Abstract

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).