Graded Bed Load Sediment Transport in a Net Degrading Channel Under Unsteady Flow Hydrographs

Le Wang, Alan Cuthbertson, An Ping Shu, Shang Hong Zhang, Gareth Pender

Research output: Contribution to journalArticle

Abstract

Regulated river channels with limited or no upstream sediment supply can be subject to net bed degradation during natural flood hydrograph events or managed water releases. Heterogeneity in the bed sediment sizes often present in these channels also means that different size classes may be transported preferentially during different parts of the flow hydrograph. A series of laboratory experiments is conducted to investigate the transport response of a graded sediment bed to a range of well-defined unsteady flow hydrographs with a zero-sediment supply condition imposed at the upstream boundary. The results show
varying temporal lag and hysteresis patterns for fractional bed load transport, defined for three size classes (fine, medium and coarse) within the graded bed sediment. The coarse size class tends to respond preferentially during the rising limb and typically exhibits a clockwise hysteresis, whereas the fine size class tends to become more active during the falling limb, typically demonstrating no/mixed or counterclockwise hysteresis. On this basis, predictions of bed load transport rates are shown to be improved by calculating separate reference threshold shear stresses for the initiation and cessation of fractional grain motions during the rising and falling limbs, respectively. Corresponding temporal variations of the median grain size within the transported bed load are shown to vary depending on the hydrograph total water work and unsteadiness, with peak values generally attained during the rising limb and an overall fining of bed load observed during the falling limb. Analysis of the three size classes also indicates that the medium-coarse and fine fractions are transported in larger relative proportions during smaller, more flashy hydrographs (i.e. lower total water work and higher unsteadiness) and larger, flatter hydrographs (i.e. higher total water work and lower unsteadiness), respectively. The resulting coarsening of the sediment bed surface layer (i.e. armouring) during the hydrographs is found to be greatest at the upstream end of the test section and decreases exponentially in the downstream direction. Two empirical models, based on combined hydrograph and bed sediment descriptors, are also shown to predict reasonably well the overall bed load yields generated under design flow hydrographs through the satisfactory collapse of experimental data from the current study, as well as similar previous datasets for both uniform and graded bed sediments.
Original languageEnglish
JournalJournal of Hydrology
Publication statusSubmitted - 30 May 2020

Keywords

  • graded sediment transport, unsteady flow hydrographs, bed load hysteresis, bed surface coarsening, net-degrading channel, bed load yield model

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    Wang, L., Cuthbertson, A., Shu, A. P., Zhang, S. H., & Pender, G. (2020). Graded Bed Load Sediment Transport in a Net Degrading Channel Under Unsteady Flow Hydrographs. Manuscript submitted for publication.