Dam upgrade means water levels down 6 metres
COOROY'S new Chamber of Commerce board and members were treated to a presentation by Seqwater on the proposed Lake Macdonald dam upgrade on Tuesday.
Seqwater staff explained the timetable and reasons for the upgrade, including the threat from earthquakes that could destabilise the earthen walls near the spillway, described as "similar to Holland's dyke walls”, necessitating the work.
Surrounding regions have experienced four minor quakes in the last six years.
The lake's water level will be lowered by six metres next June to allow work on the spillway and surrounding sand-based banks fronting the water treatment plant, and Lake Macdonald Drive, which will require studies into the impact on marine plant life and species such as platypus and Mary River Cod.
The audience was told the spillway was not big enough to handle extreme events and would need to be expanded, and flooding could occur over the earthen banks resulting in disaster.
It is understood such a situation almost occurred during a flash flood in 2012.
Excavation of the lake's floor below the spillway will need to go down as far as 15-20 metres to find rock.
The lake's water supply, which is Noosa Shire's primary water source, will meanwhile be supplemented by the Northern Connection Pipeline and supplementary sources.
Seqwater said the lake's water supply could rise from 50 per cent to 100 per cent "with one good weekend of rain”.
It was not clear which route transport would take to supply the massive works, however Seqwater said truck movements through Cooroy was not a preferred option.
This left two options: from the Bruce Highway down Elm St into Lake Macdonald Drive, and Sivyers/Gumboil roads.
Given that material supplies, including for a concrete batching plant, will likely be stored at Camp Cooroora and the Mary River Cod hatchery land, it would seem the Sivyers/Gumboil option would seem more logical.
Former hinterland councillor Ray Kelly, in the audience, queried the need for the Camp Cooroora and Mary River Cod hatchery land to be taken over, including the potential for the hatchery's pond.
However Seqwater's position is that it will primarily use the Camp Cooroora land and only fill the ponds in if no other option is available.
Alternatives to upgrading the dam, including building a smaller version in situ, or a smaller version upstream, or a weir downstream were all considered but ruled out as unviable.