Comparison of Sediment Transport and Bedforms from Steady and Unsteady Flow Hydrographs of a Similar Flow Rate

Flow rates and depths in natural rivers and streams can change rapidly with time due to runoff from rainfall and snowmelt. These changing conditions complicate the prediction of sediment transport, bed topography, and flow depth. Due to the difficulty of collecting field data during unsteady flow events, it is necessary to use laboratory flume experiments to study sediment transport and bedform properties during unsteady hydrographs. The goal of this work was to isolate the effects of hydrograph flows on bedform dimensions, water surface slope, and sediment transport relative to the same flow rates in equilibrium conditions for steady flows while also comparing results across the various flow rates. For three repeated hydrographs of 1-, 3-, and 6-h periods, quantities measured at four different flow rates during the hydrographs were compared with equilibrium conditions for each respective flow rate. The development of bedforms during rising and peak flow rates competed with the decay of bedforms during falling flow rates. When bedform growth was greater than decay, bedform sizes, water surface slopes, and transport rates during falling flow rates were typically greater than for equilibrium steady conditions at the same flow rate, whereas they were typically smaller than equilibrium bedforms for rising flow rates.