Hydrodynamics of Alluvial Channel with Downward Seepage
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Experimental investigation has been carried out in a 20 m long, 1 m wide, and 0.72 m deep ume on three sands of median diameter 1.1 mm, 0.418 mm, and 0.62 mm in two categories: no seepage experiments to check the stability of curvilinear cross-sectional channels with different top widths and when seepage is applied in the downward direction to these channels. Longitudinal and cross-sectional profiles of alluvial channels are altered by the presence of downward seepage. Available literature suggests that seepage affects the hydrodynamics of alluvial channels. In the present experimental study, downward seepage has been applied to the parabolic channels based on tractive force theory for finding its effect on the cross-sectional profiles of stable channels. It has been observed that bed shear stress increased when downward seepage was applied to the channel. Cross-sectional parabolic shape does not hold when downward seepage is applied and channels take a new shape with at-bed and two curved banks and achieve stability. An empirically derived exponential expression has also been suggested for the evaluation of bank profiles of threshold alluvial channels affected by the downward seepage which satisfactorily predicts the bank profiles at various cross-sections of the natural alluvial rivers. The relationship among hydraulic parameters for stable channel with downward seepage has also been developed on the basis of experimental observations. At high bed shear stress in alluvial channels made of the non-cohesive material, sediment transport occurs as sheet ow layer of high sediment concentration. The sediment transport in the form of sheet ow has been observed in the present study when downward seepage was applied to the non-transporting channels designed on the basis of incipient motion condition of the bed particles. The erosion of the channel banks contributed to the sheet ow because of the increased channel bed shear stress. An empirical relation for the thickness of sheet ow layer has been developed which includes seepage in the downward direction as an independent parameter along with others. Regime relationships have been developed in the non-dimensional and dimensional form for the design of alluvial channels in the presence of downward seepage. Turbulent structures of the ow have been analyzed in curved sand bed channels with and without the presence of downward seepage. Measures of turbulent statistics such as time-averaged near-bed velocities, Reynolds stresses, thickness of roughness sublayer and shear velocities were found to increase with the application of downward seepage. Turbulent kinetic energy and Reynolds normal stresses were increased in the streamwise direction under the action of downward seepage, causing bed particles to move rapidly. Analysis of bursting events shows that the relative contributions of all events (ejections, sweeps and interactions) increased throughout the boundary layer, and the thickness of the zone of dominance of sweep events, which are responsible for the bed material movement, increased in the case of downward seepage. The increased sediment transport rate due to downward seepage deforms the cross-sectional geometry of the channel made of erodible boundaries. The deformation is caused by an increase in ow turbulence and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion.
Supervisor: Bimlesh Kumar