Southern California wildfires paved the way for deadly mudslides

Last year's wildfires in Southern California paved the way for this week's deadly mudslides, which turned roadways into ruinous rivers of mud and debris.

Heavy rain unleashed flash floods and debris flow in the southern part of California Tuesday, just weeks after several wildfires torched the area. At least 17 people have died, others are missing and more than two dozen are injured in the Golden State from weather-related incidents, according to Santa Barbara County officials.

The mudslides struck the communities hit hard by the Thomas fire in December, the largest wildfire in California's modern history. That's because burned soil can act as a water repellent like pavement, so rainfall that would normally be absorbed in the ground runs off rapidly after a wildfire. Thus, much less rain is needed to cause a flash flood.

As the floodwater gushes downhill through burned areas, it can create major erosion and pick up vast amounts of ash, mud, rocks, sand, silt and scorched vegetation like trees and shrubs. Locations downhill and downstream from the burn scar left by a wildfire are susceptible to flash flooding, debris flow and mudslides, according to the National Weather Service.

The force of the rushing water, mud and debris can be catastrophic. It can damage or destroy roadways, bridges, culverts and buildings even miles away from the burned area.

The devastating Thomas fire that ignited Dec. 4 burned hundreds of thousands of acres in Southern California. Communities in Santa Barbara County, like Montecito, located below the burned area, are under siege again as mudslides crushed cars and ripped homes from their foundations this week.

Highway 101 has been shut down in both directions because of the mud, Santa Barbara County officials announced Tuesday afternoon.

A total of 0.54 inches of rain was reported at Montecito in just five minutes. The rate of rainfall in Southern California Tuesday was 18 times more than required to produce debris flow, according to an analysis by ABC News meteorologists.

Hundreds of homes in Montecito's affluent community were either damaged or destroyed as a result of the torrential conditions. At least 65 single-family homes were destroyed and another 446 were damaged. Meanwhile, eight commercial structures were destroyed and 20 commercial structures were damaged, according to the Santa Barbara County Fire Department.

The Santa Barbara County Office of Emergency Management said Tuesday night that Montecito would be without potable water, electricity and sanitation "for an extended period of time."