The effects of three repeated unsteady flow hydrographs on sand bed topography and sediment transport in a laboratory flume

Authors: Wren, Daniel; MCALPIN, TATE - Us Army Corp Of Engineers (USACE); Langendoen, Eddy; KUHLE, ROGER - Retired ARS Employee

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2024
Publication Date: 2/26/2025
Citation: Wren, D.G., Mcalpin, T.O., Langendoen, E.J., Kuhle, R.A. 2025. The effects of three repeated unsteady flow hydrographs on sand bed topography and sediment transport in a laboratory flume. Journal of Hydraulic Engineering. 151(3). https://doi.org/10.1061/JHEND8.HYENG-14138.
DOI: https://doi.org/10.1061/JHEND8.HYENG-14138

Interpretive Summary: Management of rivers and streams requires knowledge of sediment transport; however, it is expensive and difficult to make frequent measurements, so computer models are used to predict transport rates based on things like river flow depth. These predictions are meant to work in rivers where conditions do not change over time, but most rivers, and especially smaller streams, have flow rates that change due to runoff from rainfall events and snow melt. Using relationships that were developed from unchanging conditions will result in greater errors when applied to streams with conditions that change with time. In this paper, we used a laboratory flume for experiments with flows that changed in a similar manner to a small stream, with a rapid rise in flow rate followed by a more gradual decline. We measured sediment transport, the size of sand bedforms produced by the flow events, and the slope of the water surface as they changed in response to the changing flows. The flows were repeated three times to simulate streams that are subjected to multiple flow events, which is common, for example, when rain from multiple weather fronts creates repeated flow events that are above normal depths and flow rates. Longer flow events produced higher sediment transport rates and larger bedforms, but the pattern of increases was not only dependent on the length of the flow event. Two main mechanisms were found. In the first, for some flow events, conditions did not change much with repeated hydrographs. This could be caused by short events that did not allow time for changes or for long events where changes in bedforms kept up with changing flow rates. In the second, each flow event left bedform conditions that were inherited by the next flow event, so bedforms and transport rates were different for each flow event. This information will be used to improve the prediction of sand transport and the timing of bed adjustment to water flow, which will help improve sediment transport modeling. The work can also aid in flood forecasting for streams, since the bed configuration also affects water depth. The results will be used by river managers, model developers, and other researchers studying river engineering and sediment transport.

Technical Abstract: 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 where 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 United States Department of Agriculture-Agricultural Research Service-National Sedimentation Laboratory in Oxford, Mississippi, USA. The number and length of hydrographs affected the responses associated with transport rates and bedform dimensions. Two modes of response to the hydrographs were identified. In the first, bedform growth was offset by decay on the falling limb, resulting in post-hydrograph bedforms that were similar to the initial state. In the second, bedform growth that occurred during the previous hydrographs was inherited by subsequent flow events, resulting in additional growth of bedforms and changing hysteresis patterns.