Debris flows and translational landslides in the Serra do Mar mountain range: morphometric indices and TRIGRS model

Abstract

Successive mass movements have been triggered in Brazil, especially in the Serra do Mar compartment, causing several social and economic losses. In 1967, after months of significant uninterrupted precipitation over the Caraguatatuba city (state of São Paulo), numerous landslides and debris, mainly debris flows, were observed throughout the city, particularly in the Guaxinduba river basin. Thus, the questions arise: (i) what is the contribution of areas susceptible to landslides in triggering races? (ii) what is the role of morphometric indices in the susceptibility to these processes? The objective of this work was to evaluate the integration between the mechanisms responsible for the susceptibility to translational landslides and their contribution to the potential of debris flows in the Guaxinduba basin, using the TRIGRS model and morphometric indices. A joint analysis was carried out between the landslides, based on a susceptibility map generated by the TRIGRS model, validated through the superposition of scars, and its relationship with morphometric indices for the intensification of debris flows. The results indicated that 71% of the basin was classified as unstable by TRIGRS (more susceptible to landslides), with 67% of the mapped scars agreeing with these classes. The Guaxinduba sub-basins with areas less than 3 km², amplitudes greater than 300 m and circularity index greater than 0.5 presented better conditions in generating debris flows. In a conjoint analysis, the unstable areas classified by TRIGRS can be fundamental in the contribution of sediments to future events of debris flows. The use of different methods showed that the Guaxinduba river basin, in cases of intense and concentrated rainfall or extreme episodes, is susceptible to debris, mainly debris flows.