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Flood Education


Types of Floods

Flood is caused by heavy rain, snow melting too fast, dams or levees breaking, or ocean waves coming ashore. Some floods can develop over a period of several days, while other floods can develop more quickly.


Flash Floods

  • Flash Floods are the most dangerous type of flooding, and are the #1 weather-related killer in the United States.
  • A flash flood causes very strong currents that can roll boulders, take out trees and destroy buildings and bridges.
  • It happens very quickly and with little warning, typically occurring within 6 hours of a significant rain event. Rapidly rising water can reach 30 feet or more.
  • A rapid flooding of low-lying urban area areas in less than 6 hours can be caused by intense rainfall from a thunderstorm or several thunderstorms.
  • A flash flood causes rivers and streams to swell rapidly and overflow their banks.
  • It can also occur from dam or levee failure or sudden release of water held by ice jam.

Heavy Rains

  • Excessive amount of rainfall can cause cresting rivers, back up storm drains or saturate the ground.

Ice Jams

  • Long cold spells can cause surface of rivers to freeze, leading to ice jams.
  • When a rise in water level or a thaw causes the ice to break into large chunks, these chunks can become jammed at man-made and natural obstructions.

Dams

  • Dams can fail with little warning.
  • Dam failures may occur within hours of the first sign of breaching.
  • Other failures and breaches can take much longer to occur, from a few days to weeks, as a result of debris jams, accumulation of melting snow, or build up of water pressure on a dam with deficiencies after days of heavy rain.
  • Flooding can also occur when a dam operator releases excess water downstream to relieve pressure from the dam.

Levees

  • These man-made structures are designed to control or divert the flow of water.
  • Levees are designed to protect against a certain level of flooding but can fail for a number of reasons.
  • Levees can be overtopped or fail during large floods.

Tropical Storms and Hurricanes

  • Hurricanes and tropical storms can pack a powerful punch, with soaking rain, flying debris, high winds and tidal surge.
  • In addition to causing extensive damage in coastal areas, they often bring flooding hundreds of miles inland with torrential rains and high winds, posing a threat to millions of people who don't live on a shoreline.
  • Eight of the ten most costly Federally-declared disasters have been caused by hurricanes.

Mudflows

  • A river of liquid and flowing mud floods the surface of normally dry land.
  • They can develop when water saturates the ground, such as rapid snow melt or heavy or long periods of rainfall, causing a thick liquid downhill flow of earth.

New Development

  • Construction and development can change the natural drainage and create brand new flood risks. New buildings, parking lots, and roads result in less land to absorb excess precipitation from heavy rains, hurricanes, and tropical storms.

Snow Melt

  • A midwinter or early spring thaw can produce large amounts of runoff in a short period of time. Because the ground is hard and frozen, water cannot penetrate and be reabsorbed.
  • Water then runs off the surface and flows into lakes, streams and rivers, causing excess water to spill over their banks.

Spring Thaw

  • During the spring, frozen land prevents melting snow or rainfall from seeping into the ground.
  • Each cubic foot of compacted snow contains gallons of water and once the snow melts, it can result in the overflow of streams, rivers and lakes. Add spring storms to that and the result is often serious spring flooding.

Source: Floodsmart.gov