Sediment transport and bed-form characteristics for a range of step-down flows

Authors: Wren, Daniel; Kuhnle, Roger; Langendoen, Eddy

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2019
Publication Date: 12/6/2019
Citation: Wren, D.G., Kuhnle, R.A., Langendoen, E.J. 2019. Sediment transport and bed-form characteristics for a range of step-down flows. Journal of Hydraulic Engineering. 146(2): 04019060. DOI: 10.1061/(ASCE)HY.1943-7900.0001695.

Interpretive Summary: Models that are used for the prediction of sediment transport in streams assume that stream conditions do not change in time and that the bed of the stream contains bed forms that are appropriate for the existing flow conditions. This is often not the case, since large rain storms can generate high flows that create larger bed forms. Smaller flows that occur after the large event will inherit bed topography caused by the large event. In this paper, we used a laboratory flume for experiments that began with high flows that were suddenly reduced. We measured sediment transport and bed conditions, resulting in an equation that can be used to estimate the sediment transport rate following a sudden reduction in flow rate and depth in a stream with a sand bed. The results can be used to increase the accuracy of models used for sediment transport in small streams.

Technical Abstract: 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 where the first term described the reduction in bed form celerity and the second described the reworking of the bed to a new equilibrium condition. Here, flume experiments using 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 bedforms 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.