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Design Guidelines and typical Hydraulic Designs of small to large River Abstractions/Diversion works Vol 1

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Executive Summary More than a decade ago, a SA Water Research Commission (WRC) study was completed by Stellenbosch University and guidelines were generated for the hydraulic design of river abstraction works: "Considerations for the Design of River Abstraction Works in South Africa" (Basson, 2006). The guidelines have been used in the designs of many of the new abstraction works in Southern Africa since 2006 and this is probably one of the research success stories of the WRC. The guidelines of this report update the 2006 technology and also include additional components to make the designs even more sustainable. The "Design Guidelines of River Abstractions/Diversion works for Potable water use, Irrigation and Hydropower Generation in South Africa" consist of three volumes as summarised below. Volume 1: Design Guidelines and typical Hydraulic Designs of small to large River Abstractions/Diversion works The hydraulic design guidelines for small potable river abstraction works (pump capacity < 100 l/s; 8.6 ML/d) discussed in this report aim to provide sustainable, relatively low capital and low operation cost solutions, which are robust and suitable for the South African semi-arid climate with extremes of droughts and floods. The recommendations made in these guidelines are based on experience, and a combination of hydrodynamic numeral and hydraulic physical modelling. Hydraulic design guidelines were also developed for medium and large river abstraction/diversion works, with and without weirs. Medium sized abstraction works (100 l/s to 1000 l/s; 8.6 ML/d to 86.4 ML/d@24h/d) should typically have a weir and gravel trap, with pump canals as settlers or a hopper and jetpump for sediment control. Large abstraction works (pump capacity > 1000 l/s; 86.4 ML/d), could typically have a weir, boulder trap, gravel trap and pump canals/settlers for sediment control, or if a weir is not needed or not feasible, then a high intake wall and wing wall is proposed, with a submerged intake and trashrack, with a hopper and jetpump for sediment control. The site selection of river abstraction/diversion works and site inspection at a river bend is discussed in detail, with considerations for calculating the hydrology and what to specify for the topographical survey. The guidelines also deal with mechanical equipment, pipework, construction, maintenance and operational and environmental considerations. With the correct hydraulic design to overcome sedimentation problems, operational and maintenance problems could be minimized for sustainable development of river abstraction/diversion works. Design guidelines are provided for sediment traps such as sand traps, settlers and vortex settling basins. Volume 2: Case Studies of River Abstractions/Diversion works Since the University of Stellenbosch have been involved in the design and model studies of a number of river abstraction works with consultants over the years, three of these have been selected as case studies for this investigation to illustrate the design development of river abstraction/diversion works based on the above referred WRC guidelines. For the purpose of evaluating the performance of existing river abstraction/diversion schemes, of which some are designed according the above referred WRC guidelines, four case studies have been selected for this investigation. Field measurements have been performed on the Design of sustainable river abstractions/diversion works for potable water use and irrigation in South Africa March 2020 Page ii different components/aspects of these schemes (such as the location of the works on the river, orientation of the works, the weir, a boulder trap, a gravel trap, pump canal sedimentation, intake trashrack, debris, pump types and operating problems) of these case studies and feedback on performance from operators of some of these have been obtained by means of questionnaires. Guidelines for the hydraulic design of sediment management components at hydropower dams and at hydropower river diversions are provided in this report. The guidelines are based on a literature review, engineering design experience, physical and mathematical modelling. This report investigates the hydraulic design requirements of low level outlets at dams located underneath HPP intakes, for the control of sedimentation near the intakes to prevent non-cohesive sediments entering the turbines, by using pressure flushing and/or free flow water level drawdown flushing if sufficient excess flow is available. Guidelines for hydro-power plant diversion design in run-of-river schemes were prepared by literature review of the hydraulic design, sediment and debris management requirements. A new river diversion design was proposed for SA conditions with a weir (uncontrolled or controlled with gates or inflatable), a boulder trap, high intake wall with a submerged intake, gravel trap, trash racks, high-velocity canals with flow control gates, followed by a sand trap.
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Technical Report
Full-text available
Hydraulic Design guidelines (2006) by the South African Water Research Commission on the design of river abstraction works (Recently updated in 2020 by new WRC report)
Conference Paper
Full-text available
River abstraction works designed for abstracting water for potable use from alluvial rivers in Africa requires a high assurance of supply. Abstraction works that are located in alluvial rivers are susceptible to dealing with coarse sediment and therefore need to consider sediment management features in order to ensure structural and operational sustainability. The intakes of the works must be located and orientated in such a manner within the river section as to ensure the creation of necessary secondary currents for deflecting passing bedload. A physical model of the proposed abstraction works design should be built in order to test and maximize the deflection, self-scouring and flushing of sediments (i.e. entrained within the traps and canals) for different river flows. Important factors to consider during and after testing are: (1) how effective is the design in terms of keeping the intake area clean of sediment; (2) for which river flows does self-scouring of the boulder trap occur (i.e. without opening gates); and (3) for which flows does flushing of sediment occur (i.e. opening of gates). The sequence for sediment flushing to limit sediment entering the pump canals begins with the flushing of the boulder trap, followed by the two gravel traps together, and finally the pump canals individually. Flushing of sediments during floods ensure that the trapped sediment is returned to the river and assist in restoration of the sediment balance within the river. This paper uses the proposed Goodenough weir and abstraction works on the uMkhomazi River in South Africa as the basis for design.
Conference Paper
Full-text available
A vortex settling basin (VSB) offers a promising alternative to conventional sediment settling structures, such as sand traps, for the removal of fine non-cohesive sediment for potable water use at small river abstraction works of less than 100 l/s pump capacity (7.2 Ml/d at 20 h/d). The hydraulic design of a suitable VSB was carried out by numerical CFD model. The design was optimized and validated against two physical VSB models: 0.48 m diameter and 0.7 m high, as well as 0.68 m diameter and 1.0 m high, in order to optimize the hydraulic design. The simulation results indicate that a design with the following characteristics works well: inlet velocity = 0.26 m/s, í µí±í µí±œí µí± í µí±–í µí±¡í µí±–í µí±œí µí±› í µí±œí µí±“ í µí±–í µí±›í µí±™í µí±’í µí±¡(í µí°» í µí±–) í µí±í µí±¦í µí±™í µí±–í µí±›í µí±‘í µí±’í µí±Ÿ ℎí µí±’í µí±–í µí±”ℎí µí±¡ (í µí°» í µí±¡) = 0.50 − 0.88, í µí±ˆí µí±›í µí±‘í µí±’í µí±Ÿí µí±“í µí±™í µí±œí µí±¤(í µí±„ í µí±¢) í µí°¼í µí±›í µí±“í µí±™í µí±œí µí±¤(í µí±„ í µí±–) = 0.05 − 0.10, í µí° ¶í µí±¦í µí±™í µí±–í µí±›í µí±‘í µí±’í µí±Ÿ ℎí µí±’í µí±–í µí±”ℎí µí±¡ (í µí°» í µí±¡) í µí±í µí±¦í µí±™í µí±–í µí±›í µí±‘í µí±’í µí±Ÿ í µí±‘í µí±–í µí±Ží µí±ší µí±’í µí±¡í µí±’í µí±Ÿ (í µí°·) > 0.5, í µí° ¶í µí±¦í µí±™í µí±–í µí±›í µí±‘í µí±’í µí±Ÿ í µí±‘í µí±–í µí±Ží µí±ší µí±’í µí±¡í µí±’í µí±Ÿ(í µí°·) í µí°¼í µí±›í µí±™í µí±’í µí±¡ í µí±‘í µí±–í µí±Ží µí±ší µí±’í µí±¡í µí±’í µí±Ÿ (í µí°· í µí±–) = 8.2 and deflectors. It was also established that sediment size and concentration, play important roles in controlling the sediment trapping efficiency. The cone angle and the angle of the inlet effects are minimal. Two VSBs designs for removal of 75µm sediment particles at maximum inflow sediment concentration of 10,000 mg/l are proposed: (a) inflow of 5 l/s with 5% water loss at a 99% trap efficiency and (b) inflow of 10 l/s with 8% water loss at 91% trap efficiency.
Rapid growth in water requirements makes it necessary to increase the amount of water drawn from rivers. The dams necessary for capturing river water have to be built to resist damage when large floods occur, and an idea of the possible destructive power of floods is given by the front photograph. The need for protection results in thick sill structures fitted with gates, and “upstream” and “downstream” cut-off walls. Sediment transported by rivers settles forming deposits behind dams, where flow velocities decrease. On the other hand, where flow velocities are high through hydraulic machinery (pums and turbines) fed from the dam, it can be necessary to remove even fine sand from the water, and also to remove floating debris. Various hydro-mechanical installations (including gates and screens) are introduced into the flow circuits to deal with sediment and debris problems.Many emperical solutions to definition of very important details complement standard design procedures. Understanding of their use is facilitated by numerous illustrations. © 1984 Direction des Etudes et Recherches d’Electricite de France.
Sediment control at intakes: a design guide
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Avery, P. (1989). Sediment control at intakes: a design guide. Cranfield: BHRA Fluid Engineering Centre.
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Brink, C. J., Basson, G. R., & Denys, F. (2006). Sediment control at river abstraction works in South Africa (Vol. 1). Water Research Commission Report No.TT 259/06.
Rotodynamic pumps for Pump Intake Design
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Physical Hydraulic Model Investigation of Critical Submergence
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