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.
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.
|Dead storage||4,860 ML|
|Surface area||1,760 Ha|
|Full supply level||77.1 m AHD|
|Catchment area||1,891 km2|
|Maximum height||37.2 m|
|Crest length||99.2 m|
|Discharge capacity *||300,000 ML per day|
A Bulk Entitlement Order is a set of operating rules for a reservoir.
At Southern Rural Water (SRW) we deal with the following Bulk Entitlement Orders:
- Yallourn Energy
Because more than one organisation or group can have shares (or entitlement) in the water being held in a reservoir, each shareholder must follow rules about:
- the volumes that can be taken from the reservoir, system or waterway
- costs of managing the system, and how they are shared
At a number of reservoirs, SRW acts as the storage manager on behalf of all shareholders.
As a storage manager, we:
- allocate water (both increases and decreases) according to the inflow and capacity shares held by shareholders
- ensure that we meet the rules for passing flows downstream for environmental purposes
- release water to meet a shareholder’s request
Passing flows are a vital part of our Bulk Entitlement Orders and are met before allocation requests are supplied.
Environmental Entitlement Orders spell out a whole pattern of environmental flows that are required downstream of a reservoir – from how long they last, to how frequently they flow and how large they are.
Environmental flows are designed to mimic the natural conditions of rivers. It is not just about the amount of water but also the timing and quality. In each order, environmental flow requirements are low during the summer and autumn, and are much higher during winter and spring.
Environmental flows are vital to supporting the river’s ecological processes. High flows provide triggers for fish breeding and supply water for fish passage, so that they are able to move up and down rivers to appropriate habitat. They also keep estuaries open and provide recreational opportunities. Low flows in summer maintain fish refuges and connect habitats. Spring floods regenerate wetlands and floodplains and replenish the river channel.
To make sure that we comply with the rules about passing environmental flows, we:
- perform random checks at points along rivers, to ensure that customers are taking only the amount of waterthey have ordered
- maintain a number of gauging stations with alarms that alert us of unusual changes in water levels.
For more information contact us on 1300 139 510 or visit www.srw.com.au.
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?
Lake Glenmaggie boating brochure
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.
For more information, contact us on 1300 139 510 or visit www.srw.com.au