Data uncertainty and variability in modeling debris flow propagation

Abstract

We replicate the propagation of the Val Rossiga debris flow (November 2002, Central Italian Alps), a 90,000 m3 event triggered by a rapid retrogressive landslide with high water content. The rheological model combines in a linear sum the viscoplastic terms of the Bingham model and a quadratic inertial term. The model requires as input data the bulked hydrograph and the empirical coefficients which describe the exponential dependence of the rheological parameters (i.e. Bingham viscosity and yield stress) on sediment concentration. We provided these data through different methods. Alternative hydrographs were produced by simulating the propagation of the triggering landslide according to different rheologies (i.e. rigid block model, frictional material, and Voellmy material). The rheological parameters are either determined by back analyses and directly through laboratory measurements and field investigation. Laboratory measurements were performed using a Ball Measuring System and a vane apparatus connected to a rotational rheometer on three samples from different sectors of ow path (i.e. source, channel and fan deposit). The samples were analyzed at varying the solid concentration and the grain size included in the tested suspensions (maximum grain size of 0.425 mm). The alterative conditions assumed for the input data were modeled on topographies of 5 m and 10 m cell-size. The seven scenarios we obtained were optimized by back analyses of the rheological parameters. Among the condition tested, the largest uncertainty is related to the initial hydrograph, which the model is very sensitive to (particularly with respect to peak discharge). Moreover, the hydrograph input data are unknown in most of the cases and apriori derivable only with large approximation. The alternative in ow hydrographs require a variation larger than one order of magnitude in the values of the rheological parameters obtained by the back analyses. The rheological properties measured directly on samples of varying composition (e.g. origin and grain size included) fall in most of the cases within the range of uncertainty de ned by the alternative in flows considered. Over- all, the vane geometry is preferable against the Ball Measuring System. The latter suffers for more narrow testable conditions, more marked experimental limits, and produces results which are more scattered than the vane geometry.