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 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 & Water Supply Commission of Victoria, construction commenced in 1973 and was completed in 1979 by Thiess Brothers, at a cost of A$179 million.^[1] 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.^[2] The core component materials of the wall include 10.5 million m³ (370 million cu ft) of rock, 0.8 million m³ (28 million cu ft) of filter material made from crushed quarried rock, and 2.8 million m³ (99 million cu ft) of earth for the core.^[3] The reservoir has a capacity of 3,856 GL (1 trillion US gal),^[4] or approximately 6.7 times the capacity of Sydney Harbour; and can release a maximum outflow of approximately 12 GL (3.2 billion US gal) per day in normal operation.^[3]

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-metre-long (260 ft) 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.^[3]

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.^[5]

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 megawatts (240,000 hp), the largest single installed hydroelectric turbine in Australia. It is owned and operated by AGL Energy.^[6]

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.^[7] The installation shifted about 2 m within the base of the 180-metre-high (590 ft) 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 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 40-metre-high (130 ft), 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.

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.^[8]

Pondage expansion

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 Mount Beauty, 40 kilometres (25 mi) away.^[6]

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 (50 °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.^[9]

Climate

Climate data for the region are sourced at the bottom of the dam wall, in a relatively sheltered spot at 365 metres (1,198 ft) above sea level. Although rainfall records span as far back as 1918, temperature records did not commence until 1975. Winter is twice as wet as summer, and occasionally it may snow.

See also

1. ^ "Nomination of Dartmouth Dam as a National Engineering Landmark" (PDF). Engineering Heritage Australia (Victoria). 1 June 2004. p. 6. Archived (PDF) from the original on 3 January 2020. Retrieved 3 January 2019.
2. ^ "Register of Large Dams in Australia". Dams information. The Australian National Committee on Large Dams Incorporated. 2010. Archived from the original (Excel (requires download)) on 12 December 2013. Retrieved 4 March 2014.
3. ^ ^{a b c} "Dartmouth Dam". Murray–Darling Basin Authority. Archived from the original on 12 February 2014. Retrieved 10 February 2014.
4. ^ "Dartmouth Dam". Murray Darling Basin Commission. Archived from the original on 8 September 2006. Retrieved 5 November 2006.
5. ^ "Backgrounder 4: Lake Dartmouth – Overview of Operation". Murray Darling Basin Commission. Archived from the original on 11 September 2006. Retrieved 5 November 2006.
6. ^ ^{a b} "Dartmouth Power Station". AGL Energy. Archived from the original on 18 September 2006. Retrieved 13 October 2006.
7. ^ SECV, "Report of Investigation into the cause of damage of Dartmouth Power Station on 2 May 1990", State Electricity Commission of Victoria, Power Grid and Distribution Department Publications, June 1990
8. ^ "Newsletter of Engineering Heritage" (PDF). The Institution of Engineers, Australia. November 2005. Archived from the original (PDF) on 3 March 2014. Retrieved 8 February 2012.
9. ^ NSW Cold Water Pollution Interagency Group (2012) Cold Water Pollution Strategy in NSW – report on the implementation of stage one, NSW Department of Primary Industries, a division of NSW Department of Trade and Investment, Regional Infrastructure and Services, Sydney, New South Wales, ISBN 9781742563107

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