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Long-term water needs considered for Wellington

News release
20 September 2005

Long-term water needs considered for Wellington

A major new source of water may be needed for the Hutt Valley, Porirua and Wellington as early as 2011, according to a report released today by Greater Wellington Regional Council.

The report outlines how supply could be maintained as the region grows. It comes in response to the Wellington Regional Strategy, which considered a supply population for metropolitan Wellington of up to 450,000 people. New storage dams are the main option put forward for long-term growth, as there won’t be enough water from rivers or aquifers in summer to meet anticipated water use peaks.

Rex Kirton, chairman of Greater Wellington’s Utility Services Committee, said it was important to start a discussion now about how best to meet the region’s future water needs.

“Forecasts show we’ll need more storage by 2011 or shortly after, so we need to prepare for that possibility. This type of project often requires years for planning and consultation before you can start to build. A reliable water supply is too important to wait until the system isn’t coping.”

Greater Wellington’s network is designed to meet summer peaks in water use for 377,000 people at current per capita demand. That population will probably be reached in the next two or three years.
The report outlines a menu of options that would see the region’s water supply capacity increased in steps. Several lesser projects, including further development of the Hutt River, are already planned to provide for up to 390,000 people. Beyond that, an aquifer that is currently being investigated may provide another intermediate step. The next long-term development is currently timed from 2011, if water use continues to grow at the present rate.

Greater Wellington is also working with the region’s cities to agree a shared water conservation target. If peak water use could be held as the population grows, a new dam could be deferred.

“We’re approaching an important decision point”, Councillor Kirton said. “As things stand, we’ll either have to bring in more effective conservation measures or get on with the next big capacity step. That choice ultimately rests with the community.”

Early investigations have identified three possible locations for dams on Council-owned land – at Wainuiomata, Pakuratahi and Whakatikei - as the most cost-effective options to meet growing water use well into the future. The various options would hold up to 15,000 million litres of water – roughly a quarter of current annual supply – provide for a population of 450,000 or more and cost between $42 million and $72 million, depending on the option chosen.

Murray Kennedy, Greater Wellington’s acting divisional manager of Utility Services, said the dam sites put forward at this stage still needed detailed investigation.

“The options we’re proposing should all provide good quality water at a reasonable cost. However, there are unique environmental and social aspects associated with the various sites to be considered and further geological and engineering studies are needed before the best option can be identified.”

Around $1.5 million has been budgeted over the next 15 months for detailed investigation work. A report recommending one of the dam options as the region’s next major capacity step is expected to be ready for consideration by the Utility Services Committee in February 2007. Consultation with the community would follow this.

In the meantime, Greater Wellington is working with Hutt, Porirua, Upper Hutt and Wellington city councils to develop a co-ordinated water conservation plan, which is expected to include an agreed savings target. The plan will discuss a wide range of conservation measures, with projected savings and implementation costs for each. It is due to be finalised in mid 2006.


Background - Water Source Development Strategy

Under the provisions of the Wellington Regional Water Board Act 1972, Greater Wellington Regional Council (GWRC) has a statutory duty to provide adequate water to the constituent authorities, which at present are the four city councils in metropolitan Wellington.

A one in fifty-year security of supply standard has been agreed with the four councils. In effect, we plan to be able to meet all demand except during a drought with a return period of less than once in 50 years on average.

A sustainable yield model, based on historical river and climate data, is used to model the population we could supply within the one in fifty-year security standard. Resident population projections are based on data from Statistics New Zealand. The design capacity of our current network assets together with some minor enhancements is for a population of 390,000. The Wellington Regional Strategy study which GWRC has contributed to, considered supply populations of up to 450,000.

The problem that Greater Wellington faces in sourcing water for an increasing population is essentially one of meeting summer demand. The Waiwhetu aquifer and the rivers that water is currently abstracted from have considerable spare capacity to provide water for about nine months of the year, but not during hot summer periods, when heavy reliance is placed on the Stuart Macaskill Lakes.

On a simplistic basis, a population increase of 60,000 people (i.e. 390,000 to 450,000), using 550 litres per day over a 90 day summer period requires in total about 3,000 million litres (ML) of stored water. The usual way to store water is to construct a dam where untreated water is stored prior to the summer period and then treated as required. A nominal storage volume of 5,000 ML has been chosen for study as a dam will silt up over time, there may be a requirement to enhance the river flow under very low flow conditions and there is always a small amount of inaccessible storage. A lead time of eight or more years for a dam project and the high fixed costs favour building a structure that will meet storage requirements for many years in a single stage. The options summarised below reflect this. As a comparison, the Stuart Macaskill lakes at Te Marua hold 3,000 million litres (ML) of available storage.

Summary of projects and costs

Source Options - Immediate

- Te Marua River intake (including pipeline to plant) $ 6M
Or increase abstraction at Kaitoke weir nil

- Distribution network upgrading for either of the above $ 4M

Source Options – Medium Term

- Upper Hutt aquifer, as an alternative to the Te Marua intake or taking more water from the Kaitoke weir, to supply a population of approximately 400,000
o With on-site water treatment plant (WTP) $ 28M 1.8
o Piped to Te Marua WTP for treatment $ 22M 1.5

- Upper Hutt aquifer (in addition to development
at the Te Marua intake or taking more water from
the Kaitoke weir)
o With on-site WTP $ 34M 2.1
(supply to 415,000)
o Pumped to Te Marua for treatment** $ 13M 2.0
(supply to 400,000)

Source Options – Long Term

- Skull Gully (Wainuiomata) dam and pipeline to Wainuiomata WTP
o 5,000 million litres (ML) storage dam $ 42M 1.1
o 11,000 ML storage dam* $ 55M

- Pakuratahi dam and pipeline to Te Marua
o 5,000 ML storage dam $ 71M 1.8
o 15,000 ML storage dam* $ 86M

- Whakatikei dam, 40 ML/day WTP, pipeline
o 5,000 ML storage dam $ 67M 1.7
o 15,000 ML storage dam* $ 72M

* cost allows for increased storage only, without a corresponding increase in treatment and distribution costs

** this is a reduced-scale well-field compared to the $22M option

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