Editor’s note: As we head into the rainy season while current wildfires continue, we’ve updated this post to include additional post-fire debris flow facts.

Fire and Rain … and Debris Flows

That’s what happened last January in Santa Barbara County.

One of California’s largest wildfires, the Thomas Fire, was nearly 100 percent contained after five weeks. Two people died and more than 1,000 structures burned. Flames torched 281,893 acres. Then a band of thunderstorms arrived the morning of January 9, bringing a deluge. The downfall dumped just over half an inch in five minutes; 1-1/2 inches in one hour. It struck those scorched hillsides of the Santa Ynez Mountain Range above Montecito

and streamed down canyons and creeks, sending boulders and trees barreling into homes built on alluvial fans, which are water-borne deposits of mud and sediment. More than 400 homes were damaged, 41 of which were completely washed away. Twenty-three people lost their lives.

“It was a buzz saw,” is how Jeremy Lancaster, a senior engineering geologist with the California Geological Survey, described the drenching that let loose a flood of virtually inescapable debris.

Jeremy was part of a team of experts from state, federal, and local agencies that assessed the impact of the triple whammy of fire, rain, and debris flows. Analyzing weather data and field evidence at more than 1,500 impact points of the disaster, the team determined.

•  130,000 dump trucks full of debris traveled down canyon creeks to populated areas. Many of the flow paths crossed Highway 101, reaching the Pacific Ocean. The scientists found a large boulder lodged nearly 10 feet above the ground between two close-growing trees.
  
• Another boulder measuring more than 15 feet-wide was found in a Montecito yard.
• The debris was as deep as 33 feet in Montecito and Romero creeks.

• The velocity of the debris flow was estimated to range from 5 mph to more than 16 mph.
• Debris and mud overtopped the roofs of some homes while pushing walls and roofs off the concrete pads of others.
• Four major bridges were destroyed.

Local emergency officials had been aware of the heavy rain coming and the possibility of debris flows from the denuded hillsides. Warnings went out; residents were urged to evacuate. Only a small percentage did leave.

The spread of urbanization into areas subject to wildfires and other hazards is not new in California. There can be risks that sometimes turn into devastating reality, such as what happened to the Montecito community. The information gathered by California Geological Survey scientists and their colleagues will be used to improve critical efforts related to public safety, including mapping of debris flow risk. That mapping is currently non-existent.

Those who study wildfire-debris flows believe they could become more commonplace. Wildland fire and land-use practices, and a changing climate factor into that possibility.

The question now is: How will California adapt?