The Real Dangers of Dolly: Rain and Floods

Tornado Science, Facts and History

As Hurricane Dolly batters the coasts of Texas and Mexico, it's not the whipping winds that residents should be most worried about. It's the water.

Storm surge accounts for the majority of hurricane-related deaths.

As history and studies of severe weather have shown, storm surge and torrential rains can wreak havoc on coastal communities; the power of water, along with the suddenness with which floods can happen, often catches people by surprise, with potentially devastating consequences.

Flooding, which can result from both torrential rains and the ocean surf a storm can push ashore, is what officials in Texas were most concerned about after Dolly made landfall as a Category 2 hurricane at 9 am EDT on Wednesday. Residents of the Rio Grande Valley have been warned of potential severe flooding and possible levee breaks.

The National Hurricane Center predicted that Dolly could drop a total of 6 to 10 inches (15 to 25 centimeters) of rainfall in the next few days, with some isolated spots potentially getting soaked by up to 15 inches.

When an area receives such a large amount of rain in a short period of time, it can cause flash flooding. As the name implies, these floods can happen suddenly, with water rising several feet in a matter of minutes and taking unsuspecting communities by surprise.

On average over the last 30 years, more people have died as a result of flooding than from lightning, tornadoes or hurricanes, according to the National Weather Service. And almost half of all fatalities that result from flash flooding are vehicle-related, as rapidly rising waters can quickly inundate roads. Many drivers try to cross the water, only to be swept away in the current — it only takes about 2 feet of water to float a car, and 500 pounds of force pushes on a car for each foot of water rise, according the National Oceanic and Atmospheric Administration.

Pedestrians fare no better: Six inches of fast-moving water can knock a person off their feet. Not to mention the debris that can be carried along by the floods, which can often tear down trees and roll boulders.

Flash floods typically occur when a slow-moving thunderstorm or large storms such as a tropical storm or hurricane pelt an area with rain so fast that the ground can't soak all the water up. Flooding can be particularly bad in urban areas, because roads and parking lots don't soak up as much water as soil might, turning streets into rivers. (An intense thunderstorm that dumped 1.5 inches of rain over Manhattan in just one hour in August of 2007 flooded the subway system and several stations, crippling the city for several hours.)

Another watery force, storm surge, is viewed as the most destructive part of a hurricane. Storm surge can build for hours before a hurricane makes landfall as the storm's winds push against the ocean and cause the water to pile up higher than the sea's usual level.

As the hurricane makes landfall, the water is pushed onshore and can quickly wash many miles inland, destroying homes and businesses. The storm surge from Hurricane Katrina, for example, was estimated to have reached heights of 24 to 28 feet (7 to 8.5 meters) along a 20-mile swath of the Gulf Coast and washed up to 12 miles inland, devastating the Mississippi coast.

The National Weather Service warned coastal residents of Texas that storm surges of 4 to 6 feet above normal tide level could inundate the coast north of the center of the storm and that the storm would bring with it "dangerous battering waves."

While Dolly is unlikely to be the disaster that Katrina was, officials are still warning residents of low-lying areas to stay in shelters, out of the way of floods and storm surge.

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