Streambed Roughness, Flow Velocity, and Sediment Transport Rates in a Steep Mountain Channel

The interactions between channel bed morphology, flow conditions and sediment transport are important for a wide range of fluvial processes and practical applications. With increasing stream gradient, the bed morphology changes (from pool riffle, to step-pool and cascade morphology) and thus the transport characteristics of water and sediments also change. We investigated bed roughness, flow velocity and sediment transport rates in a steep, glacier-fed mountain stream (Riedbach, Ct. Valais, Switzerland) with almost flume-like boundary conditions. The streambed gradient increases along the 1-km study reach by roughly one order of magnitude (S=3-41%), with a corresponding increase in streambed roughness, while flow discharge and width remain approximately constant due to the glacial runoff regime. Streambed roughness was determined by characteristic grain-sizes and point cloud surface parameters (semi-variogram, standard deviation) derived from terrestrial laser scanning. Reach-averaged flow velocities were derived from dye tracer breakthrough curves measured with 10 Fluorometers along the channel. Sediment transport rates were determined using "Bunte bedload traps" and the "Swiss plate geophone" system, respectively. Finally, the measurement values were compared with the predictions of commonly used flow resistance- and sediment transport equations. We found that the mean flow velocity and the sediment transport rates remain approximately constant although the bed gradient increases by roughly one order of magnitude along the study reach. By accounting for the increasing bed roughness within the flow resistance and sediment transport equations, it was possible to explain the constancy of flow velocities and sediment transport rates with increasing bed gradients. This observation shows that flow resistance and sediment transport behavior across this large range of steep slopes agree with patterns established in previous studies for both lower-gradient and steep reaches and confirm the applicability of the equations also at very steep slopes.