Calculation of average landslide frequency using climatic records

Aerial photographs are used to develop a relationship between the number of debris slides generated during a hydrologic event and the size of the event, and the longterm average debris-slide frequency is calculated from climate records using the relation. For a site in California with an average of 8.3 slides km yr, a sequence of four photo sets (representing 10–15 years, 35–50 observed slides, and 4–6 large storms) is needed to estimate the long-term debris-slide frequency to within 30% of the actual value (p 5 0.90). If climatic records are used, a record length of 5–10 years (17–35 observed slides and 2–4 significant storms) is sufficient to provide the same accuracy. The climate-based model suggests that debris-slide frequency changed from approximately 1.6 to 8.3 slides km yr during the late 1930s owing to an increased frequency of high-intensity storms. The model accurately predicts the change in slide-scar density observed on sequential aerial photographs following the climatic shift.