Hydraulics of reservoir sedimentation

[摘要] 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 sedimenttransporting capacity is the dominant parameter in determining both sediment deposition and reentrainment patterns, the theoretical analysis of turbulent suspended sediment and density currentsediment transport processes has been studied in detail. The stream power theory provides the basisfor 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 outsuccessfully. The new total load equation provides the interrelationship between sedimentconcentration, energy dissipation and bed roughness in the lower and upper flow regimes.Comparison of the prediction accuracy of the new sediment transport equation with othergenerally used equations, indicates a high degree of accuracy.• A new bed roughness predictor has been developed, based on the new sediment transportequation.• Non-uniform sediment transport processes and reservoir sediment deposition patterns wereevaluated 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 theaccurate description of deposition processes. A new methodology, calibrated with canal andreservoir data, has been developed.• The density current velocity, shear stress and suspended sediment distributions were describedtheoretically in terms of stream power principles. A mathematical expression for the layerthickness of a density current was calibrated with laboratory and field data.• Sediment transport by means of density currents was verified with Chinese and South Africanreservoir data.• The formation of a density current can be predicted in terms of the minimum stream powerconcept. Verification with laboratory data was successful.• Density current velocities were described by using a Chezy type equation, which wascalibrated with laboratory data.• Densities of sediment deposits and consolidation of fine sediments were described by meansof a new method, by combining the methods of Miller (1953) and Rooseboom (1975), as wellas by incorporating an effective time approach to predict consolidation with variable sedimentyield/deposition or erosion/re-entrainment rates.• Width-depth relationships for South African reservoir flushing channels were found to besimilar 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 modelreservoir sedimentation comprehensively. Calibration and verification of the model for floodflushing at Welbedacht Reservoir were carried out successfally. Long-term sustainablereservoir capacities were also determined for changed reservoir operation rules and modifiedoutlets.• 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 storageoperation, while density current data were also gathered.

[发布机构] Stellenbosch University