Planning for regional growth

As part of the process of maintaining an adequate water supply for Hutt, Porirua, Upper Hutt and Wellington city councils into the future, Greater Wellington Regional Council is increasing its water storage at the Stuart Macaskill Lakes in Te Marua and investigating additional water source and storage options. We're also preparing to investigate land near Takapu Road, Porirua, for emergency water storage.

Questions and answers about our development options are set out below:

When might the Regional Council build new bulk water supply storage capacity?

The basis of planning

Our aim is to have a water supply system that results in a very low risk of water shortage (we use a security of supply design standard of not more than 2% annual probability of shortage, which is regarded as best practice for our combination of run-of-river, aquifer and storage).  Infrastructure is planned and developed to provide additional water and maintain the 2% standard for a growing population, dependent on the average level of water consumption per person (includes residential and commercial).

We use a water modelling programme developed by NIWA, and utilising climate, population and water use data, to assess future water demand scenarios. These inputs are reviewed and updated at regular intervals, as modelling capabilities develop, population projections change and water use trends emerge.

Current activity

The Regional Council has been planning ahead for more water storage to meet growth projections for several years. The current upgrade work on the storage lakes at Te Marua includes increasing their capacity by 13% (about 400 million litres) as well as earthquake strengthening. We are also negotiating the purchase of farm land at Kaitoke that could accommodate up to three extra storage lakes (of varying sizes and costs), with a combined volume of roughly 8,000 million litres.

Development timing

After the completion of the Stuart Macaskill lakes upgrade later this year, our modelling shows that more water supply capacity will be needed by about 2020, if water use remains at 2006-11 average levels. However, total annual water use has been decreasing steadily since 2006 despite population growth. If the level of water use seen over the last 20 months could be maintained on a reliable basis, further spending on large-scale water storage projects could be deferred past 2020.

So, we are preparing to build more water infrastructure when and if that is needed, in order to deliver on our agreed security of water supply design standard. However, if the community is prepared to offset population growth with reliable water use reduction, then it will get good value from the existing water supply infrastructure for longer.

New water storage options

We have identified two potential water storage options: a dam on the Whakatikei River or new water storage lake/s at Kaitoke.  

Investigation into a potential water storage lake/s

In 2010, GWRC began an investigation into a potential water storage lake site at Kaitoke.  These investigations were completed in mid-2011 and found:

• The site would be suitable for a water storage lake
• The proposed lake could hold up to 5,000 million litres of water
• Two embankments would be required, one at each end of the proposed lake
• Local soils could be used to build the embankments
• Limited recreation on the lake, such as kayaking, could be possible

Since August 2011, GWRC has identified that land to the east of the original investigation site could also provide additional storage in the form of two smaller water storage lakes. The Regional Council is currently negotiating with AgResearch Ltd (the owners of the land) to purchase this land. A feasibility study on this new site will be carried out in 2013.

What about the dam option?

Our preferred dam site is on the Whakatikei River, near the end of Bulls Run Road. The 2011 estimated cost for a dam and associated infrastructure, including a new water treatment plant, is $160 million.

Have we considered other dam sites?

We've also investigated Skull Gully in the Wainuiomata Water Collection Area, and a site on the Pakuratahi River. Multi-criteria analysis was used to assess the three sites on a range of attributes. The Whakatikei site has considerable advantages over the other two sites:

• It's located on the western side of the Wellington Fault, making distribution pipelines to Porirua and Wellington much less vulnerable to an earthquake on this faultline. Our existing water sources are all on the eastern side of the Wellington Fault meaning our bulk water pipe would rupture at each crossing of the fault
  
• The dam capacity could be increased substantially at a relatively low additional cost
• Water could potentially be supplied to the dam from the Akatarawa River as well
• There is a greater likelihood of getting a resource consent to build a dam on this site than on the other two sites
• It's closer to areas of projected population growth in the western side of the region

Will conserving water help?

Reducing demand for water 'per person' (by methods such as reduced leakage from city water reticulation networks, increased use of water-efficient appliances and fittings or household water metering and associated conservation education) at a rate equivalent to population growth would help to defer the need to build a new dam or water storage lake.
 
We are working with the region's city councils to develop strategies that include elements of water demand management, for public consultation. However, our projections show that continued population growth will eventually outstrip acceptable water saving measures, requiring a dam or water storage lake to be built at some point in the future.