The Transition From Bedload to Granular Debris Flow on Steep Slopes: A Force Balance Perspective

Abstract In mountainous regions, risk mitigation requires an understanding of sediment‐transport processes. We present new experiments conducted on a steep slope (33%) to study the transition from bedload to debris flow. The flume design was adapted to mimic alpine streams: instead of studying the mobility of a channel bed composed of uniform‐sediments, we generate pulses of sediment by injecting water over a self‐formed deposit of poorly sorted mixtures located at the flume entrance. The setup comprises ultrasonic sensors measuring the height of the water‐sediment mixture and a force sensor measuring normal and tangential forces exerted on the bed. The experiments show that the highest discharges generate bedload. At lower discharges, mass failure of the deposit generates two regimes: a “static‐dynamic” regime, where a granular pulse propagates without a clear water phase, and a “full‐dynamic” regime, where concentrated pulses are driven by water flows. In both regimes, pulses are vertically and longitudinally sorted: the front contains coarse particles, the tail contains finer particles, and the body a mix, with coarser particles concentrated near the surface. Force analysis shows that, in the static‐dynamic regime, mobility is governed by resistance at the front and thrust from body weight. In the full‐dynamic regime, weight alone cannot explain the observed stresses, suggesting roles for additional factors, such as non‐hydrostatic water pressure, acceleration, and vertical transfers. In both regimes, front resistance controls pulse mobility. Basal friction coefficient () analysis further shows that the transition to bedload occurs at the threshold discharge required to mobilize coarse particles alone in the channel.