Benmore Dam

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Benmore Dam from Lookout Point

Benmore Dam is the largest dam within the Waitaki power scheme on the borders of Otago and Canterbury regions in New Zealand's South Island. There are eight other power stations in the valley.

The dam is the largest earth-filled water-retaining structure in New Zealand. Its core is impermeable clay-like gravel, supported by two massive shoulders of river gravel. Lake Benmore has a volume of 12.5 million cubic metres which is about 1.5 times as much water as Wellington harbour.

The dam's spillway can cope with 3,400 cubic metres of water per second, about 10 times the mean river flow. Construction of the dam and hydroelectric station began in 1958, with it commissioned in 1965.

With a generating capacity of 540 MW, Benmore Power Station is the second largest hydro station in New Zealand. It is owned and operated by Meridian Energy.

Benmore is also the southern end of High Voltage Direct Current 500 MW (HVDC) link which joins the North and South Island electricity systems. There are two switchyards, one for AC transmission to the South Island, and the other containing the static inverter plant for the HVDC link.

Otematata is the small town that supports the dam although now with advances in technology the man power needed to maintain the dam is drastically less. Due to this the town is now a small holiday community, with only around 200 permanent residents. The dam is about 8km up the valley from the township.

Benmore Dam from south lookout, near the spillway

[edit] Key statistics

Lake Benmore area: ~ 75 kilometres square
Lake Benmore shoreline: 116 km
Dam crest length: 823 metres
Dam crest height: 110 metres
Head (hydraulic): 92 metres
Average river water flow: 340 m³/s
Nominal annual generation: 2,200 GWh
Installed capacity: 540 MW
Machinery: 6 x 93,000 W vertical Francis turbines (Dominion Engineering, Canada), coupled to 6 x 90,000 W 112,50 kVA generators (Canadian General Electric); 6 transformer banks totalling 1,150,000 kVA
Penstocks: Concrete