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Draining of water storage lake marks start of upgrade

9 September 2011

Draining of water storage lake marks start of upgrade

The draining of the southern Stuart Macaskill water storage lake at Te Marua has begun this week, signalling less stored water for Porirua, Wellington, Upper Hutt and Lower Hutt this summer.

The lake is being drained as part of Greater Wellington’s 2-3-year upgrade project to increase the resistance of the two Stuart Macaskill lakes to a major earthquake and boost their combined storage capacity by 13%.

Cr Nigel Wilson, Chair of Greater Wellington’s Social and Cultural Wellbeing Committee, says the upgrade is essential for a resilient, “future proof” water supply. (The committee’s responsibilities include water supply.)

“We’ve found that the lakes’ embankments may crack in a major earthquake, causing leakage and therefore slowing the recovery of our water supply system – seismic strengthening is a top priority to meet new dam safety guidelines. We also need to build more water storage capacity to serve our growing population.”

The upgrade works will require a lake to be empty this summer and the next – and possibly a third summer if there are any construction delays. This will essentially halve the amount of stored water available. And with one storage lake down, it’ll be harder to overcome water shortages in a warm, dry summer.

Cr Wilson says that Greater Wellington will be working closely with the Porirua, Wellington, Upper Hutt and Lower Hutt city councils to let the public know how we’re doing for water and provide simple tips on how people can help so that we don’t go short of water for essential uses. “If everyone works together and saves a bit more water, we can ease the pressure on our water supply and hopefully avoid tough restrictions.”


What’s the role of the Stuart Macaskill lakes?

Most of the treated bulk water that Greater Wellington supplies to the four cities comes from the Hutt, Wainuiomata and Orongorongo rivers and an aquifer, with the two Stuart Macaskill Lakes at Te Marua functioning as back-up to the Hutt River supply. Currently, each lake is 16 metres deep and together they have a capacity of approximately 3,000 million litres (ML), or enough to replace the average water supply from the Hutt River for about 50 days.

The lakes are filled from the Hutt River when the water is clean and there is plenty available. Stored water is pumped back to the water treatment plant when there is not enough water in the Hutt River to meet public demand, when the river is too dirty or when it is in flood and the intake is closed to prevent rocks and gravel from entering the intake pipes.

What will the upgrade involve?

The southern lake will be empty this summer (2011/12) and the northern lake empty in summer 2012/13. If there are any delays a lake may need to be empty in summer 2013/14.

Seismic strength will be increased by:

• Installing a strong, flexible liner in the lakes, which can stretch to more than seven times its length. This should prevent leakage after an earthquake even if the soil beneath the liner cracks

• Buttressing of the lakes’ external walls

Storage capacity will be boosted by:

• Raising the lakes’ walls by one metre to give an additional 400 ML (13%) combined storage capacity. That’s enough to help maintain supply for up to two weeks longer in a drought, in combination with other water sources.

• Tenders for the lakes’ upgrade are currently being considered. The lining and embankment raising works are expected to start in November.


Visit Greater Wellington’s website for a live water supply map, including a daily update on how much water Greater Wellington is supplying to the four cities: www.gw.govt.nz/live-water-supply .


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