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ThesisItem Open Access Effect of sediment particle characteristics on critical bed shear stress under different channel bed slopes and roughness conditions(G.B. Pant University of Agriculture and Technology, Pantnagar - 263145 (Uttarakhand), 2021-06) Anuradha Kumari; Akhilesh KumarThe condition for incipient motion of sediment particle i.e., critical condition of motion occurs when magnitude of drag force (shear force) exerted by flowing fluid on the sediment particle exceeds the resistance offered by the particle. The analysis of movement of sediment particles along a channel bed has been one of the most challenging problems before the hydraulic engineers for a long time as accurate measurement of flow parameters in a channel at which the incipient motion exactly occurs is a subjective criterion. Sediment transport in gravel-bottomed streams is generally through surface creep as bed load transport due to presence of larger sized particles. Ample research work has been reported on incipient motion to describe the effect of some of these variables considering sediment particles as spherical. However, a very little work is reported in the available literature for non-spherical sediment particles, which is the normal situation in natural streams, where orientation of sediment particles plays a significant role on their incipient motion. Depending on the orientation of a non spherical sediment particle, the surface area of the particle exposed to the flow gets changed and accordingly the entire force dynamics on the sediment particle is altered. This experimental study was conducted in Soil and Water Conservation Engineering Laboratory, College of Technology at Pantnagar with the focus to investigate the effects of sediment particle shape, size, unit weight, orientation and flatness along with channel bed roughness and channel bed slope on the critical bed shear stress. The experiments were conducted by using a rectangular Hydraulic Tilting Flume of 7.0m×0.6m×0.3m size with precise water supply, regulation and measurement systems and can be adjusted a forward slope up to 5%. Incipient conditions for motion of sediment particle were created and observations were recorded analyzed for 252 different combinations. The critical bed shear stress was found to be higher for non- spherical sediment particles as compared to spherical sediment particles of the same nominal diameter and increasing almost linearly with size for all shapes of sediment particles. It was also observed that spherical sediment particles of the same size were having lesser critical bed shear stress as compared to rectangular prismatic sediment particles under similar conditions. As channel bed slope increased from 0.25% to 0.5%, there was almost a 100% increase in critical bed shear stress in case of rectangular prism shaped sediment particles of the same size but about 50% increase was observed in case of spherical and cubical sediment particles under similar situations. Almost a linear increase in critical bed shear stress was observed when the bed roughness height increased from 0mm to 8.000mm for all sized sediment particles at every channel bed slope. It was observed that critical bed shear stress did not change significantly with the change in orientation of the sediment particles, while total shear force acting on the sediment particle changed with orientation in the ratio of their exposed area of non-spherical sediment particles under similar flow and channel roughness conditions. With the increasing flatness ratio, the effect of orientation of sediment particle became an important consideration to determine the magnitude of the total shear force acting on the particle under particular set of conditions. The critical bed shear stress was found to be increasing with the increase in unit weight of the sediment particle at a particular bed slope and roughness height. Relationships between dimensionless critical shear stress ‘θ’ (also called Shields parameter) to the critical particle Reynolds number (Rep) of sediment particles of noncubical and non-spherical shape along with parameters like slope of channel bed and its roughness were developed. Mathematical relationships were developed to determine critical bed shear stress and tested for their estimation efficacy using scatter plots and statistical indices for different combinations of experimental variables. The developed mathematical relationships are likely to provide an effective tool to analyze critical shear stress at incipient conditions.ThesisItem Open Access Effect of sediment particle orientation on its movement under varying channel slope and discharge conditions(G.B. Pant University of Agriculture and Technology, Pantnagar - 263145 (Uttarakhand), 2017-06) Anuradha Kumari; Akhilesh Kumar
