Rainfall threshold for the initiation of debris flows by channel bed failure in the Dolomites

The most common process of initiation of debris flows in natural channel of high mountainous areas is the channel-bed failure. In this case the runoff, rather than infiltration, becomes the most important controlling factor of the occurrence of debris flow. The estimates of the critical values of runoff can be indirectly approached by the precipitation that generated them. The aim of this study is to define the threshold rainfall conditions that result in the initiation of debris flow by channel-bed failure. Historical and rainfall data from 28 debris-flow events that occurred in six watersheds in the Dolomites (Northeastern Italian Alps) were collected. Using information on the rainfall up to the time of occurrence of the debris flows a method to identify these events using an SCS based kinematic distributed hydrological response model is proposed. Precipitation intensities plotted against durations are used to define a threshold curves for the triggering of the debris flow by channel bed-failure in the Dolomite environment. The threshold curve is compared with eleven threshold curves for the triggering of debris flow reported in the literature and with the intensity-duration-frequency curves of two gauges stations next to the five study sites. Comparisons with reported thresholds do not show a distinction between rainfall conditions that result in landslide-triggered or channel bed failure-initiated debris flows. Comparison of thresholds with intensity-duration-frequency curves lead to the identification of two different precipitation regimes for the triggering of debris flows by channel bed failure.