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<p>The Hengduan Mountains in China exhibit complex geological conditions, resulting in significant variability in post-fire debris flow behavior across distinct lithological zones. This study investigates three burned areas—Yajiang (metamorphic), Xichang (sedimentary), and Sejiao (igneous)—using integrated field surveys and laboratory analyses. The objective is to compare hazard-prone environments and debris flow characteristics across different lithologies and to decipher their initiation mechanisms. Results indicate that topography, slope structure, and soil composition are the key factors controlling debris flow initiation. In Yajiang, early- and late-season debris flows show distinct initiation mechanisms and sediment supply pathways. Shortly after the fire, debris flows were initiated by runoff-driven erosion of dry ravel deposits consisting of wind- and gravity-transported ash and loose sediment. High clay and organic matter contents promote rapid mixing of runoff with bed material, generating viscous flows that increase in velocity and volume downstream. Later in the rainy season, hillslope structure becomes more influential as sediment sources shift from dry ravel to channel bed and bank materials. In Xichang, debris flows are consistently initiated by progressive sediment entrainment during rainfall events. Although individual events are relatively small, the presence of abundant loose sediment enables frequent debris flows whenever rainfall thresholds are exceeded. In Sejiao, debris flows are predominantly driven by channel-bed erosion under a firehose effect throughout the rainy season. Steep terrain and resistant bedrock accelerate runoff and intensify scour, initiating debris flows through channel-bed failures. However, this process is constrained by the availability of mobilizable bed material and repeated runoff progressively coarsens channel deposits. As grain size increases, greater runoff intensity is required to initiate debris flow, thereby reducing debris flow susceptibility over time. This study unravels the contrasting initiation mechanisms of post-fire debris flows across lithological zones in the Hengduan Mountains and provides insights for hazard mitigation in complex mountainous terrains.</p>

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Variability of post-fire debris flow initiation mechanisms across different lithological zones in the Hengduan Mountains, China

  • 期刊论文

  • 发表日期：

    2025年9月13日

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  • Yonghao Zhou,
  • Xiewen Hu,
  • Xueqiang Gong,
  • Kun He,
  • Mi Jiang,
  • Jianqiang Yang,
  • Hao Peng

摘要

The Hengduan Mountains in China exhibit complex geological conditions, resulting in significant variability in post-fire debris flow behavior across distinct lithological zones. This study investigates three burned areas—Yajiang (metamorphic), Xichang (sedimentary), and Sejiao (igneous)—using integrated field surveys and laboratory analyses. The objective is to compare hazard-prone environments and debris flow characteristics across different lithologies and to decipher their initiation mechanisms. Results indicate that topography, slope structure, and soil composition are the key factors controlling debris flow initiation. In Yajiang, early- and late-season debris flows show distinct initiation mechanisms and sediment supply pathways. Shortly after the fire, debris flows were initiated by runoff-driven erosion of dry ravel deposits consisting of wind- and gravity-transported ash and loose sediment. High clay and organic matter contents promote rapid mixing of runoff with bed material, generating viscous flows that increase in velocity and volume downstream. Later in the rainy season, hillslope structure becomes more influential as sediment sources shift from dry ravel to channel bed and bank materials. In Xichang, debris flows are consistently initiated by progressive sediment entrainment during rainfall events. Although individual events are relatively small, the presence of abundant loose sediment enables frequent debris flows whenever rainfall thresholds are exceeded. In Sejiao, debris flows are predominantly driven by channel-bed erosion under a firehose effect throughout the rainy season. Steep terrain and resistant bedrock accelerate runoff and intensify scour, initiating debris flows through channel-bed failures. However, this process is constrained by the availability of mobilizable bed material and repeated runoff progressively coarsens channel deposits. As grain size increases, greater runoff intensity is required to initiate debris flow, thereby reducing debris flow susceptibility over time. This study unravels the contrasting initiation mechanisms of post-fire debris flows across lithological zones in the Hengduan Mountains and provides insights for hazard mitigation in complex mountainous terrains.