Il monitoraggio delle colate detritiche nel Torrente Moscardo (alpi Carniche): gli eventi dei mesi di giugno e luglio 1996

Debris flows are, in many mountain regions, the most important processes for what concerns both the morphological evolution of the basins and the hazard represented for human settlements. Field data collection is of the greatest interest, among the many research activities on debris flows, to better understand the phenomena involved and to calibrate mathematical models. Even though monitoring activities have been performed in other geographic regions, in Europe field data about debris flows are generally lacking. About ten years ago CNR IRPI of Torino started a research program, subsequently carried on in cooperation with the homonymous Institute located in Padova, in order to provide new field data regarding the characteristics of debris flows on the Alpine region. The Forest Department of the Friuli - Venezia Giulia Region also gave a fundamental support. The Rio Moscardo basin, located in the Eastern Italian Alps, was the selected site for the researches (the basin area, at the fan apex, is 4.1 km2, the elevation is 1150 m, the main channel length 2760 m). The choice of this study area was due to the high frequency of debris flow occurrence, to the easy accessibility to the fan where the monitoring equipment could be installed and to the presence of a well cut channel not subjected to significant diversions. In 1989 a monitoring system, consisting of two ultrasonic gauging stations installed along a straight torrent reach on the central portion of the fan, began to work. The time lag between the recordings of the wave at the two stations allows an estimation of the mean front velocity, while the analysis of the wave profile obtained through the ultrasonic sensors may show eventual depositions or erosions at the recording sites. A raingauge was also installed in the upper portion of the basin to characterize the rainfalls that trigger the events. The good results obtained during the first years of research (between 1989 and 1994 eleven debris flows have been recorded) suggested the increase of the monitoring activities through the addition of new equipment. By now the recording facilities consist of: oa raingauge in the upper basin, slightly outside of it. oa set of seismic sensors installed along the right channel bank in the middle-lower portion of the basin, upstream of the fan ofour ultrasonic sensors placed at three different sites in the middle portion of the fan, along a torrent reach 369 m long (at the upstream station there are two flanked sensors). oa video camera shooting a great portion of the torrent reach between the first two ultrasonic sensors. The time lag between two consecutive recordings of the sensors has been reduced to 1 second since 1995 (it was previously of 10 seconds) in order to obtain a more detailed description of the wave form. The four sensors are run by a computer that activates the video recorder when abrupt changes of the stage occur at the upstream sensors. On June and July 1996 two debris flows were recorded that were made each of different surges. The video images have been of great utility for the understanding of the examined phenomena while the availability of the seismic and ultrasonic data made it possible the estimation of the mean front velocities in different reaches of the torrent. A detailed description of the recorded debris flows is presented with particular attention to the different phases they were composed of. Both the events were preceded by hyper-concentrated flows; debris flow surges appeared with an abrupt change in stage and a front carrying many trees; in this phase turbulence was very pronounced. Then the greatest flow heights followed: in this second phase, even though a rigid plug was not visible, the phenomenon showed the common characteristics o