Upper Mangatāwhiri Reservoir

Slipping through time, we find ourselves in 1965 at the unveiling of the Upper Mangatāwhiri Dam. This marked the completion of a marvel of engineering, Auckland’s second-largest dam, the Upper Mangatāwhiri Reservoir. The reservoir is nestled in the heart of the Hunua Ranges. It’s one of a series of rolled earth dams, each contributing to Auckland’s water supply. The Upper Mangatāwhiri Dam stands as a testament to innovative design. Its construction significantly advanced rolled earth dam design in New Zealand. The dam is situated in the Moumoukai Valley, a location carefully chosen for its unique geological features. This valley was once a prehistoric lake, formed by fault-tilting and filled with gravel and silt. The eventual drainage of this lake exposed greywacke and argillite bedrock. These materials, rich in manganese oxides, proved ideal for the dam’s construction. They provided the clay and clay-rubbles for the core, and the fine-grained greywacke and argillite served as facing stone. The natural contours of the valley also played a crucial role. Spurs of country rock on either side provided perfect locations for the spillway, valve tower, intakes, and diversion culvert. The Upper Mangatāwhiri Dam shares similarities with the Cosseys Dam. It is almost identical in height and cross-section. However, it boasts roughly three times the length and volume of fill material. Construction took 276 days, a similar timeframe to Cosseys Dam. This efficiency was due to improved earth-working equipment and favorable weather. The dam impounds 15% more water than Cosseys Dam, but remarkably yields 45% more. The dam’s spillway, a curved side weir, stretches 117 meters. It is capable of discharging a massive volume of water. A 38-meter-high concrete valve tower, constructed by slip-forming, stands as another impressive feature. The Upper Mangatāwhiri Dam project holds a special distinction. It was the first to involve students from the Ardmore Engineering School. These students contributed to various aspects of the project. They gained invaluable experience, working initially as laborers and later as technicians. Upon its completion, the Upper Mangatāwhiri Dam fulfilled 50% of the Hunua catchments’ potential storage. It marked the end of the first stage of development in the ranges. It also signified a shift in responsibility for Auckland’s bulk water supply, from the Auckland City Council to the Auckland Regional Authority. Today, the Upper Mangatāwhiri Reservoir stands not just as a source of water. It’s a monument to ingenuity, a reminder of a significant chapter in Auckland’s history.