Field experiments on channel morphology and bedload interactions at near-bankfull flows in a small, step-pool stream in the Ouachita Mountains of Arkansas

Current theory provides a conceptual model of how bedload and channel morphology interact in step-pool channels during peak-flow events (Warburton 1992), however many specific details are lacking. This study provides many of these details by identifying key properties that define interactions during near-bankfull events. This is accomplished by: (1) evaluating channel characteristics that affect sediment availability, transport, and storage; (2) quantifying bedload transport processes and their relationships to hydraulic factors; and (3) assessing how channel morphology interacts with bedload processes. Data were obtained using controlled water releases from a storage tank to simulate 1.0- to 1.6-yr flow events within a natural channel in the Ouachita Mountains of central Arkansas.; Seven key properties of bedload-channel morphology interactions are identified: (1) Bed material entrainment occurs discontinuously across the channel. Cross-sectional surveys and visual observations showed no localized entrainment. (2) Bed material moves mostly through equal-mobility transport. Five of seven tests indicated such transport. (3) Channel irregularities (e.g., steps and boulder clusters) greatly reduce the shear stress available to entrain sediment. Seventy-two to 90% of total shear stress was dissipated overcoming form drag. (4) Step disruption and bank erosion are negligible. (5) Net sediment storage increases, primarily through deposition in bed patches. Patches are distinct channel areas with smaller (<16 mm), better sorted grains that occupy 4.1% of total bankfull area. Eighty percent or more responded during each event with fill being twice the volume of scour. (6) Interactions vary between different Hydraulic Unit Types (Grant and others 1990). Larger grains moved in Riffles and Rapids relative to their respective size distributions. (7) The bed reorganizes during near-bankfull events so as to limit sediment availability. Reduced exposure of smaller grains and bed material tightening best explained the decreases in bedload rates and patch volume changes between equivalent size events. Synthesis of the results with current theory provides a more complete understanding of how bedload and channel morphology interact during near-bankfull events in step-pool channels with low coarse woody debris loading such as those found in the Ouachita Mountains of Arkansas.