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What You Can Do to Prepare for Flash Floods and Debris Flows After Wildfires


Menlo Park, Calif. — Once the smoke clears from a wildfire, the danger is not over. Other hazards, such as flash floods and debris flows, now become the concern. Watch a video of the Christmas Day, 2003, debris flow in Devore, Calif., taken by local citizen Howard Davis by visiting the USGS California Water Science Center webpage.

“Wildfires remove the vegetation that acts as a rainfall sponge and make any rainfall on the area run off much more rapidly,” said USGS Research Geologist Sue Cannon. “Also, the passage of the fire itself can dry out the soil, so it makes it more difficult to absorb water.” To hear more from Cannon on debris-flow dangers after wildfires, listen to episode 22 of CoreCast, the USGS podcast.

Just a small amount of rainfall on a burned area can lead to flash floods and debris flows. The powerful force of rushing water, soil and rock, both in the burned area and downstream, can destroy culverts, bridges, roadways and structures, and cause serious injury or death.

The National Oceanic and Atmospheric Administration (NOAA) and the U. S. Geological Survey (USGS) have established a demonstration flash-flood and debris-flow early-warning system for recently burned areas in Southern California. To help prevent the loss of life and property, here are some things you can do:

Prior to Storms:


Watch the patterns of storm-water drainage near your home, and note the places where runoff water converges, increasing flow in channels. These are areas to avoid during a storm.

Contact your local authorities to learn about the emergency-response and evacuation plans for your area. Develop your own emergency plan for your family or business.

During a Storm:


Stay alert! Many debris-flow and flash-flood fatalities occur when people are sleeping. Listen to the radio for warnings of intense rainfall. The NOAA Weather Radio All Hazards tone alert will let you know of hazards in your area. Intense bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall.

If you are in an area that is susceptible to flooding or debris flows (or has been subject to flooding or debris flows in the past), consider leaving if it is safe to do so. Remember that driving during heavy rainstorms can be hazardous.

If you are near a stream or a channel, listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing mud or debris may precede larger flows. Be alert for any sudden increases or decreases in water flow and for a change from clear to muddy water. Such changes may indicate debris-flow activity upstream, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.

Keep in mind that water levels may rise much more rapidly during flash floods and debris flows and may be significantly larger in areas that have been burned by wildfires than in areas that have not burned.

Stay alert when driving. Bridges may be washed out and culverts overtopped. Do not cross flooding streams. The best advice from NOAA’s National Weather Service is Turn Around, Don’t Drown ®! Embankments along roadsides are particularly susceptible to landsliding. Watch the road for collapsed pavement, mud, fallen rocks and other debris flow and flashflood dangers.


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