CATASTROPHIC DEBRIS FLOW FAILURE OF THE LA CONCHITA HILLSIDE: LESSONS REVISITED, VENTURA COUNTY, CALIFORNIA

On January 10, 2005 a massive debris flow swept down a steep arroyo and into the coastal community of La Conchita, California, killing 10 people, and destroying 13 homes. It was initiated by heavy rains associated with an intense weather system that accounted for widespread flooding and landsliding.

La Conchita sits on a gently sloping wave-cut platform covered by old debris flow deposits at the base of a 250m-high coastal bluff. This bluff is composed of large, older landslides from which recent slumps, slides, and flows originate. The surfaces of the landslides are marked by steep-walled arroyos capable of storing large volumes of debris. The January 2005 debris flow originated in a large arroyo where a 1995 rock slump destroyed nine homes, blocked a county road, and choked the lower half of the drainage.

The long duration and high intensity of the three-day storm followed an unusually wet early winter totaling over 12 inches of rain by year's end, resulting in high antecedent moisture conditions. The debris flow initiated following nearly 10 inches of rain in the previous 72 hours. The toe of the initial stage of failure was approximately 50-70m above the La Conchita community. High pore pressures and the location of the failure high on the slope within fractured old landslide debris resulted in a rapidly moving, highly fluid debris flow. The transport zone for the debris exhibits lateral berms, thin debris cover, internal flow ridges, and a main abrasion channel. The major zone of deposition contains approximately 14,000m3 of material and flowed more than 100m into the community.

The La Conchita tragedy demands that the lessons learned from past debris flow-generating storms be revisited. California has experienced many major storm seasons that generated destructive debris flows, including 1969, 1983, 1998 and 2005. Geologists now have a relatively clear understanding of the physical conditions needed for triggering debris flows. By monitoring the short-term conditions of approaching storms we can and should forecast the likelihood that debris flows will be generated in susceptible areas, such as steep terrain underlain by potentially unstable debris and areas of previous instability. Not recognizing the hazardous geologic conditions on the La Conchita hillside appears to be a primary factor that led to the catastrophe.