Department of Civil Engineering

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Browsing Department of Civil Engineering by Subject "Abstraction works"

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    Investigation into fine non-cohesive sediment removal by Swirl/Vortex settling basins at small river abstraction works
    (Stellenbosch : Stellenbosch University, 2020-03) Kiringu, Kuria; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The often fine non-cohesive and cohesive nature of sediments in South African rivers makes sediment management at river abstraction works rather complex. Sediment removal at river abstraction works is essential for the protection of the pumps and pipelines. A wide range of sediment control design guidelines for large abstraction works are available, but these are not applicable for small abstraction works with a duty pump capacity of less than 100 l/s (7.2 Ml/d at 20 h/d), which is typical for rural potable water schemes in Africa. For sediment removal of fine non-cohesive sediment at small abstraction works, Vortex settling basins (VSBs) offer a promising alternative to conventional sediment settling structures such as sand traps, settlers, hoppers with jet pumps or primary settling tanks. VSBs have a small footprint, no moving parts, require no chemical dosing and continuously flush sediment back to the river. This study seeks to furnish the hydraulic designers with parameters for an optimized design of a VSB. Numerous computational fluid dynamics (CFD) model simulations were carried out using the software package ANSYS FLUENT 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. These tests indicated that non-cohesive sediment removal in a VSB is mainly gravity driven and centrifugal forces play an essential role in keeping particles in suspension near the outer wall, thus increasing residence time. The inlet velocity, the diameter and height of the VSB, underflow, deflectors, sediment size and concentration, the location and type of outlet structure all play important roles in controlling the sediment trap efficiency. The cone angle and the angle of inlet effects are minimal. The following design ratios are recommended: Underflow(Qu) Inflow(Qi) = 0.05-0.10, position of inlet(Hi) cylinder height (Ht) = 0.50-0.88, Cylinder height (Ht) cylinder diameter (D) >0.5, Cylinder diameter(D) Inlet diameter (Di) = 8.2 and inlet velocity of 0.26 m/s. Deflectors of length = Di extending 180° clockwise and 70° anticlockwise, inclined at an angle 1:2 (H:V), just above the inlet were found to give maximum efficiency combined with a rectangular central outlet length =1.28 Di, width = Di and height = Di, located at 180° opposite the inlet. With these findings two VSB designs are proposed: (a) for an inflow of 5 l/s with 5% water loss at a 99% trapping efficiency for sediment particles as small as 75µm in diameter and maximum inflow sediment concentration of 10,000 mg/l, and (b) for an inflow of 10 l/s with 8% water loss at a 91% trapping efficiency for sediment particles 75 µm in diameter and maximum inflow sediment concentration of 10,000 mg/l. A possible river abstraction works layout incorporating VSBs is suggested for abstraction discharges smaller than 100 l/s for use by rural small local authorities for potable use.