Debris flow triggering characterization through a comparative analysis among different mountain catchments

•Minimum critical rainfall duration provides more precise alert conditions.•Narrow range of triggering water discharges among different basins.•Second thershold discharge might amplify debris flow volume.•Different typologies of debris flow source areas. The research focuses on the assessment of the conditions for debris flow triggering in three very active mountain catchments. The study areas are the following: Rudan torrent (Vodo di Cadore, Mount Antelao in the Dolomites, Italy), Chiesa torrent (Pieve di Livinallongo, Mount Col di Lana, in the Dolomites, Italy) and Rebaixader torrent (border between the regions of Catalonia and Aragon, Axial Pyrenees area, Spain). Due to the differences among these basins and related source areas where debris flows originate, the knowledge on the link between geomorphic conditions and driving mechanisms for debris flow activation is outlined. The study of debris flow initiation is carried out through a hydrological approach and a slope stability analysis. The hydrological analysis is supported by the calibration of intensity-duration curves of triggering rainfalls coupled with a distributed rainfall-runoff model. The analysis is corroborated using literature equations to compute critical discharges for sediment motion at the different levels of terrain deformation. The main results highlight the following: (i) a quite narrow range of triggering water discharges (0.03–0.16 m3/s) and shear stresses (267–413 N/m2), promoting a new insight on the specific hazard mitigation; (ii) the quantification of a second stage of bed breakage, in some circumstances capable of amplifying the debris flow magnitude; and (iii) a minimum critical rainfall duration needed to effectively entrain the phenomenon (12–22 min) that might help to define more precise alert conditions. Finally, the geomorphologic differences among basins revealed two main typologies in terms of driving the triggering mode: flow rate dominant in the Rudan-type basins and mixed (flow rate plus slope stability conditioned) in the Chiesa-Rebaixader-type basins.