Flooding Weather

A flood is a substantial rise in water that covers areas not usually submerged. A flood occurs when water flows into a region faster than it can be absorbed (i.e., soaked into the soil), stored (in a lake, river, or reservoir), or removed (in runoff or a waterway) into a drainage basin. Common causes for floods are high-intensity rainfall, prolonged rainfall, or both.

Floods pose the greatest weather-related threat to human life, killing over 130 people each year in the United States. Not all floods are associated with thunderstorms, although most flood-related deaths in the United States result from floods caused by slow-moving thunderstorms or series of thunderstorms that move over the same region. Hurricanes can also produce flooding. Thunderstorms are of particular concern because they can produce a very dangerous condition known as a flash flood.

A flash flood is a sudden, local flood that has a great volume of water and a short duration. Flash floods occur within minutes or hours of heavy rainfall, or because of a sudden release of water from the break-up of an ice dam or constructed dam. The worst flash flood in the United States occurred in Johnstown, Pennsylvania, on May 31, 1889. A dam broke as a result of heavy rain and structural problems, resulting in a 12-meter (36- to 40-foot) wall of water that swept through the town and killed 2200 people.

Rainfall intensity and duration are two key elements of a flash flood. Topography, soil conditions, and ground cover also play important roles. Steep terrain can cause rain water to flow toward and collect in low-lying areas, causing water levels to rise rapidly. If the soil is saturated with water, it cannot absorb more and so the excess water runs off the land quickly. On the other hand, extremely dry soil conditions can also be favorable for flooding. Dry soil often can develop a hard crust over which water will initially flow as if the ground were concrete. Flash floods occur within about six hours of a rain. A flood is a longer-term event and can last weeks or months. Record flooding along the upper Mississippi River in the summer of 1993 resulted from prolonged rains in the upper Midwest of the United States and caused up to $10 billion in damage. These kinds of floods occur during blocking events (Chapter 7) in which thunderstorms develop and move over the same region repeatedly. The worst such flood in U.S. history occurred in 1927, when nearly all the tributaries of the Mississippi breached their banks because of thunderstorm rains. Seventy thousand square kilometers (27,000 square miles) of land along the lower Mississippi was flooded to depths of 30 feet, 700,000 people were left homeless, and the $1 billion in damage was nearly a third of the U.S. government's annual budget at the time!

Floods are natural phenomena and do have benefits. Large seasonal floods have resulted in productive farmland, such as in central North America and along the Nile river, by bringing nutrient-rich, fine soil to the flooded region. Floodwaters also refill wetlands and replenish groundwater. While flooding is a natural event, humans increase the likelihood of flooding by changing the character of the land through such actions as paving with asphalt and removing vegetation on hillsides. This promotes rapid runoff and flooding.

Flash Flood Safety

Stay away from streambeds, drainage ditches, and culverts during periods of heavy rain. Move to high ground when threatened by flooding. Stay out of flooded areas. Never drive your car across a flooded road, even if you think the water is shallow. Most flash-flood-related deaths occur when people drive into floodwaters. Never underestimate the power of moving water!

Keep safety in mind even after the storm passes. Stay clear of downed power lines. Do not touch them. If you smell gas or suspect a gas leak, turn off the main valve, get everyone out of the structure quickly, and open the windows. If there is a power outage, use flashlights instead of candles, which have open flames that might start a fire. After high winds use caution when walking around trees, as trees and tree limbs may be weakened and could fall unexpectedly. Deal with immediate problems, such as helping injured people, until professional help arrives.