Civil Engineering October 2022 | Vol 30 No 9

Civil Engineering October 2022 19 to the floods and droughts compared to smaller local schemes which tend to be less resilient due to the fact that they often rely on a small dam or run-of-river flow or even groundwater for their water supply. Some of the larger integrated water resource schemes are designed to handle droughts of 10 to 15 years in duration while small local schemes may struggle to handle a two-year drought event. In effect, rural water supply schemes will tend to experience regular but shorter drought events while the large inter-basin schemes will experience less frequent but potentially much longer drought events. INTERMITTENT WATER SUPPLY Intermittent water supply is the practice of cutting off the water supply into an area at certain times of the day or even for several days at a time. Typically, it is used to reduce water consumption in an area by cutting off the supply at night when consumers are asleep and not using water. In such cases, the water supply is cut from approximately 20:00 to 5:00 and is achieved by manual closing of valves or through electronic devices which can be programmed to open or close the valves at specific times of the day. Intermittent supply is usually intro- duced either as an emergency measure when the supply reservoir is close to empty, or in other cases as a measure to control water use and reduce wastage. In the first case there may be no alternative to the rationing of water as an intermit- tent supply cannot be avoided once the supply resource has been depleted. However, in the second case where it is in- troduced as a water saving measure, there may well be alternative interventions that can provide savings without the problems that tend to accompany such pressurising and depressurising of the system. In South Africa, many municipalities are now considering intermittent supply as a semi-permanent water saving measure. There are several important issues that should be taken into account when introducing such measures. HEALTH ISSUES Before looking at the actual water savings that can be achieved through intermittent supply, it is important to recognise the potential health risks associated with such practice. Figure 1 provides a simplified layout of the water supply to a property that has either water borne sewage or a basic septic tank system. In this example, the sewer network is shown to be leaking, with the result that there is some level of sewage contamination in the ground sur- rounding the water supply pipes. Under normal conditions, the water supply is fully pressurised and therefore any leakage from the water supply network is effectively clean water leaving the pipes and mixing with the surrounding ground- water which may or may not be con- taminated. As long as the system remains pressurised there is minimal opportunity for the water supply to the property to be contaminated in any way. In the event that the water supply system is depressurised, as shown in Figure 2, there is a possibility for the groundwater surrounding the water supply pipe to enter the supply since any leaks in the pipeline can allow water to move Intermittent water supply is the practice of cutting off the water supply into an area at certain times of the day or even for several days at a time. X Figure 1 Typical water supply to a property when pressurised Connection to water main Curb Curb stop Property boundary Faucet Connection Lp = 0 since meter st curb stop Water meter Kitchen Faucet Bed- room Bathroom Mixture of sewage and potable water X Figure 2 Water supply contamination when system is depressurised Connection to water main Curb Curb stop Property boundary Faucet Connection Lp = 0 since meter st curb stop Water meter Kitchen Faucet Bed- room Bathroom Mixture of sewage and potable water