Abstract :
[en] Recreational use of urban rivers, for bathing and swimming, becomes more and more popular, even when it is forbidden by local authorities, with a risk of drowning that in-creases corre-spondingly. During rescue operations, underwater searches are considerably inferred by the very harsh environment of urban rivers with turbidity, high velocities and objects on the riv-erbed such as rocks, bikes or urban scooters. Predicting the victim’s most probable position at any time during the rescue operation would thus be invaluable . The victim’s trajectory computations have to consider the dispersion due to secondary currents and turbulence. Unfor-tunately, eExisting models of this dispersion may be not adapted for the river environment, due to the absence of scale separation between a body and the flow length scales.
This work addresses this dispersion through laboratory experiments, characterizing the trajec-tory of large, neutrally buoyant spheres released in a turbulent open-channel flow. The spheres diameter to water depth ratio is in the range 0.06-0.48. Firstly, the results show that the model of a point particle undergoing a drag force reproduces quite well the mean dynamics of all the spheres, including the largest ones, in the longitudinal direction. Secondly, they show that the transverse dispersion displays two regimes. The short-range behavior follows a ballistic regime, independent of the sphere size. Conversely, the long-term behavior appears to be a diffusive re-gime for small sphere, and a sub-diffusive regime for large spheres.
