Characterizing vegetated rivers using novel unmanned aerial vehicle-borne topo-bathymetric green lidar: Seasonal applications and challenges

Topo-bathymetric surveys of stream channels and floodplains, along with their vegetation distributions, are difficult and expensive to quantify, but they are crucially important for balanced river management, addressing issues such as flood risk and ecosystem management. This study was conducted to demonstrate seasonal accuracy for the first time, appraising results of a novel cost-effective unmanned aerial vehicle (UAV)-borne topo-bathymetric light detection and ranging (lidar) used to examine the vegetated and gravel-bedded lower Asahi River, Japan. Compared with autumn, the winter findings were almost identical to those from ground-truth observations with outperformed accuracy ranging from around 2 to 12 cm of root-mean-square-error values. The results were compatible with the corresponding high-resolution aerial image. During winter, the three-dimensional laser point clouds identified details of the submerged artificial infrastructure of the river and the continuous topography of underwater (shallower than or equal to 2.08 m depth) and overland areas. In comparison, the autumn's bathymetric performance was limited to a depth of 1.20 m and turbidity of 5.50 NTU because measurements were conducted after a flooding event. Furthermore, this study applied novel approaches for accurate land cover mapping and vegetation height assessment, which can be readily applied in future riverine vegetation appraisal studies. In conclusion, because the point cloud data can be useful as input and validation data in hydrodynamic-numerical models and riparian vegetation identification, lidar can be regarded as a technique to inform the management of clear-flowing and shallow streams with vegetated floodplains.