Evolution of Upper Regime Bedforms with Grain Size and Sediment Supply

Most research on fluvial bedforms mainly focuses on the study of dunes (lower regime), and much less is known on upper regime bedforms. Studying upper regime bedforms, the conditions in which they form and how their geometry changes with flow and sediment properties is necessary for interpreting the stratigraphic record, estimate flow resistance and reconstruct flow characteristics from observed geometries. Here we present results of laboratory experiments designed to investigate the role of bed material load and grain size on upper regime bedforms. Experiments were performed in open channel flow mode in a sediment-feed glass flume with a 0.19 m wide, 0.9 m deep, 7 m long test reach. Grain sizes ranged between 0.34 mm and 1.11 mm, flow rates varied between 8 l/s and 30 l/s, and sediment feed rates between 0.5 kg/min and 20 kg/min. All experiments started from a net-depositional or net-erosional (disequilibrium) condition and continued until the bed level averaged over a series of bedforms did not change in time (equilibrium). At equilibrium, bed material load must be equal to the sediment feed rate to guarantee mass conservation. Equilibrium water surface and bed level measurements were used to characterize average flow conditions, velocity profiles were recorded to investigate the impacts of bedforms on the near bed velocity, suspended sediment concentration was measured and the near bed suspended sediment concentration was estimated. As expected, suspended bed material load significantly increased as the grain size decreased. Our results suggest that as the bed material load increases, bed configurations tend to evolve from plane bed to standing waves, upstream migrating anti-dunes to cyclic steps in the presence of significant suspended bed material. In experiments with small flow discharges we observed more stable bedforms, that is, the bed periodically became flat less often than in experiments with the same sediment supply and grain size but with high flow rates. Runs with negligible suspension (sand size larger than 0.6 mm), in contrast, showed the transition from washed out dunes, to downstream migrating anti-dunes, to upper plane bed with bedload transport in sheet flow for increasing sediment transport rates. Regardless of sediment size, as the feed rate was increased, anti-dune wavelength increased.