The effects of scour on the flow field at a bendway weir

Garfield, Mason, author
Ettema, Robert, advisor
Thornton, Christopher, committee member
Wohl, Ellen, committee member
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Bendway weirs (BWs) are rock structures commonly used for controlling the thalweg location in alluvial channels, especially bends. Although there are many studies analyzing the effect of BWs on bend flow fields, there is little known about the effects of scour on the modified flow field. Many physical and numerical models of BWs use a fixed bed with the existing river bathymetry, assuming that the effects of scour are negligible. This study analyzed the effects of the scour on the flow field using SRH-2D, a 2-dimensional numerical model solving the full St. Venant equation (also termed the dynamic wave equation). In addition, the study used small scale and large scale hydraulic models and a 3-dimensional numerical model, FLOW-3D, to validate and calibrate the 2-D model. Using FLOW-3D, the 3-dimensional flow field was used to identify areas where SRH-2D was accurate and where it was lacking. The small scale hydraulic model was used to determine the effect of streamwise blockage on scour using plates. Results from the small scale hydraulic model indicate that when the degree of streamwise blockage (L/B where L is the width of the plate and B is the width of the flume) and submergence (Y/H where Y is the flow depth and H is the height of the plate) remained the same, but the acute angle to the flume wall varied (α), the maximum scour depth did not change significantly. Accordingly, the scour morphology from the tests with α = 90° could be applied to tests with α = 30° and 60° using the 2-D model. When comparing the flow field using SRH-2D to FLOW-3D, it was found that the maximum depth-averaged velocity was closer to FLOW-3D's when Y/H = 1.25 than when Y/H = 2.0. This finding likely results from the greater effect of blockage directing the flow around the BW than the deeper case, where the flow has a higher vertical velocity over the top of the BW. The 2-D model cannot take this into account, leading to error. The pre- and post-scour maximum velocity magnitude and locations were compared to determine the effect of scour on the flow field using the 2-D model. A deep flow depth (Y/H = 2.0) and a shallower flow depth (Y/H = 1.25) were run for different BW orientations, (α=30°,60°,and 90°) to determine the applicability of the effect of scour on the flow field. The analysis found that, when Y/H = 2.0, the difference in the maximum velocity magnitude and location between pre-and post-scour were fairly negligible for all orientations, whereas when Y/H =1.25, the effects of the scour on the flow field were more prominent. Overall it was found that the scour morphology is important to take into consideration for a fixed bed numerical or physical model when analyzing flow depths slightly overtopping the BW, but has little effect when the flow is twice the size of the BW.
2019 Summer.
Includes bibliographical references.
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numerical modeling
bendway weirs
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