January 2021: Evaluation of debris flow activity in recent California Burn Areas following atomospheric river event

Science Center Objects

The January 2021 atmospheric river event produced heavy rainfall in some areas burned by wildfires in 2020. The Landslide Hazards Program (LHP) staff has evaluated the debris-flow activity in several areas. The locations are: The Carmel Fire, the CZU Lightning Complex Fire (San Mateo and Santa Cruz), the Dolan Fire, and the River Fire burn areas.

Reports by date are below. Photos will be posted soon.

person standing on step slope with debris flow deposits and burned trees in background

Rilling and a shallow landslide in CZU Fire near Eagle Rock, California. Soil ~3 cm below surface was almost completely dry after the rain storm, highlighting soil water repellancy. (Credit: Matt Thomas, USGS. Public domain.)

The Post-Wildfire Debris-Flow Hazard Maps for those areas can be found online at:

February 10, 2021 Update  

Following the late January 2021 atmospheric event in California, the Landslide Hazards Program (LHP) met with federal, state, and county officials to discuss the performance of rainfall thresholds used to issue watches and warnings for debris flows in recent burn areas of Monterey and Santa Cruz Counties. The thresholds, which were established based on USGS modeling and input from CAL FIRE watershed emergency response teams, informed decisions to issue evacuation orders for thousands of people in advance of the storm. LHP evaluated the thresholds by analyzing rain rates and surveying debris-flow activity across the four burn areas. This evaluation confirmed the threshold established for the CZU Lightning Complex (Santa Cruz County) and resulted in a slight downward revision of the thresholds for the Dolan, River, and Carmel Fires (Monterey County). Additional work to verify the accuracy of the debris-flow hazard assessments in the region is underway.

Feb 3, 2021 Summary

After a historic 2020 wildfire season, California received its first major rainfall event of the winter. This event prompted evacuation orders and flood watches and warnings for several recent burn areas in northern and southern California. The Landslide Hazards Program is rapidly evaluating debris-flow activity triggered by this storm in an effort to (1) evaluate USGS guidance on rainfall thresholds, which are used by the National Weather Service for issuing watches and warnings for the burn areas, and (2) verify the accuracy of USGS debris-flow hazard assessments for the burn areas. The biggest debris-flow impacts have been in Monterey County and include major damage along the Big Sur Coast closing Highway 1 indefinitely (Dolan Fire) and damage to numerous homes causing at least one injury (River Fire). Minor home damage occurred in the Bond Fire in Orange County, and small non-destructive debris flows have been observed in the CZU Lightning Complex in Santa Cruz and San Mateo Counties.