STORM RAINFALL CONDITIONS FOR FLOODS AND DEBRIS FLOWS FROM RECENTLY BURNED AREAS IN SOUTHWESTERN COLORADO AND SOUTHERN CALIFORNIA

Debris flows generated during rain storms on recently burned hillslopes have destroyed lives and property throughout the Western U.S. Field evidence indicates that, unlike landslide-triggered debris flows, these events have no discrete initiation source and can occur with little or no antecedent moisture. Using rain gage data and information on basin response from five burned areas in Colorado and southern California, we document the rainfall conditions that have triggered post-fire debris flows and develop empirical rainfall intensity-duration thresholds for the occurrence of debris flows and floods following wildfires in these settings. This information can provide guidance for warning systems and planning for emergency response in similar settings.

Debris flows were produced from recently burned basins in Colorado in response to convective storms. Debris flows were triggered after as little as six to 10 minutes of storm rainfall. Most of the storms that generated debris flows lasted less than one hour, ranged in average intensity between 1.0 and 32.0 mm/hr, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central Colorado are defined by I = 6.5D^-0.7, and for southwestern Colorado by I = 9.5D^-0.7, where I = rainfall intensity (in mm/hr) and D = duration (in hours).

Debris flows were generated from recently burned areas in southern California in response to frontal storms. The flows occurred after as little as two hours, and up to 16 hours, of low intensity (1-20 mm/hr) rainfall. The storms had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property-threatening floods and debris flows during the first winter season following fires in Ventura County are defined by I = 12.5D^-0.4, and for the San Bernardino, San Gabriel, and San Jacinto Mountains as I = 7.2D^-0.4, respectively.

The thresholds defined here are significantly lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on unburned hillslopes.