Investigating the influence of low-level outlet shape on the scour cone formed during pressure flushing of sediments in hydropower plant reservoirs

Dreyer, Jacob Stephanus (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The global shift to renewable energy sources has led to the exponential growth of hydropower in the 21st century. Hydropower will continue to play an important role in electricity supply in the near future and as such the sustainable development of new hydropower plants (HPPs) is of utmost importance. Major hydropower plants require a reservoir (dam or weir) to be constructed on a river to temporarily store water and generate the required head for power generation. However, due to the reduction in flow velocity and sediment transport capacity when a river enters a reservoir, all or part of the incoming sediment load is trapped. The sedimentation of a reservoir essentially decreases the live storage capacity and potentially leads to sediment-laden water entering the hydropower intakes, thereby damaging turbines and causing generation losses. Several sediment management strategies, such as watershed management, sluicing, flushing and turbidity current venting, have been employed all over the world with varying degrees of success. Pressure flushing is generally not regarded as an efficient means to regain lost storage capacity, but is often implemented in drier climates where reservoirs trap most of the incoming sediment and no additional water is available for implementing other sediment management strategies. More specifically, pressure flushing is often used in HPPs to scour out deposited sediment through a low-level outlet located below the hydropower intake. In spite of the advances in pressure flushing studies, more information on the effect of the low-level outlet shape on the scour cone geometry is needed since almost all studies have been conducted on circular outlets. The dimensions of the scour cone are essential for the proper design of low-level outlets so that sediment-free conditions can be maintained at the power intake. In this study, the scour cone geometry associated with different outlet shapes under changing water depths and sediment levels were experimentally investigated. Four outlet shapes (circular, square, flat rectangular and upright rectangular) were tested at three water depths (1.75, 1.125 and 0.5 m) and three sediment levels (0, 0.1 and 0.2 m). Fine, non-cohesive silica sand with an effective diameter of 0.09 mm was used to pack a horizontal bed. Scour cone measurements were taken using a handheld three-dimensional laser scanner and the geometrical characteristics of the scour cone were analysed. Non-dimensional equations for computing the scour cone dimensions and volume were developed using a multi-linear regression analysis. The developed equations fit the experimental data well and also take the outlet shape into account. The centreline and cross-sectional bed profiles of the scour cones were also analysed and dimensionless equations defining the bed profile shape were developed. The results indicated that an increase in water depth (and thus also discharge) led to an increase in scour cone dimensions. Increasing the sediment level was also associated with an increase in scour cone size. Overall, the outlet shape had a significant effect on the scour cone geometry. The widest and largest scour cones were generally associated with the flat rectangular outlet, especially in cases where the sediment level was above the outlet invert.

AFRIKAANSE OPSOMMING: Die wêreldwye skuif na hernubare energiebronne het gelei tot die eksponensiële groei van hidrokrag in die 21ste eeu. Hidrokrag sal in die nabye toekoms, soos vandag, 'n belangrike rol in die voorsiening van elektrisiteit vervul. Daarom is die volhoubare ontwikkeling van nuwe hidrokragsentrales van kardinale belang. Groot hidrokragsentrales benodig 'n dam of keerwal wat in 'n rivier gebou word om water tydelik op te gaar en die benodigde waterhoogte vir kragopwekking te skep. As gevolg van die afname in vloeisnelheid en sedimentvervoerkapasiteit wanneer 'n rivier 'n dam binnegaan, word al die sediment of 'n deel van die inkomende sedimentlading gedeponeer. Die toeslikking van 'n dam verminder die bruikbare stoorkapasiteit en lei daartoe dat sediment-draende water die hidrokraginlate binnedring. Dit kan skade veroorsaak aan turbines en opwekkingsverliese veroorsaak. Verskeie sedimentbestuurstrategieë (soos opvanggebiedbestuur, deurspoel van sediment, uitspoeling, deurlating van digtheidstrome, ens.) is wêreldwyd onderneem met verskillende mates van sukses. Drukspoeling word oor die algemeen nie as 'n doeltreffende strategie beskou om verlore opgaarkapasiteit te herwin nie. Dit word egter dikwels in droër klimate geïmplementeer waar damme meeste van die inkomende sediment terughou en daar geen ekstra water beskikbaar is vir die implementering van ander sedimentbestuurstrategieë nie. Drukspoeling word dikwels in hidrokragdamme gebruik om gedeponeerde sediment deur 'n laevlak-uitlaat, wat onder die kraginlate geleë is, te spoel. Ten spyte van die vordering wat gemaak is met drukspoelnavorsing, is meer inligting nodig oor die invloed van die laevlak-uitlaatvorm op die sedimentuitskuurkeël, aangesien bykans alle vorige studies met sirkelvormige uitlate uitgevoer is. Die afmetings van die uitskuurkeël is noodsaaklik vir die behoorlike ontwerp van laevlak-uitlate, sodat sedimentvrye toestande by die hidrokraginlaat gehandhaaf kan word. In hierdie studie is die uitskuurkeëlafmetings van verskillende uitlaatvorms onder veranderende waterdieptes en sedimentvlakke, eksperimenteel ondersoek. Vier uitlaatvorms (sirkelvormig, vierkantig, plat reghoekig en regop reghoekig) is getoets op drie waterdieptes (1.75, 1.125 en 0.5 m) en drie sedimentvlakke (0, 0.1 en 0.2 m). Fyn, nie-kohesiewe silika sand met 'n effektiewe deursnee van 0.09 mm is gebruik om 'n horisontale sediment bed te pak. Die skuurkeëls is opgemeet met behulp van 'n drie-dimensionele laserskandeerder en die geometriese eienskappe van die keëls is ondersoek. Dimensielose vergelykings vir die berekening van die uitskuurkeëlafmetings en volume is ontwikkel met behulp van 'n multi-lineêre regressie-ontleding. Die vergelykings wat ontwikkel is pas die eksperimentele data goed en neem ook die uitlaatvorm in ag. Die lengte- en dwarssnitte van die skuurkeëls is ook ontleed en dimensielose vergelykings wat die bodemprofielvorm definieer, is ontwikkel. Die resultate het getoon dat 'n toename in waterdiepte (en dus ook deurstroming) lei het tot 'n toename in uitskuurkeëlafmetings. Die verhoging van die sedimentvlak is ook gekoppel aan 'n toename in die uitskuurkeëlgrootte. Die uitlaatvorm het 'n beduidende invloed op die uitskuurkeëlafmetings. Die wydste en grootste uitskuurkeëls is oor die algemeen verwant aan die plat reghoekige uitlaat, veral in gevalle waar die sedimentvlak bo die uitlaat was.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104910
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