SAN JOSE, Calif. - The mudslides in Santa Barbara County were so powerful they moved entire houses, crushed automobiles and pushed huge boulders down hillsides. That has prompted many people to ask about the science of mudslides and what areas are most vulnerable.
"The fires had a direct impact on what happened in Montecito and Santa Barbara County," said San Jose State University professor Laura Sullivan-Green who serves as chair of the Civil and Environmental Engineering department.
She says normally rainwater is absorbed into the earth, but fire changes the ground in two major ways.
Burned vegetation can leave behind a residue that mixes with ash and the soil, creating an oily, waxy layer on the ground.
"It can change the chemical composition of the soil and it makes the soil repel water, so all of that rain that's fallen has nowhere to go, but run off the surface," said Sullivan-Green.
Fires also burn vegetation and that destroys the network of roots that keep soil in place. The Thomas fire left behind more than 280-thousand acres of scorched earth
The bare hillsides were visible, just above homes in Montecito.
Tuesday's rain turned thousands of gallons of water into heavy mud.... hitting homes with full force.
The north bay fires in October also have left thousands of acres in a similar state, with scorched earth that could erode in a storm.
"The fire scars that are in Napa and Sonoma County are susceptible to this exact same type of event," said Sullivan-Green, who adds that similar sedimentary rock and geological characteristics mean bay area residents should be prepared as winter progresses.
"Making sure that water can drain easily and sandbagging to prevent erosion are really the key things right now," said Sullivan-Green, "It can take up to two years for the soil to recover significant vegetation as well as reduce that water-repelling chemical change."