Entrainment-driven changes in runout deposition of debris flows at small scale

Entrainment plays a vital role in shaping debris flow deposits, influencing their morphology and dynamics. Our study utilized a small-scale flume experiment to investigate the effects of water content (w/c), sediment composition, and bed morphology on granular flow behavior. Sixteen experiments were conducted with varying w/c levels (20–50%) and erodible bed configurations, analyzing deposit morphology in terms of width, thickness, and runout length. The results revealed distinct morphological patterns across different w/c levels. At low w/c levels (20–24%), deposits formed broad, shorter lobes with minimal scouring, resulting in cone-shaped structures. Moderate w/c (~28%) increased flow mobility, leading to thicker deposits near the flume bed due to reduced entrainment. At higher w/c levels (30–50%), deposits shifted farther downstream, characterized by greater entrainment volumes and extended runout distances. While higher w/c reduced deposit thickness, it significantly increased deposit width, highlighting the combined effects of w/c and entrainment. The study identified a clear relationship between entrainment and flow mobility, with greater entrainment volumes producing wider and flatter deposits. Water content was found to be the primary factor influencing deposit thickness, emphasizing its critical role in sediment transport dynamics. The deposits were poorly sorted and exhibited a bedding structure similar to natural debris flows, validating the experimental approach. This research presents an effective and scalable method for studying granular flow behavior over erodible beds, offering valuable insights into sediment transport processes and bridging mesoscale experiments with practical applications in natural hazard mitigation and geotechnical engineering.