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Dartmouth Dam
Dartmouth Dam wall, pictured in April 2003
Dartmouth Dam is located in Victoria
Dartmouth Dam
Location of the Dartmouth Dam in Victoria
Country Australia
Location Victoria
Coordinates 36°33′29″S 147°31′25″E / 36.55806°S 147.52361°E / -36.55806; 147.52361
Status Operational
Construction began 1973
Opening date 1979
Construction cost A$139 million
Owner(s) Murray Darling Basin Authority
Operator(s) Goulburn–Murray Water
Dam and spillways
Type of dam Embankment dam
Height 486 m (1,594 ft) AHD
Height (thalweg) 180 m (591 ft)
Length 92 m (302 ft)
Width (crest) 670 m (2,198 ft)
Width (base) 700 m (2,297 ft)
Dam volume 14.1×10^6 m3 (498×10^6 cu ft)
Spillway type Uncontrolled chute
Spillway capacity 2,750 m3/s (97,115 cu ft/s)
Creates Dartmouth Reservoir
Total capacity 3,856 GL (848×10^9 imp gal; 1,019×10^9 US gal)
Inactive capacity 71 GL (16×10^9 imp gal; 19×10^9 US gal)
Catchment area 3,600 km2 (1,390 sq mi)
Surface area 6,300 ha (15,568 acres)
Power station
Name Dartmouth Power Station
Operator(s) AGL Hydro
Commission date January 1981
Type Francis-type
Turbines 1 x 180 MW (241,384 hp)
Installed capacity 150 MW (201,153 hp)
Annual generation 310 GWh (1,116 TJ)
Dartmouth Dam at the Murray-Darling Basin Authority

Dartmouth Dam is a large rock-fill embankment dam with an uncontrolled chute spillway across the Mitta Mitta, Gibbo, and Dart rivers, the Morass Creek and a number of small tributaries. The dam is located near Mount Bogong in the north-east of the Australian state of Victoria. The dam's purpose includes irrigation, the generation of hydro-electric power, water supply and conservation. The impounded reservoir is called Dartmouth Reservoir, sometimes called Lake Dartmouth. The Dartmouth Power Station, a hydro-electric power station that generates power to the national grid, is located near the dam wall.

A smaller pond, called the Dartmouth Dam Regulating Pond or Banimboola Pondage, approximately 9 kilometres (6 mi) downstream of the main dam, also across the Mitta Mitta River, is located adjacent the Banimboola Hydroelectric Power Station and forms part of the Dartmouth Dam complex of facilities.

Location and features

Designed by the State Rivers and Water Supply Commission of Victoria, construction commenced in 1973 and was completed in 1979 by Thiess Brothers, at a cost of A$179 million. The embankment dam wall is constructed with an earth core and rock fill, rising to a height of 180 metres (591 ft) from the lowest part of the foundation to the roadway across the top of the dam; making the dam wall the highest in Australia. The core component materials of the wall include 10.5 million cubic metres (370×10^6 cu ft) of rock, 0.8 million cubic metres (28×10^6 cu ft) of filter material made from crushed quarried rock, and 2.8 million cubic metres (99×10^6 cu ft) of earth for the core. The reservoir has a capacity of 3,856 gigalitres (1.018647433893×1012 US gal), or approximately 6.7 times the capacity of Sydney Harbour; and can release a maximum outflow of approximately 12,000 megalitres (3.170064628×109 US gal) per day in normal operation.

The crest of the uncontrolled spillway is 486 metres (1,594 ft) AHD  and is approximately 92 metres (302 ft) long. When full, flood flows spill over the crest and down an 80 metres (262 ft) long concrete chute. The water then returns to the river via an open rock cascade which gradually widens to 300 metres (984 ft) at river level. Once Dartmouth Reservoir reaches 99% capacity, it is considered to be 'operationally full' and releases are then set to pass inflows downstream to prevent the level rising further. Releases are passed through the outlet works and power station whenever possible and water will only flow over the spillway if significant flood inflows enter from upstream when the storage is close to full. This approach reduces the chance of downstream flooding, maximises operating flexibility for hydro-power generation and protects the spillway.

The Dartmouth Dam stores water from the Victorian "High Country's" snow fields for summer release into the Mitta Mitta (and the downstream Lake Hume) and subsequently into the greater Murray River for irrigation. The reservoir's inflow and outflow capacity is quite small considering its size, meaning that its levels vary little compared with some other dams on the Murray and their tributaries.

The reservoir is a popular recreational trout fishery, being regularly restocked by the Victorian Department of Primary Industries.

Hydro-electric power generation

Designed by the State Electricity Commission of Victoria, and constructed by Lewis Constructions Limited, and commissioned in January 1981, Dartmouth Power Station has one Francis turbine-generator, with a generating capacity of 180 megawatts (240,000 hp), the largest single installed hydroelectric turbine in Australia. It is owned and operated by AGL Hydro. The turbine casing and concrete machine block surrounding the power station were destroyed in May 1990 when two steel beams entered the turbine. The resulting force ruined the power station and the dam's control systems, making it impossible to gradually release water from the near-capacity dam by conventional means. An improvised system, placing large pipes over the spillway to siphon water over it, was soon installed, but the inflow from an unusually wet spring was such that the dam would have overflowed anyway, leading to a spectacular cascade over the huge rock steps formed when the rock used for the dam itself was quarried from the valley walls. The station was subsequently re-built and recommissioned in 1993.

The capacity of the regulating pondage was increased in 2003 to further optimise the power station's generation flexibility. The station is connected to the electricity grid via a 220 kV transmission line to Mt Beauty, 40 kilometres (25 mi) away.

Power Station Accident

On 2 May 1990, the 180MW Francis turbine-generator running at full speed was instantaneously stopped by a foreign body left in the penstock following maintenance. The installation shifted about 2m within the base of the 180m high earth and rock fill gravity dam wall of the 3,906GL reservoir. After initial consternation regarding the integrity of the wall (declared safe after lengthy assessment), the hydro installation was repaired/replaced but was off-line for several years. A breach of the wall would have obliterated only a couple of small towns and a sparsely settled agricultural area in the relatively narrow 120 km Mitta Mitta valley below the dam, but more significantly, would have resulted in the over-topping and probable failure of the earthen walls of the 40m high 3,038GL Lake Hume, 200 km downstream on the Murray River. This is immediately upstream of the regional cities of Albury and Wodonga and a much more intensively settled irrigation area, and consequences would have been disastrous.

Ecological impact on the Murray-Darling Basin

The construction and operation of Dartmouth Dam has caused significant changes to the flow patterns and ecology of the Mitta Mitta and Murray rivers. In particular, the unnaturally cold water released from the dam, up to 10 °C (18 °F) colder than it naturally should be, contributed directly to the disappearance of the Murray Cod, Trout cod and Macquarie Perch from the Mitta Mitta River within the first few years of the start of the dam's existence. Cold water pollution caused by Dartmouth Dam is also considered to have contributed to the disappearance of the freshwater catfish from the upper-reaches of the Murray River.

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