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Stellenbosch University (2020)

HYDRAULIC OPTIMISATION OF A POSTULATED SEDIMENT FLUSHING SYSTEM IN LOW-LEVEL DAM OUTLETS

Van der Spuy, Liam

Titre : HYDRAULIC OPTIMISATION OF A POSTULATED SEDIMENT FLUSHING SYSTEM IN LOW-LEVEL DAM OUTLETS

Auteur : Van Der Spuy Liam

Université de soutenance : Stellenbosch University

Grade : Master of Engineering in Civil Engineering 2020

Résumé partiel
Reservoir sedimentation is an ongoing critical concern worldwide with a recorded global average of 33% of reservoir storage capacity already been lost therefrom, which is expected to exceed 50% by 2050 (i.e. not considering new dam projects). Drawdown or pressure flushing at hydropower intakes are common mitigation measures for removing locally deposited sediment from reservoirs. This is periodically necessary to especially keep hydropower intakes free of coarse non-cohesive sediment, as sand fractions will typically damage turbines in hydropower conduits. The aim of this study was to provide design guidelines for a postulated configuration for a sediment flushing system of a low-level outlet at a dam for control of reservoir sedimentation to protect the hydropower intakes. The postulated design, which comprised of a hybrid low-level outlet conduit and intake structure, was optimised by means of numerical and physical modelling for effectively flushing sediments (i.e. sand, gravel and boulders when the settled sediment delta has reached the dam) during different flood and water level scenarios. The intake structure (a semi-circular low-weir and ogee-type spillway with wing-walls) was designed and positioned in the vicinity of the low-level intake inside of the typical sediment scour cone that forms during pressure flushing. The main aim of the spillway was to produce desirable supercritical flow conditions upstream of the conduit intake, as well as through it. This was necessary to help optimise the flushing of sediments during free-surface flow conditions as well as to prevent deposition of coarse sediment near the outlet gates, thereby preventing closure. The submerged weir was, however, also designed for pressure flushing where the aim was : (1) local sediment removal ; and (2) preventing main reservoir sediments from depositing near the outlet gate and hindering gate closure. For this study, 3D CFD modelling in ANSYS Fluent v19.1 was used to hydraulically compare four different proposed flushing system designs, from which the best design option could be chosen and further refined. Flow vectors (representing the degree of streamlined flow) and flow depths and velocities (representing the degree of supercritical flow) along the model during free-flow flushing conditions were considered for comparisons. A 1:40 scale physical model of the postulated flushing system design, which evolved from the findings of the numerical model simulations, was built for further testing and refinement. The main aim of the physical model was to test the robustness, reliability and actual flushing capability of the design.

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