Anatomy of a Storm Surge

The greatest natural disaster in the history of the United States was caused by storm surge. In September 1900, an unnamed hurricane brought sustained winds over 120 mph and a 15 1/2-foot storm surge to Galveston Island, Texas. No part of the island was left dry as even the highest elevation home only rested at 8-9 feet above sea level. The massive storm surge gathered everything to form a moving wall of debris and water over 2 stories high, demolishing everything in its path. Between 6,000-12,000 people lost their lives and no one was left unaffected by either loss of family or property.

Hurricanes are defined by their wind-driven potential for damage to life and property. Many people think that wind is the only concern in a hurricane. Water that is pushed onto land by hurricane force winds is called storm surge. It is responsible for 9 out of 10 fatalities during a hurricane. Surges often cause uncontrollable flooding in low-lying coastal areas and are considered to be the most dangerous aspect of any hurricane.

Storm surges aren’t simply wind driven waves. Many factors play into the creation of monster storm surges that wreck shorelines and lives. Surge is created when the water is churned up, creating friction between the air and water that creates a piled up wave. This wave can increase in size with onshore winds, high tides, and low atmospheric pressure during a storm. In addition, the slope of the underwater topography or continental shelf plays a roll in the height of a storm surge. The gently sloping shelf of the Atlantic and Gulf coasts allows water to pile up and overflow across low lying coastal areas.

In 1900, Galveston Island was devastated by just such a storm surge from a hurricane. Picture a rolling wall of water, almost riding along the surface of the ocean. Only the strongest buildings survived the impact with a storm surge. Many building only survived when the rolling wall of water and debris finally stopped, forming a barrier of protection. Since most of the Atlantic and Gulf coasts lie at less than 10 feet above sea level, these areas are considered at great risk during hurricanes.

The shape of the shoreline can also affect the height of a storm surge. Bays and inlets are particularly susceptible because surging water is funneled into small areas, producing an even greater buildup of water that rushes ashore. Storm surge can inundate a low-lying area in record time. Many forecasters recommend evacuations of coastal areas based on projected storm surges.

For perspective, the worst storm surge occurred in Bangladesh in 1970. The wall of water inundating the low-lying coastal areas was over 13 meters high. Hurricane Katrina, a recent U.S. storm, exhibited a storm surge of 7.6 meters at St. Louis Bay, Mississippi. Basic storm surge is measured, without including winds and tides, at 1 meter. Adding in the influence of winds and tides often takes the surge height above 8 meters.

Storm surge is measured specifically to an area. Tidal influences must be taken into account to accurately measure the volume of water rushing onshore. The measurement is the difference between the projected tide and the highest observed rise in the level of water. Because of the immense danger involved the National Weather Service now issues storm surge warnings in addition to providing continually updated hurricane wind speed and path information.