Mapping in river geomorphology

Land laser, drone, LIDAR, which tool to choose, where, when, how?

Dimitri Lague (CNRS, Géosciences Rennes) and Baptiste Feldmann (CNRS, OSUR UMS) publish in May 2020 two chapters/articles on the contribution of remote sensing in geomorphology,one in particular on the use of airborne LIDAR, the other on the use of the terrestrial laser scanner, and their comparative merits compared to the drone: these 3 tools are adapted to the study of river geomorphology, but each with its own qualities and thus its complementarity. These two contributions are part of a book published under the direction of Paolo Tarolli and Simon Mudd that reviews and explores all the latest remote sensing techniques in the field of geomorphology, to serve as both a reference for experienced practitioners and a guide for geomorphologists who wish to use these remote sensing techniques for their research. Of course, these contributions are based on the experience gained in particular from the use of the LIDAR Aéroporté Topo-Bathymétrique Nantes-Rennes, Joint Research Platform of the Observatoires of the Sciences of the Universe of Nantes (OSUNA) and Rennes (OSUR).

The 2nd chapter of the book «Topo-bathymetric airborne LiDAR for fluvial-geomorphology analysis» is specifically dedicated to the analytical capabilities and performance of the Topo-Bathymetric Airborne LIDAR in a context of stream geomorphology. The "conventional" airborne topographic LIDAR, using a near-infrared laser, cannot penetrate the water. To overcome this technical limitation, a new generation of topo-bathymetric sensors – including the LIDAR Aéroporté Nantes-Rennes – incorporates a green laser to measure bathymetry at shallow depths. These new sensors are particularly useful in the context of river geomorphology. They allow a synoptic mapping of the topography and bathymetry with a vertical accuracy better than 10 cm and an ability to solve details of 20 to 30 cm The maximum measurable depth can be up to 5 m in the river and 15 m in the coastal range depending on the turbidity of the water and the reflectance of the bottom. On the basis of a survey of 55 km of the Ain (France), the study highlights the potential of this type of LIDAR with regard to the level of detail obtained and the potential of the analysis of the complete waveform (i.e., the entire backscattered signal); but it also shows the limitations and difficulties of detecting individual bathymetric points that require a massive data dig to perform a refraction correction of a fraction of the billions of 3D points. Dimitri Lague and Baptiste Feldmann also present various applications, including synoptic measurement of erosion/sedimentation, estimation of water clarity from lidar data, and large-scale, high-resolution mapping of flood models.


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