Intense bedload associated with unsteady dam-break surges

Sediment transport has traditionally been tackled with vastly phenomenological approaches, and the proposed empirical relations contain calibration coefficients that have been adjusted on the basis of a combination of flume and field data. Since recently, the study of bedload is benefiting from the progress made in granular physics, aimed at accounting for the collisional and frictional interactions between grains, especially for bedload layers several grains thick. The development of a specific rheology requires a deeper understanding of the internal structure of bedload layers, in particular in terms of velocity and concentration profiles. We investigate these profiles experimentally, by looking at intense bedload associated with unsteady dam-break surges in a laboratory flume. We resort to a novel technique to simultaneously measure such profiles by imaging the flows with a fast camera combined with a normal-to-wall laser light-sheet. Velocities are obtained by tracking particle positions and concentration is obtained by an original method measuring the penetration depth of the laser light into the granular assembly. Over the wide range of flow conditions covered by our dam-break experiments, from the sparse grain-by-grain movements upstream of the wave to the hyperconcentrated bedload invading the whole flow depth near the downstream wavefront, observations suggest a linear variation of both velocity and concentration throughout the transport layer, though with a concentration at its base that may be noticeably lower than the reference motionless bed value. We extend to unsteady flow a theory for intense bedload recently proposed by Capart and Fraccarollo (2011), and compare predictions with our experimental measurements. The theory lets the granular concentration at the base of the bed-load layer vary with flow conditions, regulating both the frequency of collisions with the bed and the density stratification across the layer. Predictions include longitudinal profiles of bed, water and transport layer boundaries, as well as vertical profiles of velocity and concentration. For the first time, we obtain such predictions using a theory that is free from adjustable coefficients, and provide detailed comparisons with local velocity and concentration measurements, with excellent agreement.