Listen closely. I’m about to share the story of Wairoa Dam. It’s a vital part of Auckland’s water supply.
The Wairoa Dam sits in the Otau Valley. This is within the Hunua Ranges, southeast of Auckland. The Auckland Regional Authority built it in the early 1970s. Today, Watercare Services Ltd owns it.
Originally, Wairoa Dam was planned as the fourth dam here. A tunnel system already ran past the site. It carried water to the Ardmore Treatment Plant. An interim intake weir started supplying water in 1960.
Mangatangi Dam was next in line. However, construction faced delays. Auckland’s water demand was growing rapidly. Eric Scanlen suggested building Wairoa Dam sooner. It was completed before Mangatangi.
Don Wilson led the design team. Eric Scanlen handled hydrology. John Crabtree worked on the spillway. Garry Law focused on the delivery system. Graeme Barnard managed geotechnical aspects. Remi Gobet oversaw structures. All calculations were done by hand.
Construction started with the diversion tunnel. The Authority provided all equipment and materials. Downer and Co won the main construction contract. An ARA pipe gang installed the delivery pipeline.
The Wairoa Dam was completed in 1975. The lake area spans 98 hectares. It can hold 11.6 gigalitres of water. The dam is 47 meters high. It uses 685,000 cubic meters of earth fill. The bellmouth spillway can handle 122 cubic meters per second. The project cost $3.9 million.
The valve tower stands 70.1 meters tall. It was built in just six days. Aluminium powder was added to the concrete mix. This prevented leakage and corrosion.
Earthworks took place over two seasons. The contractor and ARA had a good working relationship. Modern machinery helped with compaction.
The bellmouth spillway was completed in 1974. Filling began seven months ahead of schedule. The official opening was on May 23, 1975.
The dam has a crest length of 213 meters. Its catchment area covers 32,580 hectares. It can safely yield 66,700 cubic meters of water per day. The reservoir can hold 16,550,000 cubic meters when full.
The Wairoa Dam and reservoir can be seen from the Wairoa Loop Track.
Sadly, vandals stole the bronze plaque. It commemorated the opening by Authority Chairman Tom Pearce. It also contained information about the dam’s construction.
The lake area was mostly farmland before. It had reverted to scrub by the 1970s. The vegetation was removed for water quality. Staff cut a waterline road. They felled and burned the vegetation. One fire burned a small area above the waterline.
The Wairoa Dam stands as a testament to innovative engineering. It ensures a reliable water supply for Auckland. Its construction overcame challenges. The dam is a vital asset for the region.