River ice analyses and roughness calculations using underwater drones and photogrammetric approach

Abstract. In the Northern Hemisphere, freshwater ice forms a significant part of the cryosphere during winters. River ice cover strongly affects the hydrology and flow characteristics of northern rivers, and the effect can last for several months a year. The magnitude of this effect is on the other hand dependent on characteristics of the ice, especially on subsurface ice roughness. However, ice-covered areas have commonly remained unexplored due to challenging conditions and difficult access. This study focuses on developing an improved approach in studying river ice by applying cost-efficient underwater drone platform and camera solutions in studying the ice underside. Furthermore, the developed methodology utilises a photogrammetric approach, Structure from Motion. One key result of the study is a workflow for reconstructing a digital elevation model of the ice underside. It was found that applied photogrammetric approach also enables calculating roughness coefficient for the ice underside. The results of this study show that underwater drones enable studying river ice in more comprehensive and detailed way compared to conventional methods. Additionally, it is noted that applying Structure from Motion in mapping the ice underside can offer feasible approach in determining subsurface ice roughness, which has wider application potential in modelling fluvial processes in subarctic rivers under changing environmental conditions.