Debris flow phenomena: a short overview

Summary: On the day of 29 August 2003 in the extreme northern part of Italy, the Friuli-Venezia Giulia region was invested by a violent and intense rainfall that caused several instability phenomena. Along the sides of the Val Canale valley were mobilized over a thousand of landslides, most of whom were of first generation. The rainfall started at midnight, firstly affecting the areas belonging to the upper sector of the mountains around Cucco, Malborghetto and Ugovizza, then it gradually moved downwards with increasing intensity (Calligaris et al., 2010). A total value of 293 mm rainfall was recorded by the Pontebba rain gauge from 02h00 to 18h00. Around 18h30, some impressive waves plunged with unexpected violence, power and noise, an avalanche of water, big rocks and trunks crashed against courtyards, houses and went inside windows (Russo, 2003). This event caused the death of two people, 300 lost their homes, 260 buildings were damaged and substantial damages occurred to infrastructures that remained out of action for several days. The event was so extreme and particular that the return time has a considerable variation in its value depending on the period: between 1 and 24 h, the time of delivery is between 50 and 100 years; for 12 h it is between 200 and 500 years, while, for a period from 3 to 6 h, return period varies between 500 and 1000 years (Borga et al., 2007; Calligaris et al., 2010). This area has been chosen as test site due to the important amount of data availability. Many studies have been realized to characterize the debris flow occurred. The Geological Survey of FVG Region had the necessity to better understand the occurred phenomena in order to prevent future disasters and to proceed to a better and more coscientious territorial planning. In this context, many approaches were used. Back analysis simulation thought commercial and innovative software permitted to delimit the flooded areas. The debris flow is often considered to be a mixture of viscous slurry, consisting of finer grain sizes and water, and coarse particles (Scotto di Santolo, 2008). The volume and the composition of the mixture are the main factors that contribute to determine the hazards associated with such phenomena, since they govern the mobility and impact energy of the debris (Iverson, 1997; Jakob, 2005). During the last years, several simulation models and approaches have been implemented (Cesco Bolla, 2008; Pirulli, 2005; Rickenmann, 1999) and created to reconstruct a debris-flow phenomena, but a believable scenario can be obtained only by resorting to real parameters that are suitable to characterise the involved material (Sosio et al., 2006). Thus, it is necessary to calibrate those computational codes through back-analysis simulations and laboratory analysis (Tecca et al., 2006). The input variable needed by the codes are so aleatory that is a strongly felt need to isolate each one of them and to go deep inside their meaning. Rheological parameters of viscous debris flows are one of these variables; they are influenced by a great amount of factors and are therefore extremely difficult to estimate. Viscosity and yield stress are the one that define the debris flow behaviour. For these reasons laboratory rheometer analysis have been realized in order to better characterize them, to apply the obtained results at the available codes and to proceed to new outlines of the invested areas.