Dartmouth Dam

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 km 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 & 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 m 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 e6m3 of rock, 0.8 e6m3 of filter material made from crushed quarried rock, and 2.8 e6m3 of earth for the core. The reservoir has a capacity of 3856 GL, or approximately 6.7 times the capacity of Sydney Harbour; and can release a maximum outflow of approximately 12 GL per day in normal operation.

The crest of the uncontrolled spillway is 486 m and is approximately 92 m long. When full, flood flows spill over the crest and down an 80 m concrete chute. The water then returns to the river via an open rock cascade which gradually widens to 300 m at river level. Once Dartmouth Reservoir reaches 99% capacity, it is considered to be 'operationally full'. 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. 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 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, and commissioned in January 1981, Dartmouth Power Station has one Francis turbine-generator, with a generating capacity of 180 MW, the largest single installed hydroelectric turbine in Australia. It is owned and operated by AGL Energy.

Power station damage
On 2 May 1990, the 180 MW Francis turbine-generator running at full speed was instantaneously stopped by a foreign body left in the penstock following maintenance. The installation shifted about 2 m within the base of the 180 m earth and rock fill gravity dam wall of the 3,906 GL 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 a couple of small towns and a sparsely settled agricultural area in the relatively narrow 120 km Mitta Mitta valley below the dam. More significantly, it would have resulted in the over-topping and probable failure of the earthen walls of the 40 m, 3,038 GL 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.

In May 1990, the turbine casing and concrete machine block surrounding the power station were destroyed, 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. The inflow from an unusually wet spring meant 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 was quarried from the valley walls. The station was re-built and recommissioned in 1993.

Pondage expansion
In 2003, the capacity of the regulating pondage was increased to further optimise the power station's generation flexibility. The station is connected to the electricity grid via a 220 kV transmission line to Mount Beauty, 40 km away.

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 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.

Climate
Climate data for the region are sourced at the bottom of the dam wall, in a relatively sheltered spot at 365 m above sea level. Rainfall records began in 1918. Temperature records began in 1975. Winter is twice as wet as summer, and occasionally it may snow.