Effects of Three Repeated Unsteady Flow Hydrographs on Sand Bed Topography and Sediment Transport in a Laboratory Flume

Natural rivers and streams are influenced by runoff from rainfall and snowmelt, resulting in flow conditions that change over timescales that are related to the size and location of contributing watersheds. Predictive relations for sediment transport are based on steady, uniform conditions, which are only applicable during periods in which flow conditions do not change over time. To study relationships between flow rates, sediment transport rates, and bedform development during hydrographs, we initiated a series of experiments with three repeated unsteady-flow hydrographs in a laboratory flume at the USDA Agricultural Research Service–National Sedimentation Laboratory in Oxford, Mississippi. The results include a detailed analysis of the sand-bed data collected during a series of three repeated nonsymmetric flow hydrographs of 1-, 2-, 3-, 4-, 5-, and 6-h duration created using scaled versions of the same natural hydrograph in a laboratory flume. For all hydrograph lengths and repetitions, transport hysteresis was counterclockwise. The magnitude of transport hysteresis, found by calculating the difference between the transport on rising and falling hydrograph limbs, did not have a clear pattern with either the hydrograph period or the number of repetitions, although the magnitude was variable, especially for hydrograph periods greater than 2 h.