How much rain does it take to make a flood?

The amount varies widely for different areas. In some U.S. western deserts, or in some large urban areas, just a few minutes of strong rain will cause a flash flood in canyons and low-lying areas. In areas prone to greater rainfall amounts, it often takes quite a bit more rain (sometimes a few days' or weeks' worth) to cause rivers to overflow and dams to fill up, raising concerns of those who live downstream. Areas that normally receive more rainfall have better natural drainage systems and are usually home to plants that readily absorb the extra water.

What have been some of the most destructive floods in history?

In the United States, the failure of a dam in 1889 upstream from the community of Johnstown, Pennsylvania, killed 2,200 people. Some of the world's most catastrophic flooding takes place in China. A flood on the Huange He River in 1931 killed 3.7 million people.

What is a floodplain?

A floodplain is the area surrounding a river that, when unmodified by human structures, would normally be flooded during a river flood. A floodplain can be a few feet or many miles wide, depending on the river flow as well as the local terrain. Even though

Why do people live in floodplains?

People have lived in floodplains for thousands of years. Fertile land for agriculture lines the floodplain, and the nearby water source makes life easier. Unfortunately, when the river does flood, these communities are severely damaged and people suffer. Hazard mitigation, such as levees, dams, dikes and other structures, attempt to limit damage during floods. Sometimes, when the structures fail (such as a levee breaking), large areas are inundated with water. Inhabitants of floodplains must balance the risks with the rewards of living in such an unpredictable environment.

levees and flood walls can be built (with homes and businesses built just behind them), the floodplain does not vanish. If the structures break or are damaged, the water from a flood can fill a floodplain, just as it did before humans occupied it.

What is a 100-year flood?

A 100-year flood refers not only to the size of a flood, but also to the odds of it occurring. A 100-year flood has a one percent (or 1 in 100) chance of occurring in any given year. It has no relationship to the frequency of occurrence. The magnitude of such a flood is relative to the frequency of occurrence, so a 100-year flood is much larger than any run-of-the-mill annual flood. A 500-year flood only has a one in 500 (0.2 percent) chance of occurring in any given year and would be much larger and more devastating than a 100-year flood.

What is the National Flood Insurance Program?

The National Flood Insurance Program (NFIP) was established by the U.S. federal government in 1956 as a subsidized insurance program for home and business owners. The government began the program by creating Flood Insurance Rate Maps (FIRM) showing the boundaries of 100-year and 500-year flood zones. The cost of the insurance is based on the flood risk. The Federal Emergency Management Agency (FEMA) oversees the program and requires the purchase of flood insurance by any owner affected by a disaster before they can be provided with disaster assistance. This way, the next time a flood occurs, they will be insured.

How can I obtain a flood map of my community?

The best way to see a Flood Insurance Rate Map (FIRM) for your area would be to contact your local government. Their planning or emergency management agency should have the FIRM maps available. Purchasing them from FEMA is not recommended because the maps change often and are best interpreted by a planning or emergency expert.

What should I do in the event of a flood?

If a flood is expected, turn on your battery-powered radio and listen for information about when and where to evacuate. If a flood or flash flood is coming toward you, move quickly to a higher elevation—but don't ever try to outrun a flood. Also, don't drive through standing water, as it can quickly rise and stall your vehicle, possibly trapping you among swift water.

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