Thesis (MScEng (Civil Engineering)--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: A study of the state of reservoir sedimentation in South Africa based on reservoir sediment deposit data, has shown that a considerable number of reservoirs have serious sedimentation problems. The analysis of the reservoir sediment deposit data showed that almost 25% of the total number of reservoirs have lost between 10 to 30% of their original storage capacity. The average storage loss due to sedimentation in South African reservoirs is approximately 0.3% per year while the average annual storage loss for all the reservoirs in the world is 0.8%. The aim of this research was to develop sediment yield prediction methods based on analytical approaches and mathematical modelling. The sediment yield prediction methods can be used in planning and management of water resources particularly in reservoir sedimentation control. The catchment erosion and sediment yield modelling methods can be applied in temporal and spatial analysis of sediment yields which results are essential for detailed design of water resources, particularly in the identification of critical erosion areas, sediment sources and formulation of catchment management strategies. Current analytical methods for the prediction of sediment yield have been reviewed. Nine sediment yield regions have been demarcated based on the observed sediment yields and catchment characteristics. Empirical and probabilistic approaches were investigated. The probabilistic approach is based on analysis of the observed sediment yields that were calculated from reservoir sediment deposit, river suspended sediment sampling data and soil erodibility data. The empirical equations have been derived from regression analysis of the variables that were envisaged to have a significant effect on erosion and sediment yields in South Africa. Empirical equations have been developed and shown to have accurate and reliable predictive capability in six of the nine regions. The probabilistic approach has been recommended for the prediction of sediment yields in the remaining three regions where reliable regression equations could not be derived. The predictive accuracy of both the probabilistic and empirical approaches was checked and verified using the discrepancy ratio and graphs of the observed and calculated data. While the analytical methods are needed to predict the sediment yield for the whole catchment, mathematical modelling to predict sediment yields is applied for more detailed analysis of sediment yield within the catchment. An evaluation of available catchment sediment yield mathematical modelling systems was carried out. The main criteria for the choice of a numerical model to be adopted for detailed evaluation was based on the following considerations: the model’s capabilities, user requirements and its application. The SHETRAN model (Ewen et al., 2000) was therefore specifically chosen because of its ability to simulate relatively larger catchment areas (it can handle catchment scales from less than 1km2 to 2500km2), its ability to simulate erosion in channels, gullies and landslides, its applicability to a wide range of land-use types and ability to simulate land use changes. Another model, ACRU (Smithers et al., 2002) was also reviewed. The aim of the model evaluation was to provide a conceptual understanding of catchment sediment yield modelling processes comprising model set up, calibration, validation and simulation. The detailed evaluation of the SHETRAN model was done through a case study of Glenmaggie Dam in Australia. The flow was calibrated and validated using data from 1975 to 1984, and 1996 to 2006 respectively. The results for both the calibration and validation were reasonable and reliable. The sediment load was validated against turbidity derived sediment load data from 1996 to 2006. The model was used to identify sources of sediment and areas of higher sediment yield. The land use of a selected sub-catchment was altered to analyse the impact of land use and vegetative cover on the sediment yield. Based on the results, the SHETRAN model was confirmed to be a reliable model for catchment sediment yield modelling including simulation of different land uses. / AFRIKAANSE OPSOMMING: ‘n Studie van die stand van damtoeslikking in Suid-Afrika toon dat daar ernstige toeslikkingsprobleme by baie reservoirs bestaan. ’n Ontleding van die toeslikkingsyfers gegrond op damkomopmetings toon dat omtrent 25% van die totale getal reservoirs tussen 10 en 30% van hulle oorspronklike opgaarvermoë verloor het. Die gemiddelde tempo van damtoeslikking in Suid-Afrika is 0.3%/jaar, wat laer is as die wêreld gemiddeld van 0.8%/jaar. Die oogmerk met hierdie navorsing was om sedimentlewering voorspellingsmetodes te ontwikkel deur gebruik te maak van analitiese metodes en wiskundige modellering. Die sedimentlewering voorspellingsmetodes kan gebruik word vir die beplanning en bestuur van waterbronne en veral vir damtoeslikking beheer. Die opvangsgebied erosie en die sedimentlewering modelleringsmetodes kan toegepas word in tydveranderlike en ruimtelike ontleding van sedimentlewering. Hierdie inligting word benodig vir die detail ontwerp van waterhulpbronne en veral vir die identifisering van kritiese erosiegebiede, bronne van sediment en die formulering van opvangsgebied-bestuur strategië. ‘n Literatuuroorsig oor die huidige metodes vir die voorspelling van erosie en sedimentlewering is gedoen. Nege sedimentasie streke is afgebaken in Suid-Afrika, gegrond op waargenome damtoeslikkingsdata en opvangsgebied-eienskappe. Proefondervindelike en waarskynlikheidsbenaderinge is ondersoek. Die waarskynlikheidsbenadering is gegrond op die ontleding van waargenome damtoeslikking wat bereken is uit reservoir opmeting data en rivier gesuspendeerde sediment data, asook data oor gronderosie. Die proefondervindelike metode se vergelykings is afgelei vanuit regressie ontleding van die veranderlikes wat ‘n belangrike invloed het op die erosie en sedimentlewering in Suid-Afrika. Daar is bevestig dat die ontwikkelde proefondervindelike (empiriese) vergelykings ‘n akkurate en betroubare voorspellingsvermoë in ses van die nege streke het. Die waarskynlikheidsbenadering is aanbeveel vir die voorspelling van sedimentlewering in die ander drie streke, waar betroubare regressie vergelykings nie afgelei kon word nie. Die voorspellingsakkuraatheid van albei metodes is nagegaan en bevestig deur gebruik te maak van die teenstrydigheidsverhouding en grafieke van die waargenome en berekende data. Analitiese metodes van sedimentleweringsvoorspelling is nodig vir ‘n volle opvangsgebied, terwyl wiskundige modellering om sedimentlewerings te voorspel gebruik kan word om ‘n meer in diepte ontleding van die sedimentlewering binne ‘n opvanggebied te doen. ‘n Evaluasie van beskikbare wiskundige modelle wat opvangsgebied sedimentlewering kan voorspel, is gedoen. Die hoofkriteria vir die keuse van ‘n model vir gebruik by gedetailleerde ontleding is gegrond op die volgende: die vermoëns van die model, wat verbruikers benodig en die aanwending van die model. Die SHETRAN model (Ewen et al., 2000) is spesifiek gekies weens sy vermoë om relatief groter opvangsgebiede te simuleer (dit kan opvangsgebiede van 1km2 tot 2500km2 wees) asook om erosie in kanale, dongas en grondverskuiwing simuleer. Dit kan toegepas word op ‘n wye reeks grondtipes en kan ook die gevolge simuleer as die gebruik van die grond verander. ‘n Ander model, ACRU (Smithers et al., 2002) is ook ondersoek. Die doel van die modelevaluering was om ‘n konseptuele begrip te kry van sedimentlewering modelleringsprosesse wat die opstelling, kalibrasie, toetsing en simulasies insluit. Die volledige evaluasie van SHETRAN is gedoen deur middel van ‘n gevalle-studie van die Glenmaggiedam in Australia. Die riviervloei is gekalibreer en getoets deur gebruik te maak van data wat strek van 1975 tot 1984, en van 1996 tot 2006 onderskeidelik. Die resultate van beide die kalibrasie en die toetswas redelik en betroubaar. Die sedimentlading is gekalibreer teen velddata van 1996 tot 2006. Die model is gebruik om bronne van sediment te identifiseer, asook gebiede met ‘n hoër sedimentlewering. Die gebruik van die grond op ‘n gekose sub-opvangsgebied is verander om die impak van grondgebruik en plantbedekking op sedimentlewering te ontleed. Die resultate bewys dat die SHETRAN model ‘n betroubare model is vir groot opvangsgebied sedimentlewering modellering, asook vir die simulasie van verskillende grondgebruike.

A previsão hidrológica é um processo estocástico dependente de fatores aleatórios, e, por si só, possui incertezas, porém, como a descarga sólida de um rio se correlaciona com a vazão por uma relação do tipo potência, na previsão do assoreamento de reservatório estas incertezas são maximizadas. Métodos empíricos e semi-empíricos de previsão de assoreamento negligenciam importantes fenômenos hidráulicos que regem o comportamento instantâneo do transporte de sedimentos. Incertezas e erros na estimativa da vida útil de reservatórios na fase de estudo de viabilidade é prejudicial ao processo decisório de qual reservatório e/ou se o reservatório deve ser construído. A pesquisa identificou os mecanismos de como a variabilidade hidrológica influencia a previsão do assoreamento de reservatórios por meio de casos idealizados e validação das hipóteses em estudo de caso, também foi proposto um fluxograma de atividades para a previsão de assoreamento capaz de reduzir incertezas inerentes a estes estudos, pautado na modelagem hidrodinâmica de transporte de sedimentos, ferramentas de hidrologia estatística e uso de múltiplos cenários, o Procedimento Metodológico para Redução de Incertezas na Previsão do Assoreamento de Reservatórios (PRIPAR - NH). Identificou-se que: séries de vazões com maior variabilidade resultam em um aporte de sedimento maior quando comparado à somatório de seu valor médio, e quanto maior a variabilidade maior o aporte resultante; séries com mesmo aporte de sedimentos e maior parte nos primeiros anos do reservatório resultam em assoreamento mais rápido devido a relativa alta Eficiência de Retenção de sedimentos nos primeiros anos e sua redução com a evolução do assoreamento; e, comprovou-se que modelos estocásticos para geração de séries sintéticas produzirão séries ligeiramente diferentes caso aplicados mais de uma vez, mesmo quando utilizando parâmetros de calibração iguais, causando significativa diferença na previsão do assoreamento. O PRIPAR - NH foi aplicado no estudo de caso do reservatório da PCH Mogi-Guaçu. Os 50 cenários analisados suportam a hipótese, apresentando amplitude de 12,4% do volume assoreado calculado, com média de 48,1% de seu volume em 50 anos. / The hydrologic forecast is a stochastic process that relays on a random factor. It present uncertainties by itself, but when used for reservoir sedimentation studies this uncertain are maximized due the fact that sediment load relates with flow discharge by a power type function. Empirical and semi-empirical methods for reservoir sedimentation forecast, widely used in Brazil, neglects important hydraulic phenomena that governs the instantaneous sediment transport behavior. Uncertainties and errors in reservoirs useful life estimation during feasibility studies are detrimental for the process of choosing which reservoir should be build and/or if the reservoir should be build. This study identified how hydrologic forecast and flow variability affects reservoir sedimentation studies, by means of hypothetical ideal scenarios and validations of the hypothesis in a study case. It also proposed a process flowchart for reservoir sedimentation studies able reduces uncertainty by the use of sediment transport modelling, statistical tools applied in hydrology and multiple scenarios, the PRIPAR-NH. It was identified that: as the degree of variability in flow data series increases, sediment loads also increase in comparison with the value calculated by the average flow; for the same sediment income, data series that presents most part of the sediment loads in the early years of the reservoir operation presents a faster sedimentation, it is explained by the fact the sediment Trap Efficiency reduces along the time due the reservoir storage capacity loss; and, it was proved that stochastic model will produce different data series if applied more than once, even when using the same calibration parameters, and it affects significantly the sedimentation forecast results. The PRIPAR-NH was applied to assess the sedimentation in Mogi-Guaçu Hydropower reservoir. The 50 analyzed scenarios support the study main hypotheses, presenting results for sedimentation volume ranging in 12.4%. The average sedimentation volume found was 48.1% for a 50 years scenario.

A previsão hidrológica é um processo estocástico dependente de fatores aleatórios, e, por si só, possui incertezas, porém, como a descarga sólida de um rio se correlaciona com a vazão por uma relação do tipo potência, na previsão do assoreamento de reservatório estas incertezas são maximizadas. Métodos empíricos e semi-empíricos de previsão de assoreamento negligenciam importantes fenômenos hidráulicos que regem o comportamento instantâneo do transporte de sedimentos. Incertezas e erros na estimativa da vida útil de reservatórios na fase de estudo de viabilidade é prejudicial ao processo decisório de qual reservatório e/ou se o reservatório deve ser construído. A pesquisa identificou os mecanismos de como a variabilidade hidrológica influencia a previsão do assoreamento de reservatórios por meio de casos idealizados e validação das hipóteses em estudo de caso, também foi proposto um fluxograma de atividades para a previsão de assoreamento capaz de reduzir incertezas inerentes a estes estudos, pautado na modelagem hidrodinâmica de transporte de sedimentos, ferramentas de hidrologia estatística e uso de múltiplos cenários, o Procedimento Metodológico para Redução de Incertezas na Previsão do Assoreamento de Reservatórios (PRIPAR - NH). Identificou-se que: séries de vazões com maior variabilidade resultam em um aporte de sedimento maior quando comparado à somatório de seu valor médio, e quanto maior a variabilidade maior o aporte resultante; séries com mesmo aporte de sedimentos e maior parte nos primeiros anos do reservatório resultam em assoreamento mais rápido devido a relativa alta Eficiência de Retenção de sedimentos nos primeiros anos e sua redução com a evolução do assoreamento; e, comprovou-se que modelos estocásticos para geração de séries sintéticas produzirão séries ligeiramente diferentes caso aplicados mais de uma vez, mesmo quando utilizando parâmetros de calibração iguais, causando significativa diferença na previsão do assoreamento. O PRIPAR - NH foi aplicado no estudo de caso do reservatório da PCH Mogi-Guaçu. Os 50 cenários analisados suportam a hipótese, apresentando amplitude de 12,4% do volume assoreado calculado, com média de 48,1% de seu volume em 50 anos. / The hydrologic forecast is a stochastic process that relays on a random factor. It present uncertainties by itself, but when used for reservoir sedimentation studies this uncertain are maximized due the fact that sediment load relates with flow discharge by a power type function. Empirical and semi-empirical methods for reservoir sedimentation forecast, widely used in Brazil, neglects important hydraulic phenomena that governs the instantaneous sediment transport behavior. Uncertainties and errors in reservoirs useful life estimation during feasibility studies are detrimental for the process of choosing which reservoir should be build and/or if the reservoir should be build. This study identified how hydrologic forecast and flow variability affects reservoir sedimentation studies, by means of hypothetical ideal scenarios and validations of the hypothesis in a study case. It also proposed a process flowchart for reservoir sedimentation studies able reduces uncertainty by the use of sediment transport modelling, statistical tools applied in hydrology and multiple scenarios, the PRIPAR-NH. It was identified that: as the degree of variability in flow data series increases, sediment loads also increase in comparison with the value calculated by the average flow; for the same sediment income, data series that presents most part of the sediment loads in the early years of the reservoir operation presents a faster sedimentation, it is explained by the fact the sediment Trap Efficiency reduces along the time due the reservoir storage capacity loss; and, it was proved that stochastic model will produce different data series if applied more than once, even when using the same calibration parameters, and it affects significantly the sedimentation forecast results. The PRIPAR-NH was applied to assess the sedimentation in Mogi-Guaçu Hydropower reservoir. The 50 analyzed scenarios support the study main hypotheses, presenting results for sedimentation volume ranging in 12.4%. The average sedimentation volume found was 48.1% for a 50 years scenario.

In this study, a qualitative and comparative investigation on sediment transport equations used in prediction of rserevoir sedimentation is presented. 32 sediment transport equations, which are selected by literature review on sand and gravel size ranges, grouped according to the median particle sizes on which their derivation based. In order to compare these equations computer program DELTA, which is a one dimensional simulation program developed by Graf and Altinakar (1998) for the prediction of delta formation in resrvoirs, is used. Computer simulation is performed within each group of sediment transport equations in order to determine the most suitable equation for corresponding median diameter of sediment particles. 8 of the equations gave simulation results that are in good agreement with average values related to delta deposition extent, height and location in the reservoir. The effects of river slope change and median diameter change on delta deposition also investigated and simulation results are compared with previous model studies.

Orito Field occupies an area of 31 mi² (80 km²) in the west-central portion of the Putumayo Basin, Colombia and forms part of an extensive littoral system that dominated sedimentation during Albian-Aptian time. The Caballos Formation represents the oldest Cretaceous unit, and was deposited at the beginning of a retrogradational episode immediately above the eroded Triassic-Jurassic surface. The Caballos Formation has an average thickness of 240 ft (73 m) and is largely composed of fine grained, highly compacted quartzarenites, cemented by quartz and kaolinite. A geologic model integrating all the available information allows the definition of four depositional events in the Caballos. The lowest depositional unit is composed of fluvial deposits with minor tidal influence. These fluvial sands grade upward into estuarine deposits formed in tidal channels and tidal flats, that are in turn overlain by tidal channel deposits, and are finally eroded and overlain by tidal mouth bars deposits. The vertical facies association is the product of a retrogradational episode and represents deposition in a tide-modified estuary, inside which diagenetic processes acted differently modifying the petrophysical properties of the facies that compose the Caballos reservoir in Orito field. Historical production trends of the Caballos reservoir correlate with the major depositional axes defined in this study and allow to delineation of high potential areas for future development, by means of targeted infill drilling and workovers.

Submitted in fulfilment of the requirements of the degree of Master of Engineering , Durban University of Technology, Durban, South Africa, 2016. / Reservoirs are designed to specific volume called the dead storage to be able to withstand the quantity of particles in the rivers flowing into it during its design period called its economic life. Therefore, accurate calculation of the quantities of sediment being transported is of great significance in environment engineering, hydroelectric equipment longevity, river aesthetics, pollution and channel navigability. In this study different input combination of monthly upstream suspended sediment concentration and upstream flow dataset for Inanda Dam for 15 years was used to develop a model for each month of the year. The predictive abilities of each of the developed model to predict the quantity of suspended sediment flowing into Inanda Dam were also compared with those of the corresponding developed Sediment Rating Curves using two evaluation criteria - Determination of Coefficient (R2) and Root-Mean-Square Error (RMSE). The results from this study show that a genetic programming approach can be used to accurately predict the relationship between the streamflow and the suspended sediment load flowing into Inanda Dam. The twelve developed monthly genetic programming (GP) models produced a significantly low difference when the observed suspended sediment load was compared with the predicted suspended sediment load. The average R2 values and RMS error for the twelve developed models were 0.9996 and 0.3566 respectively during the validation phase. The Genetic Programming models were also able to replicate extreme hydrological events like predicting low and high suspended sediment load flowing into the dam. Moreover, the study also produced accurate sediment rating curve models with low RMSE values of between 0.3971 and 11.8852 and high R2 values of between 0.9833 and 0.9962. This shows that sediment rating curves can be used to predict historical missing data of the quantity of suspended sediment flowing into Inanda Dam using existing streamflow datasets. The results from this study further show that the predictions from the Genetic Programming models are better than the predictions from the Sediment Raring Curve models, especially in predicting large quantities of suspended sediment load during high streamflow such as during flood events. This proves that Genetic Programming technique is a better predictive tool than Sediment Raring Curve technique. In conclusion, the results from this study are very promising and support the use of Genetic Programming in predicting the nonlinear and complex relationship between suspended sediment load and streamflow at the inlet of Inanda Dam in KwaZulu-Natal. This will help planners and managers of the dam to understand the system better in terms of its problems and to find alternative ways to address them.

The October 2007 removal of Marmot Dam, a 14.3-m-tall dam on the Sandy River in northwestern Oregon storing approximately 730,000 m3 of impounded sediment, provided an opportunity to study short- and long-term geomorphic effects of dam removal. Monitoring reservoir morphology during the two years following dam decommissioning yields a timeline of reservoir channel change. Comparison of a pre-dam survey in 1911 with post-removal surveys provides a basis from which to gage the Reservoir Reach evolution in the context of pre-dam conditions. Analyses of time-lapse photography, topographic surveys, and repeat LiDAR data sets provide detailed spatial and temporal documentation of a release of sediment from the reservoir following dam removal. The majority of morphologic changes to the reservoir largely took place during the first few days and weeks following removal. Channel incision and widening, along with gradient changes through the Reservoir Reach, exhibit diminishing changes with time. Channel incision rates of up to 13 m/hr and widening rates of up to 26 m/hr occurred within the first 24 hours following breaching of the coffer dam. Although channel position through the Reservoir Reach has remained relatively stable due to valley confinement, its width increased substantially. The channel reached an average width of 45 m within two weeks of breaching, but then erosion rates slowed and the channel width reached about 70 to 80 m after one and two years, respectively. Diminishing volumes of evacuated sediment were measured over time through quantitative analysis of survey datasets. About 15 percent of the initial impounded sediment was eroded from the Reservoir Reach within 60 hours of breaching; after one and two years, 50 and 58 percent was eroded, respectively. Grain-size analysis of terraces cut into reservoir fill following dam removal show that bed material coarsened over time at fixed elevations and vertically downward as the channel incised. Overall, these findings indicate valley morphology and local in-channel bedrock topography controlled the spatial distribution of sediment within the reservoir reach while variability in river discharge determined the timing of episodic sediment release. Changes within the Reservoir Reach shortly after dam removal and subsequent evolution over the two years following removal are likely attributable to 1) the timing and intensity of flow events, 2) the longitudinal and stratigraphic spatial variations in deposit grain-size distributions initially and over time, and 3) the pre-dam topography and existing valley morphology.

Thesis (M.Tech.:Civil Engineering)-Dept. of Civil Engineering and Surveying, Durban Institute of Technology, 2005 v, 124 leaves, Annexures 1-10, Appendices 1-4 / Lesotho is a land locked country, entirely surrounded by the Republic of South Africa. Maseru is the capital of Lesotho and the country’s main centre for commerce and industry. The study area is located on the North-Eastern outskirts of the Maseru urban area. The catchment occupies an area of 44km2 with a length of about 13 km and channel slope of 0.4 km/km. The Maqalika Reservoir was built in 1983 to meet the water demands for Maseru city up to 1995, and its storage capacity was 3.7 Mm3. The storage is gradually decreasing as sediment, carried by the natural run-off accumulates in the reservoir. Moreover, water pumped into the reservoir from the Caledon River (which is heavily sedimented) adds its own contribution of silt. The reservoir is located in a very densely populated area, and is heavily polluted leading to high purification costs. The study was motivated by the fact that Welbedacht Dam was constructed in 1973 in the Caledon catchment but downstream of Maqalika. After 20 years, 85% of the volume of the dam was silted. The study was intended in finding whether the positioning of the Maqalika reservoir is acceptable and to find its remaining capacity as a water body supplying a fast growing city. Consideration was also given to the effect of land use practices on the water quality of the Maqalika reservoir, including the cost incurred during purification. The water quality data on physico- chemical was collected from the Water and Sewerage Authority and was analysed using excel spreadsheets. Results obtained were compared with WHO, SABS and National Standards of Lesotho. It was found that nitrates, phosphates and faecal coliforms levels were by far above minimum standards rendering water to be very contaminated and the source being leaking sewers, defeacation in dongas and leachate from Tsosane and Lower Thamae dumping site. Iron levels were also high with mean values beyond 0.3mg/l and the source being leachate from dumping sites, poor disposal of scraps and minerals from soil. Conductivity levels were high and the suspected source is waste solid disposal having a maximum of 442mS/m in March 2001. Hardness, temperature and alkalinity do not pose much danger to Maqalika water since recorded results were almost within limits. Turbidity levels were very high and the main source was found to be catchment sedimentation through run-off. For determination of the impact of sedimentation through pumping, hydrological data was obtained from the Department of Water Affair (DWA) and analysed using Excel spreadsheets to get sediment concentrations. A linear regression graph was plotted using discharge against sediment concentration that yielded y = 0.0007x – 0.0019. This was used in the Rooseboom mathematical equation for estimation of volume occupied by sediment from 1983 - 2002 and was found to be 6789 m3. For determination of the impact due to catchment run-off, a map method of estimating sedimentation from ungauged catchments developed by Rooseboom was used and a volume of 4.598 x 106 m3 was obtained showing that the main contributor of sedimentation in the reservoir is catchment run-off. The chemical costs employed during purification were also compared between WASA and Umgeni Water of Kwazulu- Natal and WASA was found to be expensive with 9 cents/kl while Umgeni spent only 5.24 cents/kl.

Thesis (M.Tech.:Civil Engineering)-Dept. of Civil Engineering and Surveying, Durban Institute of Technology, 2005 v, 124 leaves, Annexures 1-10, Appendices 1-4 / Lesotho is a land locked country, entirely surrounded by the Republic of South Africa. Maseru is the capital of Lesotho and the country’s main centre for commerce and industry. The study area is located on the North-Eastern outskirts of the Maseru urban area. The catchment occupies an area of 44km2 with a length of about 13 km and channel slope of 0.4 km/km. The Maqalika Reservoir was built in 1983 to meet the water demands for Maseru city up to 1995, and its storage capacity was 3.7 Mm3. The storage is gradually decreasing as sediment, carried by the natural run-off accumulates in the reservoir. Moreover, water pumped into the reservoir from the Caledon River (which is heavily sedimented) adds its own contribution of silt. The reservoir is located in a very densely populated area, and is heavily polluted leading to high purification costs. The study was motivated by the fact that Welbedacht Dam was constructed in 1973 in the Caledon catchment but downstream of Maqalika. After 20 years, 85% of the volume of the dam was silted. The study was intended in finding whether the positioning of the Maqalika reservoir is acceptable and to find its remaining capacity as a water body supplying a fast growing city. Consideration was also given to the effect of land use practices on the water quality of the Maqalika reservoir, including the cost incurred during purification. The water quality data on physico- chemical was collected from the Water and Sewerage Authority and was analysed using excel spreadsheets. Results obtained were compared with WHO, SABS and National Standards of Lesotho. It was found that nitrates, phosphates and faecal coliforms levels were by far above minimum standards rendering water to be very contaminated and the source being leaking sewers, defeacation in dongas and leachate from Tsosane and Lower Thamae dumping site. Iron levels were also high with mean values beyond 0.3mg/l and the source being leachate from dumping sites, poor disposal of scraps and minerals from soil. Conductivity levels were high and the suspected source is waste solid disposal having a maximum of 442mS/m in March 2001. Hardness, temperature and alkalinity do not pose much danger to Maqalika water since recorded results were almost within limits. Turbidity levels were very high and the main source was found to be catchment sedimentation through run-off. For determination of the impact of sedimentation through pumping, hydrological data was obtained from the Department of Water Affair (DWA) and analysed using Excel spreadsheets to get sediment concentrations. A linear regression graph was plotted using discharge against sediment concentration that yielded y = 0.0007x – 0.0019. This was used in the Rooseboom mathematical equation for estimation of volume occupied by sediment from 1983 - 2002 and was found to be 6789 m3. For determination of the impact due to catchment run-off, a map method of estimating sedimentation from ungauged catchments developed by Rooseboom was used and a volume of 4.598 x 106 m3 was obtained showing that the main contributor of sedimentation in the reservoir is catchment run-off. The chemical costs employed during purification were also compared between WASA and Umgeni Water of Kwazulu- Natal and WASA was found to be expensive with 9 cents/kl while Umgeni spent only 5.24 cents/kl.

Cette thèse a pour objectif de caractériser les processus dominants dans l'évolution morphologique d'une retenue de forme allongée. Ces retenues sont caractérisées par une dimension longitudinale prédominante par rapport à la dimension transversale. La thèse s'est appuyée sur le cas de la retenue de Génissiat, située au cœur d'une série d'aménagements hydro-électriques sur le Haut-Rhône et soumise à des opérations régulières de chasses hydrauliques. Une analyse hydro-morphologique basée sur l'interprétation des évolutions morphologiques et des conditions d'écoulement a été réalisée. A partir de cette analyse, un découpage de la retenue en tronçons a permis de mettre en évidence la dynamique spatiale des sédiments au sein de la retenue. Une analyse de la dynamique spatio-temporelle du transport des sédiments a permis de quantifier et de caractériser les flux de transportés et leurs incertitudes. La contribution des processus associés au transport des différentes classes granulométriques (sables et sédiments fins) a alors été évaluée. Enfin, deux modèles numériques unidimensionnels ont été développés afin de simuler les processus dominants pour le transport des sédiments fins et des sédiments grossiers. Les résultats de modélisation ont montré la capacité des modèles à reproduire les évolutions morphologiques et les flux transportés, avec des valeurs simulées comprises dans les plages d'incertitude de mesure. La partie amont de la retenue apparait ainsi nettement dominée par le transport de sable alors que la partie aval est dominée par le transport de fines. Finalement, les modèles ont été appliqués pour tester différents protocoles de gestion de la retenue dans le but d'établir des règles d'exploitation permettant de limiter le comblement de la retenue et les impacts environnementaux à l'échelle d'évènements de chasse et sur le long terme / This PhD thesis aimed to characterize the main processes responsible for the morphological evolution of an elongated reservoir, characterized by predominant longitudinal dimensions. It was based on the case study of the Genissiat reservoir, located in a series of hydropower plants on the French Upper Rhone River and regularly subjected to flushing operations. A fluvial geomorphological analysis based on morphological evolutions and hydraulic conditions was performed. A longitudinal delineation of the reservoir allowed to highlight the spatial dynamics of the reservoir. Sediment fluxes in the reservoir and their uncertainties were quantified and qualified and evidenced the contribution of transport processes associated to sand and fine sediments. Two one-dimensional numerical models were developed to simulate the main processes for fine sediment and coarse sediment transport, respectively. Numerical results showed that these models were able to reproduce morphological evolutions and sediment fluxes, with differences between simulated and measured values lower than the measurement uncertainties. It appeared that the upstream part of the reservoir is dominated by sand transport while the downstream part of the reservoir is dominant by silt and clay sediment transport. Finally, the models were used to simulate predictive scenarios and to evaluate the impact of the operating rules on sediment dynamics. Some enhancements to current operating rules were proposed to limit reservoir sedimentation and downstream ecological impacts related to flushing events and long-term management