Debris flows generated during rainstorms present a greater risk of death and injury to southern California residents than all other kinds of slope failure combined. During the years 1962-1971, twenty-three people in the greater Los Angeles area died from being buried or struck by debris flows, all of which probably originated as soil slips. Soil slips and debris flows are recurring major natural geomorphological processes in the region.
Soil slips are reconstituted into debris flows when the initial movement (sliding failure) of slabs of soil and wedges of ravine fill causes remoulding of the saturated moving mass into viscous, debris-laden mud, which then flows down available drainage courses. This change of state results in a marked reduction in resistance to shear, permitting masses to accelerate down the same slopes on which, only moments earlier, the slabs of soil mantle had barely overcome the resistance to sliding. Many flows reach avalanche speeds and do not begin to deposit significant amounts of detritus until they reach lower gradients far from their sources. The deposits form steep ‘alluvial’ fans at the mouths of short, steep drainage basins tributary to broad valleys, and form debris trains in and along narrow trunk canyons below the mouths of short, steep tributaries.
The exceptional storm period of January 18th to 26th, 1969, was accompanied by thousands of soil slips and provided an unusual opportunity to determine the times of occurrence of numerous debris flows, establish their origin from soil slips, and compare the times of those events with rainfall records from an extensive network of continuously recording rain gauges and a sequential set of radar weather maps. An empirical association between soil slips and rainfall suggests that a 10 inch antecedent rainfall is required to bring most of the colluvial soil of the area to field capacity, and 0.25 inch-per-hour is the minimum rate at which surface infiltration exceeds subsoil drainage for most of the colluvial soils of the area so that pore pressures are raised in a zone above the less permeable parent materials. Soil moisture of this level also appears to be sufficient so that most of the colluvial soils of the area will become at least partly liquid when disturbed.