Channel form, flow and sediment transport in a step-pool stream

The influence of channel morphology and hydraulics on sediment transport within steep upland streams is investigated. Step-pools are the most common bedform in such streams. Bedload transport processes operating in a perennial step-pool stream in central England are established using magnetically-tagged particles and bedload pit samplers. Water-surface slope in two step-pool streams is monitored in detail. Water-surface slope measurements show that step-pool bedforms cause large spatial variations in flow depth during floods, and that the temporal variation of watersurface slope during a flood wave differs diametrically from the pattern found in lowgradient rivers that have little roughness. These results demonstrate that it is difficult to estimate the forces acting on the streambed in steep, rough streams and, therefore, conventionally established flow variables cannot be used to predict bedload transport . rates in step-pool channels. Particle tracing experiments are used to determine the control that step-pool morphology and flow hycfraulics have on particle displacements. Step-pool streams exhibit smaller particle travel distance per flood event than low-gradient rivers. This difference reflects the greater flow resistance and bed rugosity of step-pool streams. Bed morphology dictates flood particle displacement at step-pool bedform scale. It is shown that entrainment, pool export rates and transport distances of coarse sediment are dependent on downstream step dimensions, step permeability and pool form. Large variations in bedload transport rates at any given flow strength are associated with variable sediment supply, and particle tracing demonstrates that sediment availability is controlled at unit pool scale. Bedload consists of fine gravel moving over a stable bed surface at low discharges, while, at higher discharges, bedload coarsens, approaching the size of the pool bed material as the bed surface is partially mobilized. Step framework clasts are stable features of the streambed. Results from this study are incorporated into a descriptive model of discharge-related bedload transport phases in step-pool streams.