Floods occur when a normally drier land area is temporarily submerged in water overflowing from rivers, dams (barrier to contain the flow of water), runoff, or tides. Runoff is water that accumulates and flows after heavy rainstorms or snowmelts.
Floods occur in all fifty states and around the world. Floods can be caused by several factors: heavy rainfall over a short period, moderate rainfall over a long period, melting snow, hurricane storm surge (a dome of water that builds up as a hurricane moves over water), ice or debris jams on rivers, and dam failures.
Floods can cause great harm to people and property. Floods are the deadliest form of natural disasters, killing more Americans every year than tornados, lightning, earthquakes, and forest fires combined. Due to the potential harm, government agencies work to prevent and predict floods.
Scientists describe floods according to three criteria: the maximum height of the water above normal levels during the flood, the time period required for the flood waters to rise and fall, and the size and frequency with which similar floods are likely to occur.
Height of the floodwater. In describing the maximum height of a flood, scientists refer to the crest of a river (its maximum height during a flood) or the height of the floodwaters over the floodplain. A floodplain is a low-lying area near a water source that is normally dry, but is subject to overflow by a river, lake, or water from a man-made water barrier.
In Florence, Italy in 1966 for example, the Arno River crested at about 20 feet (6 meters) over flood stage (the level at which water leaves its banks and begins to flood), and filled the nearby Santa Croce church, home to countless masterpieces of art, with over 8 feet (2.4 meters) of water and mud. Watermarks indicating the height of the flood are still visible today along the walls of the church.
When describing floods according to the time period required for the water to rise and fall, scientists often refer to flash floods. Flash floods are floods that occur in only a few hours or even minutes, usually due to heavy rainfall or a dam break.
Flash floods can also occur when ice or debris to obstruct the flow of river water, causing water to back up upstream. When the ice or debris breaks loose, a wall of water rushes downstream and can cause flash floods.
Speed of the flood. Flash floods are dangerous because they are usually accompanied by fast moving water. Fast moving water usually occurs in drainage ditches, canyons, and in rivers and creeks. Flash floods can produce fast-moving walls of water up to 20 feet (6 meters) high.
However, it does not take 20 feet of water to cause death or major damage. Two feet of fast moving water can wash away cars, and flash floodwaters only 6 inches (15 centimeters) deep have knocked down people.
Weather forecasters are especially alert to notifying the public about conditions that are favorable for the development of flash floods, but people should also be wary of low-lying roads and other areas during heavy rains.
Floods that are slower to develop than flash floods can also be just as deadly and damaging. These floods occur when an area receives moderate rainfall over a period of days or weeks, and streams and rivers accumulate more water than they can handle.
As the water has nowhere else to go, it spills over the banks of the river and onto the floodplain. Flooding can even occur where no rain has fallen. Bulging rivers can force floodwaters tens or even hundreds of miles (kilometers) downstream.
Size and frequency of the flood. Scientists also categorize floods according to the size of the flood and the likelihood of another similar flood in the same place within a one-year time-frame.
For example the 1966 flood in Florence, Italy was described as a 100-year flood, which means that the chance of a similar flood occurring within the same year in Florence was 1 in 100.
A 5-year flood has a 5 in 100, or 20% chance of a similar flood happening in the same place within a year. The classifications of 5-year flood, 10-year flood, 25-year flood, 50-year flood, 100-year flood, or 500-year flood, actually refer more to size of the flood than to predictions when a similar one will happen again in the future.
The water volume of the flood increases along the scale as the frequency decreases. A 5-year flood is almost always a mild occurrence, whereas a 500-year flood tends to have very high and violent water flow, and covers a wide area of the floodplain.
Since the 1970s, an average of 140 Americans have died in floods every year. The deadliest flood in American history occurred in Johnstown, Pennsylvania, a town that was built on the floodplain near the fork of the Little Conemaugh and Stony Creek Rivers. On May 31, 1889 an upstream dam on the Little Conemaugh ruptured.
The wall of water that rushed downstream resulted in a flash flood that killed 2,200 people when it reached Johnstown. In 1972 232 people died in Rapid City, South Dakota, during a flash flood. Many of the people that were killed were spectators who were swept away while admiring the floodwaters.
Floods can also cause significant property damage. Flood damage includes damage and destruction of houses, businesses, automobiles, crops, and personal property. Floods are responsible for over $2 billion of damage every year in the United States.
The Great Midwest Flood of the Mississippi and Missouri Rivers in 1993 caused between $15 and $20 billion in damages and 50 deaths. This slow-forming flood covered thousands of square miles (kilometers) with water in most Midwestern states.
Residents downstream placed sandbags along the banks of the river to create a barrier before the floodwaters reached them, but the river engulfed large parts of farmland and cities all along its floodplain. The floodwaters remained for 144 days.
Flood watches and warnings
The National Weather Service issues four types of flood warnings: flood watches; flash flood watches; flood warnings; and flash flood warnings. The National Weather Service is a government agency that predicts the weather and warns the public of dangerous weather situations. Knowing what these watches and warnings mean can save lives.
A flood watch means that weather conditions are favorable for floods to form. A flood watch is usually issued during long periods of moderate rainfall. A flash flood watch means that flooding could occur within six hours or less after the rain ends. Flash flood watches are usually issued during periods of heavy rainfall when waters may rise quickly.
A flood warning means that flooding is occurring. Rivers and creeks are rising. People in low-lying areas should be prepared to evacuate if waters in their area begin to rise.
A flash flood warning means that water levels are rising quickly and will threaten life and property within six hours. Upon receiving a flash flood warning, people in low-lying areas or areas near creeks and rivers should move to higher ground immediately.
Forecasters advise people to remain indoors during a flood watch or warning and go out only if necessary to evacuate. Keep a weather radio nearby and have an escape plan. Never walk through running water; find a way around it. Even a small creek or drainage ditch can be deadly during a flood.
Also, never drive across a road that has water on it, even if it looks safe to do so. Judging the depth of water over a road is difficult, especially at night. In the United States more flood victims die in their cars than anywhere else.
Given the awesome and destructive power of flood waters, humans have long sought to tame rivers and streams to prevent future flooding. Rivers that have many streams that feed them are more likely to flood than rivers with few streams.
In the United States, the Mississippi River drainage basin accommodates more water than any other drainage basin. A drainage basin is an area of land in which all of the creeks, streams, and rivers drain into a common source. The Mississippi River drainage basin is the third largest drainage basin in the world.
Approximately 41% of all water in the lower 48 states drain into the Mississippi River. The Mississippi River drainage basin covers over 1.2 million square miles (3.1 million square kilometers) and all or part of 31 states.
This results in the Mississippi River handling a great amount of water. Flood waters anywhere in the drainage basin will eventually all flow into the Mississippi River, potentially causing it to flood as well.
Spanish explorers recorded flooding of the Mississippi River in 1543. The floodwaters remained for 80 days. Flooding in the early twentieth century devastated the lower Mississippi River drainage basin. In 1927 floodwaters covered over 26,000 square miles (67,300 square kilometers).
This huge flood led Congress to pass a law called the Flood Waters Control Act of 1928, which sought to control the waters of the lower Mississippi River. The result of this law was the Mississippi River and Tributaries Project.
The project sought to allow the Mississippi River to handle a flood 11% larger than the flood of 1927. The project was designed to prevent flooding along about 600 miles (957 kilometers) of the river from Cape Girardeau, Missouri to New Orleans, Louisiana.
The project led to the construction of a series of dams and levees (walls built to hold back and channel flood water) to control the Mississippi River. Deep channels called floodways also provide a path for floodwaters.
Dams can be opened to allow water out of the river and into these floodways. Most floodways are located near areas that flood easily (low-lying floodplains).
The Tennessee Valley Authority, a government agency that controls dams and generates electricity, also runs a flood control program. The Tennessee Valley Authority controls 34 dams that prevent flooding along the Tennessee River, Ohio River, and Mississippi River.
The dams release water from reservoirs before flood season. A reservoir is the water that is backed up behind a dam. Lowering the reservoirs allows them to hold more water during the flood season.
The floodwaters are then slowly released from behind the dams to prevent flooding downstream. The Tennessee Valley Authorities flood control system prevents over $200 million in flood related damage every year.