Municipal water storage - How much is enough
Annual Conference
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Extreme events and their aftermath such as the Christchurch earthquake as witnessed by New Zealand since 22 February 2011 have served to highlight the importance of reliable water supply to serviced communities, and in particular the need to have sufficient water in storage to cope.
Christchurch was fortunate in some senses in that most of their water supply reservoirs survived the earthquake and retained water due to the response of seismic isolating valves, while other reservoirs were manually isolated in the wake of the earthquake. Ultimately these actions contributed to the relatively quick recharge of water supply networks in restoring up to 95% service to Christchurch two weeks after the quake.
Elsewhere various approaches have been taken to the problem of bulk water storage requirements in an emergency. In New Plymouth District Council’s case several components have been incorporated into their standard reservoir design to allow retention of a minimum bulk volume of water for subsequent rationing and distribution through a small bore outlet on the site.
A review of New Plymouth’s overall water storage requirements and allowing for both operational and extreme demands has highlighted where storage provisions are adequate, and where they are not. New Plymouth District Council has in the past ten years built eight new reservoirs to augment their network storage, and to provide redundancy and emergency storage in the case of extreme events.
A comprehensive hydraulic model of the network including bulk storage also allows a number of scenarios to be tested and validated.
The combination of the above engineering design and operational management tools gives the network manager a higher level of confidence in determining the present and future water supply needs for their respective communities. This paper will examine some of the tools and techniques used in reservoir tank design around New Zealand, and present some options for incorporation into bulk water storage design for water supply managers.