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Rainfall intensity and duration of storms has been shown to influence the triggering of debris flows. After examining storm records of the San Francisco Bay region, documenting when debris flows occurred, and measuring piezometric levels in shallow hillside soils, continuous high-intensity rainfall was found to play a key role in building pore-water pressures that trigger debris flows.

Debris flows in 10 storms between 1975 and 1984 in a 10-km2 area near La Honda, California, were examined, and their rainfall records compared to the records of other storms to determine the antecedent conditions and the levels of continuous, high-intensity rainfall necessary for triggering debris flows. No flows were triggered before 28 cm of rainfall had accumulated each season, which suggests that prestorm soil-moisture conditions are important. After this sufficient antecedent rainfall, a threshold of rainfall duration and intensity—which accounted for triggering at least one debris flow per storm within the study area—was identified. The number of debris flows increased in storms with intensity and duration characteristics significantly above this threshold.

By studying where debris flows initiated in storms of different intensity and duration, debris flow susceptibility was found to depend on soil thickness and hillside concavity and steepness. Moderate intensity storms of long duration triggered complex soil slump/debris flows in thick soils on concave slopes below large drainage areas, whereas high-intensity storms of short duration caused complex soil slide/debris flows in thinner soils without respect to size of drainage area. From these observations, an empirical model based on geology, hydrology, and topography is proposed to account for the triggering of debris flows at selective sites by storms with different combinations of intensity and duration once the antecedent and intensity-duration thresholds are exceeded.

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