Flash Flooding

Flash Flood: #1 weather-related killer in the United States!

What is a flood?

What is a flood?A flood results from

days of heavy rain and/or melting snows, when rivers rise and go over their banks.

The "State Street River" (floodwater from City Creek) rushes through downtown Salt Lake City, Utah, in May 1983. Rapid melting of a record-water-content snowpack caused the most widespread flooding in the State's history, resulting in damages totaling $621 million.

State Street River

What is a flash flood?

A flash flood is a life-threatening flood that begins within 6 hours —and often within 3 hours—of the rain event.

Flooding is a longer term event and may last a week or more.

What is a flash flood?

A flood is typically associated with a defined body of water, but flash floods can occur in normally dry areas with no visible stream channel.

What is a flash flood?

What is a flash flood?

Consequently, included in the flash flood definition is the rapid inundation of normally dry areas, roads, underpasses, and buildings.

Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms.

What is the cause of flash flooding?

Midwest flooding caused by rain storms, August 2007

Hurricane induced flood

Katrina, 2005: New Orleans, LA

Floyd,1999: North Carolina

Hurricane Floyd,1999

Hurricane Katrina, 2005

Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Panther, WV,

May 8, 2002 -- Heavy silting happened on many creeks and rivers as a result of the violent flash flood that struck southern West Virginia on May 2, 2002.

Clear Fork Road, WV, July 14, 2001 -- This house was torn apart by the flooding of the nearby Clear Fork Creek.

How dangerous could flash floods be?

Flash flood-producing rains can also trigger catastrophic mud slides. You will not always have a warning that these deadly, sudden floods are coming.

How dangerous could flash floods be?

Flooding TypesRiver Flood: Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.

Coastal Flood: Winds generated from tropical storms and hurricanes or intense offshore low pressure systems can drive ocean water inland and cause significant flooding. Escape routes can be cut off and blocked by high water. Coastal flooding can also be produced by sea waves called tsunamis , sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity.

Urban Flood: As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water.

Flash Flooding in Arroyos/Washes: An arroyo is a water-carved gully or normally dry creek bed. Arroyos can fill with fast-moving water very quickly. Flash flooding at this arroyo in Arizona took only 58 seconds to develop.

Ice Jam: Floating ice can accumulate at a natural or man-made obstruction and stop the flow of water.

How do flash floods occur?

Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.

Rainfall and Runoff Relationship

Flash floods are rapid-onset hydrologic events that can be very difficult to forecast. A combination of high rainfall rate with rapid and often very efficient runoff is common to most flash flood events. Ice jam formation on rivers can also play a role in rapid-onset flash flooding, particularly upstream of the ice jam.

Soil InfluencesThe three critical soil properties to consider when assessing the risk of flash flooding are soil moisture, soil texture, and soil profile.

Soil moisture is often considered the most important soil factor for rapid runoff and flash flooding. There is some validity to this reasoning in that if the soil is saturated there is no room for additional rainfall to infiltrate, and all rainfall becomes runoff regardless of the soil texture.

Flash floods occur with intense rainfall rates that often exceed the soil's infiltration capacity even when the soil is dry.

When rain is very intense...

Soil Influences: Soil Texture

Certain soil textures such as clay, and to some extent silt, can be associated with low infiltration rates and result in rapid runoff during intense rainfall. Therefore, runoff from intense rainfall is likely to be more rapid and efficient with clay soils than with sand.

Soil Influences: Soil Texture

Although sandy soils permit greater infiltration of intense rainfall, rapid runoff can occur if there is only a thin layer of soil. For example, if an impermeable layer of rock underlies a thin layer of soil, that soil layer can saturate quickly and result in large amounts of runoff.

Flash flooding facts

1. Flood can occur nationwide.

2. Flash floods are the #1 weather-related killer in the U.S.

3. Nearly 80% of all flash flood fatalities are auto related!

4. Many flash floods occur at night....be prepared to take quick action.

5. A mere 2 feet of water can float a large vehicle or even a bus. This is why you should never drive through flooded roads. Just 6 inches of rapidly moving flood water can knock a person down.

FLOOD WATCH - means that an overflow of water from a river is possible for your area.

FLASH FLOOD WATCH - means that flash flooding is possible in or close to the watch area. Flash Flood Watches can be put into effect for as long as 12 hours, while heavy rains move into and across the area.

FLOOD WARNING - means flooding conditions are actually occurring in the warning area.

FLASH FLOOD WARNING - means that flash flooding is actually occurring in the warning area. A warning can also be issued as a result of torrential rains, a dam failure or snow thaw.

Watch and warning

What should we do in case of flooding?

The rule for being safe in a flooding situation is simple: HEAD FOR HIGHER GROUND AND STAY AWAY FROM FLOOD WATERS!

When a flash flood WATCH is issued Be alert to signs of flash flooding and be ready to evacuate on a moment's notice.When a flash flood WARNING is issued for your area, or the moment you realize that a flash flood is imminent, act quickly to save yourself. You may have only SECONDS!

Go to higher ground Climb to safety!