UAV-based tracking of floating wood during a flood 

Floods in mountain rivers cause significant geomorphic changes and may entrain and transport large quantities of sediment and wood (uprooted trees). Although large wood contributes to healthy river ecosystems by providing and forming habitats for various species, it can also pose additional risks. In particular, the wood can block critical infrastructure. Governed by its density, the buoyancy of wood allows it to float and generally move at a velocity similar to the river flow. However, wood movement is complex due to interactions with the flow and the influence of turbulence and drag forces. Moreover, contact with other wood pieces can alter the forces on the floating wood. Observations of wood transport during floods are very rare, and this study aims to address this by monitoring the motion of wood during large-scale experimental floods in the Spöl River.  The Spöl River, partially situated in the Swiss National Park, is controlled by two upstream dams, the Punt dal Gal and Ova Spin. As part of a restoration project, the company responsible for the dams releases environmental floods annually. This study monitored the movement of floating wood pieces entrained during these releases using a combination of methods.  During the flood in June 2023, three drones were flown simultaneously. Each drone flew a maximum of 15 minutes, which was done five times between 09:30 and 11:45 in the morning. Each drone captured 24 frames per second with a resolution of 3840x2160 pixels. At an average height of 60 meters, this resulted in an average pixel size of 2.3 centimetres. Also, 6 topographical cross-sections, distributed over the observed river section were taken before and after the flood.  The flood caused important geomorphic changes along the riverbed. In some of the surveyed sections, erosion was significant, whereas in others, aggradation was the dominant process. The video footage included approximately 90 pieces of floating large instream wood with and average length of 3 meters. When comparing the wood movement with the cross-sections, we observed that the wood pieces did not necessarily follow the river thalweg or deepest part of the channel. Local morphology, flow velocity and turbulence played a large role in the wood trajectory. In addition, the size, trajectory and rotation of wood are being analysed and compared with the flow field and local topography.    This study provides unique insights into the behaviour of individual floating wood pieces transported during a flood. The findings will be valuable for comparison with numerical models and will help improving our understanding of large wood dynamics in rivers.    This work is funded by the Swiss National Science Foundation project PCEFP2_186963, and supported by the University of Lausanne, the Swiss Academy of Sciences, the Swiss National Park, and the Engadiner Kraftwerke.