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

Interactions between hydraulics and morphology in step-pool channels were investigated using physical modeling and field studies. Flow resistance dynamics in step-pool channels were examined by means of over 400 laboratory flume runs in which variables contributing to flow resistance were manipulated using a factorial design. Measurement of resistance partitioning between grains, steps, and large woody debris (LWD) showed that LWD and spill over steps were responsible for the largest components of total resistance, and grain roughness was a small component of resistance. Additive approaches to resistance partitioning 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. Measurement of the effects of changes in LWD configurations, step geometry, discharge, and slope on flow resistance documented significant interaction effects between steps, grains, and LWD. Discharge strongly influenced resistance dynamics: it had the largest effect on total resistance of all variables tested; altered resistance partitioning; and had highly significant interactions with all other variables, thereby mediating the effects of bed roughness configurations 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. 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.; The combined results of the flume and field investigations illustrate that, 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 than in low-gradient channels. Further, LWD produces substantial hydraulic effects in these systems as a result of both form drag from LWD pieces and spill resistance contributions from step-forming LWD.