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Tropical Cyclone Hazards: Inland Flooding

In addition to the storm surge and high winds, tropical cyclones threaten the United States with their torrential rains and flooding. Even after the wind has diminished, the flooding potential of these storms remains for several days.

Flooding from Tropical Storm Allison - ©2002 Harris County Flood Control DistrictSince 1970, nearly 60% of the 600 deaths due to floods associated with tropical cyclones occurred inland from the storm's landfall. Of that 60%, almost a fourth (23%) of U.S. tropical cyclone deaths occur to people who drown in, or attempting to abandon, their cars.

Also, over three-fourths (78%) of children killed by tropical cyclones drowned in freshwater floods. In fact, more people are killed by floods than any other weather related cause. Most of these fatalities occur because people underestimate the power of moving water.

It is common to think the stronger the storm the greater the potential for flooding. However, this is not always the case. A weak, slow moving tropical storm can cause more damage due to flooding than a more powerful fast moving hurricane. This was very evident with Tropical Storm Allison in June 2001.

Allison, the first named storm of the 2001 Atlantic Hurricane Season, devastated portions of Southeast Texas, including the Houston Metro area and surrounding communities, with severe flooding. Allison spent five days over Southeast and East Texas and dumped record amounts of rainfall across the area. Allison deposited up to three feet of rain to the east and northeast of Houston, Texas during a 5-day period. This map of Harris County shows the heaviest rainfall distribution.

In addition to the storm surge, tropical cyclones can, and usually do, cause several types of flooding.
Flash flooding
Flash floods are rapid occuring events. This type of flood can begin within a few minutes or hours of excessive rainfall. The rapidly rising water can reach heights of 30 feet or more and can roll boulders, rip trees from the ground, and destroy buildings and bridges.
Urban/Area floods
Urban/Area floods are also rapid events although not quite as severe as a flash flood. Still, streets can become swift-moving rivers and basements can become death traps as they fill with water. The primary cause is due to the conversion of fields or woodlands to roads and parking lots. About 10% of the land in the United States is paved roads. So, water that would have been absorbed into the ground now runs into storm drains and sewers.
River flooding
River floods are longer term events and occur when the runoff from torrential rains, brought on by decaying hurricanes or tropical storms, reach the rivers. A lot of the excessive water in river floods may have began as flash floods. River floods can occur in just a few hours and also last a week or longer.
Next: Tropical Cyclone Hazards - Tornadoes

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