Hunua Ranges Water Supply Rolled Earth Dams

Development of Auckland's bulk water supply

The development of the bulk water supply system for Auckland began with storage, pumping, transmission and water treatment facilities constructed in the Waitakere Ranges.

This phase of the development ended with the construction of the first rolled-fill dam in New Zealand, the Lower Nihotupu Dam.  Designed and supervised by CW Firth, this dam, and the construction and testing techniques developed during its construction, were the forerunner of the sequence of dams built in the Hunua Ranges under the supervision of Firth during his tenure as Chief Engineer Water with the Auckland City Council.

The need for continuous development of storage facilities in the Hunua-Wharekawa Ranges was a direct result of the growth of Auckland City and projections of water demand. To carry out the long term plan, a Water Supply Division within the City’s Department of Works and Services was established and managed by Firth. This efficient and well-integrated unit continued its work until 1965, when the entire fixed bulk assets of the City were handed over to the Auckland Regional Authority (established 1960).

Development of the Hunua catchments continued until the 1980s, when further investigations of options for the supply of bulk water to Auckland’s ever-increasing population included a scheme for drawing water from the Waikato River. This idea had been proposed much earlier, in the 1940s, by Arthur Mead, then Chief Engineer Water for the City, and was put into practice when it became clear that alternatives, such as further dam building in the Hunua and Waitakere Ranges were limited in their ability to sustain the supply to the city. Dam building ceased with the development of the Waikato River Supply Scheme.

Hunua Ranges catchment rolled earth dams

Development of the catchments in the Hunua Ranges commenced in 1946. The selection of dam site locations and the associated catchment capacities were significantly influenced by the characteristics of the geology in the area.

The catchment reserves are situated more or less in the centre of the Hunua-Warekawa District, with its distinctive north-northwest trending Wairoa Fault. This fault divides the upland into two main blocks – the Bombay-Hunua block on the western and downthrow side of the Fault and the Hunua-Warekawa block on the eastern upthrow side. The Hunua Hills are drained by the Wairoa River and its tributary, Cosseys Creek; the Wharekawa Hills by the Mangatangi and Mangatawhiri Streams. These latter streams drain independently to the Waikato River, while the Wairoa River flows south at first and then turns north and flows along the front of the fault to Clevedon where it drains into tidal waters from the Firth of Thames. In so doing the Wairoa has formed a spectacular gorge across the end of the hills near Ardmore.

All the dam sites and associated reservoirs are located to allow for gravity feed to the Ardmore Filter Station.

The Hunua Ranges dams are the:

• Cosseys Creek Dam (built 1951-1955)
  
• Upper Mangatawhiri Dam (built 1961-1965)
  
• Mangatangi Dam (built 1972-1977)
  
• Wairoa Dam (built 1972-1975)
  
• Hays Creek Dam (completed 1967)
  

Tunnelling works

All of the dams of the Hunua Range catchments relied for their operation on a series of tunnels through each of the ridges within the area for the conveyance of bulk water to the new Filter Station at Clevedon. Most of these tunnels were constructed by gangs employed directly by the Auckland City Council Waterworks Department during its existence, and a considerable specialised expertise grew out of this work.

Timing of construction ensured that progressive discharge of bulk water from the dams as they were built was possible, culminating in the connection of the Mangatangi Dam in 1977 as the last dam to be completed in the second stage of catchment development in the Hunuas.

Engineering importance

Taken as a whole, this programme of dam building represents a remarkable achievement in the history of civil engineering in New Zealand.

Adopting rolled earth techniques for building these dams was an innovative approach, requiring at its outset the importation and use of specialised earth-moving machinery never before used in the country. Dam design involved state-of-the-art soil testing and analysis. The University of Auckland Engineering Faculty and its specialists were involved in model testing of hydraulic structures and field work during construction, which provided opportunities for engineering students like no other in the country before on this method of dam building.

The variety of geological and foundation conditions produced technical challenges in the design and construction of the works which required innovation, excellent management and a high level of workmanship. All of which was provided to bring about successful completion.

Planning of the overall scheme in response to the projected demands for bulk water with the growth of Auckland City, meant that a secure supply was ensured during the years of its development. Experience gained by contractors in the use of heavy machinery, and construction management for this type of work was clearly invaluable for ongoing projects. The foundation was laid for the continued expansion of Auckland’s Bulk Water Supply with the advent of the Waikato River Scheme.

The overall scheme of dam building in the Hunua Ranges is an outstanding example of a cost-effective, timely, well-engineered, environmentally sound and publically beneficial civil engineering project. As a whole, this project deserves a status as one of the best examples of sound civil engineering and planning in New Zealand.

More information

Access

The dams are within the Hunua Ranges Regional Park. Access information is available from the Auckland Council website.

References

‘Water Supply Headworks,’ Auckland Regional Council leaflet.

CW Firth, ‘A Century of Water Supply for Auckland , New Zealand,’ Auckland Regional Authority (December 1967).

CW Firth, ‘The Auckland Water Supply System,’ New Zealand Engineering12:9 (1957), 292-306.

GJ Murdoch, ‘The Water Supply of Metropolitan Auckland,’ unpublished report, 1992.