Cotter Dam

The Cotter Dam is a concrete gravity and rockfill embankment dam across the Cotter River, located in the Australian Capital Territory, Australia. The impounded Cotter Reservoir is a supply source of potable water for the city of Canberra and its environs.

Cotter Dam
Cotter Dam, 2013
Location of the Cotter Dam in the ACT
LocationAustralian Capital Territory, Australia
Coordinates35.31955°S 148.93853°E / -35.31955; 148.93853
PurposePotable water supply
StatusOperational
Opening date1912 (1912); 2013 (2013)
Owner(s)ACTEW Corporation
Dam and spillways
Type of damGravity dam
ImpoundsCotter River
Height87 m (285 ft)
Length330 m (1,080 ft)
Elevation at crest550.8 m (1,807 ft) AHD
Dam volume380×10^3 m3 (13×10^6 cu ft)
Spillways1
Spillway typeUncontrolled
Spillway capacity5,670 m3/s (200,000 cu ft/s)
Reservoir
CreatesCotter Reservoir
Total capacity78 GL (1.7×1010 imp gal; 2.1×1010 US gal)
Catchment area192.387 km2 (74.281 sq mi) + 287.688 km2 (111.077 sq mi) in Corin and Bendora Dam Catchments
Surface area285 ha (700 acres)
Website
Cotter Dam at ACTEW Water

Original dam completed in 1915
The Cotter Dam in December 2005, surrounding country still showing the effects of the 2003 bushfires.

The original concrete gravity Cotter Dam was started in 1912 and finished by 1915 when the city of Canberra was being established. The height of the dam wall was raised to a height of 31 metres (102 ft) in 1951 in order to increase capacity of the reservoir. The 26-thousand-cubic-metre (920×10^3 cu ft) dam wall was 118 metres (387 ft) long and created a reservoir with a surface area of 500 thousand cubic metres (18×10^6 cu ft). The uncontrolled spillway was capable of discharging 850 cubic metres per second (30,000 cu ft/s). At that time, the top water level of the dam was 501 metres (1,644 ft) above sea level and the reservoir had a capacity of 3,856 megalitres (848,000,000 imp gal; 1.019×109 US gal).[1] A subsequent review in October 2006, using more accurate mapping methods, resulted in capacity being re-estimated downwards from the previous estimate of 4,700 megalitres (1.0×109 imp gal; 1.2×109 US gal). Additional galleries and drains were constructed between 1984 and 1986.[1] In order to supply the city with potable water, water from the reservoir was pumped to Mount Stromlo, and from there the water flowed by gravity to fill the city's reservoirs.

From the 1960s onwards, better quality water could be supplied without pumping using the newly completed Bendora and Corin dams, and Cotter Dam was only used when water was in short supply. However, in December 2004, ACTEW Corporation brought the dam back on line in response to the ongoing drought.[2]

Enlarged dam completed in 2013

Completed in 2013, the enlarged Cotter Dam comprises a new 87-metre (285 ft) high roller compacted concrete dam wall built downstream from the old 26-metre (85 ft) high dam wall, along with two auxiliary embankment dam walls along low-lying adjoining valleys. Constructed on rock foundations by Abigroup–John Holland joint venture, with engineering design by GHD, the main dam wall is 330 metres (1,080 ft) long, with the two rockfill embankments 340 metres (1,120 ft) and 300 metres (980 ft) long and 15 metres (49 ft) and 18 metres (59 ft) high respectively, both with internal earthen cores.[1] The enlarged dam walls increased the storage capacity of the Cotter Reservoir from the previous 3.9 gigalitres (860,000,000 imp gal; 1.0×109 US gal) to 78 gigalitres (1.7×1010 imp gal; 2.1×1010 US gal).[3]

The old dam wall remains, inundated by the water held behind the new dam wall and acting as a sediment trap for the new dam's intake tower. The old dam may only be visible in exceptional circumstances of drought. Completion was originally scheduled for the end of June 2011,[4] however construction was delayed until August 2013[5] due to heavy rainfalls in the summer of 2010/2011,[6] the discovery of an unexpectedly large rock seam at the site of the foundations in 2011,[7] and severe flooding in March 2012.[8] The uncontrolled spillway is capable of discharging 5,670 cubic metres per second (200,000 cu ft/s) with a high water level approximately 550.8 metres (1,807 ft) above sea level.[2]

Aerial view (June 2011)

Engineering heritage

The dam precinct received a Historic Engineering Marker from Engineers Australia as part of its Engineering Heritage Recognition Program.[9]

See also

References
  1. "Register of Large Dams in Australia" (Excel (requires download)). Dams information. Australian National Committee on Large Dams. 2010. Retrieved 4 April 2014.
  2. "Cotter Dam". ACTEW Water. ACTEW Corporation. 2011. Archived from the original on 14 April 2014. Retrieved 4 April 2014.
  3. "Enlarged Cotter Dam". ActewAGL. Archived from the original on 29 September 2013. Retrieved 20 October 2013.
  4. "Enlargement of the Cotter Reservoir - Development application". ACTEW Corporation. July 2009. Archived from the original on 19 April 2013. Retrieved 4 January 2013.
  5. "$405m dam operational, but more work to be done". The Canberra Times. Retrieved 6 September 2013.
  6. "Rain delays Enlarged Cotter Dam by three months" (Press release). ACTEW Corporation. 14 February 2011. Archived from the original on 17 October 2013. Retrieved 4 April 2014.
  7. "Enlarged Cotter Dam construction and schedule update". ACTEW Corporation. Archived from the original on 28 April 2013. Retrieved 4 January 2013.
  8. Kretowicz, Ewa; Downie, Graham (2 March 2012). "Cotter spills as deluge hits region". The Canberra Times. Retrieved 13 February 2013.
  9. "Cotter River Dam Precinct, 1918-". Engineers Australia. Retrieved 27 April 2020.

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