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The Main Causes of Floods

Floods were the most devastating natural disaster to affect America in the 20th century in terms of property damage and lost lives, according to the U.S. Geological Survey. Areas that experience flooding are usually dry, broad and flat, and often situated below sea level or close to a body of water that is susceptible to overflowing. Each flood is unique, developing in its own way and bringing unknown debris, dangers and duration.

Excessive Rains
- Heavy rains can occur in almost any area unexpectedly, causing what is known as flash floods. In August 1969, Tropical Storm Camille combined with a large system of thunderstorms and caused extensive flooding throughout the Tye and Rockfish River valleys in Virginia. Some areas saw more than 31 inches of rain in just six hours, and widespread flooding occurred as rain and overflowing rivers filled the low valleys.
River Runoff
- Rivers overflow due to heavy, sudden rainfall, excessive snowmelt, or a breached dam or levee. When a river or stream reaches its defined capacity, excess water runs off into surrounding low-lying areas. More than 200 square miles along the Trinity River in Texas flooded in May 1990 due to river runoff. Each summer the Nile River in Egypt floods, depositing fertile silt and soil among its extended network of riverbanks.
Storm Surge
- Wind and offshore storms can cause the ocean to overflow onto dry land, creating a flooding phenomenon known as storm surge. Water from storm surge can reach depths greater than 20 feet. These floodwaters are often the result of intense, low-pressure storm systems and hurricanes moving slowly along coastlines. In September 1900, Galveston, Texas, experienced one of the nation's worst natural disasters when storm surge flooding from an unexpected hurricane killed more than 6,000 people.
Tsunamis
- Giant tidal waves, or tsunamis, can cause widespread flooding, often with little or no warning. Generated by underwater landslides, volcanoes and earthquakes, tsunamis can travel thousands of miles at high speeds without losing any energy. When tsunami waves reach shallow water and shorelines, they slow down and build in height, from 50 to 100 feet, before breaking on land, causing shoreline damage and inland flooding. The Indian Ocean tsunami in December 2004, triggered by a 9.0 magnitude earthquake off the coast of Indonesia, inundated coastal communities and killed more than 225,000 people in 14 countries.

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