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.

Debris-flow deposits in Cable Canyon, 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.

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.

About the Warning System

Image of Southern California real-time rain gage locations
Southern California real-time rain gage locations (red dots) and radar coverage (blue regions).

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.