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Lake Glenmaggie

History

The first stage of Glenmaggie was built between November 1919 and 1926 and held 132,000 ML when full.

After World War 2, extra water was required to enable returning soldiers to settle on land and farm within the district. This was achieved by raising the wall 3.6 metres, by placing 14 radial gates along the top of the original wall. This increased its capacity to 177,640 ML.

In 1987 the dam was strengthened to withstand major floods and earthquakes by drilling through the wall into the bedrock and installing 70 re-stressable steel cables. This effectively tied the wall to the ground to resist overturning.

Purpose

The lake harvests water for use in our gravity irrigation systems to properties in the Macalister Irrigation District, near the towns of Maffra, Heyfield and Stratford and the City of Sale.

The Glenmaggie dam wall, which creates the lake, is a mass concrete overfall dam with irrigation outlets on both sides of the river serving irrigation channels. The central portion is an overfall spillway.

The Macalister Irrigation District covers about 53,000 hectares around the Macalister and Thomson Rivers, extending from Lake Glenmaggie to Sale. It comprises two areas: the Maffra-Sale Irrigation Area to the north of the Thomson River and the Central Gippsland Area (including the Nambrok-Denison Soldier Settlement district) to the south.

To distribute the water from Lake Glenmaggie, three main channels have been constructed: the Main Northern (capacity 500 ML/d), the Main Southern (capacity 1,460 ML/d) and the Main Eastern (capacity 600 ML/d). The Main Southern is supplemented by flow from the Cowwarr Channel diverted from the Thomson River at Cowwarr Weir.

Statistics
RESERVOIR
Capacity 177,628 ML
Dead storage 4,860 ML
Surface area 1,760 Ha
Full supply level 77.1 m AHD
Catchment area 1,891 km2
EMBANKMENT
Maximum height 37.2 m
Length 296 m
SPILLWAY
Crest length 99.2 m
Discharge capacity * 300,000 ML per day

* Glenmaggie Dam was upgraded in 2003 to enable it to withstand overtopping.

Power generation
In 1994 Pacific Hydro installed a small generation station at Lake Glenmaggie with twin turbines, giving a total capacity of 3.8 megawatts, which supply power to the state network.

These are driven by releases to the Macalister River and Southern Channel, and provide an environmentally friendly source of power generation. The intake and offtake pipes into and out of the turbine house are clearly visible from the dam wall recreational area.

For more information please visit Pacific Hydro.

Location

Lake Glenmaggie is located on the Macalister River in Central Gippsland, Victoria – VicRoad Reference Map 82 E8
Glenmaggie wall cracks and joins

Why are there cracks in the weir wall?
When the dam was built in the 1920s, it was done in a series of interlocking concrete blocks. Over time cracks have appeared on the wall; these are a mix of actual joints between the blocks, and cracks which have opened up. The dam undergoes changes in stress over a year.

In summer, the concrete tends to expand and the blocks push against each other, which may cause cracking to occur to relieve the stress. In winter, the concrete will contract and these joints and cracks will open up and may leak water from the reservoir.

The reservoir level also impacts on the stresses. When it is full, the dam tends to move slightly downstream. When the level is low the dam will “relax” back in the upstream direction. This movement may also induce cracking.This behaviour is considered to be normal for concrete dams and is not a concern.

Why doesn’t Southern Rural Water repair them?
The cracks are not causing any concern in the operation of the reservoir, and any repair would be likely to simply open up again. This is because the dam is a massive structure and has its own movement patterns.

We can repair a crack, but we can’t significantly change the dam’s movement pattern. So the dam will move and the crack repair will open up again or a new crack will be created to relieve stress.

How long have they been there for?
This is uncertain, but most likely soon after construction. There are no recent cracks.

The dam has now been stressed in virtually all possible combinations so the cracks (which are effectively a means of stress relief) are well established.

How does Southern Rural Water monitor them?
The cracks are monitored in two ways.
For cracks which are larger and more visible, we regularly check the width of the crack. The width of the crack changes with different conditions (reservoir level, and temperature) but if we had an unexpectedly large reading that could not be explained by seasonal conditions, we would further investigate and possibly carry out repairs.

How does Lake Glenmaggie compare?
This illustration shows the catchment size and storage size of Lake Glenmaggie in comparison with the Thompson Dam in Victoria, and the Wivenhoe Dam in Queensland.

catchment_comparison_large_72dpi

Lake Glenmaggie managing floods

 

The Macalister Irrigation Area (MIA) is a vital part of Gippsland’s local economy. The dairy industry produces some 400,000,000 litres of milk, returns around $150 million at the farm gate and grosses approximately $500 million annually. The MIA also supports other important industries including vegetable and beef production and fodder cropping. It is estimated that these activities generate almost $650 million into the local economy. 

The success of the irrigation area requires a secure and reliable water resource. The primary source of water for the MIA is Lake Glenmaggie, which sources its water from the Macalister River catchment, located to the north of the storage. Lake Glenmaggie is relatively unique amongst large storages in Victoria, in that it fills and spills on average in nine out of ten years. As a comparison, the Thomson reservoir, Melbourne’s primary water source, has a catchment area only 25% of Lake Glenmaggie but has a storage capacity five times greater.

Lake Glenmaggie was built in the 1920s and its height was raised in the 1950s with the establishment of 14 flood gates. These gates can only be operated when the reservoir level reaches 119,174 ML, or about 70% of capacity.

Large river flows leading to floods are not uncommon on Gippsland rivers and they occur regularly. Flood severity is increased when bushfires reduce the vegetation in the catchment, as we experienced in the 2006/07 bushfires.

When floods occur, we have a number of objectives to meet – ensuring the safe operation of Glenmaggie Dam, retaining water for future irrigation and to manage the effects of flooding downstream. Lake Glenmaggie’s large catchment means that river flows can be quite extreme, so even when the reservoir is only partly full we can have limited ability to reduce the impact of floods downstream. At such times, we work with the emergency services agencies to help ensure that downstream communities are well informed.

Where the risk of a rainfall event is identified, we maintain close contact with the Bureau of Meteorology to understand what is expected and the maximum rainfall that is forecast. We also review different scenarios for the runoff and likely inflows from the expected and maximum rainfall. At this stage we also undertake the necessary staffing arrangements to run our operations 24 hours a day.

During a rainfall event we are constantly planning for releases from the storage. These are based on actual and expected rainfall levels, stream flow measurements taken upstream from Lake Glenmaggie and calculated inflows into Lake Glenmaggie, based on the actual storage level. Our objective is always to reduce the severity and duration of any flood. 

Leading up to and during flood events we work closely with the Bureau of Meteorology, State Emergency Service and other emergency services to provide regular updates on the current and projected situation, and to assist them with setting actions, such as warnings, public meetings and evacuations where required. We also support the Macalister flood warden system – set up for the community, by the community following the 1971 floods.

We strongly recommend that if you are in a flood prone area that you and your family should familiarise yourself with the SES’s “Flood Safe information brochure” and take all necessary precautions.

More information
For more information, contact us on 1300 139 510 or visit www.srw.com.au

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Southern Rural Water acknowledges and recognises Aboriginal people as the Traditional Owners and Custodians of the land and waters on which we work and live, and we respect their deep and ongoing connection to Country. For more about the First Nations peoples on whose Country we work, click here.