Hydrologic Factors Triggering a Shallow Hillslope Failure

In February, 1983, an intense rainstorm triggered a shallow, rapid slump/debris-flow on a monitored hillslope of coastal central California. Discontinuous records of rainfall and maximum ground-water levels within the slide mass were collected before and after the event. These water levels show the development of a shallow perched water table in soil overlying an older, low permeability landslide deposit. Although this perched water table had an overall downslope hydraulic gradient, the slope failure occurred in an area of localized gradient decrease or mounding. The cause of this localized mounding was not apparent from field observations. We used a mathematical model of variably saturated water flow to investigate the pre-failure hillslope hydrology and to demonstrate a possible cause of the localized ground-water buildup. Simulations indicate that only a slight decrease in soil hydraulic conductivity, relative to the overall soil variability, would have been needed to create the observed buildup.