Particle image velocimetry scalar measurements were carried out on the body of a stably stratified density current with an inlet Reynolds number of 2,300 and bulk Richardson number of 0.1. These measurements allowed the mass and momentum transport between the current and the less dense ambient fluid to be investigated. Reynolds stress, Reynolds flux, and shear production of turbulent kinetic-energy profiles revealed local maxima at the bed, as well as at the interface with the ambient fluid. Profiles of excess density variance and buoyancy production of turbulent kinetic energy revealed only local maxima at the interface with the ambient. These maxima decreased downstream as the stable density gradient reduced the turbulent intensities, until turbulence collapsed. A two-dimensional, unsteady, Reynolds-averaged Navier-Stokes (2DV-URANS) simulation was also performed on this density current. Good agreement was found between the modeled and measured normalized mean flow profiles. A comparison was also made between the measured and modeled outer flow scales of the density current.

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. Keywords: sediment yield, analytical, probabilistic method, empirical method, ungauged catchments

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 m(2)/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 m(2)/s. The innovative Flaring Gate Pier design, which was adapted on existing spillways in China, with reported design prototype unit discharges exceeding 200 m(2)/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 m(2)/s with an X-shape Flare Gate Pier (FGP) on the spillway crest.

The Nqweba Dam is one of the oldest dams in South Africa and was commissioned in 1925 for irrigation. The original reservoir depth at the dam wall was 31 m, but over the years sedimentation has reduced the water depth to 9.8 m. The original storage capacity of 79 million m3 has decreased to 46 million m3 by 2020. The dam supplies the town of Graaff-Reinet and due to the growing population at 1.9% pa it was realised in 1995 that the water use should change from irrigation to 100% potable use. For the past 25 years the town has been supplied from the dam when it has water and from ground water when the dam runs empty during droughts. The current water requirement of the town is 3.3 million m3/a and during 2019 the dam ran dry, while the ground water resource only supplied less than 50% of the demand. Urgent short and medium term measures were sought to solve the water crisis. Dam raising was found not to be beneficial, but options to increase storage capacity such as dredging or a new off-channel dam, and reduce evaporation such as floating balls/solar panels on part of the reservoir will restore the dam’s firm yield. Le barrage de Nqweba, l’un des plus anciens barrages d’Afrique du Sud, a été mis en service en 1925 pour l’irrigation. La profondeur initiale de la retenue était de 31m, mais au fil des ans, l’alluvionnement a réduit la profondeur de l’eau à 9,8m. La réserve totale initiale de 79 millions de m3 a été réduite à 46 millions de m3 en 2020. Le barrage alimente la ville de Graaff-Reinet et en raison de l’augmentation de la population (1,9 % par an), il a été établi en 1995 que l’utilisation de l’eau devait passer de l’irrigation à une utilisation 100 % potable. Depuis 25 ans, la ville est alimentée par le barrage lorsqu’elle a de l’eau et par les eaux souterraines lorsque le barrage se vide durant les sécheresses. Les besoins actuels en eau de la ville s’élèvent à 3,3 millions de m3 par an. En 2019, le barrage s'est asséché tandis que la ressource en eau souterraine a fourni moins de 50% de la demande. Des mesures urgentes à court et moyen terme ont été recherchées pour résoudre la crise de l’eau. L’élévation du barrage n’a pas été bénéfique, mais des options pour augmenter la capacité de stockage sont étudiées, tel que le dragage ou un nouveau barrage hors chenal, ainsi que l’utilisation de ballons flottants / panneaux solaires sur la retenue pour rétablir le rendement ferme.

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. Key words: Berg River Dam, emergency bottom outlet gate, air blowback, constricted roof, high-headed outlets

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.