Automatic detection of river bankfull parameters from high density lidar data

The European Water Framework Directive (WFD), adopted in 2000, set out requirements for abetter understanding of aquatic environments and ecosystems. In 2006, following the transposition ofthe WFD into French law (LEMA), France began work on a field protocol for the geomorphologicalcharacterization of watercourses, as part of a partnership between the Centre National de la RechercheScientifique (CNRS) and the Office Français de la Biodiversité (OFB). This protocol, known as "Carhyce"(For « River Hydromorphological Caracterisation »), has been tested, strengthened and approved overthe last 15 years at more than 2500 reaches. It consists of collecting standardised qualitative andquantitative data in the field, essential for the caracterisation of a watercourse: channel geometry,substrate, riparian vegetation... However, certain rivers that are difficult to survey (too deep or toowide) pose problems for data collection.To address these issues, and to extend the analysis to a wider scale (full river section), usingremote sensing, and in particular LiDAR data, was considered. The major advantages of LiDAR overpassive optical sensors are better geometric accuracy and especially under vegetation. For a long time,LiDAR data rarely exists at national scale with data density similar to passive imagery. Today, the FrenchLiDAR HD dataset (10 pulses per meter square) program run by the French mapping agency offers anunprecedented amount of data at this scale. Thanks to them, a national 3D coverage of the ground canbe used, and numerous geomorphological measurements can be carried out on a more or less largescale. This is the case for hydromorphological parameters such as water level and width.The aim of this study is therefore to use this high-density lidar to automatically determine thehydromorphological parameters sought in the Carhyce protocol. In particular, we have developed alidar-based algorithm to reconstruct the topography from point cloud and automatically identify thebankfull level at reach scale. Designed to be applicable to every French river, the method must berobust to all river features such as longitudinal slope, width, sinuosity, multi-channel etc... Forvalidation purposes, the bankfull geometry calculated by the algorithm has been compared with fieldmeasurements at some twenty Carhyce stations across France. To determine the test stations, welooked for the diversity of situations in terms of river characteristics describe above to observed theinfluence of this features on the results.