Internal structure of intense collisional bedload transport

Abstract

AbstractIntense bed load refers to a regime which is responsible of most of sediment transport in torrents and rivers during floods. Even if restricted to uniform steady conditions and well sorted particles, the liquid and solid phases interact in a rather complex way over the plane mobile bed. We focus on flow conditions where the sediment grains in the flow are supported by mutual contacts, primarily collisions, and not by the fluid turbulence. Nonetheless, the turbulent liquid stresses are important everywhere, but the top of the bed, and may not be neglected. Being particles and fluid reciprocally affecting at various scale, mechanical interpretations are challenging and not consolidated, yet. In this picture, it is clear that experimental evidences provide a fundamental, still lacking, piece of information.We contribute with new experiments in a tilting flume to face this need. Compared to previous laboratory tests, we apply various measuring techniques, getting to a robust validation of the acquired dataset. We exploited stereoscopic imaging techniques to capture the solid concentration and the three components of grain velocity, and extract the velocity fluctuations from the local mean values. Measured velocity distributions are independently checked by using an Ultrasonic Doppler Velocimetry technique.With all the needed quantities available, we exploited the constitutive relations based on the kinetic theory for granular flow, to highlight how the comparison between predictions and measurements works, deploying some accuracy deficiencies in the measurement and/or interpretation limits. Our results also address the attention to interfaces that separate layers affected by different rheological mechanisms. Along with pure experimental and theoretical aspects, we grasp the chance to use the data to check how the relations involving global quantities (discharges, friction factor, bed slope, flow depth), so common in hydraulic practice, work when the sediment transport is intense. © 2019 John Wiley & Sons, Ltd.