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 is increasing its water storage at the Stuart Macaskill Lakes in Te Marua and investigating additional water source and storage options.
Questions and answers about our development options are set out below.
Why do we need to make decisions now?
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). 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.
In 2000, modelling of water use showed our system would achieve this standard for up to 377,000 people; a level of population not then forecast as likely until at least 2020. However, since 2001 and particularly following the 2006 Census, Statistics NZ has increased its population projections for the Wellington region. Higher than expected growth saw the last estimate of urban population in the four cities reach almost 390,000 (30 June 2010).
In 2010, our water modelling program developed by NIWA was upgraded to include water consumption by city, and updated with more recent water consumption figures to reflect the reduced consumption levels over the previous 5 years. The upgraded model has confirmed that the 2% security of supply standard was still being achieved in 2011 as a result of reduced water consumption per person.
As the population increases into the future our security of supply will deteriorate, unless demand for water reduces at least at the same rate as the population rises, or additional water sources are developed. Greater Wellington encourages water conservation, reducing wastage and more efficient use of water, but we also need to make decisions now on future water sources.
New water storage options
We have identified two potential water storage options: a dam on the Whakatikei River or a new water storage lake at Kaitoke.
How much will a new dam cost?
Our preferred dam site is on the Whakatikei River, near the end of Bulls Run Road. The 2011 estimated cost for a dam and the associated infrastructure, including a water treatment plant, is $160 million.
Have we considered other dam sites?
We’ve also investigated Skull Gully in the Wainuiomata Water Catchment 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 a Wellington fault movement. Our existing water sources are all on the eastern side of the Wellington Fault
- The dam capacity could be increased substantially at a relatively low cost
- Water could potentially be supplied to the dam from the Akatarawa River as well
- There is 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
Update on the proposed Whakatikei Dam
In early 2012, we engaged consultants to look at the proposed dam on the Whakatikei River. They will be looking at what the size of the dam should be as well as what would be the best location for the dam, taking into account its visual impact and any recreational opportunities.
Investigation into a potential water storage lake
Greater Wellington has investigated a potential water storage lake site at Kaitoke. The investigations began in November 2010 and were completed in mid-2011. During 2012, the potential water storage lake will be evaluated against the proposed Whakatikei dam site (the council's current preferred option).
Questions and answers about the investigation
URS New Zealand Limited completed a feasibility study to determine the potential for creating a water storage lake. They 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
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.