Rainfall thresholds for the initiation of debris flows at La Honda, California

In order to study the relation between heavy rainfall, shallow pore pressures, and slope stability in hillslopes susceptible to debris flows, we have been observing debris flows and measuring rainfall and hillslope pore pressures in a 10-km² study area in the central Santa Cruz Mountains near La Honda, California. A simple numerical model, based on the physical analogy of a leaky barrel, can simulate significant features of the interaction between rainfall and shallow-hillslope pore pressures. In the model, the barrel is filled at a rate equal to the rainfall intensity and drained at a rate equal to the product of the water level retained in the barrel, Z, times the drainage coefficient, k_d. If the retained rainfall exceeds a critical water level, Z_c, the slope becomes unstable. Thus, the threshold for the intensity and duration of storm rainfall required to initiate debris flows is determined by the k_d and Z_c values of the most susceptible slopes in the study area.