Storage lake upgrade 2011
We're upgrading the Stuart Macaskill water storage lakes in Te Marua to improve their strength in an earthquake and increase their capacity. The upgrade will take place over the next two to three years, requiring a lake to be empty each summer.
One of the water storage lakes at Te Marua is empty for upgrading work
Having one lake empty will halve the stored water available to Lower Hutt, Porirua, Upper Hutt and Wellington. There's more chance of a water shortage if we get a warm and dry spring or summer, so we're asking everyone to do a bit more to save water. We'll keep you updated here with how we're doing.
The seismic strengthening is needed to meet national dam safety guidelines and give greater certainty that in a large earthquake the lakes will keep their water.
At the same time, we will increase the lakes’ water storage capacity by 13%, by raising the embankment walls slightly. Construction work to reinforce the outer embankment walls started in mid January 2011. Installing a flexible liner inside each lake and raising the embankment walls is scheduled to start from November 2011, once the southern lake is empty.
The 2-3 year project will require the southern lake (closest to Upper Hutt) to be empty in the summer of 2011/12 and the northern lake (closest to the treatment plant) in summer 2012/13 – there’s also the possibility that one of the lakes may be out of service in summer 2013/14 if construction is delayed.
We have been granted a variation to our consent to take water from the Kaitoke Weir, to reduce the risk of a summer water shortage while the lakes are upgraded.
Te Marua water storage lakes upgrade - Q&As
Why do we need to increase the seismic strength and capacity of the Stuart Macaskill lakes?
Why do we need to do this work over summer?
Why not delay the upgrade until you have built more storage elsewhere?
But hasn’t water use dropped? Why do you need more water when use is going down?
How much more water will there be if the lakes’ capacity is increased?
How much will the lakes upgrade cost and how will it affect ratepayers?
Why do we need to increase the seismic strength and capacity of the Stuart Macaskill lakes?
A feasibility study for increasing lake capacity in 2009 found that in a Wellington Fault earthquake (using the latest GNS energy estimate) significant cracking in the lakes’ lining could occur, ultimately resulting in the loss of stored water and erosion of the lakes’ embankments. In order to comply with NZSOLD Dam Safety Guidelines, we are required to reduce this possibility by increasing the lakes’ seismic strength.
Increasing the lakes’ capacity is an important short-term measure to boost our overall water supply capability in a dry year. The lakes’ embankments need to be reinforced regardless of whether we increase the storage capacity.
Why do we need to do this work over summer?
The earthquake strengthening involves exposing the clay lining on the embankment’s internal face and installation of a plastic liner on top of the clay. This will require machinery to work over the top of the clay lining, which may get damaged if it is wet. Sections of the plastic liner will be welded together, which also needs warm, dry conditions. This means the work needs to be carried out over summer. Depending on the weather, it may be possible to start work in November.
Why not delay the upgrade until you have built more storage elsewhere?
As a responsible large dam owner (the Stuart Macaskill Lakes are classified as dams) and legislative regulator, Greater Wellington has decided that the upgrade work should be carried out within an acceptable timeframe. Our assessment of an acceptable timeframe is up to five years, and the programme for the work meets that criterion. Investigations into a new storage lake are at an early stage, and, if built, could take around eight years. A dam may take longer. It would not be responsible to delay for that length of time.
But hasn’t water use dropped? Why do you need more water when use is going down?
Actual water use has reduced over recent years and if our summer weather and water usage remain moderate, river flows may not fall below 600 L/s. Weather conditions are a key influence on the demand for water and the water supply available. We don’t know in advance the level of rainfall or demand for water that we will get each summer and with one storage lake out of service, we want to ensure we have options available to maintain an adequate water supply. It’s all about managing risk.
How much more water will there be if the lakes’ capacity is increased?
When complete, the storage capacity upgrade will provide some 400 million litres (ML) more storage (13%).
In early 2008 (the most recent year when we used the lakes extensively to supplement river flows), we supplied an average of 32 ML/day from the lakes on the days that we used them. The increased capacity would allow us to supply 32 ML/d for an extra 12 days. A drought management plan, in place since 2008, is likely to improve that situation further.
How much will the lakes upgrade cost and how will it affect ratepayers?
The cost of the upgrade is $13 million. Upgrading the Stuart Macaskill lakes' levels has already been budgeted in Greater Wellington's annual plan so will have no significant additional rates impact.