Impact of Wildfire on Triggering Mudslides—A Case Study of 2018 Montecito Debris Flows

Although wildfires are common globally, California has been experiencing significant loss due to the wildfires. Examples of such losses include frequent landslides and debris flow events. Although most of such post-wildfire debris flow events do not always cause human casualties, the 2018 Montecito debris flow event not only caused over $200M property loss, but also caused a death toll of 21 people in addition to full or partial damage of over 500 buildings. A few bridges were washed away due to the blockade of their waterways by debris and tree branches. The debris basins overflowed and the debris mass was diverted from the natural flow channels. The main reason for the debris flow was a short duration but concentrated rainfall that the area got even before a full containment of the Thomas Fire, one of the worst wildfires in the history of California. This study includes geotechnical properties of the debris flow soil, comparison of several wildfire induced debris flows in Southern California, statistical evaluation of the rainfall threshold for triggering wildfire induced landslides in Southern California, and analysis of rainfall pattern for a few years preceding the disaster. The Montecito debris flow disaster can be attributed to the high intensity rainfall prior to the debris flow event and the low residual shear strength of the ash deposited on the soil matrix.