An experimental investigation of sediment kinematics and multi-scale propagation for laboratory bed-load dunes

The dynamics of river bedforms is still not well understood and detailed experimental investigation may provide significant insight into the process mechanisms. In this study, results are presented for a laboratory experiment that returned long time series of sediment kinematics for bed-load dunes. The run was performed in a closed conduit but, in the light of prior literature, the results are also representative of dunes under open-channel flows. A flow rate of 1.4 times the threshold for sediment transport was used, and the experiment lasted long enough to measure the sediment kinematics for four full dunes. This enabled a stability of mean values to be obtained for the key properties; concentration of moving sediment, sediment velocity and sediment transport rate, that were measured over a Eulerian grid. At a measuring location, all the properties present an oscillation pattern that resembles the passage of the dunes, also including higher-frequency oscillations (due to the flow turbulence) superimposed onto the low-frequency ones related to the dunes. The time evolution of the sediment transport rate is more similar to that of the sediment concentration than to that of the sediment velocity, which varies comparatively less. A multi-scale propagation is demonstrated considering the propagation of the dunes, of the sediment grains, and of quick ‘sediment gusts’ triggered by the flow field. Taking advantage of a Taylor-like hypothesis, that is assumed to be valid as a firstorder approximation, a mean temporal evolution of the sediment–kinematics properties is determined. Although limited to a single hydro-dynamic condition, and thus in need of further testing, the present results are of interest for a number of applications, including bedform analysis, live-bed scour processes, and quantification of sediment transport rates by measuring surrogate quantities.