Remember back to 1975. The Wairoa Reservoir officially opened. Authority Chairman Tom Pearce marked the occasion on May 23. Sadly, vandals removed the bronze plaque commemorating the opening and dam construction details.
The Wairoa Reservoir lies in the Otau Valley. It is nestled within the Hunua Ranges southeast of Auckland. Watercare Services Ltd owns it now. The Auckland Regional Authority originally built it.
Interestingly, the Wairoa Dam was planned as the fourth dam in the ranges. A tunnel system already existed. It carried water to the Ardmore Treatment Plant. This system connected to the second dam, Upper Mangatawhiri. A flow gauging weir was built above the proposed Wairoa Dam site. An interim water supply started in 1960.
Mangatangi Dam was slated next. But construction delays risked Auckland’s water supply. Resident engineer Eric Scanlen suggested building Wairoa Dam concurrently. Thus, Wairoa Dam entered service two years before Mangatangi.
Don Wilson lead the design. Eric Scanlen handled hydrology and spillway arrangement. John Crabtree worked on the spillway. Garry Law designed the spillway and delivery system. Graeme Barnard managed geotechnical aspects. Remi Gobet oversaw structures. All calculations were done by hand. Early electronic calculators assisted. Computer-aided design was in its infancy then.
Construction started with a labor-only contract for the diversion tunnel. The Authority provided all equipment and materials. A concrete batching plant was established on site. John Crabtree supervised this work. Downer and Co won the main work tender through a conventional contract. An ARA pipe gang laid the delivery pipeline. An on-site earthworks control laboratory ensured quality.
While building the dam, ARA staff investigated the Lower Mangatawhiri Dam site. They also duplicated the Ardmore supply tunnel.
The diversion tunnel included the spillway up to the elbow. Tunnel lining followed excluding the elbow. The delivery pipe trench was dug in the tunnel floor. The pipe was laid and concreted integrally with the tunnel floor.
Site stripping began. The valve tower base was built. It was tendon anchored into the rock. The temporary supply diverted through the supply pipe. The stream then diverted through the diversion tunnel.
Stripping of the dam site commenced. The valve tower reached full height. The bridge to the valve tower was under construction. The dam neared full height in the second earthworks season.
The valve tower is 70.1 meters high. It was constructed in six days. The contractor used slip-forming methods. A new mix design included aluminum powder in the concrete. This eliminated leakage into the tower. It also prevented corrosion problems.
The embankment construction was smooth. Borrow areas were close. Good relations existed between the contractor and the ARA. Modern machinery was used.
The bellmouth spillway and stilling basin completed in late 1974. All other works finished by May 1975. The reservoir filled seven months ahead of schedule.
Completed in 1975 the Wairoa Reservoir has a lake area of 98 hectares. It can hold 11.6 gigalitres of water. The catchment area spans 1303 hectares. The dam height is 47 meters. Its type is earth fill with a fill volume of 685000 cubic meters. The service spillway is a bellmouth. It has a spillway capacity of 122 cubic meters per second. The mean yield is 32 million litres per day. The construction cost $3.9 million.
Before the dam, the lake area was mostly scrub. Staff removed vegetation for water quality. They cut a waterline road. Then they felled and burned the vegetation.
Today, the dam and reservoir are visible from the Wairoa Loop Track.
The Wairoa Reservoir stands as a testament to engineering ingenuity. It highlights collaborative spirit. It ensures Auckland’s water supply for generations to come.