The influence of extreme events on sediment transport, channel bed conditions, and the accuracy of bedload transport calculations
Event details
| Date | 09.06.2011 |
| Hour | 12:15 |
| Speaker | Assistant Prof. Elowyn YAGER, Department of Civil Engineering, Center for Ecohydraulics Research, University of Idaho, USA |
| Location |
GC C30
|
| Category | Conferences - Seminars |
The sediment supply to steep streams is highly episodic and is partially a function of the magnitude and timing of landslides and debris flows. In these channels, clusters and steps of large boulders move only during extreme flow events, and significantly alter the flow hydraulics and the transport of the more mobile gravel. Bedload flux predictions are often inaccurate in these streams because they do not account for the influence of the immobile boulders and the highly variable sediment supply. We measured bedload transport rates, grain-size, and the boulder-step characteristics (e.g. protrusion) in the Erlenbach torrent (10% slope), Switzerland, over a period of 6 years. This period encompassed two extreme events that reorganized boulder steps. Bedload fluxes increased and bed grain sizes fined after each extreme event because of greater hillslope sediment supply, reduced bed armoring and particle interlocking. The boulder step protrusion was a power function of the bedload flux; protrusion increased with lower sediment availability and with time elapsed since the last extreme event. We previously developed a bedload transport equation that accounts for the influence of boulder steps on the flow hydraulics. We now incorporate a function for protrusion, to estimate the sediment availability during any flow event. Sediment transport predictions were within an order of magnitude of the measured values if they used a time-dependent protrusion. Use of a constant protrusion caused the predicted bedload fluxes to systematically over- or under- estimate the measured values. This suggests that protrusion may be used as a proxy for the relative sediment availability and that bedload flux predictions may be improved using stochastic functions for bed roughness and sediment supply. Elowyn Yager obtained her PhD in Geology at the University of California, Berkeley in 2006. After a postdoc at Arizona State University, she joined the faculty of Civil Engineering at the University of Idaho, where she is currently assistant professor at the Center for Ecohydraulics Research. Her research focuses on understanding the mechanics of sediment transport, hillslope erosion, and channel morphology using field measurements, laboratory flumes and numerical models. She studies the interaction between flow turbulence and sediment motion, the influence of vegetation on bedload transport and flow hydraulics, and the coupling between hillslope sediment supply and channel conditions.
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- Giovanni De Cesare