How much water supply resilience is enough? – A discussion paper

Abstract

Resilience for water supply systems is the ability to maintain supply when the system is impacted by a disruption from the norm. It is not just about source reliability. It includes all manner of infrastructure failures and natural disasters, and even terrorist and war events. There is a spectrum of failure scenarios ranging from relatively short local supply area outages through to city wide total losses of supply, and the associated consequences range from inconvenience to catastrophic. Whatever the nature of a supply disruption, there will be a resilience measure which could be taken to reduce its impact, but could it be justified particularly when there is a small probability of it ever being needed. It is proposed guidance about how much resilience is needed could be obtained from existing level of service standards, even though those standards might not be used locally or be directly related to water supply. The paper investigated two ends of the resilience spectrum, frequently and exceptionally infrequently resilience needed events. It was found that most Australian water supply authorities are providing a similar level of service, and consistent with a 10 to 15-year standard for frequently occurring resilience events. Storage tanks are the main source of resilience for many water supply areas for such events and have historically been sized using qualitative methods. This paper discusses the more effective use of quantitative methods. At the other end of the spectrum resilience is capable of maintaining supply in events with average recurrence intervals of 1,000 to <10,000 years is specified by published emergency risk guidelines. A high-level review of Queensland urban centres found resilience to meet all types of failure scenarios, not just droughts, approaching a 10,000-year standard could be provided in conjunction with high level of water restrictions if independent water source and treatment capabilities from adjoining supply areas were interconnected.