Large scale debris-flow hazard assessment: a geotechnical approach and GIS modelling

Abstract

Abstract. A deterministic distributed model has been developed for large-scale debris-flow hazard analysis in the basin of River Vezza (Tuscany Region -Italy). This area (51.6 km 2 ) was affected by over 250 landslides. These were classified as debris/earth flow mainly involving the metamorphic geological formations outcropping in the area, triggered by the pluviometric event of 19 June 1996. In the last decades landslide hazard and risk analysis have been favoured by the development of GIS techniques permitting the generalisation, synthesis and modelling of stability conditions on a large scale investigation (>1:10 000). In this work, the main results derived by the application of a geotechnical model coupled with a hydrological model for the assessment of debris flows hazard analysis, are reported. This analysis has been developed starting by the following steps: landslide inventory map derived by aerial photo interpretation, direct field survey, generation of a data-base and digital maps, elaboration of a DTM and derived themes (i.e. slope angle map), definition of a superficial soil thickness map, geotechnical soil characterisation through implementation of a backanalysis on test slopes, laboratory test analysis, inference of the influence of precipitation, for distinct return times, on ponding time and pore pressure generation, implementation of a slope stability model (infinite slope model) and generalisation of the safety factor for estimated rainfall events with different return times.Such an approach has allowed the identification of potential source areas of debris flow triggering. This is used to detected precipitation events with estimated return time of 10, 50, 75 and 100 years. The model shows a dramatic decrease of safety conditions for the simulation when is related to a 75 years return time rainfall event. It corresponds to an estimated cumulated daily intensity of 280-330 mm. This value can be considered the hydrological triggering threshold for the whole Vezza basin.

A model for triggering mechanisms of shallow landslides

“…Important contributions on the matter, referring to the Italian territory, are the data collection carried out by the Piedmont Region after the 1994 flood (Sutera Sardo et al, 1996;Campus et al, 1998) and the scientific notes, concerning similar types of events, in other regions: Campania in 1998 (Cascini et al, 2000(Cascini et al, , 2003Olivares 2001;Olivares and Picarelli, 2001), Umbria in 2004 (Cardinali et al, 2006), Tuscany in 1996 (Delmonaco et al, 2003) and Emilia Romagna in 2000 (Emilia Romagna ReCorrespondence to: L. Montrasio (lorella.montrasio@unipr.it) gion, Report 2002). Soil slips are portions of soil of small thickness that, in consequence of rainfalls, detach from the shallow part of a slope and move in different ways.…”

“…The second method, tends to consider, even if in a simplified way, the role played by the soil, in foreseeing the occurrence of a soil slip both on small and large scales. These kinds of models, which can be considered deterministic, take into account not only external factors, such as the rainfall amount, but also the geometrical features of the slope, the physical and mechanical behaviour of the soil and the mechanism of rainfall infiltration (Haefeli, 1948;Campbell, 1975;Montgomery and Dietrich, 1994;Bandis, 1996;Alonso et al, 1995;Delmonaco et al, 2003;Iverson and Major, 1987;Iverson, 2000;Salciarini et al, 2006).…”

Assessment of debris-flow susceptibility at medium-scale in the Barcelonnette Basin, France

“…It is characterised by (1) a mountain climate with a marked inter-annual rainfall variability (735 ± 400 mm over the period and 130 days of freezing per year, (2) a continental influence with significant daily thermal amplitudes (> 20 • ) and numerous freeze-thaw cycles and (3) a Mediterranean influence with summer rainstorms yielding more than 50 mm h −1 on occasion Flageollet et al, 1999). Heavy spring rains on melting thick snow layers also lead to high discharges (Flageollet et al, 1999). Meso-climatic differences on a small scale emerge due to the east-west orientation of the valley (Remaître, 2006).…”

“…By means of a combination of environmental parameters chosen on the basis of experience, primarily slope angle, upslope area and planar curvature, the debris-flow susceptibility is computed. For physically-based source identification a common option is to couple hydraulic models with the calculation of the safety factor (Delmonaco et al, 2003;Carrara et al, 2008).…”

Combination of statistical and physically based methods to assess shallow slide susceptibility at the basin scale

“…In addition, landslide inventory information is frequently used for calibrating stability models through back-calculation (e.g. Delmonaco et al, 2003;Teixeira et al, 2015).…”