Part 6Part 6

DEBRIS FLOW DEBRIS FLOW HAZARD MAPSHAZARD MAPS

• USGS landslide hazard maps typically assess debris flows and deeper-seated landslides.

• Landslide hazard maps for Contra Costa County, CA

Debris Flow Hazard Map

Landslide Hazard Map

• The Dec 31, 1933-Jan 1, 1934 Montrose-LaCresenta debris flows damaged or destroyed 600 homes and killed 44. 600,000 cubic yards of material was deposited on the fan in one evening.

• Left: DRG overlay of Left: DRG overlay of 1934 Montrose-LaCresenta debris flows1934 Montrose-LaCresenta debris flows, , which occurred several weeks after a series of brush fires in the which occurred several weeks after a series of brush fires in the watersheds at upper right. This is known as a watersheds at upper right. This is known as a fire-flood fire-flood sequence.sequence.

• Right: DEM overlay of same map. Note flow paths along streetsRight: DEM overlay of same map. Note flow paths along streets

Flow quantities generated using 3X bulking factor with Q100 flow using HEC-RAS; then overlain on digital topo base using ArcGIS.

Project-specific Debris Flow Project-specific Debris Flow Hazard MapHazard Map