Flow Resistance Partitioning in Step-Pool Channels

Increased understanding of controls on flow resistance and of how resistance is partitioned between different sources is essential for advancing understanding of physical processes in steep channels. Step-pool stream channels are an important category of steep channels in which flow resistance is created by large clasts, spill over step-pool bedforms, large woody debris, and other factors. In order to measure resistance partitioning between grains, steps, and woody debris, we manipulated variables contributing to flow resistance in step-pool channels via a series of laboratory flume runs. A factorial design, whereby total resistance was measured for flume runs with and without grains, steps, and woody debris, and at multiple slopes and discharges, was employed. This provided a means of quantifying the relative contributions of grain resistance, spill resistance, and debris resistance to total resistance, represented here by Darcy-Weisbach friction factor. Independent estimates of resistance partitioning were developed based on calculations of grain resistance, from Keulegan log-law relations, and of debris resistance, based on calculations of drag force associated with the cylinders used here to represent woody debris. 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. 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. The large contributions of resistance associated with woody debris and spill over steps suggests that Keulegan-based methods of calculating flow resistance may substantially underestimate total resistance in step-pool channels. In addition, significant interaction effects observed between steps, grains, and debris in a factorial ANOVA provide insight into the potential errors in simple additive approaches to resistance partitioning.