Early post-fire debris-flow initiation following the March 2024 Yajiang Fire, Sichuan, China

Abstract

Burned slopes, with reduced vegetation cover, enhanced soil water repellency and decreased infiltration capacity, are highly susceptible to post-fire debris flows and can pose immediate threats to infrastructure and communities in steep mountain regions. However, the initiation mechanisms of the first set of debris flows after a fire remain poorly understood, hindering effective hazard management. Following the 15 March 2024 Yajiang Fire in Sichuan, China, which burned approximately 278.8 km², 506 post-fire debris-flow events occurred during the same year and caused severe damage. Within this broader context, this study focuses on the early post-fire stage and uses the Baima catchment as a detailed study site to investigate how ash-rich hillslope sediment was redistributed, stored in channels and subsequently involved in debris-flow initiation. By combining rainfall and wind records, soil water repellency tests, repeated measurements of ash thickness on hillslopes, surveys of dry-ravel storage within channels, unmanned aerial vehicle (UAV) mapping, repeated field investigations and sediment analyses, we show that wind and gravity rapidly transferred ash from slopes to channels. Within 11 days after the fire, approximately 93.8% of the ash in the Baima catchment had accumulated in channels as dry-ravel deposits, which became the dominant source material for the earliest debris flows. Field evidence indicates that these early debris flows were initiated when rainfall mixed with channel-stored dry-ravel deposits to form a high-viscosity, ‘concrete-like’ mixture, which was then driven downslope by continued inflow and gravity. At the regional scale, early post-fire debris flows were concentrated in small, steep catchments, most of them smaller than 1 km². In the Baima catchment, about 86.6% of the solid material in the debris flow was derived from loose ash-rich sediment. Coarse particles were preferentially deposited at catchment outlets as a granular phase, whereas finer ash-rich material extended farther downstream as a muddy phase. These findings provide direct field evidence for the initiation of early post-fire debris flows in the Hengduan Mountains, showing that rapid hillslope-to-channel sediment transfer can preload small, steep burned catchments and strongly influence their earliest debris-flow response. This study therefore provides a process-based basis for early hazard recognition and emergency response to post-fire debris-flow hazards in the Hengduan Mountains and other similar steep mountain regions.