Characterization of reach velocity and detailed geometry in well-vegetated, high-gradient streams

We developed field procedures for relating complex spatial patterns in channel geometry and roughness elements to flow resistance of mountain streams. High gradient, forested stream reaches in the U.S. Forest Service's Fraser Experimental Forest, of Colorado, were characterized using Rhodamine WT dye tracing for average reach velocity and LiDAR scans for reach geometry. The dye tracing technique consists of an injection of 20 percent rhodamine WT dye solution, with concentrations measured every second at the upstream and downstream limits of the reach using two Turner Designs Cyclops 7 fluorometers linked with two Campbell Scientific CR10X dataloggers and a calibration relating voltage output with concentration. Reach-averaged velocity was computed with the thalweg distance and both the time between peaks and the time between center of mass of the tracer pulse. Geometric data for points above the low flow water level were collected using a tripod-mounted Leica HDS LiDAR scanner. Reaches were scanned from multiple directions to minimize shadow. Targets were set up over at least two control points, for each scan, to register the scan into a consistent coordinate system. The LiDAR scans were combined with detailed total station surveys of features below the water surface. With the joining of multiple data sources, proper survey control was essential for data compatibility. The LiDAR and total station bed surveys were merged into a common pointcloud and the resulting data were cleaned to improve clarity. From this merged and cleaned pointcloud, a wide variety of geometric characteristics can be measured in the office, as the need arises. Specific characteristics will support such analyses as the determination of reach section hydraulic variables, and the use of spatial-statistical, tracer skewness and bank/bed form as indicators of roughness.