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Tuesday, September 22: Floods and Flash Floods

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...Flooding is the #1 Weather Killer...

What is the difference between a flash flood and river flooding? Flash flooding occurs within a few hours of the heavy rain event and ends a few hours after it starts. River flooding takes half a day or longer to develop and frequently rivers will remain above flood stage for days at a time.
flood photo

Flooding and flash flooding occur every year across California . Since 1998…over 1000 floods and flash floods have been reported causing 28 fatalities. This is an average of over 100 flood and flash flood events per year. In a bad year more than 200 such events can occur.

In southern California most flooding is the result of heavy precipitation over periods of one or two days. The short streams and steep watersheds emptying onto lowlands that may be heavily populated, produce large volumes of water within short periods and damage is often severe. The problem is sometimes compounded by the denuding of large areas of watershed by fire during the previous season.

The west slopes of the coastal ranges in the central and northern parts of the state also experience flooding as a result of heavy precipitation over a period of only a few days. These streams are usually longer than those of southern California and require a longer time to build up a flood potential. In these streams a flood buildup may extend over a period of one week or longer.  

flash flooding photo

The streams of the Sierra Nevada and Cascades overflow either as a result of rainfall or snowmelt, or from a combination of these.  

The extreme southern portion of the San Joaquin Valley has no direct drainage to the sea . Excessive runoff from the southern Sierra Nevada into this area can result in the temporary enlargement of Buena Vista Lake and Tulare Lake .

Flash flooding is commonly thought to be a threat only during the summer months due to heavy downpours from thunderstorms. Flash floods may be more common in the summer months but flash floods have been reported in every month of the year.

A Flash flood refers to a dangerous sudden rise in water along a stream...river... wash ...or over a normally dry land area . Flash floods result from heavy rainfall...river ice jams ... snowmelt... and dam or levee failures. Flash floods can occur within a few minutes or hours... and can move at surprisingly high speeds...striking with little warning. They can erode an entire mountain side ... roll boulders the size of trucks...tear out trees...destroy buildings... wash out roads and bridges and cause loss of lives. Rain weakened soils can also result in mud slides capable of closing interstates.

To stay informed about a flash flood listen for:

A Flash Flood Watch

This tells you that flash flooding is possible within the watch area .  You should remain alert and be ready to evacuate on a moment's notice.

A Flash Flood Warning

This tells you that flash flooding has been reported or is imminent. When a flash flood warning is issued for your area ...act quickly to save yourself.  If advised to so immediately. Go to higher ground or climb to safety. Move to a safe area before access is cut off by flood water.

An Urban and Small Stream Flood Advisory

This tells you that flooding of small streams...streets and low lying areas...such as railroad underpasses and urban storm occurring...but rainfall is not expected to produce a flash flood situation.

The following basic flash flood safety rules should be observed when you see flooding or hear about a flash flood warning…

Do not camp or park your vehicle along streams and washes...particularly during threatening conditions.

If you are near a river be aware of water levels and be prepared to take action to move to higher ground if river levels rise.

Do not enter areas that are already flooded.

Do not try to cross a flowing stream on foot when the water is at or above your knees.

If walking or fishing along a aware that erosion from swift running water can cause river banks to collapse.

Never let your children play around high water...storm drains...viaducts or arroyos.

If you live near a burn area from a recent wildfire, you may have a higher risk for flash flooding.

Be especially cautious at night when it is harder to recognize flood dangers.

Nearly half of all flash flood fatalities are auto related.  While driving your vehicle look out for flooding at highway dips...bridges and low areas.  Two feet of water will carry away most automobiles.  Never attempt to drive over a flooded road .  The road bed may be washed out under the water and you could be stranded or trapped.  If the vehicle stalls...leave it immediately and seek higher ground.  Rising water may engulf the vehicle and sweep it away.

The best advice if you are in a vehicle…turn around…don’t drown.

Stay informed.  You can receive forecasts and immediate notification of warnings on NOAA weather radio.

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page last updated: September 24, 2007