Prototype Post-Fire Debris Flow Warning System in Southwest California: History, Lessons Learned, and Partnership Building
Eric Boldt, NOAA/NWS Los Angeles/Oxnard, Oxnard, CA
Jayme Laber, NOAA/NWS Los Angeles/Oxnard
Dennis Staley, USGS
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Abstract
Rapid runoff of heavy rainfall on a recent burn scar can quickly become a life-threatening debris flow in mountain drainages near urban-wildland interfaces across southern California. In the winter season of 2005-06, the National Weather Service (NWS) and the U.S. Geological Survey (USGS) established a prototype flash flood and debris flow warning system for recently burned areas in an eight-county area of southern California. This prototype system combines existing NWS flash flood prediction and monitoring tools with USGS rainfall intensity-duration thresholds and burn area-specific debris flow threat and volume maps, to assist in flash flood and debris flow warning decision-making. Debris flows of varying severity have occurred across southwestern California during the wet seasons since the initial 2005-06 winter, with numerous small debris flows that were redirected by mitigation and captured in debris catch basins or stopped short of reaching property in upper reaches of burn areas. However, in the last 12 years there have been three major debris flow events that caused severe property damage but fortunately only minor injuries and no loss of life.

This research documents the history of the NWS and USGS debris flow warning system, including lessons learned in seeking advanced and accurate warnings, and a focus on the three major debris-flow producing rainstorms since the initial 2005-06 winter season. In using this system for over 60 burn areas in the NWS Los Angeles four-county warning area, a key finding has been that intense rainfall rates of less than one-hour duration have resulted in the most damaging debris flows, whether antecedent moisture was high or non-existent. In addition, southern California NWS offices have played an integral and growing role in preparedness for debris flow threats by providing expertise as part of Burn Area Emergency Response (B.A.E.R) teams to determine key parameters that the USGS uses to estimate potential debris flow response (even before the fire is officially out). NWS also works closely with partner agencies on notification methods and development of operational plans in advance of significant weather events, and by presenting at community preparedness meetings. This outreach is aimed at educating communities on the science of debris flows and how they differ from mud or landslides, the danger they pose to life and property, and how residents can be prepared to take protective action if debris flows threaten or occur.  We show how the NWS Los Angeles worked closely with the Glendora city manager, public works department, police, Los Angeles County Fire Department, and the USGS prior to, during, and after storm events in complete partnership to ultimately protect life and property.  Finally, we highlight a commendable community response plan developed after the 2013 Colby Fire that uses a “stoplight” color protocol to alert their community about the impending debris flow threat levels.