Hydraulics of reservoir sedimentation

Basson, Gerrit Roux (1996-12)

Thesis (PhD)--University of Stellenbosch, 1996.

Thesis

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.

AFRIKAANSE OPSOMMING: Damme het 'n beperkte leeftyd as gevolg van toeslikking wat 'n wereldwye probleem skep. Daar bestaan dus die behoefte om damtoeslikking te beperk. Die hidroulika van damtoeslikking is in hierdie proefskrif bestudeer, met die Idem op sedimentvervoer aangesien dit die dominante faktor is in damtoeslikkingprosesse. Turbulente gesuspendeerde sedimentvervoer sowel as digtheidstroming sedimentvervoer-prosesse is in detail ondersoek. Die stroomdrywingteorie vorm die basis vir die akkurate beskrywing van hidrouliese prosesse betrokke by damtoeslikking. Die hoofresultate van die navorsing is: • 'n Nuwe sedimentvervoerformule, gebaseer op aangewende stroomdrywing, is ontwikkel. Kalibrasie en stawing met laboratorium-, rivier- en damdata is suksesvol uitgevoer. Die nuwe sedimentvervoerjormule beskryf die verwantskap tussen sedimentkonsentrasie, energiedissipering en bodemruheid, beide vir laer en hoër vloeiregimes. Vergelyking van die akkuraatheid van die nuwe vergelyking met die van ander algemeen gebruikte vergelykings, dui op die hoë relatiewe betroubaarheid. • 'n Nuwe bodemruheidvergelyking is ontwikkel wat op die nuwe sedimentvervoervergelyking gebaseer is. • Nie-uniforme sediment-vervoerprosesse en sediment neerlating-prosesse is ontleed. Die belangrikheid van nie-uniforme sediment-vervoer en die verwantskap met sediment-kohesie, konsolidasie, uitskuring, ens. is bevestig. • Nie-ewewigs sedimentvervoer van fyn sediment is ondersoek en het geblyk om baie belangrik in die akkurate beskrywing van sedimentneerlatingprosesse in damme te wees. 'n Nuwe met ode is ontwikkel om nie-ewewigs sedimentvervoer te beskryf wat gekalibreer is met kanaal en damdata. • Die vloeisnelheid, sleurspanning en gesuspendeerde vertikale sedimentkonsentrasie verspreidings van 'n digtheidstroom is teoreties beskryf met stroomdrywingbeginsels. Die wiskundige beskrywing van diktes van digtheidstrome is met laboratorium- en damdata bevestig. • Sedimentvervoer in digtheidstrome is met Chinese en Suid-Afrikaanse damdata geverifieer. • Die vorming van 'n dighteidstroom kan beskryf word volgens die beginsel van minimum stroomdrywing, soos gestaaf met laboratoriumdata. • Digtheidstrome se vloeisnelhede is beskryf met 'n Chezy-tipe vergelyking en gekalibreer met laboratoriumdata. • Digthede en die konsolidasie van fyn sediment afsettings is beskryf met 'n nuwe metode soos ontwikkel uit die metodes van Miller (1953) en Rooseboom (1975). 'n Effektiewe tydskaal is ook gebruik om konsolidasie te voorspel met wisselende sedimentleweringlsedimentneerlating en erosie/uitskuringtempo 's. • Wydte-diepte-verhoudings vir Suid-Afrikaanse dam spoelkanale het ooreengestem met soortgelyke data vir Chinese damme. • Kritiese toestande vir massa-uitskuring van kohesiewe sediment is in verband gebring met sediment-skeursterkte, - digtheid en -klei-inhoud. • Die teorie is saamgevoeg in 'n bestaande rekenaarmodel (MIKE 11) wat dit moontlik maak om verskeie aspekte van damtoeslikking saam te ondersoek. Kalibrasie en stawing van die model is suksesvol uitgevoer vir sediment spoeling uit Welbedachtdam tydens vloedtoestande. Langtermyn damkapasiteite wat behou kan word onder veranderde bedryfstoestande en met aangepaste damuitlate is ook ontleed. • 'n Databasis vir sedimentvervoer deur damkomme is opgebou wat verskillende bedryfs- en vloedtoestande insluit.

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