Stuart Macaskill Lakes Hero

Stuart Macaskill Lakes: securing Wellington’s water supply into the future

The Stuart Macaskill Lakes, at Te Marua, act as a back up to the Hutt River water supply and are used when levels are low or the river is too dirty. The lakes currently hold enough water to replace the Hutt River supply for 50 days.

In 2009, Greater Wellington Regional Council's (GWRC) water group began a review of their water storage sites to assess their water capacity in case of a large Wellington earthquake. In the case of Stuart Macaskill Lakes, it found that significant cracking in the lakes’ lining could occur, possibly resulting in the loss of stored water and internal erosion of the lakes’ embankments. In order to comply with NZSOLD Dam Safety Guidelines and to retain stored water for use following an earthquake, GWRC decided to increase the lakes’ seismic strength. It also decided to increase the lakes’ capacity by 13% to boost overall water supply capability in a dry year.

The upgrade project began in December 2011 with construction work to strengthen the outer embankment walls of both lakes.

GWRC’s aims:

  • To increase the Stuart Macaskill lake capacity by around 13%, an important short-term measure to boost our overall water supply capability in a dry year
  • To increase the lakes’ seismic strength in order to reduce the possibility of significant cracking in the lakes’ lining and to comply with NZSOLD Dam Safety Guidelines.

What we did:

We were appointed lead contractor on this project. The upgrade included:

  • Seismic mitigation works at both lakes to prevent loss of containment in an earthquake
  • The raising of the lake embankment crests by 1 metre to increase storage capacity
  • Works to minimise excessive seepage.

There were two separate components (known as separable portions) to this project:

The first involved works on Lake 2, with a contract start date of 6 December 2011 and a contract end date of 30 August 2013. Hawkins successfully met this target.

The second involved works on Lake 1. The initial contract end date was 14 August 2012 but we agreed a time extension with the client due to approved variation works. The new end date, which we met, was 19 September 2012.

The entire project, including the sealing of crest roads and landscaping works was due to be completed in January 2014.

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 storage capacity of the lake was increased at the same time.

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.

Unique challenges:

There were several environmental aspects that needed to be managed. As a result we implemented an environmental management plan, which was approved by GWRC and the engineer. Risks included open drains that drain directly into the Hutt river (these drains collect any rain water or runoff from the crest road), and the runoff from the existing slopes and the lake bed into the river through a scour pipeline for draining purposes. We installed hay bales and silt socks as well as a sediment pond to minimise the silt runoff from earthworks leaching into the river via the scour pipe.

Because the storage lakes are located in a Regional Park we needed to make the public aware of site activities. We did this by installing project and road signs at both the entrance to the park and the entrance to the site. Neither Hawkins nor GWRC received any public complaints about the project.

The outcome:

The Stuart Macaskill water storage lakes are now much less susceptible to major damage in the event of an earthquake. They also have increased capacity, helping to boost Wellington’s overall water supply capability in a dry year.