THE BIG ROCK MESA LANDSLIDE, MALIBU , CALIFORNIA, A CASE HISTORY

The Big Rock Mesa is located in Malibu, California, on the south flank of the Santa Monica Mountains, and bordered on the south by the Pacific Ocean. The Big Rock Mesa area includes about 190 acres of which approximately 136 acres are involved in landsliding. The Big Rock Mesa landslide is composed of faulted blocks and deep-seated landslides that have undergone different movement histories. There are 206 developed parcels and a major transportation/utility corridor(Pacific Coast Highway)within the landslide.

Deformation was documented in late 1971 to early 1972 following exceptionally-heavy precipitation during the winter 1968 to spring 1969. The average annual rainfall totals for the period July through June of 1968 to 2006 is approximately 16.3 inches. During the winter of 1977-1978 the area received about 34 inches of precipitation over a 4-month period. By spring of 1980, four areas of distress had been identified. By about September 1983, movement of the main portion of the Big Rock Mesa Landslide was widely recognized.

There is a strong correlation between the rate of landslide movement and groundwater levels. Dewatering wells were installed between 1971 and 1974,with additional wells installed in 1983 and later. Survey data from seven monuments indicate that the rate of landslide movement between September 1983 and July 1984 ranged from 1.6 to 4.3 feet per year. The rate of landslide movement between July 1984 and July 1985 ranged from 0.10 to 1.12 feet per year, indicating a reduction in the rate of landslide movement following installation and operation of the second round of dewatering wells. Based on inclinometer data, the headscarp region of the landslide experienced 0.22 inches of displacement, and the Central Mesa Region experienced 0.10 inches of displacement during 2005-2006 monitoring year.

The City of Malibu currently operates 22 dewatering wells and 34 hydraugers on behalf of the homeowners as part of an assessment district. Since 1983, the facility's 12-month average discharge has ranged from a low of 58,226 gallons per day (gpd) in 1991-92 to a high of 215,808 gpd in 1984-85. Discharge varies due to climatic conditions (rainfall), domestic water usage, and the interception of groundwater pockets. The effectiveness of the dewatering system is dependant upon the landslide structure, material properties, proximity to other dewatering installations.