Research Articles (Civil Engineering)

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Browsing Research Articles (Civil Engineering) by Author "Basson, G. R."

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    Artificial aeration of stepped spillways by crest piers and flares for the mitigation of cavitation damage
    (South African Institution of Civil Engineering, 2019) Koen, J.; Bosman, D. E.; Basson, G. R.
    ENGLISH ABSTRACT: Stepped spillways are one of the oldest spillway designs dating back to 500 B.C. With technical advances in Roller Compacted Concrete (RCC) construction, the stepped spillway has become increasingly popular over recent decades. However, the use of this spillway is limited to a maximum safe unit discharge of 25 m2/s due to the risk of cavitation. In order to increase the discharge capacity on stepped spillways, various crest pier designs were introduced for flow aeration, thereby reducing the risk of cavitation damage. These pier designs were investigated on two physical models, constructed on a scale of 1:15 and 1:50, both with a standard ogee crest profile which transit to a stepped spillway chute. Air concentration was recorded along the pseudo-bottom, while pressures were measured at the step riser. The results of the 1:15 scale model indicated that the implementation of a short bullnose pier increased the safe unit discharge capacity to 30 rm/s. The innovative Flaring Gate Pier design, which was adapted on existing spillways in China, with reported design prototype unit discharges exceeding 200 m2/s, was investigated on the 1:50 scale model. Based on the experimental results of the current study, the safe unit discharge capacity (i.e. a discharge satisfying the relevant criteria defined for this study) was increased to 50 m2/s with an X-shape Flare Gate Pier (FGP) on the spillway crest.
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    The effect of aeration through an internal gallery of a dam on the cavitation risk of Roberts splitters
    (South African Institution of Civil Engineering, 2018) Calitz, G.; Basson, G. R.
    ENGLISH ABSTRACT: Roberts splitters is an effective means of energy dissipation for dam spillways. Roberts' (1943) standard unaerated splitter design procedure is, however, limited to a spillway head (H) of 3.0 m (q ~ 12 m²/s). In order to avoid cavitation at higher design spillway heads, this study investigated the artificial aeration of the flow by local air vents positioned on the splitters. A 1:20 scale hydraulic model of an ogee spillway equipped with Roberts splitters was constructed. Two aerated models, with differently sized air vents, were compared to an unaerated control model in order to determine the effect that the proposed aeration system has on the cavitation risk of the splitters at prototype unit discharges (q) of up to 50 m²/s. At the maximum tested spillway head of 7.6 m (q = 50 m²/s) the minimum pressures and air concentration around the splitters of both aerated models increased considerably. It was further observed that the unaerated splitters were prone to drowning at high spillway heads, leading to unfavourable hydraulic conditions. Based on the results of this study, the addition of aeration through an internal aeration gallery can increase the unit discharge capacity of Roberts splitters to at least 50 m²/s, up by 43%, from the unaerated limit of 35 m²/s.
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    Evaluation of empirical equations to predict bridge pier scour in a non-cohesive bed under clear-water conditions
    (South African Institution of Civil Engineering, 2019) Vonkeman, J. K.; Basson, G. R.
    ENGLISH ABSTRACT: Pier scour has been cited as the main mechanism responsible for the failure of bridges spanning rivers. Despite extensive research since the 1950s, there is no universally agreed upon procedure to accurately predict the equilibrium scour depth. Experimental data was generated by 48 tests with four flows and three pier shapes to evaluate the capability of 30 empirical equations to predict the local scour depth. Fine sand and crushed peach pips were used to address the scaling challenges of the equations by means of an equivalent movability number. The equations yielded a wide range of mostly unreliable results, particularly for the non-cylindrical pier shapes. Nevertheless, the HEC-18 models are recommended, in conjunction with Shen et al (1969), and Ali and Karim (2002), because they rely on the pier Reynolds number, a parameter which is significant in the vortex formation. Prediction models taking the horseshoe vortex into consideration could offer better scour depth predictions. Field data was analysed to improve the HEC-18 equation with new factors for pier shape and armouring for different confidence intervals. The armouring factor is based on the particle Reynolds number as opposed to the widely adopted critical velocity, and achieves considerably less scatter about the line of equality despite under-predictions for the cylindrical piers. Alternatively, a diagram comparable to the Modified Liu Diagram has the potential to predict bridge pier scour even though the pier structure parameters are omitted. Further research and improved prediction models should be considered, particularly advanced numerical models which are becoming increasingly feasible.
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    Hydraulic model study of the blowback behaviour of the bottom outlet of the Berg River Dam, South Africa
    (South African Institution of Civil Engineering, 2016) Bosman, A.; Basson, G. R.; Bosman, D. E.
    The Berg River Dam is equipped with the first multi-level draw-off environmental flood release outlet in South Africa and can release flows up to about 200 m3/s. The outlet is controlled by a radial gate at the outlet end, and is protected by a vertical emergency gate near the inlet end. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled, instead of the expected air entrainment into the air vent, designed to reduce expected negative pressures in the conduit during emergency gate closure. This paper describes the testing of a 1:14 physical model representing the outlet works of the Berg River Dam to determine the reasons for the unexpected release of air from the outlet work's air vent, as observed in the field during the commissioning tests of the emergency gate in the outlet conduit. Simulations of continuous gate closure on the as-built physical model of the Berg River Dam outlet showed predominant inflow of air into the air vent during emergency gate closure, with intermittent short duration high-speed air releases during the stages of emergency gate openings between 37% and 25% open. The problem was determined to be one of intermittent air blowback from the outlet conduit via the air vent during the latter stage, rather than continuous air release for all stages of the gate opening operation. The cause of the blowback was found to be the constriction of flow due to a reduction in the conduit cross-section at the radial gate chamber located at the downstream end of the outlet conduit.
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    Revised regional sediment yield prediction methodology for ungauged catchments in South Africa
    (South African Institution of Civil Engineering, 2017-06) Msadala, V. C.; Basson, G. R.
    ENGLISH ABSTRACT: This paper presents the research on the revision of the regional probabilistic method (Rooseboom et al 1992) for the prediction of catchment sediment yields in South Africa. The determination of sediment yields using probabilistic or empirical methods is suitable for ungauged catchments in the absence of observed data. The prediction of sediment loads is a key component in the quest to deal with reservoir and river sedimentation which is a potential threat to the sustainability of water resources in southern Africa. The revision was necessitated by increased sediment data availability and improved data analysis tools. Ten new sediment yield regions were demarcated in South Africa and Lesotho. Two analytical approaches were developed, namely probabilistic and empirical. The probabilistic approach is applicable in sediment yield Regions 3, 6 and 9. The empirical approach is applicable in sediment yield Regions 1, 2, 4, 5, 7 and 8. The estimation of sediment yields in Region 10 (Lesotho Highlands) needs to be based on direct measurements and locally observed data since no adequate analysis of sediment loads was possible due to limited data. GIS and electronic portable document file (pdf) copies of maps were produced for the retrieval of catchment data.