Controls over debris flow initiation in glacio-volcanic environments in the Southern Andes

Abstract. The Southern Andes is an active zone of mass wasting processes with unknown constraints for public policies. Several conditioning factors could have an impact on the generation of debris flows, being controlled by water accumulation. This study investigates the generation of the Ñisoleufu debris flow, an active area of debris flow generation in Southern Andes, reviewing the interplay between geomorphological, geotechnical and hydrometeorological controls in debris flow dynamics, focusing on the effects of soil properties, slope characteristics and precipitation events. Our results highlight significant changes in soil moisture content on critical days associated with debris flow events. We revealed that the combination of areas with high water accumulation capacity from local runoff and slopes that capture precipitation effectively were crucial in the generation of debris flows. Areas with granular volcanic soils acted as storage mediums for water, which, coupled with decreased shear strength, facilitated debris flow initiation. The thin and fine-grained layers of glacial deposits located beneath the volcanic soil, characterized by low hydraulic conductivity, created localized accumulation zones that reinforced the storage capacity of adjacent areas, particularly in pyroclastic volcanic deposits in the release zone. The hydraulic properties of the volcanic deposits suggest that water storage capacity and high hydraulic conductivity play a critical role in rainfall-induced debris flow initiation. Additionally, we observed that the debris flow of the Ñisoleufu event has evidence of reworked lapilli-sized particles (> 5 mm), being consistent with the surface and shallow water movement that reduces the slope stability within the area. Analysis of ERA5-land dataset showed abrupt changes in soil moisture content at various depths and time periods, correlating with intense or prolonged rainfall events. These results underscore the role of geomorphological features in modulating soil moisture and thereby affecting the stability and movement of debris flows. Our results provide a comprehensive understanding of how geomorphology interacts with hydrological factors to influence debris flow behaviour in volcanic areas of the Southern Andes for the first time. Overall, the research highlights the critical role of geomorphological and hydrological factors in debris flow generation and dynamics. It emphasizes the need for incorporating detailed soil and slope characteristics into models for predicting debris flow risks. By understanding the combined effects of water accumulation, soil properties, and slope dynamics, this study contributes valuable insights into managing and mitigating debris flow hazards in vulnerable regions. These findings enhance the predictive capacity for rainfall-induced debris flows and provide practical criteria for hazard assessment in post-glacial volcanic terrains.