Flash Flood & Debris Flow Warnings
System Tested Provides Early Flooding Alerts for Wildfire Burn Areas
NOAA and USGS demonstrate an early warning system for flash floods and debris flow events and offers safety suggestion for residents living near burned areas
Wildland fires in California have highlighted the dangers posed to residents by flash floods and debris flows. Working together, the National Oceanic and Atmospheric Administration (NOAA) and the United Stated Geological Survey (USGS) have demonstrated an early warning system that can alert residents in recently burned areas of southern California to the presence of flash flood and debris flow events. The agencies also offer safety suggestions for residents that live near recently burned areas.
Post Wildfire Hazards - Wildfires strip the ground of protective vegetation and leave it vulnerable to the effects of heavy rainfall. Vegetation holds the soil during rain storms, adsorbs moisture, and slows the free flow of surface water. After a wildfire, just a little rainfall can trigger flash floods or debris flows. The rush of water and debris across a burn scar can destroy infrastructure and kill aware people in its path.
Operation of the Warning System - The NOAA/USGS warning system depends on precipitation prediction and monitoring combined with rainfall threshold modeling. The National Weather Service forecasts and monitors rainfall constantly with forecasts issued twice daily. Researchers have developed peak rainfall thresholds for burned areas in southern California that compare know storm events that caused flash floods and debris flows to storms that did not cause a flooding event. Thresholds are tailored to each areas unique vegetation, topography, surface materials, and prevailing weather. When rain prediction forecasts exceed the rainfall threshold, a flooding and debris flow advisory outlook, watch or warning will be issued based on the level of predicted rainfall. Agencies are also experimenting with the use of webcams to compliment the warning system.
Actions Residents Near Burned Areas Should Take - Residents should become familiar with the patterns of storm water runoff around their homes and identify areas to avoid during a heavy rainstorm. It is a good idea to learn about local emergency evacuation plans and develop personal emergency plans before the occurrence of a flooding or debris flow event. NOAA/USGS offer these suggestions for actions to be taken during a rainstorm near a burned area:
- Remain vigilant for NOAA Weather Radio All Hazards tone alert on your weather radio
- Intense bursts of rain can be very dangerous after a long period of heavy rainfall
- Residents near river or stream beds should listen for signs of moving debris like boulders striking each other. Watch for major variations in water flow and color that could indicate debris flow upstream. Be prepared to evacuate quickly
- Water and debris flows coming off of burned areas will be significantly greater than those experienced by an unburned watershed
- Be aware of bridge failures and overtopped culverts while driving in heavy rain. Don't ford flooded crossing with your car. Watch for failed paving, fallen rocks and landslides that cross the roadway
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