# Cotter Dam

The Cotter Dam is a concrete gravity and rockfill embankment dam across the Cotter River, located in the Australian Capital Territory, Australia. Both the dam and river are named after early settler in the area Garrett Cotter. The impounded Cotter Reservoir is a supply source of potable water for the city of Canberra and its environs.

## Original dam completed in 1915

The original concrete gravity Cotter Dam was designed by Harry Gustav Connell (The Department of Home Affairs supervising engineer in Canberra from 1912 to 1916). The construction 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; 1.019×10$^9$ US gal). 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 (1.0×10$^9$ imp gal; 1.2×10$^9$ US gal). Additional galleries and drains were constructed between 1984 and 1986. 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.

## 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 an 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. The enlarged dam walls increased the storage capacity of the Cotter Reservoir from the previous 3.9 gigalitres (860,000,000 imp gal; 1.0×10$^9$ US gal) to 78 gigalitres (1.7×10$^{10}$ imp gal; 2.1×10$^{10}$ US gal).
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, however construction was delayed until August 2013 due to heavy rainfalls in the summer of 2010/2011, the discovery of an unexpectedly large rock seam at the site of the foundations in 2011, and severe flooding in March 2012. 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.

## Engineering heritage

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