“Hydro-morphological impact of a ten-year flood on the Ardèche gorges: contribution of pre- and post-flood Lidar topo-bathymetric data and numerical simulation” par Philippe Steer (Géosciences Rennes)

Discharge variability and the occurrence of large discharge events are thought to control erosion and sediment transport of rivers. However, few direct evidence exists due to the difficulty of monitoring and measuring erosion and sediment transport induced by a single flood. Here, we present one of the first topo-bathymetric lidar dataset obtained just before and after a decadal flood, which occurred the 3^rd to 4^th of October 2021, along the Ardeche river. The resulting 3D point clouds, covering the whole Ardeche gorges over ~30 km with a sub-meter resolution, gives access to the topographic changes induced by this flood,  below and above the water line. We first explain how the point clouds were processed, including the processing of the bathymetric data, point cloud classification, and cloud-to-cloud change detection. We then present the resulting topographic changes and discuss them in term of erosion processes and sediment transport. We also discuss these results at the light of hydraulic conditions during the flood. Water depth, specific discharge and basal shear stress are computed using the GraphFlood 2D hydrodynamic model. We also discuss future work and how to use this unique experiment to foster the development of new landscape evolution models. We also discuss the results of this study with respect to historical floods (e.g., 1890) along the Ardeche river and discuss the already visible impact of climate change on the frequency of large floods.