Combining Geodetic and Geomorphic Methods to Monitor Restored Side Channels: Feedback from the Upper Rhine

Producing accurate and rapid geomorphic surveys is a key issue for the growing scientific and operational area of physical river restoration. A geodetic survey using 3-D modelling (lasergrammetry, photogrammetry, tacheometry) was combined with a geomorphic survey (RFID bedload tracking, survey of grain size, geomorphic units evolution and geometrical changes) in order to monitor morphodynamic adjustments in a restored anastomosing channel of the Upper Rhine. On this basis, functioning indicators were developed to survey morphological changes at different spatio-temporal scales. Because Structure from Motion (SfM)-photogrammetry is a fast and low-cost method able to produce high-resolution point clouds, the method is particularly well-suited for monitoring complex fluvial environments that have been subject to rapid and intense changes. A hybrid method was developed to complement bank point-clouds with bathymetric data obtained by Total Station leveling. By using the Multiscale Model to Model Cloud Comparison method (M3C2), volumes of erosion-deposition were determined for all surveys. Combining geodetic and geomorphic approaches resulted in a detailed assessment of channel adjustments. This was achieved by creating a set of indicators (e.g., related to vertical and longitudinal evolutions, sediment budget, etc.) that allowed us to both characterize geomorphic adjustments and identify morphodynamic limiting factors. These indicators may be used in the future in a wide range of restoration surveys.