An experimental study on the displacement of large wood in river channels

Large wood is an essential component of river systems, often considered as the third leg of riverine fluxes (together with water and sediment). Large wood can provide beneficial effects to river restoration and natural flood management (NFM) measures. At the same time, large wood can obstruct bridge openings and increase risk of failure to structures and risk of flooding to adjacent areas. The transport of large wood in rivers crucially affects all the above processes, but to date the importance of factors affecting displacement of large wood in rivers is still poorly understood. Past theories postulated that flow secondary cells may drive large wood trajectories, but have never been confirmed. In this work, we experimentally tested at the flume scale the hydrodynamic factors influencing the displacement of large wood at the river surface. Results showed that past theories were inconclusive, whereas large wood elements tend to follow well-defined trajectories mostly driven by localised changes of the flow velocity. Furthermore, large wood elements are very sensitive to changes in their trajectories at the onset of motion, although are much less prone to change once motion has fully developed. The results from this work will pave the way for better-defined motion models of floating large wood, and will be used to test and calibrate smart sensors for field-based applications.