Lake Waikaremoana
Lake Waikaremoana means rippling ocean
Geography, natural history and climate
Waikaremoana, the North Island's deepest lake (256 m deep), has its surface at 600 metres above sea level. A huge landslide dam about 250 metres high formed the lake around 2,200 years ago.[1] Before the landslip was sealed, around 1950, much of the lake outflow flowed through the landslip rather than out of an overflow at a low point in the slip.[1]
Other geographical features include Panekiri Bluff and Puketukutuku Peninsula, which is the site of a kiwi-conservation programme. Surrounded by mountains clad with native forest which has never been logged, Waikaremoana is regarded[by whom?] as the North Island's most attractive lake. Many native bird-species scarce in most other parts of the North Island occur in the area. A possum-hunting programme operates in the area to help protect the forest. Numerous understory species grow within the forested area of the catchment basin, crown fern, Blechnum discolor, for example.[2] Since at least the early 1900s lake-bottom molluscs have been studied by Colenso (1811-1899) and others.[3]
The climate of Lake Waikaremoana is temperate during the summer months and cool in the winter where snow events are not unheard of. Heavy rains affect the region, especially about late winter and early spring. The weather in the area is very changeable, trampers in the region need to be on the look out for dangerous weather conditions.
Hydroelectric power scheme
.JPG)
The Waikaremoana Hydroelectric Power Scheme appears to be the only example of a hydroelectric power station being built on a natural landslide dam.[citation needed]
Modifying the natural dam
The stability of the natural dam has been the subject of intense engineering review, both at the time of construction and subsequently. Construction of an outlet tunnel through the slip, which commenced in 1935, required extensive grouting around the control structures and throughout tunnel construction. Work was suspended at the end of 1936 because Bob Semple, the newly elected Minister for Public Works, wanted the tunnelling project reconsidered for "risk, cost and value". A new tunnelling scheme was devised in 1941 based on what had been learned from initial exporatory tunnelling and work recommenced in 1943 and continued for about 5 years because of continual problems with dewatering the tunnels. After the tunnels and intake headworks had been completed the natural dam was sealed for leaks on the lake side by removing submerged timber, a task that took a year and then applying 40,000 cubic metres of crushed rock and clay-like pumice in 6 layers, then covering those layers with a top layer of larger rock and spalls to protect the material from wave action. This sealing reduced the natural flow by about 80%. The sealing of the lake was only done after tunnelling was completed otherwise it would have caused the lake level to rise and make tunnelling more difficult. As it was, the lake level had to be lowered by temporary syphons to enable the construction of the headworks and make sealing of the natural dam easier.[1]
Power stations
Although the Waikaretaheke River carries a flow of about 17 m³/s from Lake Waikaremoana, the head of water through the 3 power stations, Kaitawa, Tuai, and Piripaua is around 450 metres, allowing the stations to potentially generate 138 megawatts all up. The 250m head of water for the Kaitawa station is the highest for a dam in New Zealand and among the highest in the world.[1]
See also
References
- C. Michael Hogan. 2009. Crown Fern: Blechnum discolor, Globaltwitcher.com, ed. N. Stromberg
- New Zealand Department of conservation. 2009. Te Urewera National Park
- Royal Society of New Zealand. 1905. Transactions of the Royal Society of New Zealand, v. 1-76; v. 77, pts. 1-5; v. 78-88
Line notes
Gallery
External links
