Thesis
Effectiveness of widely used critical velocity and bed shear stress equations for different types of sediment beds
Washington State University
Master of Science (MS), Washington State University
2010
Handle:
https://hdl.handle.net/2376/100244
Abstract
Critical bed shear stress and velocity estimations of sediment particles are the two fundamental properties that help to understand sediment transport, scour, and deposition under different flow conditions. Six widely used bed shear estimation methods (i.e., Shields, Log Profile (LP), Prandtl's Seventh Power (PSP), Turbulent Kinetic Energy (TKE), Reynolds Stress (RS), and Kim et al. 2000) were compared using flume study. Two widely used sediment transport rate estimation methods (i.e., Ackers and White, and Engelund and Hansen) were applied for known sediment transport rates for calculating bed shear stresses and then compared with bed shear stress estimation methods. An Acoustic Doppler Velocimeter (ADV) for velocity and turbulence measurements and a Laser Displacement Meter (LDM) for sediment transport rate measurements were used in a re-circulating, low turbulence linear flume (3.66 m (12 ft) long, 0.884 m (2.9 ft) wide and 0.61 m (2.0 ft) deep). A consistent log layer of 2.0 cm was found for all the experiments. Velocity data 0.4 cm above the sediment bed which was the closest possible accurate velocity measuring points to the sediment bed was used for the turbulence analysis based bed shear estimation methods. RS method showed good agreement with the TKE (Slope = 0.779, R2 = 0.788) and LP methods (Slope = 0.773, R2 = 0.861) for sediment beds with uniform sands. The Shields and Kim et al. (2000) methods estimated very low critical bed shear stresses and the PSP method estimated the highest critical bed shear stresses for sediment bed with uniform sands. The bed shear stress estimated from Engelund and Hansen's method showed considerable agreement with the TKE and RS methods and Ackers and White's method showed large discrepancies with all the bed shear estimation methods for the existing flow condition. Experiments performed with sand mixture sediment beds showed very interesting results. Sediment beds with different sizes of sand particles tended to develop a bed shear stress similar to sediment beds with the dominant particle size in the mixed sediment beds. Moreover, discrepancies among the bed shear stress estimation equations were higher for sediment beds prepared by different particle sizes. Different bed shear estimation methods compared in this study using advanced velocity and sediment transport rate measuring instruments will add confidence to previous and current research.
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Details
- Title
- Effectiveness of widely used critical velocity and bed shear stress equations for different types of sediment beds
- Creators
- S. M. Helalur Rashid
- Contributors
- Michael E. Barber (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Civil and Environmental Engineering, Department of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; Pullman, Wash. :
- Identifiers
- 99900525297001842
- Language
- English
- Resource Type
- Thesis