South East Queensland dams were built to high safety standards, but most were built some decades ago. Methodologies and data used to estimate extreme rainfall events continue to improve and industry engineering standards and understanding of risks can progress over time. 

Across Queensland and Australia, there are multiple dam upgrades occurring to meet modern engineering standards. 

Seqwater’s Dam Improvement Program is ensuring the region’s dams comply with the latest safety standards and continue to function safely during extreme weather events in line with regulatory requirements.  

Since 2008, Seqwater has delivered 13 upgrades, including Leslie Harrison Dam at Capalaba, Sideling Creek Dam north of Brisbane and Ewen Maddock Dam on the Sunshine Coast.  

Planning work is underway for the next tranche of upgrades at Somerset, Wivenhoe and North Pine dams. Lake Macdonald Dam site establishment works are underway.

Seqwater undertakes regular monitoring and engineering assessment to make sure the region’s dams continue to operate safely. 

Lake Macdonald – also known as Six Mile Creek Dam – was originally constructed in 1965. 

There have been significant advances in dam design since the dam was built, as well as new ways of estimating extreme rainfall and flood events, and a greater understanding of the size and severity of earthquakes.

This will be the first major upgrade of the dam since the wall was raised in 1980. The Lake Macdonald Dam Improvement Project aims to:

•    increase the spillway capacity to safely pass floods 
•    protect the dam wall in extreme flood events
•    efficiently control water flowing out of the dam 
•    reduce risks to the dam structure during earthquakes
•    meet modern design and engineering standards
•    comply with dam safety guidelines.

The new dam will occupy the same footprint as the current structure, but the spillway will be a split-level concrete spillway. The primary ‘ogee’ spillway is set at the original lake level, and an upper labyrinth spillway will be constructed for large floods. No spillway gates are included in the new structure; and the earth embankments will be reconstructed to their original level on either side of the primary spillway. 

The primary spillway is a flat concrete structure, similar to the current dam wall.   

The labyrinth spillway has a zigzag shape when viewed from above and is made from concrete. In the event of a very large inflow, the water will flow over the upper labyrinth spillway and into Six Mile Creek downstream.  

The current earth fill embankment on the Lake Macdonald Drive side of the spillway does not meet today’s design and safety standards – that is, it is too steep and at risk of erosion. The new embankment at this location will be the same size and height of the existing embankment and finished grass and rocks.  

The right earth embankment on the other side of the dam's spillway, closest to the water treatment plant, will also be re-constructed and finished with grass (looking much the same as it does now).   

Rocks will be placed downstream of the new spillway, called scour protection, and will assist in preventing erosion.  

An outlet tower will be constructed to the left side of the spillway which will have mechanical pipework and electrical components to facilitate both emergency release of water, to lower the dam reservoir, and environmental flows to the downstream Six Mile Creek.  

A temporary cofferdam structure is, engineered to manage inflows after each rain event. This structure will hold back the water in Lake Macdonald during construction of the new dam wall. This is a temporary measure for dam safety and once the new dam wall is complete, the cofferdam will be removed, and Lake Macdonald Dam will be able to naturally refill to its original full supply level.  

We expect the project to take around five years to complete, subject to weather and construction conditions.  

As part of the planning and delivery, Seqwater has undertaken extensive environmental investigations by way of both desktop searches and field surveys focusing on protected plants, important habitats for plants and animals, waterway environments, and significant local species. 

This has helped to inform reduced water levels, locations to avoid around Lake Macdonald, where possible, as well as inform any mitigations needed to be in place during construction. 

Lake lowering and construction works will have some impacts on the flora and fauna, both in and around the lake, and Seqwater is committed to minimising and mitigating these impacts. A range of surveys and studies have been completed to identify the species of aquatic fauna, including those of conservation significance in the lake. An environmental management plan will be in place during construction, which outlines the measures required to minimise the impact of the natural environment.  

Click here for the link to the Coordinator-General’s evaluation reports on this project.

Investigations are underway into the measures needed to manage aquatic fauna at the 42% or 93m reduced water level. Seqwater will develop a recovery and relocation plan, in consultation with key stakeholders, to manage any aquatic fauna impacted during the lowering and throughout construction. 

It is expected there will be a period when the habitat available for aquatic species, including those of conservation significance such as the Mary River cod, Australian lungfish, Mary River turtle, white-throated snapping turtle, and platypus, is limited. To minimise the impacts to these important species, Seqwater is proposing an initial aquatic fauna relocation program.  

We will aim to relocate species of conservation significance before construction begins. Some fish are sensitive to handling or changes in water temperature and may suffer injury or mortality if relocated. Therefore, appropriate measures will need to be identified and incorporated into the management approach. 

All relocation will be delivered by expert fish biologists, via a purpose-built fish transport vehicle to Cooloolabin Dam.

Lake Macdonald is expected to remain open to all on-water recreation activities during the construction period including motorless boating, canoeing, and fishing. These activities will be permitted outside of the construction zone, which the cofferdam will isolate. We expect there will be areas around the lake for shoreline fishing. Once the upgrade is complete, Lake Macdonald will continue to be used for water supply as part of the SEQ Water Grid, and the lake will remain open to the public for shared recreation.  

Heavy vehicles will use Elm Street and Lake Macdonald Drive. A traffic management plan will be implemented, including traffic control as required to manage the safe flow of traffic, on the affected roads. More information on project impacts, including traffic, noise and vibration, will be provided prior to construction commencing. 

During construction there may be partial road closures and temporary changes to traffic conditions at times for the safety of road users. We will provide advance notification of any road closures, including details of any detours. 

The project team will be in contact with residents who are directly affected throughout the project to advise of noise impacts in advance. Every effort will be made to minimise impacts on the local community and environment while this work is undertaken. We thank the community in advance for your patience and understanding while these necessary works are carried out. 

Construction activities will include, but are not limited to, demolition works, heavy vehicle movement, concrete batching and pouring, rock breaking, drilling, piling, excavation and stockpiling of raw materials. 

There will be times of weekend and nightworks for time critical construction activities, which we will be sure to notify the local community in advance.

Seqwater has been able to work through a new construction approach and Lake Macdonald is expected to remain open to on-water recreation, and parks and facilities recreation throughout the project. This means that both Lake Macdonald and Lake Macdonald Park (off Lake Macdonald Drive), and the Noosa Botanic Gardens will remain open.

There will be closures to the scouts campground, the Mary River Cod Park and the Mary River Cod Park boat ramp, the fishing platform on Lake Macdonald Drive, and walking, riding and horse trails.

To maintain local water security during the construction of the project, a minimum Full Supply Volume of 42% will be retained for Lake Macdonald Dam (which equates to a level of 93m AHD), and the Noosa Water Treatment Plant will continue to operate. 

To the community, this will look approximately two (2) metres lower than the dam's full supply level. 

This is the level the lake will sit at throughout construction unless dry conditions create a naturally lower level.

All Seqwater dams within the Dam Improvement Program continue to operate safely. Dam improvements are undertaken to meet modern guidelines for these life-long assets – many of which were built some years ago. 

Seqwater undertakes regular monitoring and engineering assessments to ensure all our dams continue to operate safely. 

Dams in Queensland were built to high, internationally accepted safety standards. However, the methodologies and data used to estimate extreme weather events and flooding impacts have significantly improved, and engineering standards and risks have progressed. 

All these factors are considered as part of Seqwater’s Dam Improvement Program so our dams continue to operate as they should, now and into the future.

In line with the water license for Six Mile Creek, Seqwater manages continuous environmental releases from Lake Macdonald to maintain a healthy water supply to help protect the habitat downstream.

Environmental releases, which will continue during the project, are adjusted based on a range of factors including rainfall, inflows, losses and releases to ensure we meet our release obligation.

Seqwater has ongoing long-term source water protection partnership programs with both the Mary River Catchment Coordinating Committee (MRCCC) and the Noosa District Landcare Group (NDLG). As part of these partnerships, Seqwater works with MRCCC and NDLG to improve water quality and broader catchment health.

Seqwater will continue taking steps to improve overall catchment resilience in the region for water supply and quality benefit, including during periods of low rainfall.  

Seqwater is committed to the responsible management of the environment, including caring for flora and fauna and complying with relevant legislation and environmental requirements. A Species Management Plan has been developed which addresses platypus preservation, including specific mitigation and management actions for each phase of the project. Measures include riparian vegetation management and considering the timing and volume of water releases to maintain flows in line with the natural creek flow regime for platypuses living downstream.

There is no existing fish passage (a fish ladder is a specific type of fish passage) at Lake Macdonald Dam, and no proposed fish passage included in the Lake Macdonald Dam Improvement Project. The reasons for this are:   

•  A fish passage could allow for pest species such as Tilapia to enter Lake Macdonald reservoir.  

• A more suitable location for a fish passage has been identified at the Gympie Weir. A fish passage in this location will allow fish to pass up and down the main Mary River, allowing passage between the upper tributaries and reaches. 

• The Coordinator-General, as part of the Project Approval Conditions, have stated that Seqwater upgrade the Gympie Weir to include a fish passage.  

The coffer dam will be constructed from sheet piles, tie-bars, structural steel members, rock bags, and graded rock fill. Cranes equipped with vibratory and impact hammers will drive the sheet piles into place. The same cranes will be used to install the tie-bars, structural members, and rock bags. Rock fill will be delivered by trucks and positioned using earth-moving equipment.

John Holland, the construction contractor responsible for sourcing materials from business suppliers and quarries, has not yet determined the specific sources for the construction materials. We will keep the community informed on this matter as it progresses.