Soil slips, debris flows, and rainstorms in the Santa Monica Mountains and vicinity, southern California

On the record of the past decade, debris flows generated by soil slips 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 1962-71, 23 people in the greater Los Angeles area died as a direct result of being buried or struck by debris flows that probably originated as soil slips. Soil slips are shallow failures of colluvial soil and ravine fill. They have in common several characteristics and associations with rain storms that set them apart from other classes oflandslides, such as rotational slumps and block glides. The latter, for example, depend more upon deep percolation of ground water and may not respond to the effects of heavy rainfall until long after a storm. In contrast, the shallow soil slips occur during, and only during, heavy rainfall. Soil slips appear to occur only on steep slopes, whereas other classes of landslides may occur on low slopes as well. Moreover, unlike the more deeply rooted landslides that damage structures situated on them by differential movement of the foundations, damage is due chiefly to inundation by, or high-velocity impact of, debris flows generated by the shallow slides. Debris flows are generated when the initial movement of slabs of soil and wedges of ravine fill causes a reconstitution of the sliding masses into viscous, debris-laden mud, which then flows down available drainage courses (in some accelerating to avalanche speed) until reaching gradients gentle enough for deposition to begin. Soil slips require a combination of three conditions: (1) A mantle of colluvial soil or a wedge of colluvial ravine fill, on (2) a steep slope, where (3) soil moisture is equal to or greater than the liquid limit of the remolded colluvial soil. The most common range of slopes for soil slips that give rise to destructive debris flows is from about 56° (150 percent) to about 27°(50 percent). Slopes steeper than 56° generally do not have a continuous mantle of colluvium; most commonly they are bare bedrock. Soil slips on slopes of less than 27° are less common; moreover, the debris flows that they generate do not tend to accelerate downslope, though flows originating on steeper slopes above may be transported across with little or no loss of destructive power. The lowest slope on which a soil slip has been reported in coastal California is 15°. Most of the debris flows begin to deposit their coarser material on slopes of 12° (about 20 percent) or less, as indicated by the many fanhead slopes of from 10° to 12°, though transport on much gentler slopes is common. The soil moisture is almost entirely a direct result of seasonal rainfall. Because of the long dry season, the soil moisture at the beginning of the rainy season is generally well below field capacity. Once field capacity has been reached, further increase in the moisture content of the slope mantle requires rates of rainfall high enough so that water is added at a faster rate than it can drain away through the underlying subsoil. The exceptional storm period of January 18-26, 1969, presented 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 gages, as well as with a sequential set of radar weather maps. An empirical association between soil slips and rainfall was noted for that storm period; the same association seems applicable to the less severe storms of February 1969, February 1962, November 1965, and December 1965, during which smaller numbers of soil slips occurred. In all cases where the times of soil slip failure could be documented or reliably inferred, the rainfall intensity at nearby recording gages exceeded 0.20 inch per hour, and nearly all exceeded 0.25 inch per hour. Moreover, there were few reports of debris flows, interpreted to have originated as soil slips, that occurred before the total seasonal antecedent rainfall has reached 10 inches. A 10-inch antecedent rainfall appears to represent the total required to bring most of the colluvial soil of the area to field capacity, and a 0.25 inch per hour intensity apparently represents the minimum rate at which surface infiltration exceeds subsoil drainage for most of the colluvial soils of the area. With radar weather maps showing the distribution of high-intensity rainfall and slope maps showing the distribution of slopes of the most susceptible steepness, the empirical association may provide a means of recognizing areas where the hazard is greatest at any given time during a storm, and warnings to residents could be more specific and reliable.