NOAA/USGS Demonstration Flash-Flood and Debris-Flow Early-Warning System
| Current Outlooks | Statement of Problem | Warning System Operation | Rainfall Thresholds | What Can You Do If You Live Near A Recently Burned Area? | Other Links |
Current Outlooks
Outlooks issued for areas burned by 2005 fires, and detailed maps of these areas
Statement of Problem
Once the smoke clears from a wildfire, the danger is not over!! Other hazards, such as flash floods and debris flows, now become the focus. Areas recently burned by wildfires are particularly susceptible to flash floods and debris flows during rainstorms. Just a short period of moderate rainfall on a burn scar can lead to flash floods and debris flows. Rainfall that is normally absorbed by vegetation can run off almost instantly. This causes creeks and drainage areas to flood much sooner during a storm, and with more water, than normal. Additionally, the soils in a burn scar are highly erodible so flood waters can contain significant amounts of mud, boulders, and vegetation. The powerful force of rushing water, soil, and rock, both within the burned area and downstream, can destroy culverts, bridges, roadways, and structures, and can cause injury or death if care is not taken.
Figure 1 Debris-flow deposits in Cable Canyon in San Bernardino County, California. Cable Canyon was burned during the Grand Prix-Old Fire of 2003 and experienced debris flows and floods in response to the December 25, 2003 rainstorm. Two people lost their lives at this site.
Warning System Operation
The National Oceanic and Atmospheric Administration (NOAA) and the United States Geological Survey (USGS) have established a demonstration flash-flood and debris-flow early-warning system for recently burned areas in southern California. The demonstration project covers eight counties within southern California, and utilizes the National Weather Service's (NWS) Flash Flood Monitoring and Prediction (FFMP) system. FFMP identifies when both flash floods and debris flows are likely to occur based on comparisons between radar precipitation estimates and established rainfall intensity-duration threshold values. Beginning in autumn 2005, advisory Outlooks, Watches and Warnings are disseminated to emergency management personnel through the NWS Advanced Weather Information Processing System.
Figure 2 Southern California real-time rain gage locations (red dots) and radar coverage (blue regions).
Rainfall Thresholds
Warnings and Watches for post-fire flash flood and debris flow are based on rainfall intensity-duration thresholds. Such thresholds have been developed for recently burned areas in southern California by comparing characteristics of storms known to have produced flash floods and debris flows with those that did not. Thresholds are defined by identifying those combinations of rainfall intensity and duration that are unique to flash flood and debris-flow producing storms. Where available, information on known times of flood or debris-flow occurrence is incorporated into the thresholds. Threshold lines delineate a range of rainfall combinations - from short duration, high intensity to longer duration, lower intensity - any of which can result in flash-flood or debris-flow activity. Post-fire threshold conditions change with time as sediment supplies are depleted and vegetation recovers, and so in addition to thresholds that are useful for immediately after a fire, thresholds for the following rainy season have also been developed.
Figure 3 Rainfall intensity-duration thresholds for recently burned, mountainous areas of southern California. Rainfall conditions above the thresholds are those likely to trigger debris flows and flash floods from recently burned areas. Different areas require different thresholds because of variations in surficial materials, vegetation, topography and prevailing meteorologic conditions.
What Can You Do If You Live Near A Recently Burned Area?
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 flood fatalities occur when people are sleeping. Listen to the radio for warnings of intense rainfall. NOAA Weather Radio All Hazards tone alert will let you know of hazards in your area. Be aware that intense bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall.
- If you are in an area susceptible to flooding or debris flow (or has experienced flooding or debris flow 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 rises in water levels during flash floods and debris flows may occur much more rapidly, and may be significantly larger, than those produced when the watershed is not burned.
- Be particularly alert when driving. Bridges may be washed out, and culverts overtopped. Do not cross flooding streams!! Turn Around, Don't Drown ®! Embankments along roadsides are particularly susceptible to landsliding. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of debris flow.
Other Links
Warning system Report and Fact Sheet:
- Fact Sheet
- Report
NOAA's Weather Radio:
- http://www.nws.noaa.gov/nwr/
- http://www.warnings.noaa.gov/wxradio.html
Post-Wildfire Landslide Hazards
Other USGS fire information:
- http://www.usgs.gov/themes/wildfire.html
- http://firescience.cr.usgs.gov
