Laser bathymetry on rough riverbed channels: State‐of‐the‐art and future prospects

Abstract The riverbed topography provides habitat for aquatic organisms and influences flow dynamics and thus sediment transport. Understanding these processes in rivers with rough riverbeds is not only relevant for research into mountain rivers, but also for measures to restore rivers. Rough‐bed rivers with protruding boulders, shallow flows with dynamic water surfaces and turbulence with whitewater rapids restrict most survey techniques. Airborne topo‐bathymetric LiDAR, as a non‐invasive and active survey technique, is a state‐of‐the‐art method for sand‐gravel bed rivers. The miniaturisation of the sensors now enables the use of unmanned aerial vehicles (UAV's), reducing the illuminated footprint diameter and opening up the possibility of detecting rough riverbed topography. This review focuses on three research questions: i) Is topo‐bathymetric LiDAR capable of reproducing the underwater topography of rough riverbeds? ii) Does the achievable point density allow the reproduction of the submerged topography on a grain‐scale (decimetre resolution)? iii) Are there any sensor limitations with regard to the characteristics of these river types? Based on a literature review, we found eight studies of seven different river reaches that examined rough riverbed sections. Three studies used the same sensor operated on a fixed‐wing aircraft and reported good overall results due to its medium footprint diameter of 0.5 m and a reported point density of approximately 90 points/m 2 . Compared to the gravel scale resolution of topographic LiDAR, topo‐bathymetric sensors were able to reproduce morphological structures and enabled the detection of large boulders. The limiting factors in whitewater rapids and shallow water were only partially investigated. However, none of the studies used UAV‐borne sensors on rough riverbed sections. This review revealed the potential for future studies testing UAV‐borne sensors on rough‐bed rivers, as well as for investigating signal penetration in turbulent whitewater rapids and water surface and bottom signal separation in very shallow water.