Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Author "Basson, Gerrit Roux"

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    Hydraulics of reservoir sedimentation
    (Stellenbosch : Stellenbosch University, 1996-12) Basson, Gerrit Roux; Rooseboom, A.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Reservoirs generally have a limited life span due to sedimentation. The replacement of lost storage capacity is a worldwide problem and the need therefore exists to limit reservoir sedimentation as much as possible. The hydraulics of reservoir sedimentation has been studied in this dissertation. Since sediment transporting capacity is the dominant parameter in determining both sediment deposition and reentrainment patterns, the theoretical analysis of turbulent suspended sediment and density current sediment transport processes has been studied in detail. The stream power theory provides the basis for accurate descriptions of all the hydraulic processes involved in reservoir sedimentation. Key theoretical developments and findings from the study are: • A new sediment transport equation has been derived, based on applied stream power. Calibration and verification with laboratory, river and reservoir data has been carried out successfully. The new total load equation provides the interrelationship between sediment concentration, energy dissipation and bed roughness in the lower and upper flow regimes. Comparison of the prediction accuracy of the new sediment transport equation with other generally used equations, indicates a high degree of accuracy. • A new bed roughness predictor has been developed, based on the new sediment transport equation. • Non-uniform sediment transport processes and reservoir sediment deposition patterns were evaluated and found to be important as the sorting process is interrelated with re-entrainment, consolidation, cohesion etc. • Non-equilibrium sediment transport of fine sediment was found to be very important in the accurate description of deposition processes. A new methodology, calibrated with canal and reservoir data, has been developed. • The density current velocity, shear stress and suspended sediment distributions were described theoretically in terms of stream power principles. A mathematical expression for the layer thickness of a density current was calibrated with laboratory and field data.• Sediment transport by means of density currents was verified with Chinese and South African reservoir data. • The formation of a density current can be predicted in terms of the minimum stream power concept. Verification with laboratory data was successful. • Density current velocities were described by using a Chezy type equation, which was calibrated with laboratory data. • Densities of sediment deposits and consolidation of fine sediments were described by means of a new method, by combining the methods of Miller (1953) and Rooseboom (1975), as well as by incorporating an effective time approach to predict consolidation with variable sediment yield/deposition or erosion/re-entrainment rates. • Width-depth relationships for South African reservoir flushing channels were found to be similar to those in China. • Critical conditions for mass erosion of cohesive sediments can be related to shear strength, sediment density and clay content. • By combining the theory in an existing mathematical model (MIKE, 11), it is possible to model reservoir sedimentation comprehensively. Calibration and verification of the model for flood flushing at Welbedacht Reservoir were carried out successfally. Long-term sustainable reservoir capacities were also determined for changed reservoir operation rules and modified outlets. • A database on reservoir sediment transport through a number of South African reservoirs was . established. The data were obtained under conditions of flushing, sluicing and storage operation, while density current data were also gathered.