The geomorphology of some debris flows in the southern Sierra Nevada, California

Debris flows are one of the natural hazards present in the southern Sierra Nevada of California. Historic debris flow activity is documented at a USDA Forest Servuce research facility in the Kings River drainage related to a 1937 storm event. No subsequent study of this phenomenon was undertaken until 1982. Observations of debris flows over the succeeding 10 years offer an initial assessment of the physical geomorphology of debris flows in this area. This information provides a starting point for future efforts to avoid or limit the effect of this natural hazard. Observations were made in the Tuolomne, Merced, San Joaquin, and Kings River drainages. Of the twenty-six debris flows observed, six were examined in detail to provide specific data on this phenomenon. Triggering events for debris flows in the southern Sierra Nevada include intense rainfall, rain-on-snow storms, and seasonal melting of heavy snowpacks. Movement typically occurs at depths between 0.3 and 5 m below ground surface. This is representative of depths for the three interfaces associated with initiation of movement: (1) at the base of the root zone, (2) at the contact of well-weathered and less-weathered soil, and (3) at the contact between soil and unweathered bedrock. Measurement of debris flow velocity based on indirect methods found values ranging from 2.6 m/s to 7.2 m/s (9 km/h to 26 km/h). Recurrence intervals based on radiocarbon dates are between 425 and 500 years BP. Roads and other land use investments have suffered damage from debris flow activity during the 1982 to 1993 period. Stump Springs road in the San Joaquin river drainage required US$1.4 million to repair damage primarily from 1982 debris flows. The threat to life arising from debris flows is illustrated by the consequences of vegetation losses from wildfire near El Portal, California. The projected population growth with associated increased infrastructure raises concern for greater impacts from future debris flow occurrence in the southern Sierra Nevada.