Interactions between flow hydraulics and channel morphology in step-pool streams

Abstract

This research investigated interactions between hydraulics and morphology in step-pool channels, an important class of steep mountain stream channels, using physical modeling and field studies. Flow resistance dynamics in step-pool channels were examined through physical modeling in a laboratory flume configured to resemble a step-pool channel. Over 400 flume runs were completed using a factorial design in which variables contributing to flow resistance in step-pool channels were manipulated in order to both measure resistance partitioning between grains, steps, and large woody debris (LWD) and to quantify the effects of changes in LWD configurations, step geometry, discharge, and slope on total flow resistance. Analysis of partitioning between LWD, spill, and grain roughness showed that LWD and spill over steps were responsible for the largest components of total resistance and that grain roughness was a small component of total resistance. Flume experiments documented significant interaction effects between steps, grains, and LWD, illustrating the synergistic effect of roughness features on hydraulics in these channels and providing insight into the errors in simple additive approaches to resistance partitioning. Approaches to resistance partitioning that assume that sources of resistance are isolated and additive were found to inflate the values of components that are quantified by subtraction from measurable components, likely as a result of interactions between roughness features. Discharge strongly influenced resistance dynamics: it had the largest effect on total resistance of all variables tested; altered resistance partitioning between spill, LWD, and grains; and had highly significant interactions with all other variables, thereby mediating the effect of LWD configuration and other factors on resistance. LWD position, density, orientation, and step geometry also had highly significant effects on flow resistance. Spatial and temporal patterns of hydraulics and energy dissipation in step-pool channels were also examined in a field setting using measurements of three-dimensional velocity and turbulence structure. Contributions to overall velocity vector magnitudes and especially to turbulence intensities from vertical and cross-stream flow components were substantial, creating three-dimensionality in overall flow characteristics compared to lower-gradient systems. Variation in hydraulics both spatially, between positions upstream and downstream from steps, and temporally, with changing discharge, resulted largely from changes in the streamwise velocity component. Spatial variations in hydraulic characteristics in the study reach reflected the form drag and spill resistance generated by step-pool sequences consisting of LWD and/or large clasts. The combined results of the flume and field investigations illustrate several aspects of the interactions between hydraulics and channel morphology in step-pool channels. Whereas bed roughness features in these channels create very large flow resistance values at lower discharges, the drag created by LWD and step-pool sequences is diminished substantially with increasing discharge, suggesting a greater sensitivity of hydraulics to discharge variations in step-pool channels than in low-gradient channels. Further, the hydraulic effect of LWD is substantial in these systems and results from both form drag from LWD pieces and spill resistance contributions from step-forming LWD, suggesting that forced step-pool systems containing LWD have higher flow resistance and morphologic and hydraulic complexity compared to steep channels lacking LWD. Reductions in LWD abundance as a result of land-use activities may have significantly altered flow resistance dynamics and associated morphologic characteristics in these channels.