From cylinder-scale drag to reach-scale resistance in debris-flow routing: A flume-based framework

Study region Jiuzhaigou Valley is a UNESCO World Heritage Site renowned for its outstanding scenic and ecological values, preserving important forest ecosystems and high biodiversity, and it is also recognized as a UNESCO Biosphere Reserve. Study focus To provide a mechanistic resistance parameter applicable to obstacle-rich channel reaches (tree stems and pier-like elements), we conducted long-duration quasi-steady flume experiments of debris flows passing a single circular cylinder using a recirculating system. Drag forces were measured under controlled mixture properties and flow regimes and converted to drag coefficient C d . In addition, we provide an application framework that links the experimentally derived mean drag coefficient to an equivalent reach-scale Manning roughness for obstacle- or forest-belt-rich reaches, enabling implementation in depth-averaged routing models, pending field-scale calibration and validation. New hydrological insights for the region The results show that Froude-number-only descriptions are insufficient for debris-flow interactions with cylindrical obstacles when geometric effects become important. The drag coefficient depends systematically on the depth-to-diameter ratio, reflecting a transition from front-dominated loading to flow diversion and shear-influenced interaction along the cylinder. Incorporating depth–diameter scaling and viscous control yields a practical, physically based drag closure for a single cylinder under quasi-steady motion. This closure provides an element-scale basis for a preliminary upscaling framework for obstacle-induced resistance in channel-routing calculations in Jiuzhaigou-type catchments.