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Dams in the Hunua and Waitakere Ranges

Auckland’s water supply dams are constructed to capture and store water that will be treated and supplied to our customers connected to the Auckland metropolitan water supply system. Streams and tributaries feed water to fill the reservoirs created by these dams. 

​Tthe Hunua and Waitakere ranges provide the ideal environment and elevation for the majority of Auckland's water sources.

Dam operation

The operation of a water supply reservoir is quite different to reservoirs used for the generation of hydro-electric power. Water supplied from our reservoirs flows to a water treatment plant, is treated to ensure that it is safe to drink, and then distributed to consumers. The water stored in a hydro-electric generation reservoir, passes through a dam to generate power. This water is then discharged to a downstream water course. Hydro-electric dams also have ‘release gates’ to control reservoir levels. 

Our water supply dams do not have release gates.

The function of our dams is to store water for supply. Our aim is to try to capture as much water as possible so that Aucklanders can use and enjoy water on demand, and so there is enough water available for firefighting if required.  

Aside from storing water, Auckland’s reservoirs provide other benefits:

- By storing water in a  lake, sediments have time to settle on the lakebed.

- Capturing water during heavy rainfall events minimises the likelihood and impact of downstream flooding. When the dams do reach capacity, they are designed to spill into an engineered spillway. When this happens, the volume of water spilling reflects the volume of natural rain falling at that time. 

To make sure our dams do not adversely impact the downstream environment, we release a small flow of water from each dam via a ‘free discharge’ valve, to maintain flora and fauna habitats. We call this a ‘compensation flow’ and this supports the environment when our dams are not full and spilling.

The valves are also required for dam safety purposes so we operate the valves periodically to ensure they operate as required, for environmental purposes and to meet resource consent requirements.  

Living downstream of dams

During heavy rainfall events, the dam will provide attenuation to minimise the likelihood and impact of downstream flooding. However, the dam is allowed to spill and excess water flows downstream through a spillway as the storage level increases and the dam reaches full supply level.

Residents living downstream and in close proximity to dams are encouraged to keep informed and prepared for extreme weather. In particular, the summer period is considered a cyclonic season where extreme weather is likely to eventuate.

Here are some tips to keep informed and prepared:

  • Keep informed on developing extreme weather (weather warnings) and keep up-to-date with weather reports
  • Be prepared for an emergency situation by developing a 'Get Ready Get Thru' plan which involves preparing an emergency plan and kit 
  • Keep informed on dam storage levels on Watercare's website
  • Understand how Council and Local Disaster Management Group manage emergency situations in your local area
  • More information​ on flooding and methods

Catchments

The dams only capture water from the upper parts of the catchments.  Some of our dams have extensive areas below them which contribute to the total stream or river flows. Occasionally there are extensive storms that do not pass over the part of the catchment above the dam.

 

Dam location

It makes sense to locate dams in high rainfall areas close to cities. The Hunua and Waitakere ranges have a high elevation and receive around 1.8 metres of rain annually, about 50 percent more than central Auckland gets. The high location of the dams also makes it easy for water to flow to the city by gravity. Watercare also operates the Mangakura Dams in Helensville which supplies only the Helensville and Parakai area.

Dam structure

Auckland’s dams are constructed with either compacted earth or concrete. The earlier dams in the Waitakeres were made of concrete while the later, post-war dams were built with earth and rock.

Dams in the Hunua Ranges

There are five earth dams in the Hunua Ranges which supply about 65 percent of Auckland’s water. Each was constructed between 1951 and 1977 on the headwaters of the river it is named after, apart from Cosseys, which is on the headwater of the Wairoa River. The water from each dam is piped to the Ardmore Water Treatment Plant. It is then stored and transported through pipes, which are up to 1.9 metres in diameter, into reservoirs nearer the city.

 
Mangatangi Dam
Completed:                    1977
Lake area:                      185 hectares
Capacity:                        35.3 gigalitres
 
Mangatawhiri Dam
Completed:                    1965
Lake area:                      128.5 hectares
Capacity:                        16.2 gigalitres
  
Cosseys Dam
Completed:                    1955
Lake area:                      123 hectares
Capacity:                        14.03 gigalitres
  
Wairoa Dam
Completed:                    1975
Lake area:                      98 hectares
Capacity:                        11.6 gigalitres
 
Hays Creek Dam
Completed:                    1967
Lake area:                      18.2 hectares
Capacity:                        1.1 gigalitres
   

Dams in the Waitakere Ranges 

The Waitakere Ranges is home to five dams which supply about 26 percent of Auckland's water. The dams were constructed between 1907 and 1971, three of concrete and two of earth. Each dam is named after the stream that feeds into its reservoir. Raw water from these dams is piped to treatment plants in Huia, Titirangi and Swanson.

 
Waitakere Dam and Waitakere Saddle Dam
Completed:        1910
Lake area           25.1 hectares
Capacity:            1.76 gigalitres
 
Upper Nihotupu Dam
Completed:        1923
Lake area:          12.5 hectares
Capacity:              2.2 gigalitres
 
Lower Nihotupu Dam
Completed:        1948
Lake area:          52.9 hectares
Capacity:              4.6 gigalitres
 
Upper Huia Dam
Completed:        1929
Lake area:          18.9 hectares
Capacity:              2.2 gigalitres
 
Lower Huia Dam
Completed:        1971
Lake area:          50.3 hectares
Capacity               6.4 gigalitres