Roxburgh Dam

Roxburgh Dam
Roxburgh Dam machine hall
Location of Roxburgh Dam in New Zealand
Location	Central Otago, New Zealand
Coordinates	45°28′33″S 169°19′21″E / 45.475811°S 169.322555°E / -45.475811; 169.322555Coordinates: 45°28′33″S 169°19′21″E / 45.475811°S 169.322555°E / -45.475811; 169.322555
Dam and spillways
Type of dam	Concrete gravity dam
Impounds	Clutha River / Mata-Au
Height	76 m (249 ft)
Length	358 m (1,175 ft)
Width (crest)	10.7 m (35 ft)
Width (base)	61 m (200 ft)
Reservoir
Creates	Lake Roxburgh
Surface area	6 km2 (2.3 sq mi)
Power station
Operator(s)	Contact Energy
Commission date	1956 - 1962
Turbines	8
Installed capacity	320 MW (430,000 hp)
Annual generation	1,650 GWh (5,900 TJ)

The Roxburgh Dam is the earliest of the large hydroelectric projects in the southern South Island of New Zealand. It lies across the Clutha River / Mata-Au, some 160 kilometres (99 mi) from Dunedin, some 9 kilometres (5.6 mi) to the north of the town of Roxburgh. The settlement of Lake Roxburgh Village is close to the western edge of the dam.

In 1944 the State Hydro Department estimated that even with the power stations currently under construction they would only be able to meet projected South Island load up until 1950 or 1951 and that a new large power station was required. Detailed investigations by the Public Works Department identified two alternatives, Black Jack's Point on the Waitaki River (where eventually Benmore Power Station would be built) and Roxburgh Gorge on the Clutha River. The Roxburgh location had the advantage of being less remote, requiring less geological investigation, half the materials for the same power output and a better climate in which to undertake construction work, which were important considerations at a time of serious shortages of labour and cement. Historical records showed that the long term flow of the river was 17,650 cusecs (499.8 m3/s) and that a controlled flow of 15,000 cusecs (424.7 m3/s) would be possible through the power station. To avoid exceeding the height of the 1878 flood at Alexandra, the maximum retention level of the lake which would be impounded behind the dam was fixed at 430 ft (131.1 m) above sea level. The designers estimated that with a overall efficiency of 85% the mean output would be 160 MW and assuming a annual power factor of 50% the station could deliver a maximum output of 320 MW.

The Clutha River is fed from lakes Hawea, Wakatipu and Wanaka. There were already existing control gates on the Kawarau River at the outlet of Lake Wakatipu and it was decided to control the flows from the remaining lakes. After investigation found that the soil conditions were unsuitable at Wanaka a control structure was only built at Lake Hawea. This was commissioned in 1958 and consists of four radial gates housed in an earth dam. The dam raised the existing lake level and currently provides approximately 290 GWh of storage.

In 1947 the Labour Government approved plans to build a hydro power station on the Clutha River. The Clutha River between Alexandra and Roxburgh runs through a deep gorge which offered a number of locations for a power station. Detailed investigations found that Tamblyn's orchard where the river exited the Roxburgh Gorge near the town of Roxburgh, offered the most power output, better tailwater conditions, the best access and would be closest to suitable locations for both construction and permanent villages. A concrete design was selected as due to the geology of the site this was a better option than a earth dam at the wider Pleasant Valley location. The Ministry of Works also had limited experience with earth dam construction and its only engineers with the necessary experience where engaged on the Cobb Power Station. Many of the design decisions were based upon results from studies undertaken from 1949 to 1954 on a 1:80 scale model of the dam at the DSIR's Hydraulics Laboratory at Gracefield, Lower Hutt.