Assessment of Flow Resistance of Steep Mountain Rivers Measuring Streambed Topography by Range Imaging Technology

Characterizing streambed morphology is crucial to better understand fluvial systems, since the streambed is an important control on stream dynamics, particle motion and bed stability. Measurement apparatus like terrestrial laser scanners or airborne Lidar systems are difficult to successfully apply in such environments, because they need free sight, elevated positions and good aerial or road access, which is generally not the case in mountain streams. Moreover, such streams are typically complex in shape and have a wide grain size distribution which complicates measuring with a single method. We present recently available Range Imaging cameras as a novel tool to obtain three-dimensional coordinates of rough surfaces. Mounted on a lightweight crane above the streambed, the cameras generate distance images without significant shadowing effects of large boulders. This experimental study evaluates the precision and the range of applicability of this method. We carried out laboratory and field measurements to get 3D data of artificial and natural surfaces. In the field we realized mean spatial resolution of ~1 cm with distance standard deviations of 7 mm (1σ). Measurement precision degrades with low reflective surfaces or strong illumination contrast at direct sunlight exposure. However, Range Imaging has the potential to generate high resolution data of streambed topography, which is suitable for developing roughness measures.