LIVING WITHIN A DYNAMIC LANDSCAPE - LEGACY CHALLENGES FROM THE ST. FRANCIS DAM FAILURE

The study area is located in San Francisquito Canyon, Southern California, and includes an active landslide complex adjacent to a U.S. Forest Service fire station, which is about one mile downstream from the former site of St. Francis Dam that had tragically failed in 1928. The hill slopes along the canyon are inherently unstable, and there are numerous paleolandslides developed in the Pelona Schist. The Pelona Schist is the bedrock in this area with bedding planes dipping 35 to 45 degrees out of the slope face. Overlying the bedrock are layers of colluvial and residual deposits of 10- to 15-ft in thicknesses, which are prone to move along the contact surface with the bedrock. During the winter of 2004-2005, the wettest winter on record, rain storms triggered numerous landslides in the area, including the landslide complex on the mid-slope above the fire station. The complex has several nested slumps with main scarps coalescing for about 1000-ft in total length.

The active landslides, which place the fire station at risk, are sensitive to failure in response to heavy rainfall. This study’s goal was to find a threshold level that could be used to predict when the hillslope will fail again. A deterministic analysis using a limit equilibrium method with the infinite slope equation was performed with a USFS computer model, DLISA to back analyze the amount of groundwater levels needed to replicate the 2005 failure event.

Based on this analysis, the estimated the amount of water that is needed to reactivate the landslide movement is 2.6 inches of water to be percolated into the soil above the slip surface. Considering the steep slope and lack of vegetation, resulting in low percolation rate of rain water into subsurface; the predicted threshold is 13-inches of continuous rainfall, if the soil has not been wetted previously. The threshold also helps to explain why no measurable slope movement has occurred since 2005. Our advice is to watch the local weather forecasting closely, and be aware of the pending rainfall levels that may reactivate the landslides. This study did not consider the effect of a possible earthquake that could trigger and/or intensify the slope failure.