Abstract :
[en] This research aims to provide new knowledge about the inception of motion in rivers with coarse sediments and, possibly, macro-roughness flow regime, for which, so far, civil and environmental engineers face much uncertainty in the determination of threshold hydraulic conditions for sediment transport. The threshold conditions have to be clearly defined, at the theoretical level, and well identified, in experimental tests. Therefore, a specific program of laboratory experiments has been designed. It relies on two complementary laboratory flumes of, respectively, 6m and 20m in length (discharge up to 100 l/s, slope between 0 and 4%). Three different thresholds have been defined to distinguish between the following four states: no motion, vibration of stone, local motion, and large scale motion. Based on experimental tests conducted so far in the smaller flume, velocity profile and turbulence intensity measured in quasi-uniform and non-uniform flow conditions have been correlated with the inception of motion. Flow velocity was measured using UVP transducers. In this paper, the results of a first phase of the study are presented, with the aim of validating the experimental setup, procedure and instrumentation. They focus particularly on the feasibility of collecting the relevant flow variables (shear velocity, turbulence) from a limited number of pointwise velocity measurements.
