Sediment Transport and Bed-Form Characteristics for a Range of Step-Down Flows

Ephemeral streams with mobile beds represent a challenging environment for sediment transport prediction. Bed topography from relatively infrequent high flow events may persist after hydrograph recession, resulting in sediment transport over bed topography that is not in equilibrium with flow conditions. Previous research has shown that an equilibrium sand bed formed at a high flow rate followed by a near-instantaneous reduction in discharge and depth produced a gradual reduction in sediment load that could be modeled with a two-term exponential equation in which the first term described the reduction in bed-form celerity and the second term described the reworking of the bed to a new equilibrium condition. In this paper, flume experiments with a range of flow rates that produced dune bed forms were conducted to establish a more general predictive relationship for sediment load following rapid reductions in discharge and depth from equilibrium bed and flow conditions. It was found that the period of adjustment, as larger bed forms were reworked into smaller ones, depended on the final reduced rate of sediment transport. The rate constant for the term of the exponential function, which sets the gradual rate of sediment load reduction after the initial rapid drop, was shown to also describe the gradual reduction in bed-form amplitude that occurred after the discharge and depth were reduced. The results of this work may be used to estimate declining sediment transport rates in a sand-bedded channel resulting from rapid reduction of discharge and depth. (C) 2019 American Society of Civil Engineers.