Storage lakes upgrade
We're upgrading the Stuart Macaskill water storage lakes in Te Marua to improve their strength in an earthquake and increase their capacity.
The project requires the northern lake (closest to the water treatment plant) to be empty in summer 2012/13. The southern lake (closest to Upper Hutt) was upgraded last summer. A lake may be out of service during the summer 2013/14 if construction is delayed.
Having one lake empty has halved the stored water available to Lower Hutt, Porirua, Upper Hutt and Wellington.
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're increasing the lakes’ water storage capacity by 13%, by raising the embankment walls slightly.
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.
Work on the southern lake has been completed.
The northern lake was emptied in December 2012. The upgrading work on the northern lake started in January and approximately 20% of the plastic liner has now been installed. Work on the northern lake is scheduled to be complete by August 2013.
The upgrade project began in January 2011 with construction work to strengthen the outer embankment walls of both lakes. This strengthening work was completed in December 2011. In 2012, work began on upgrading the southern lake. The upgrade work consisted of draining the lake so that a plastic liner could be laid underneath the rip-rap (the rocks that protect the embankments of the lake). The northern lake will be upgraded in 2013.
The liner is made of plastic that can stretch to seven times its original length without breaking and is designed to stop leakage of stored water and erosion of the lakes' earth embankments after an earthquake.
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.
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.
With an empty lake, there's more of a chance of a water shortage than usual if we get a warm and dry spring or summer. We're running summer water conservation campaigns with the region's city councils throughout the upgrade project to encourage people to save a bit more water so that there's enough to go around for essential uses.
We have also been granted a change to our consent to reduce the minimum flow over the Kaitoke Weir from 600 litres per second to 400 litres per second. We would only use the lower minimum flow if it's needed to meet the demand from water users.
During the summer of 2011/12 we didn't need to use the change of consent condition that allows us to temporarily reduce the minumum flow at Kaitoke Weir - so the minimum flow over the Kaitoke Weir remained at 600L/s.
Our consent application contained scientific research on the impact of the change that we were proposing. The impact was judged to be no more than minor.
Four other major river sources feed into the Hutt River downstream of Kaitoke (where water is sourced for our drinking water). Modelling shows that if the river level over the weir dropped to 400L/s, this additional allowance would be measureable in just a few millimetres of the river level at Taita.
Algal blooms can occur in rivers across the region during periods of warm weather and consistently low flows and the absence of flushing from heavy rainfall. During the 2011/12 summer, there was an increase in coverage of cyanobacteria (toxic algae) in the Hutt River, which coincided with a period of relatively dry weather and a lack of heavy rainfall from late January to mid-February.
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.
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.
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.
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.