How do brush fires in the San Gabriel Mountains aggravate the downhill debris problem in Los Angeles County?

When there's a brush fire up in the San Gabriel Mountains, the chaparral burns intensely and leaves oily ash on the ground. When it rains, this ground is slippery and the oil keeps the water from being absorbed into the soil. Rain water slides down the mountain, bringing with it oily ash and other debris, including rocks and boulders. So after a fire, the debris down a mountain increases during a significant rainfall.