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.
The standard design and cost estimation for a sewer network involves considerable time and financial investment. There are, however, many cases where a rapid assessment of the sewer infrastructure or related costs associated with a service zone might be required. Although there are numerous approaches to rapid sewer infrastructure estimation in the literature, to date, no widely available tool has been developed that can be applied to reliably estimate the expected sewer pipeline infrastructure associated with a service zone in South Africa. The aim of this study was to develop a method for estimating the sewer pipeline infrastructure required for a service zone, based on limited information, that could be applied to future developments. A database of South African sewer network data was used in the development of three major study outcomes. Study Outcome I involved developing regression models for estimating the total sewer pipeline length using only basic service zone characteristics. Models were developed for different categories of land use and area size, allowing for estimation of the total pipeline length as a function of the service zone area size, relief, and the density of contributing users. Study Outcome II involved determining the average pipeline diameter distributions for different types of service zones, enabling disaggregation of the total pipeline length into lengths per diameter. Study Outcome III involved determining the average number of manholes per kilometre of sewer pipeline. Combined, the three study outcomes form an infrastructure estimation tool that enables the sewer pipeline length per approximate diameter and the number of manholes associated with a service zone to be estimated, applicable to service zones smaller than 450 hectares. This study illustrates how the same methodology can be followed to develop similar tools which are applicable to other specific regions or development types, provided an appropriate dataset is obtainable.
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