An Overview of the January 9, 2018 Extreme Flash Flood and Debris Flow Event in Montecito, CA

The Thomas wildfire broke out near Santa Paula, CA on Monday December 4, 2017 and subsequently became the largest wildfire in recorded history in the state of California. The fire burned 281,893 acres within Ventura and Santa Barbara Counties impacting Federal, State, County, and public lands. The fire destroyed 1,063 structures and damaged 280 more before it was finally declared 100% contained on January 12, 2018.

In the wake of the fire, attention quickly turned to post wildfire flooding risks. Federal, State, and Local agencies and jurisdictions worked to undertake and implement post wildfire projects and mitigation measures.

After a dry start to the water year (October to December 2017), a powerful winter storm system took aim at southern California on January 8^th and 9^th 2018 resulting in some very intense rainfall rates (5-minute, 10-minute, 15-minute, and 30-minute) over the Thomas burn area in and adjacent to the Montecito, Summerland, and Carpinteria communities in coastal southeastern Santa Barbara County. Immediately after this rainfall, reports of major flash floods and debris flows were reported in several creeks in and near Montecito. These floods and debris flows resulted in 21 fatalities and 2 additional residents missing and presumed deceased. There were a total of 128 single family residences destroyed and 307 damaged. Additionally, there were 6 commercial properties destroyed and 17 that were damaged. U.S. Highway 101 – a major transportation route along the Santa Barbara south coast – was closed for a total of 12 days due to severe inundation from debris and flood waters.

This presentation will provide an overview of this catastrophic event as well as provide examples of the extreme rainfall that was observed in and near Montecito on the morning of January 9, 2018. A summary of the ensuing flash flooding and debris flows will be covered, including discussion on the scientific processes of post-fire debris flows and the challenges associated with predicting these oftentimes deadly natural hazards.