Influence of sand supply and grain size on upper regime bedforms

Experimental Overview Laboratory experiments were conducted in the Hydraulics Laboratory at the Department of Civil and Environmental Engineering, University of South Carolina. The main objective was to observe and quantify how bedform type and geometry are affected by sediment supply rate and grain size. Sediment supply and flow discharge were chosen with the intended goal to obtain equilibrium bed configurations that evolve from lower regime to upper regime bedforms, with upstream bedforms migrating in the upstream and downstream directions. Sediment geometric mean grain sizes ?? ranged between 0.22 mm and 0.87 mm, flow rates ?? varied between 5 l/s and 30 l/s, and sediment feed rates varied between 0.5 kg/min and 20 kg/min. Sediment sizes were chosen based on the range where sand waves occur and what material is locally available. Experimental Setup Experiments were conducted in a glass wall sediment feed flume. The flume is 13 m long, 0.5 m wide and 0.9 m deep. A custom sediment trap is placed 9 m downstream of the flume entrance and a tailgate controls the downstream water surface level. A calibrated orifice plate and a Dwyer series 490 wet-wet manometer were used to measure the flow rate from the head tank. To decrease the sediment supply needed for experiments and the occurrence of three-dimensional bedforms, the cross-section of the test reach was narrowed to 0.19 m with the use of marine plywood. In the first 2 m of the flume, the cross-section is gradually narrowed to 0.19 m to obtain a 7 m long experimental test reach. Experimental Procedure 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, suspended sediment concentration was measured at the downstream end of the test reach and the experiment terminated. Experiment 1-SS started with a 10 cm thick flat layer of sediment with ?? equal to 0.43mm. The equilibrium bed of one experiment was used as the initial condition for the next experimental run. For example, the initial deposit in experiments 2-SS and 5-SS, was the equilibrium bed of experiments 1-SS and 4-SS respectively. After experiment 3- SS, a ~3 cm layer of sediment mix with ?? equal to 0.34 mm was sprinkled over the existing deposit to perform experiments with a finer sediment grain size. This process was repeated once more before experiment 7-SS to use coarser sediment with ?? equal to 7 0.62 mm. The duration of an experiment varied between 45 minutes to two hours depending on the initial condition and the feed rate. As the sediment size decreased, the time it took to reach equilibrium conditions decreased. On the glass wall and at the top of the flume there are vertical and horizontal rulers which indicate the distance from the flume entrance and the elevation above the flume bed. Bed and water surface elevations were measured at every 10 cm interval moving downstream with ruler readings. The first measurement was recorded at 3.50 m from the flume entrance and the last measurement was taken at 8.90 m from the flume entrance. During the experiments, measured values were reported in a spreadsheet, plotted, and the slopes of the best fit lines were computed to estimate the bed slope and the water surface slope. When the bed and water surface slopes did not significantly change in time, the flow and the sediment transport were deemed to be at equilibrium.