Rain delays Enlarged Cotter Dam by three months

February 14, 2011

Extreme wet weather and flooding has resulted in at least a three month delay to the Cotter Dam enlargement. ACTEW Corporation Managing Director Mark Sullivan said the project was due to be completed in December 2011 but the construction schedule has been adversely affected by multiple severe rainfall events in September to December 2010 and early in January 2011. More heavy rainfall could result in further delays.

Mr Sullivan said: "The extremely heavy rainfall we have experienced over spring and summer has hit the Cotter particularly hard".

The bridge providing access to the Cotter Dam Discovery Trail was destroyed.
The bridge providing access to the Cotter Dam Discovery Trail was destroyed.

"On seven occasions areas in the Cotter had to be closed to the public due to flooding and there have been four separate flood events since September 2010 which caused significant damage to the worksite and the Precinct, including severe damage to the Cotter Dam Discovery Trail".

Mr Sullivan said the biggest impact on the construction schedule related to excavation work which had been delayed since the existing 4GL Cotter Dam reached capacity.

"The amount of rain we have experienced has made excavation works downstream in the valley where the new wall will be positioned quite difficult and at times impossible - work in the valley has had to be halted on multiple occasions over this time to ensure safety of our workers," said Mr Sullivan.

Construction was halted in the valley as water gushed over Cotter Dam on 9 December 2010.
Construction was halted in the valley as water gushed over Cotter Dam on 9 December 2010.


Constructions works continue in early February 2011 in the valley where the new dam will be built.
Constructions works continue in early February 2011 in the valley where the new dam will be built.


As a result of increased flood risk the existing diversion arrangement within the Cotter River which diverts water away from the construction area where the new dam will be positioned has also required a reassessment and redesign.

"The diversion was designed to manage conditions which were expected to be much drier and more like the weather conditions of the past decade. However, the prolonged wet weather and almost constantly overflowing dams have forced a major redesign of the diversion works," Mr Sullivan said.

The new diversion measures will be much more significant and include construction of a large culvert in the base of the new dam to better manage flood risk. The constructors are also adjusting methodology to manage the foundation excavations and concreting in segments which, while not as productive, involves less exposure to flood events.

Mr Sullivan said the three month delay would not have cost implications due to cost savings made by ACTEW's alliance partners Abigroup, GHD and John Holland and the use of contingency which was allowed in the budget for unforeseen events such as this. "ACTEW and its Alliance partners continue to seek to minimise delays and to manage and contain costs on the Enlarged Cotter Dam," he said.

The Cotter catchment is now very wet with all three dams upstream of the works at 100 per cent capacity, meaning there is very little opportunity to manage even a modest rainfall event without flooding occurring.
Debris caught at the top of Cotter Dam as it continues to overflow.
Debris caught at the top of Cotter Dam as it continues to overflow.

"The next four to five months are critical as excavation work in the valley where the new dam will be positioned will be taking place. This area is especially prone to flooding so if we see more heavy rainfall events in this period then further delays are highly likely," he said.

Mr Sullivan said reports the Queensland recovery effort will threaten the Cotter Dam enlargement were overstated and impacts are likely to be manageable. He said: "Work on our new dam is more than a year underway with labour contracts in place and not only ACTEW Corporation but our partners ActewAGL, GHD, Abigroup and John Holland committed to delivering the project by autumn next year".

"Construction projects starting now will be highly impacted, but that is not the case for us. Our thoughts are with Queenslanders at this difficult time and we will happily work through any limitations resulting from the recent devastation to see the rebuild happen as quickly as possible," said Mr Sullivan.

The Enlarged Cotter Dam project involves enlarging the Cotter Dam by almost 20 times its current size from 4-78GL. It is one of three important water security projects ACTEW and its alliance partners are delivering to help provide a secure future water supply for the ACT region. Despite the wet weather, significant progress has been made on the project with one of two saddle dams completed, the successful installation and operation of a rock crushing plant and batch plant, which produces the concrete for the dam, and the majority of excavation works complete.