Roughness determination of coarse grained alpine river bed surfaces using Terrestrial Laser Scanning data

Channel bed roughness principally controls fl ow resistance in gravel bed riv ers. Direct measurement of streambed surface roughness however remains a diffi cult problem in fl uvial geomorphology. The conventional methods for measuring bed roughness often require an exact knowledge of grain size distributions throughout a given stream reach and thus rigorous grain size analysis. This may be impractical for large catchments, and systems containing a large degree of form roughness. Surface characterization using terrestrial laser scanning (TLS) addresses these problems. The ultimate goal of this research is to improve upon the methods for roughness length determination in gravel-bed rivers using TLS. To this end, two principle methodological considerations were examined. 1.) The infl uence of the number of scan positions on roughness calculation. 2.) The infl uence of grid-cell size on roughness calculation during post-processing. Scan data were furthermore compared to sediment samples to relate TLS-data to conventional roughness calculation methods. Several test sites in the Reintal valley, Bavaria, Germany were scanned from multiple orientations. The results from these experiments show that despite minor particle shading, roughness length determination does not depend signifi cantly on the number of scan orientations used. However, results clearly show that roughness length determination depends highly on the choice of grid cell size during post processing. This study supports the use of TLS as the most appropriate and versatile method for roughness analysis in gravel bed rivers.