COMPARISON OF TWO LANDSLIDE EVENTS IN NORTH COASTAL CALIFORNIA

Mass movement processes are a dominant means of supplying sediment to mountainous rivers of north coastal California, but the episodic nature of landslides represents a challenge to interpreting patterns of slope instability. Two major landslide events occurring in 1964 and in 1997 in the Redwood Creek basin in north coastal California are compared. In 1997, a moderate-intensity, long-duration storm with high antecedent precipitation triggered 317 landslides in the 720 km² Redwood Creek basin, compared to about 1500 slides in a larger storm in 1964. The rainfall intensity-duration threshold for landslide initiation in both periods was consistent with previously published values. The sizes and locations of the 1997 landslides were compared to the 1964 distribution of landslides. During both periods, inner gorge hillslopes produced many landslides, and a few large landslides dominated sediment delivery to rivers. However, the volume of landslide material produced by the 1997 storm was an order of magnitude less than that generated in the earlier period. In 1997, the volume of landslides (on a per unit area basis) was five times greater along a fault zone than in areas underlain by other lithologic units. Clusters of landslides were associated with intense rainfall cells detected from Doppler radar data. The slope stability model SHALSTAB identified general areas susceptible to failure. The 22% of the watershed area classified as moderately to highly unstable by the SHALSTAB slope stability model included locations that generated almost 90% of the landslide volume during the 1997 storm.