A Flume Study of Hydraulic Resistance in Steep Channels

Understanding the role of large woody debris (LWD) and other controls on hydraulic resistance in step-pool channels is critical to developing insight into the mechanics and morphology of these channels. We manipulated variables contributing to flow resistance in step-pool channels via a series of flume runs in order to allow measurement of the relative contributions of grain roughness, form roughness, and debris roughness to total resistance. For each of 86 flume runs, total resistance (represented here by Darcy-Weisbach friction factor) was calculated based on measurement of reach-averaged velocity and flow depths. Values for grain resistance were also calculated using a modified form of the Keulegan (1938) relation. The flume was configured to represent a steep step-pool channel by fixing slope at 0.14, constructing step-pool sequences using plywood and two-by-fours, gluing fine gravel to the bed, and fixing polyvinyl-chloride (PVC) tubing to the bed and/or banks to represent LWD. Thirty-six different woody debris configurations were established by varying LWD density, length, shape, and orientation. These experiments found that form roughness (from steps) and debris roughness (from LWD) were responsible for the largest components of total resistance, and that grain roughness was a small component of total resistance when steps and/or debris were present. The relative contributions of form roughness and debris roughness depended on debris density, with similar contributions from these components at low debris densities and greater debris roughness at higher debris densities. Measured friction factors ranged from 0.04 (smooth plane-bed, low flow) to 27 (step-pools with grains, low flow, high density of vertically stacked pieces of LWD oriented perpendicular to flow). Resistance was positively correlated with debris density and inversely correlated with discharge. Piece length had minimal effect on resistance for a given debris density, and debris pieces oriented perpendicular to flow created greater resistance than ramped pieces. In comparisons of runs with and without grains, the increase in total resistance caused by adding grains was much greater than the calculated grain resistance based on the Keulegan relation, suggesting either that the Keulegan relation is not applicable to steep channels and/or that total resistance is not additive.