Triggering factors, behavior, and social impact of the January 2021 hail-debris flows at the Central Valley of Chile

An extreme precipitation event produced catastrophic debris flows in central Chile during 29–31 January 2021 (austral summer). Our study focuses on the triggering factors and dynamic behavior of hail-debris flows affecting the small commune of Malloa (Central Valley), which caused 200 injured individuals and 73 damaged houses. We carried out a post-event detailed field mapping of the local geology, the erosional features on the ravines, and its related hail-debris flow deposits. In parallel, the study involved a socio-cultural analysis of vulnerability to debris flows, with a particular focus on the disaster experience of the local community. Our results indicate that these hail-debris flows were likely conditioned by extended drought, local geomorphology, bedrock weathering/alteration, and water-oversaturated soil by two antecedent precipitation pulses. Soil erosion triggered by a hailstorm during a third precipitation pulse initiated hail-debris flows from small basins (< 1.2 km 2 ). Basin concentration times were estimated in 6–8 min, while hail reduced flow resistance by interparticle lubrication, promoting peak flow velocities near 2.4 to 5.5 m/s. Debris flow risk management should focus on developing suitable infrastructure and installing capacities at the local level as an essential condition for implementing subsequent inter-sectoral actions (for prevention, mitigation, and design risk scenarios).