Debris flows can be considered as highly hazardous hydrological processes very common in the whole Alpine environment, such as in the Friuli Venezia Giulia Region located in the extreme northeastern side of Italy. The progressive increase in socio-economic activities during these last years has created, in the mountain regions, the potentiality for an increased risk related to this kind of destructive phenomena. After the critical alluvial event which occurred on 21–22 June 1996 in the Fella watershed and the alluvial event of 29th August 2003 in the 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 deeply investigate the interpretation of the rainfall thresholds and on their redefinition. The analyzed examples show that there is a considerable increase in the intensity of the rainfall with a duration equal to or less than 24 hours. Above all – very critical for debris flow – are the rains of 1 to 6 hour duration having an intensity greater than 50 mm/hour.