REOLOGIA E DINAMICA DI PROPAGAZIONE DELLE COLATE DETRITICHE: ANALISI E MODELLAZIONE A DIVERSA SCALA

This research focuses on small scale laboratory tools to characterize the rheological behavior of the matrix (maximum diameter of sediment analyzed equal to 19 mm) of real debris flows affecting Eastern Italian Alps. The main aim is to define novel, easily reproducible and low-cost procedures that make available to a wider audience, in particular to the end users of numerical models, this kind of information. The research seeks therefore to delineate a connection between the laboratory data and the numerical models because, as mentioned by Calligaris and Zini (2012), the current main objective of debris-flow modeling is to clearly describe the input variables in order to better understand the formation of debris flow fans and to predict, mitigate or control the hazard posed by these phenomena to communities situated into the mountain areas. This research takes into account monophasic numerical models because of their widespread and proven efficiency (Rickenmann et al., 2006; Bisantino et al., 2009; Calligaris and Zini, 2012). Summarizing, the detailed aims of this research are: - Debris-flow behavior investigation through small scale laboratory tools and original and easily reproducible procedures; - Identify methodologies for applying the laboratory data to the numerical modeling and to calibrate the rheological parameters; - Verify the methodologies identified through a performance analysis of numerical simulations of real and well documented debris flows. Initially, the objectives have been pursued through a literature review focused, in particular, on the methodologies for analyzing debris-flow behaviors. In the following laboratory investigation, the results of two different tools are displayed: the vane spindle rheometer Brookfield DVIII Ultra and the tilting plane rheometer of the Institute for Hydrological and Geological Protection of the Italian Research Council (CNR IRPI) (D’Agostino and Cesca, 2009-a; D’Agostino et al., 2010). Moreover, a standard and easily repeatable procedure, based on the two-dimensional simplification suggested by Hungr (1995), is proposed to estimate the mean basal shear stress, which develops during the runout phase. Interesting considerations about flow regime and depositional features have been achieved thanks to the velocity recorded in the tilting plane laboratory test. Considering the definition of procedures to calibrate the rheological parameters, two approaches have been proposed. The first is based on the tilting plane rheometer results while the second uses the relation between magnitude and debris-flow mobility (Rickenmann, 1999; Lorente et al., 2003) and a specifically-shaped index. In the last part of the research work, a performance analysis about two monophasic numerical models is presented. The compared models are FLO-2D (O’Brien et al., 1993) and RAMMS (Rapid Mass MovementS, Christen et al. 2010). The models have been tested in the simulation of some debris flows with heterogeneous characteristics: different rheological behavior, magnitude and topographic situation. The efficiency of the laboratory tests in the calibration of the rheological parameters of the model has been also investigated through this performance analysis. The output of the model has been assessed through two indexes: the first analyzes the positive accuracy (Scheidl e Rickenmann, 2010), whereas the second expresses the model efficiency quantifying the excess deposit simulation.