Location: Watershed Physical Processes ResearchTitle: Detailed bed topography and sediment load measurements for two stepdown flows in a laboratory flume
|MCALPIN, TATE - Us Army Corp Of Engineers (USACE)|
|ABRAHAM, DAVID - Us Army Corp Of Engineers (USACE)|
|JONES, KEATON - Us Army Corp Of Engineers (USACE)|
Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2021
Publication Date: 11/17/2021
Citation: Wren, D.G., Kuhnle, R.A., Mcalpin, T.O., Abraham, D.D., Jones, K.E. 2021. Detailed bed topography and sediment load measurements for two stepdown flows in a laboratory flume. Journal of Hydraulic Engineering. 37(3): 287-298. https://doi.org/10.1016/j.ijsrc.2021.11.002. 2022.
DOI: https://doi.org/10.1016/j.ijsrc.2021.11.002. 2022
Interpretive Summary: Sand beds in streams and rivers rarely achieve stable conditions when flow rates vary. Technology used to predict sediment transport in streams assumes that conditions do not vary in time and that bed forms evolve based on existing flow conditions. Large storm events can generate high stream flows that create bed forms that are large enough to remain in the channel, while smaller flows inherit the bed forms that were produced by previous large flow events. The inherited bedforms are then gradually worn down, making sand movement difficult to predict. In this study, a laboratory flume was used to create experiments with high flows that were suddenly reduced to measure the impact on sediment transport. These tests were repeated by changing the flow rate and monitoring the shape of the sand bed. The results provide a better understanding of how sand beds respond to changing flow rates and validate previously developed relationships for predicting the rate of sand movement as the bed evolves from one state to another.
Technical Abstract: Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bed forms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bed forms respond to rapid changes in flow rate, a series of flume experiments were performed where the flow was reduced rapidly from equilibrium conditions. Sediment transport rates and detailed bed data across the flume and over a 15-meter-long test section were collected throughout the experiments to allow detailed examination of bed form dynamics. For the experiments in this series, it was found that relict bed forms stopped moving completely after flow reductions and the mode of sediment transport was shifted to small bed forms that arose rapidly over dune stoss sections throughout the test section. Changing sediment transport with time after the flow reductions was found to agree with predictions based on the relations proposed in Wren et al. (2020).