Entrainment-driven morphological changes in debris flow deposits by varying water content at laboratory scale

Abstract

Entrainment is crucial in shaping debris flow deposits, influencing their morphology and dynamics. Understanding deposition driven by entrainment is vital for improving hazard mitigation and sediment management strategies. This study employs a small-scale flume setup to examine the interplay between water content (w/c), sediment composition, and bed morphology on granular flow behavior. Sixteen experiments were conducted, varying w/c (20–50%) and erodible bed configurations, with deposit morphology analyzed for width, thickness, and runout length. The findings revealed distinct patterns in deposit morphology across w/c levels. At lower w/c (20–24%), deposits exhibited broad, shorter lobes with minimal scouring, forming cone-shaped structures. Moderate w/c (~ 28%) enhanced flow mobility, producing thicker deposits near the flume bed due to reduced entrainment. At higher w/c (30–50%), deposits shifted further downstream, driven by greater entrainment volumes and longer runout distances. While higher w/c levels decreased deposit thickness, they significantly widened the runout deposits, demonstrating the dual influence of w/c and entrainment. A clear relationship emerged between entrainment and flow mobility, as increased entrainment volumes widened and flattened deposits. Additionally, water content dominated entrainment in controlling deposit thickness, underscoring its critical role in sediment transport dynamics. The deposits were poorly sorted, with a distinct bedding structure akin to natural debris flows, validating the experimental setup. This study provides an efficient and scalable methodology for analyzing granular flow behavior in erodible beds. The results offer insights into sediment transport processes, bridging the gap between mesoscale experiments and real-world applications in natural hazard mitigation and geotechnical engineering. © The Author(s) 2025.