Speaker: Dr. Jim Dungan Smith of USGS at the University of Colorado. Sponsored by the Miles Lowell and Margaret Watt Edwards Endowment, College of Engineering. Seminar begins at 4:00, followed by social and refreshments at 5:00. http://cce.oregonstate.edu/
During the past century rivers were treated primarily as conveyances for water and were modeled by empirical equations developed for irrigation canals. This thinking still serves as the unstated basis for most field investigations and modeling studies of natural channels. Unfortunately, the overwhelming characteristic of most rivers is the irregularity of their beds, banks, paths, and floodplain surfaces. When concerned only with water discharge, this irregularity was treated using a single empirically adjusted roughness parameter, such as Manning’s coefficient. The use of a quantitatively unpredictable empirical roughness coefficient, however, makes accurate predictive modeling of discharge impossible. It also makes high-flow sediment transport and geomorphic adjustment calculations too inaccurate to be of much scientific or engineering value. This talk will describe how predictive models for water discharge can be constructed using basic fluid mechanical techniques and simple field measurements of channel shape and the physical and biological roughness elements on the beds, banks and floodplains of rivers. It also will describe how these measurements make possible the accurate modeling of the component of boundary shear stress responsible for sediment movement.
The presentation will then show comparisons of empirically generated and predictive-model-derived rating curves with data from many USGS gaging stations in the Western United States and describe the multitude of scientific and engineering advantages of model-derived rating curves over empirical ones. These advantages include substantially increased accuracies and significantly lower costs per gaging station. Our predictive flow model makes it possible to calculate (1) velocity fields, (2) turbulence fields, (3) bed-load transport fields, (4) bed, bank, and floodplain erosion fields, and (5) bed and floodplain morphological rearrangement, if any, throughout the measurement reach at a full range of river stages. It also makes it possible to calculate (1) size-by-size suspended sediment concentration and flux fields from a single time series of size-by-size sediment concentrations procured at a near-bed point, (2) size-by-size sediment discharges from that single near-bed time series, and (3) species-by-species sorbed contaminant discharge from chemical analyses of the sorbed material on the various sizes of sediment in near-bed sediment samples. Owing to the lower cost of model-based water-discharge gaging stations, they can be used in large numbers to gage river networks (1) for flash-flood prediction and, (2) in conjunction with multi-parameter radar rainfall measurements, for comprehensive scientific regional hydrologic and hydrometeorological investigations, such as ultimately will be motivated by CUHASI and supported by NSF.
In flood-consequence-prediction and river-restoration problems it is essential to be able to calculate accurately (1) cut-bank erosion rates resulting from high flows, and (2) deposition on floodplains resulting from the fluid-mechanical drag on floodplain plants with specific allometries (particularly those with woody stems and branches), verses unbounded downward erosion due to above critical boundary shear stresses produced by overbank flows. Fully predictive procedures for calculating overbank flows on vegetated floodplains will be described and reconstructions of two multi-century-recurrence-interval floods using these fully predictive algorithms will be presented. In one case, dense willows penetrating the overbank flow caused approximately 0.3 meters of highly contaminated mine tailings to be deposited over many tens of kilometers during a five-day flood in Montana, and in the other case, downstream of beaverpond- supported dense willow carrs, a cottonwood-tree and small-shrub-covered floodplain was completely washed away. In the first case, the river channel was hardly affected by the flood and in the second case the river channel became almost straight, its width increased by a factor of ten, and antidunes planed the new channel bottom (now composed of sand from the old floodplain) into an exceedingly flat surface. Ten years ago accurate reconstructions of such floods were impossible owing to the lack of fully predictive river channel and floodplain flow models.
Memorial Union (campus map) |
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Powell Leadership Room |
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Prof. Harry Yeh |
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541-737-8057 |
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Civil & Construction Engineering |
