In late 2004 and early 2005, an intense series of rainstorms impacted southern California, causing flooding and innumerable landslides throughout the region. The January 10, 2005 La Conchita landslide was the deadliest single event triggered by the 2004-2005 storm sequence. The landslide, which occurred about 130 km northwest of Los Angeles, California, mobilized over 40,000 cubic yards of wet debris into a large scale debris flow (commonly referred to in the media as a mudslide) that flowed into a residential community at the foot of the slope, killing 10 persons and damaging or destroying 36 residences. Ten years earlier, in March 1995, a large rotational landslide had occurred in the same area following a period of heavy rainfall. This event damaged or destroyed seven residences, destroyed an access road that traversed the slope, and covered a major street in the community, Vista Del Rincon Avenue, with up to about 20 ft of debris. In 2000, the County of Ventura constructed a temporary soldier pile wall consisting of steel H-piles and wood lagging along the northern margin of Vista del Rincon Avenue to allow the removal of debris from the street. The wall was 270 feet long and stood between 5 and 23.5 feet above the road. Litigation triggered by the 2005 debris flow focused in part on the role played by the temporary wall in affecting the path taken by the flow as it entered the community.
Exponent performed an investigation that included an assessment of the design and construction of the temporary retaining wall, as well a causation analysis of the landslide. The causation analysis included an assessment of site geology as well as the morphology, sedimentology and kinematics of the landslide. Additionally, the flow of the wet landslide debris was modeled using the commercial FLO-2D software package to quantify the effects of the temporary wall on the movement rate, travel direction, and area inundated by the debris flow (click here to view poster presentation providing details of the geological characterization and FLO-2D modeling). The FLO-2D analyses offered a good approximation of the actual behavior of the La Conchita landslide, as validated by means of aerial photographs, a news crew video of the event, and field investigations.
The results of the investigation indicate that the temporary wall played no role in either triggering the landslide or in redirecting the major lobe of wet landslide debris that impacted the community. Construction of the temporary wall and the accompanying removal of 1995 slide debris from Vista del Rincon did, however, prove instrumental in protecting at least one home from destruction in the event.