Abstract.To set an approach for the future territorial planning, the Geological Survey of Friuli Venezia Giulia Region, through the researchers of Trieste University, started a program of debris-flow risk analysis using Flo-2D software as tool to delimit the hazardous areas. In the present paper, as a case study, a debris flow, called Fella sx, occurring in a torrent catchment was analyzed. The choice was due to the abundance of information about past events, inundated areas, rain fall, geology and to its representativeness. An initial back-analysis investigation identified a couple of representative rheological parameters. Riverbed samples were collected, sieve analyses were performed and rheological tests were carried out on the fraction finer than 0.063 mm using a rotationally controlled stress rehometer equipped with the serrated parallel plate geometry. The shear dependent behaviour was examined at different concentrations ranging from 33 to 48%, by weight. Viscosity data treatment was performed to determine the most suitable rheological model to provide the best approximation of the debris-flow behaviour. The rheological parameters, derived from experimental data, were used and compared with those obtained through the back-analysis and with the real inundated area. Data obtained through rheological analysis are useful in constructing scenarios of future events where no data for back-analysis are available.

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On August 29th 2003 many debris flows were activated along different streams of the Fella river basin (North-East Italy). After that huge event a program of debris flow risk analysis was started and through the use of FLO-2D software, a new outline of the hazardous areas was obtained. In the last years, part of a PhD research project was addressed to define a quick field and laboratory survey methodology suitable to obtain reliable rheological parameters and to reproduce debris flow scenarios in the absence of back analysis. An example of the methodology is presented, including grain-size and rheological analysis on samples of a watershed located in left side of the Fella river, the reconstruction of its DTM starting from detailed laser scanner data (Civil Defense of Friuli Venezia Giulia Region) and the updating of the morphometric parameters in virtue of the more detailed data. For the present research, a comparison between previous simulated scenarios and those obtained through the introduction of new basic data was provided. In particular, the simulations based on the parameters collected after the 2003 event on the DTM of 2003 were used as back analysis, whereas new simulations were performed using the rheological parameters derived from samplings and laboratory analyses realized during 2013 on the DTMs of 2008, in order to create future possible flooding scenarios on the actual morphology and to test the effectiveness of the mitigation measures, built up after the 2003 event.

Debris flows are rapid gravity-induced flows of high-concentration granular-liquid mixtures, consisting of clay, silt, sand and boulders with a variable quantity of water.\ud Due their high speed and energy, they represent a severe hazard in mountain regions. In this study, we investigate the properties and the dynamic of a debris flow occurred on 29th August 2003 in Valcanale valley (Friuli Venezia Giulia, Italy). Among the many watersheds affected by debris flow during the catastrophic alluvial event, we have chosen a dolomitic one, called “Solari” stream. Grain size, mineralogy and rheological behavior were characterized through laboratory analysis; later two-dimensional simulations, using the Flo-2D software, were performed on a 5 5 m grid obtained by the extrapolation of elevation values of the Regional Technical Map (CTRN), surveyed in 2003, in order to replicate the event, and on a 5 5 m grid obtained by the resampling of a LiDAR DTM, surveyed in 2008, to test the efficiency of the mitigation works. This work points out the safety of the infrastructures downstream, protected by an adequate deposition basin. At the same time, it underlines some issues during the rheological analysis and the overestimation of the depositional extent of the debris flow, when the laboratory data were used to run the\ud simulations