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

Abstract

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.