Characterization of flow resistance in high-gradient, forested streams

Velocity and flow resistance of high-gradient, forested stream reaches in the Fraser Experimental Forest, Colorado, were characterized using Rhodamine WT dye tracing, ground-based LiDAR scans and laser theodolite surveying. Travel time and average reach velocities were computed using a spatial harmonic mean travel time, and compared to values computed using time between peaks and time between centroids. Peak velocities differed from harmonic by an average of 12.4 percent while centroid velocities differed by -2.8 percent. Manning’s n and Darcy-Weisbach ff were computed and related to numerous potential explanatory variables. Variables that quantified bed variability, such as the relative ratio of average maximum depth to standard deviation of the residuals of the bed profile regression, were the best predictors of flow resistance in these cascade, step-pool, plane-bed streams. Individual bed variability parameters explained 72 to 80 percent of the variance in the resistance coefficients. Spatial characterization of the resistance elements in relation to the resistance coefficients was also performed.