Experimental study of the transient motion of floats reproducing floating wood in rivers

The flow of large wood among hydraulic structures in rivers, especially in urban areas, can cause many problems. Despite many statistical, morphological and hydrodynamical studies on this phenomenon, little information is available on the transient motion of floating wood pieces. In this study, we investigate theoretically and experimentally the transient motion of floating particles under a simple acceleration. From a standard advection model we identify a particle characteristic response distance to the flow, noted λ. This key parameter is then measured for different floating particles reproducing wood in rivers (logs without and with idealized roots). We show here the typical value of this parameter as a function of particle streamwise body length for different particle geometries. The influence of roots can be well captured by an equivalent frontal area, regardless of the root pattern. This response distance could provide useful information on the probability of impact on hydraulic structures depending on the floating wood characteristics