Enlarged Cotter Dam

An enlarged Cotter Dam is being built downstream of the existing one to increase the Cotter Reservoir’s capacity from the current 4 gigalitres (GL) to around 78 GL – almost 20 times its current size. ACTEW Corporation, working with ActewAGL, formed an alliance in 2008 with GHD, Abigroup and John Holland to design and construct the dam. The Enlarged Cotter Dam forms part of ACTEW’s continued response to ensuring a secure water supply for the ACT and to address drought, climate change and variability.

Construction is well underway and completion is expected in mid 2012.

Construction progress

Since August 2011, when pouring of Roller Compacted Concrete (RCC) to construct the new dam began, over 175,000 cubic metres (m3) of RCC has been placed. This takes the new wall to around 38m in height.

Good progress has been made on the intake tower, which is the structure that enables water to be drawn from the reservoir.  To intake tower has now reached it's full height of 65m and the internal fitout and valve installation continues. The intake tower will have seven intake locations. Each intake location is 7.5m apart going up the tower and this allows water to be drawn from the reservoir at varying depths with different temperatures and oxygen levels.

Weather remains the biggest risk to construction and the recent rainfall has already had an impact on the site and expected schedule for the completion of the dam. If there is too much rain then the placement of the RCC must stop, and if there is a risk of flooding then we also need to be sure to remove as much equipment as possible from the flood zone. This of course results in lost time, not just from stopping the concrete placement but also from moving in and out all of the equipment.

But the clean up following a big rainfall event, like the ones we experienced over the last month, has the greatest impact. Even after the sun comes out our team needs to spend several days cleaning up the site and preparing the surface of the new wall so we can start placing concrete again. So although the rainfall is great for keeping Canberra green, it can have a very significant impact on the progress of the Enlarged Cotter Dam.

Placing Roller Compacted Concrete

The Roller Compacted Concrete (RCC) mix is prepared at the twin batch plants and then moved via a specialised conveyor belt system down the right hand abutment. This is loaded onto trucks and is then placed and compacted in continouis layers to construct the new wall.

The placement of RCC requires the continuation of construction work 24 hours a day, 7 days a week, until the dam is complete. Our excavation, preparation works as well as RCC methodology and placement are overseen by expert geotechnical engineers and renowned dam safety experts.  Approximately 1,000,000 tonnes of concrete will be placed to construct the dam, making it the highest dam of its type in Australia.

What else has been happening?

Cotter Avenue

Due to its proximity to the construction site, Cotter Avenue remains closed until completion of the new dam. The area will also be rehabilitated, upgraded and reopened once construction is complete. An important part of the Cotter Avenue history was the original swing bridge constructed in the early 1900’s to provide access across the Cotter River. Sadly, the bridge was destroyed in the 2003 bushfires. The replacement of this historically significant bridge will form a key element in the reinstatement of Cotter Avenue, and will provide access across Cotter River to the Cotter Dam Discovery Trail. Work on this upgrade will begin in 2012.

View plans for the upgrade 

Cotter Dam Discovery Trail

The Cotter Dam Discovery Trail is the ideal location for an ‘up-close’ look at the work underway and as the new 80m high wall is constructed layer by layer. The trail commences near the new car park opposite Cotter Avenue and continues along the Cotter River and through the surrounding natural environment. The Trail is a 1.4km circuit and is lined with stories of Canberra’s water history and features a 16-metre wide, amphitheatre style viewing platform which provides direct views of the Cotter Dam construction work and is situated just 350 metres southeast of the new dam. 

Visitors and locals are also encouraged to enjoy nearby Casuarina Sands which was upgraded to provide an alternate recreation area, with parking BBQs and shade, while Cotter Avenue is closed.

How will the new dam affect the environment?

One of the main advantages of the Enlarged Cotter Dam is that it will have relatively minimal impact on the environment. This is because it will be constructed in an area already impacted by the existing reservoir, on land that was badly affected by the 2003 fires and previous forestry plantations.

Protection of the environment is integral to the project. A range of environmental plans manage the potential impacts associated with construction activities. This includes many sediment basins onsite and a series of conservation and revegetation strategies, such as the careful relocation and future reintegration of Xanthorrhoea trees and hydro-seeding to stabilise the natural vegetation of disturbed areas.

The dam design also includes measures to protect fish populations, particularly to allow safe upstream migration of the endangered Macquarie Perch. Artificial rock reefs are being built to provide new habitat for the Macquarie Perch when the enlarged reservoir fills. More than 6km of rock reef has been constructed at carefully selected sites around the Cotter Reservoir.

More Information

Join our Friends of the Cotter community group to stay informed about projects taking place at the Cotter, and to share your memories of the Cotter Precinct with others. Sign up by clicking here. The ACTEW website also has daily time lapse images, live DamCam and YouTube footage of the dam construction as well as the Cotter Dam Education Kit. For more information on this or other water security projects, please contact the information line on 02 6248 3563 or email watersecurity@actew.com.au