CONTROLS ON HYDRAULIC RESISTANCE IN STEP-POOL CHANNELS

Step-pool stream channels are characterized by complex hydraulics, with flow resistance created by large woody debris (LWD), clasts, spill over step-pool bedforms, and other factors. In order to allow quantification of variations in total resistance caused by changes in LWD configurations, discharge, and slope, as well as measurement of resistance partitioning, we manipulated variables contributing to flow resistance in step-pool channels via a series of flume runs. For each of over 200 flume runs, total resistance (represented here by Darcy-Weisbach friction factor) was calculated based on measurement of reach-averaged velocity in a flume configured to resemble a step-pool channel. A factorial design was used to facilitate analysis of interactions between resistance features and estimates of the partitioning of resistance between grains, steps (spill), and LWD. The variables that were tested for their effect on flow resistance included discharge, slope (0.10, 0.14 m/m), presence/absence of steps and grains, and LWD density, orientation, piece length, and arrangement.

Discharge had the largest effect on resistance of all variables tested, and the effect of LWD configuration on roughness tended to be drowned out at high discharges. Debris density, debris orientation, and slope also had highly significant effects on total friction factor, whereas piece length (including presence or absence of rootwads) and arrangement were not significant at a=0.05. In a factorial ANOVA on a subset (n=108) of the runs, significant 3-way interactions at a=0.05 were present for discharge*orientation*slope and discharge*orientation*length, and a significant 2-way interaction was also present for discharge*density. Calculations of resistance partitioning indicated that spill resistance and debris resistance 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 grain, spill, and debris resistance depended on discharge, with debris resistance dominating at higher discharges, and debris density, with similar contributions from spill and debris components at low debris densities and greater debris roughness at higher debris densities.