Flood and Flash-Flood Basics
Floods affect most communities and cause more damage than any other natural disaster in the United States (excluding droughts). Floods can be caused by one or more of the following:
- Prolonged rainfall over a long period
- Intense rainfall over a short period
- Dam failure
- Ice or debris jam — debris or ice floating in a river or stream can act like a dam and cause flooding.
- Development of wetlands, which prohibits absorption of water — paved areas increase runoff by two to six times.
- Disturbance of a natural bank of a river or stream
What Is the Difference Between a Flood and a Flash Flood?
Floods usually develop over a period of hours or days. They can be fast-rising or slow-rising. Flash floods are sudden events that can occur without warning. They are caused by a dam break or when water is suddenly released from an ice or debris jam. They also can be caused by an excessive amount of rain from a thunderstorm, tropical storm or hurricane. A flash flood occurs within six hours of a rain event.
When and Where Do Floods Occur?
Floods occur throughout the world, except Antarctica. They affect all 50 states and most communities in the United States. About 7% of the country, an area equal to the size of Texas, is at risk. The peak time for flooding in the United States is from April through September, but flooding can happen at any time.
Why Is Flooding Dangerous?
Water is heavy (62 pounds per cubic foot) and can move at speeds of 6 to 12 miles per hour. Just 6 inches of fast-moving water can knock a person down. Water also can move vehicles. For every foot of water, 1,500 pounds are displaced. Therefore, 2 feet of water is enough to move a car. Floodwater can reach 10 to 20 feet deep. Drowning is the No. 1 cause of death during a flood.
Flooding is also dangerous because it can wash out roads and cause dangerous debris flows, causing serious injuries for those who attempt to cross floodwater. Downed power lines under the floodwater are also a hazard; electrocution is the second leading cause of death during floods.
Floodwater is also likely to be contaminated by chemicals (such as pesticides), oil, and other runoff from roads, sewage and silt.
Bacteria, viruses and other microorganisms contaminate floodwater and cause diseases such as shigellosis, hepatitis and cholera.
Additionally, floods can contaminate the tap water supply, making it dangerous. You cannot know if water is contaminated just by
seeing, smelling or tasting it.
Resources for Floods and Flash Floods