of the land just off the coast also plays a part: Water will more easily flood a shallow coast than a steep one.

Storm Surges and Coastal Communities

Tropical cyclones, and the storm surges they generate, are a serious hazard for coastal areas in tropical and subtropical regions of the world. Developing in the late summer months (July-August in the Northern Hemisphere, January-February in the Southern Hemisphere), when the waters are warmest, tropical cyclones hit regions as far apart as the Gulf Coast of the United States, northwestern Australia and Bangladesh.

When a cyclone hits land, the accompanying storm surge will most often flood the surrounding coastal area. Flooding is responsible for most deaths and economic damage associated with tropical cyclone landfalls. When a hurricane hit Galveston, Texas, in 1900, the storm surge was responsible for approximately 6,000 deaths. In East Pakistan (now Bangladesh), the Bhola cyclone killed as many as 500,000 people in 1970. The storm surge from the Bhola cyclone was estimated to be 10 meters (33 feet) high.

Improvements in forecasting cyclones and issuing early warnings to the public have become indispensable as both coastal populations and the occurrence of extreme storms continue to rise.

However, even sophisticated meteorology and storm warnings do not always protect against devastating storm surges. Hurricane Katrina’s storm surge flooded the U.S. coastal communities of Louisiana and Mississippi, as well as the urban areas of New Orleans, Louisiana, and Biloxi, Mississippi, in 2005. The flooding killed more than 1,500 people in New Orleans alone, and caused millions of dollars in damage. Homes, businesses, schools, and hospitals were destroyed.

Still, improvements in forecasting greatly benefit regions like the Chesapeake Bay, in the U.S. states of Maryland and Virginia. The Chesapeake Bay suffered severe damage from Hurricane Isabel in 2003. Emergency managers failed to predict Isabel’s extreme storm surges, which caused widespread flooding in the region.

Now, meteorologists and emergency managers monitor the storms forming in the southeast more closely. New computer simulations by the Federal Emergency Management Agency (FEMA) examined the effects of a Category 4 hurricane (131-155 mph winds) landing in the U.S. states of North or South Carolina, hundreds of miles south of the Chesapeake. The simulation showed the hurricane could produce storm surges as high as 5 or 6 meters (18 or 20 feet) along the Chesapeake shoreline. FEMA used the latest version of its computerized SLOSH model to predict the surge. An acronym for "Sea, Lake, and Overland Surges from Hurricanes,'' the SLOSH program is used by the National Weather Service and the National Hurricane Center.

The Maryland Emergency Management Agency is now working with the U.S. Army Corps of Engineers and local officials to turn the SLOSH simulation data into updated maps for emergency planning. The new maps show how far inland flooding could stretch under certain conditions. With this new forecasting data, emergency planners and citizens will be better prepared for storm surges.

Government agencies can arrange for residents to evacuate. Some residents don’t have cars and may require transportation to safer ground. Others may not have a place to go and require emergency shelter. Many residents, such as those in hospitals and prisons, are especially at risk.

Advance warning of a strong storm surge also allows homes and businesses to prepare for damage. Business owners can relocate expensive machinery or tools to safe areas, and homeowners can board up windows to protect against strong floods or move furniture and other valuables to a second story.

Wetlands

Coastal residents can reduce the damage done by a storm surge by protecting local wetlands. Wetlands, such as swamps, estuaries, and mud flats, act as sponges for tropical cyclones. As the cyclone makes landfall, the marshy land and plants absorb the water and the energy of the storm surge. Silt and swamp vegetation prevent the most intense part of the storm surge from hitting homes and businesses.

Estuaries are such an important part of our natural environment that the U.S. Environmental Protection Agency (EPA) created the National Estuary Program in 1987. This program seeks to improve the quality of estuaries to protect important wildlife habitats, public water supplies, and coastal regions affected by severe storms.

The development of coastal wetlands for housing, industry, or agriculture reduces the natural barrier that wetlands provide. Communities can protect themselves against storm surges by maintaining healthy coastal wetland ecosystems.