Data From: 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; Kuhnle, Roger

Submitted to: Ag Data Commons
Publication Type: Database / Dataset
Publication Acceptance Date: 11/5/2024
Publication Date: 11/5/2024
Citation: Wren, D.G., Mcalpin, T.O., Langendoen, E.J., Kuhnle, R.A. 2024. Data From: The effects of three repeated unsteady flow hydrographs on sand bed topography and sediment transport in a laboratory flume. Ag Data Commons. https://doi.org/10.15482/USDA.ADC/25328815.
DOI: https://doi.org/10.15482/USDA.ADC/25328815

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 results include detailed analysis of sand-bed data collected during a series of three repeated non-symmetric flow hydrographs of 1-, 2-, 3-, 4-, 5-, and 6-hour duration that were 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 transport on rising and falling hydrograph limbs, did not have a clear pattern with either hydrograph period or number of repetitions, although the magnitude was variable, especially for hydrograph periods greater than 2 hours.