Debris follow rainfall thresholds in Val Canale Valley: first steps into their redefinition.

Debris flows can be considered as highly hazardous hydrological processes really common in the whole Alpine environment as in the Friuli Venezia Giulia Region located in the estreme northeastern side of Italy. The progressive increase in the socio-economic activities during these last years, created, in the mountain regions, the potentiality for an increase risk related to this kind of destructive phenomena. After the critical alluvial event occurred on 21-22 June 1996 in the Fella watersheld and the alluvial event of 29th August 2003 occurred in Val Canale valley when more than 293 mm of rain were recorded by Pontebba’s rain-gauge in four hours (the total influx of the meteorological event, which lasted about 12 h, was equal to 389.6 mm) the need arose to deep investigate on the meaning of the rainfall tresholds and on their ridefinition. The analyzed examples show that there is a considerable increase in the intensity of the rainfall with a duration equal or less than 24 hours. Above all are very critical for debris flow, the rains of 1 to 6 hour duration having an intensity greater than 50 mm/hour.