Airborne and UAV-borne Laser Bathymetry applied to a mountain river reach

Abstract. Mapping mountain river bathymetry poses significant challenges due to low flow depths, variable bed topography, and whitewater rapids, which hinder most survey techniques. This study explores the use of airborne and Unmanned Aerial Vehicle (UAV) laser bathymetry to address these challenges. Two topo-bathymetric RIEGL sensors, VQ-880-G (aircraft) and VQ-840-GL (UAV), were applied to a 500m section of the Fischbach River in the Ötztaler Alps (Austria), characterized by step-pool morphology and high bed roughness. The surveys carried out under low flow conditions showed the superior performance of the VQ-840-GL in capturing detailed submerged topography due to its smaller footprint and closer point spacing. The VQ-880-G could capture broader features but lacked detail, in particular under water. Both systems failed to penetrate whitewater rapids. Despite these limitations, UAV-based laser bathymetry marks a significant step forward, enabling spatially continuous bathymetry data collection in complex mountain river settings, critical for advancing hydraulic and sediment transport research.