2004-2005 EL NINO DEBRIS FLOWS IN SOUTHERN CALIFORNIA: SOME COMPARISONS WITH PREVIOUS WINTERS

The moderate El Nino conditions of the 2004-2005 winter in southern California produced widespread hillside soil-slip debris-flows during rainstorms. Early rains in October falling on dry ground produced debris flows in some semiarid areas. These were atypical in being generated by only about 10 cm of rainfall, about 16 cm less than the typical antecedent rainfall required to produce debris flows in this area. This unusually early rainfall fell on dry surfaces and may be more akin to summer flash-flood generated debris flows in the desert.

Prolonged and intense rains, typical of El Nino, occurred from January 7th to 10th. Rain from this storm produced a large number of debris flows including the lethal La Conchita debris flow. At Laguna Beach only a few debris flows contrasted with numerous 1998 lethal debris flows. The Lake Cachuma area had numerous debris flows in 1998 but few in 2005. The foothill area of the southern San Gabriel Mountains, essentially devoid of debris flows in 1998, had widespread debris flows in 2005. Tertiary rocks in the Santa Clarita area were the source for more debris flows than in 1998.

A number of areas had about the same frequency of debris flows in 2005 as 1998, including the Ventura-Santa Paula, Sunland, and the Trabuco Formation in the foothill area of the Santa Ana Mtns.

February storms, resulted in few new debris flows. The only exception observed was in the Puente Hills where a small area of the Puente Formation was the site for more debris flows than in January.

One area in the San Bernardino Mountains, which burned in 2003, and is underlain by granitic bedrock, was the site of numerous hillside soil-slip debris-flows; elsewhere most debris flows in the burned area were limited to channels.

Rapid germination during moist conditions between the January and February storms resulted in some debris flow scars and deposits being largely overgrown with new vegetation and obscured.

The expected, deep-seated (bedrock) landslides following the rains occurred, with the destructive Bluebird Canyon landslide attracting the most attention.