SANTA BARBARA — Picture it: a flood roaring down from foothills above this scenic coastal town, carrying room-sized boulders hurtling at 50 mph.
One of the largest such debris flows in Southern California history may have done just that only a few thousand years ago, according to researchers from the University of California, Santa Barbara.
Geology professor Ed Keller and graduate student Amy Selting say they have found evidence of the flow in Rattlesnake Canyon above the site of the Santa Barbara Mission.
The flood dumped about 10 million cubic yards of boulders and mud, or about 300,000 truckloads along a two-mile route, they estimate. It left deposits 30 feet deep in places.
The debris flow was about a thousand times bigger than those that have followed wildfires in the area during the last century, Keller said.
“It would have been terrible — a snapping of trees and breaking of rocks bouncing off each other. It would have sounded like a thousand thunders,” Keller said. “If such an event were to occur again today, many homes and buildings ... would be destroyed, and the loss of life would be catastrophic.”
And one really wet year, a big earthquake or a soil-baring wildfire might be all it takes to send another one roaring down from a canyon onto the town 80 miles north of Los Angeles.
“It’s scary because this flow covered a third of a square mile of Santa Barbara. That’s a large space that got inundated with water, boulders and sediment,” Selting said.
“You need to be aware that these things happen and have some plans about what you would do,” Keller said.
Their findings were presented earlier this month at the annual meeting of the Geological Society of America in Boston.
The researchers used computerized topographic maps of downtown and mapped the deposits to estimate the volume of mud, water and rock that hurtled downstream.
Radiocarbon dating will be used to pin down the age of the flood next month, but the boulders are relatively unweathered, pointing to a young geographical age, the researchers said.