Relations between rainfall and triggering of debris-flow: case study of Cancia (Dolomites, Northeastern Italy)

Abstract. Debris-flows occurring in the area of Cancia (Dolomites, Northeastern Italy) in recent years have exposed the population to serious risk. In response to the recurring hazard, an alarm and monitoring system was installed to provide a sufficient level of safeguard for inhabitants and infrastructures. The data recorded at three rain gauges during debris-flow events has been analysed, taking into consideration the different elevation of the gauges to delineate the storm rainfall distributions. Rainfall data is compared with the occurrence of debris-flows to examine relations between debris-flow initiation and rainfall. In addition, the data is compared with that recorded during debris-flows which occurred under similar or different geological settings in the Eastern Italian Alps, in order to define triggering thresholds. A threshold for debris-flow activity in terms of mean intensity, duration and mean annual precipitation (M.A.P.) is defined for the study area The normalised rainfall and the normalised intensity are expressed as a per cent with respect to M.A.P. This threshold is compared with thresholds proposed by other authors, and the comparison shows that a lower value is obtained, indicating the debris-flow susceptibility of the area. The threshold equations are: 

R/M.A.P. = - 1.36 · ln(I) + 3.93  where I > 2 mm/h 
I /M.A.P. = 0.74 · D^-0.56. 

The determination of a debris-flow threshold is linked to the necessity of a fast decisional phase in a warning system for debris-flow protection. This threshold cannot be used as a predictive tool, but rather as a warning signal for technicians who manage the monitoring/warning system.