Dr. Jason Kean, USGS, Boulder, CO

In situ measurements of post-fire debris flows: Comparisons of the timing and magnitude of 24 debris-flow events in southern California 

Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. This talk will give an overview of the first known measurements of flow stage, rainfall, and hillslope soil-moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). Post-event field observations show that channel erosion and hillslope rilling, not shallow landslides were the primary sources of debris-flow material. The timing of all recorded debris-flows coincided with short-duration (< 30 minutes) bursts of intense rainfall, and the time of peak stage for 79% of the debris-flow events was within just 10 minutes of peak 15-minute rainfall intensity.  An index of debris-flow volume was correlated with 15-minute rainfall intensity, but poorly correlated with cumulative rainfall and hillslope soil water content. The observed rapid response to rainfall means that adequate warning must come before a storm is over a burn area.