Flash Flood
Causes
The desert sand doesn't soak up water quickly, so heavy rains can produce flood conditions very quickly
and without warning. Dry channels, ditches and lake beds will fill quickly and the water can be strong
and violent -- sometimes creating a wall of water 10 to 30 feet high. Remarkably, more people drown in
the desert than die of thirst. These storms are rough, but usually short-lived.
Flash floods can be produced when slow moving or multiple thunderstorms occur over the same area.
When storms move faster, flash flooding is less likely since the rain is distributed over a broader area.
Highly populated areas have a high risk for flash floods. The construction of buildings, highways,
driveways, and parking lots increases runoff by reducing the amount of rain that the ground can absorb.
This runoff increases the flash flood potential. Sometimes, streams through cities and towns are routed
underground into storm drains. During periods of heavy rainfall, storm drains may become
overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking
garages and basements can become death traps.
Water flowing over an earthen dam can cause the dam to weaken or fail, sending a destructive wall of
water downstream.
Flash floods are usually caused by excessive rainfall but ice jams suddenly breaking up, dam failures, or
levee breaches can also cause flash flooding. Due to the rapidly changing weather patterns that cause
flash floods, predicting them is very difficult to say the least.
Flash floods can occur under several types of conditions. Flash flooding occurs when precipitation falls
rapidly on saturated soil or dry soil that has poor absorption ability. The runoff collects in gullies and
streams and, as they join to form larger volumes, often forms a fast flowing front of water and debris.
Flash floods most often occur in normally dry areas that have recently received precipitation, but may be
seen anywhere downstream from the source of the precipitation, even many miles from the source. In
areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been
melted by the intense heat. Flash floods are known to occur in the highest mountain ranges of the
United States and are also common in the arid plains of southwestern United States. Flash flooding can
also be caused by extensive rainfall released by hurricanes and other tropical storms, as well as the
sudden thawing effect of ice dams. Human activities can also cause flash floods to occur. When dams,
constructed for hydro-electricity, have failed, large quantities of water can be released and can destroy
everything within its path.
Damage
Because of the threat of a flash flood, you should never rest or sleep in ditches or dry creeks -- even if it
doesn't look like rain. Desert thunderstorms come on quickly and without warning and can uproot trees
and move boulders. A rain storm in Las Vegas in 1999 swept cars away, killed two people, injured many
others and caused millions of dollars in property damage
Flash flood damage and most fatalities tend to occur in areas immediately adjacent to a stream or
arroyo. Additionally, heavy rain falling on steep terrain can weaken soil and cause mud slides, damaging
homes, roads and property.
Flash floods can develop quickly, sometimes in just a few minutes and without any visible signs of rain.
Flash floods often have a dangerous wall of roaring water that carries rocks, mud, and other debris and
can sweep away most things in its path. Overland flooding occurs outside a defined river or stream, such
as when a levee is breached, but still can be destructive. Flooding can also occur when a dam breaks,
producing effects similar to flash floods.
flood was at least 6 feet deep for at least 25 miles downstream, and wiped out the town of Kelly which
was 6 miles downstream. Six people died, and many others lost everything they owned.
Fifteen miles downstream in Wilson,WY, 6 feet of water flowed into the town and hundreds of farm
animals died. In narrower sections of the Snake River near Hoback, flood waters rose as much as 50 feet.
The next day the waters reached Idaho Falls,ID and covered lowland sections there. When all was said
and done, damages totaled over 27 million dollars.
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. Flash flood-producing rains can also trigger
catastrophic mud slides.
 Death or serious injury
The very nature of flash floods makes them fast and very difficult to predict. Since they can
occur without warning, it is no surprise that people can be seriously injured or killed by these
natural disasters. Also, many flash floods occur at night while people are sleeping adding to
the risk. Flash floods are the #1 weather-related killer in the U.S. according to the National
Weather Service.
 Immediate property damage.
In addition to the force of the water, flash floods can carry large debris such as boulders. This
combination can cause heavy structural damage to homes making them uninhabitable and can
carry away large pieces of property such as vehicles.
 Long-term property damage.
Rooms, vehicles, and other property that came in contact with flood waters may not be
salvageable. Homes that were flooded can be susceptible to harmful molds. Vehicles that were
submerged usually have irreparable water damage to the engine and other critical
components.
 Loss of critical infrastructure.
Large debris and floodwaters can cause structural damage to bridges and roadways, making
travel impossible. Power, telephone, and cable lines can be taken out by flash floods as well.
Flood waters can disrupt or contaminate ground water, making tap water unfit for
consumption.
 Deposited sediment and silt.
Flood waters carry along with them sand, silt, and other debris as they travel. When the
velocity of the flash flood slows, it begins depositing this debris. Floods can leave behind large
amounts of silt and other debris that can make travel difficult and can be costly to remove.
Description/types
A flash flood is a quick flood caused by a sudden cloudburst or thunder storm. Huge amounts of water
fall in a short time and in cities and towns the drains overflow and roads become flooded. Flash floods
also happen in mountainous areas, where steep slopes cause the water to travel at high speeds. The
rushing water erodes the soil, washing it away down the slopes. Flash floods often occur rapidly and
with little warning.
Flash floods are short-term events, occurring within 6 hours of the causative event (heavy rain, dam
break, levee failure, rapid, snowmelt and ice jams) and often within 2 hours of the start of high intensity
rainfall. A flash flood is characterized by a rapid stream rise with depths of water that can reach well
above the banks of the creek.
Flash floods can occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a
sudden release of water held by an ice jam. Flash floods often have a dangerous wall of roaring water
carrying rocks, mud and other debris.
There are several different types of flash flood including intense rainfall floods, glacial lake outburst
floods, landslide dam outburst floods, and floods caused by rapid melting of snow and ice or failure of
dams and other hydraulic structures.