Below is very helpful information by various agencies regarding how to best prepare for floods and debris/mud flow after a hillside fire:
National Weather Service: Post Wildfire: Flash Flood and Debris Flow information
From Ready.gov: Landslides and Debris Flow
Dept. of Building & Safety: Homeowners Guide for Flood, Debris Flow, and Erosion Control
Emergency Survival Program, LA County: Landslides and Mud Slides.
Sandbag Use:
LA City Bureau of Street Services: Homeowners Guide for Flood, Storm Debris, and Erosion Control Preparedness