Managing Water Storages

Releases are meeting normal operating requirements, including delivery of Broken IVT to the Goulburn system.

Cairn Curran: Releases are meeting normal operating requirements.

Tullaroop: Releases are meeting normal operating requirements.

Laanecoorie: Releases are meeting normal operating requirements.

Releases are meeting normal operating requirements.

Releases are meeting normal operating requirements, including delivery of water for the environmental water holder.

The spillway at Lake Eppalock is a fixed crest spillway, which means GMW is unable to release more water than the capacity of outlet valve of 1,600 ML/d. During spill events, downstream flows are governed by how high the storage level exceeds the full supply level.

While GMW storages are not specifically for flood mitigation, Lake Eppalock does provide some flood mitigation by storing water above the full supply level and water passing over the spillway.

For more information on how the October 2022 floods occurred across the Campaspe River, visit Floods in Focus: Campaspe River System 

Flow and release rates for the Campaspe River system

GMW has created a simple dashboard overview of flow and release rates for the Campaspe River system from Lake Eppalock to Rochester since October 2022.

The inflow rate at the storage includes flows from tributaries upstream of Lake Eppalock, such as the Coliban and Upper Campaspe rivers and McIvor and Wild Duck creeks.

As well as releases from Lake Eppalock, inflows from Mount Pleasant, Forest and Axe creeks contribute to flow rates along the Campaspe River downstream of the storage and Rochester. 

Click the button below to be taken to the dashboard.

Campaspe River system flow and release rates dashboard

*Actual releases from Goulburn Weir may vary from those forecast at any time without notice.

What do changes in river flows mean for river levels?

The table below provides a summary of the river levels at various flow rates as measured at the gauges on the Goulburn River at Murchison, Shepparton and McCoy Bridge.

Please note that the table below is specific for the river gauges at Murchison, Shepparton and McCoy Bridge and the relationship between flows and river level will vary along the river as the profile changes.

This information does not relate to absolute water depth in the river however the table provides an indication of what various flows mean for river levels. For example, if flows increased from 2,000 ML/day to 3,000 ML/day, the river level at Murchison would increase by around 0.5 m.

Please note: the table above provides an indication of the scale of typical river flows and flows may exceed 10,000 ML/day in response to rainfall in the catchment.

Yes. GMW operates dams with two different types of spillways, fixed crest spillways or gated spillways.

Fixed crest spillway dams generally have relatively small outlet works and operate on a ‘fill and spill’ basis. As inflows enter the dam, the water level in the dam increases until it reaches the spillway crest level where it will overflow into the downstream river channel. The fixed crest level is at the Full Supply Level (FSL) of the dam and is the point at which the dam is 100% of capacity. The embankment of the dam is designed to allow the water level to naturally surcharge above 100% of capacity and allow the spillway structure to ‘throttle’ outflow. Dams of this type provide some flood mitigation by attenuating the flood peak. There is no operational control from the dam operator in passing floods through these types of dams.

Gated spillway dams are configured with large gates in the spillway that enable water to be stored above the spillway crest level. As the dam’s FSL is above the spillway crest level, the gates must be closed to allow the dam to fill to 100% of capacity. During winter and spring, water is harvested in the dam by following a target filling curve that balances resource increases against a degree of flood mitigation. Flood water is passed through the spillway by opening the spillway gates. The embankments on GMW’s gated spillway dams are not designed to be surcharged in the same way fixed crest spillway dams operate. The flood operating procedures for gated spillways aim to pass inflows through the dam while maintaining the level within the dam at or below FSL. Generally the operation of gated spillway dams involves using pre-releases as any air space (the available volume between the lake level at the beginning of an event and FSL) to delay the onset of flooding and reduce the flood peak. The amount of flood mitigation that can be provided depends on the amount of air space available and the size of the flood. Only 3 of GMW’s 16 dams have gated spillways – Eildon, Buffalo and Cairn Curran.

GMW Flood Operations Policy, opens in a new window [80 KB]

A pre-release can manage the rate of a storage filling and be a valuable management tool to balance GMW customer’s water entitlement (that is stored in water storages) and downstream flood impacts. It’s a little like letting the plug out of a bath for a moment when it is nearly full.

GMW Flood Operations Policy, opens in a new window [80 KB]

Pre-releasing from Eildon is normal operating procedure for the dam and has been done in the past where the dam has been near full during winter and spring. Pre-releases are also made from our other gated spillway dams, Buffalo and Cairn Curran. Due to the design of the spillway, there is limited opportunity to pre-release from our other dams. Where possible GMW tries to maintain some "airspace" in all the dams during winter and spring.

The pre-release of water from any storage will not minimise the effects on downstream users, it provides a balance between resource harvesting and flood mitigation. If we receive a large inflow to the water storage, there is still the risk of flood downstream.

Unfortunately it’s not that simple. Water in a storage is owned by customers and a storage simply holds it for them until they are ready to use it. The level in the water storage during winter and spring is managed in accordance with rules and procedures that GMW must adhere to. The rules limit the opportunity for GMW to pre-release significant volumes of water for flood mitigation. There are also potential downstream impacts that have to be considered and communicated.

The impact of rain in a catchment is not an exact science – a small amount of rainfall can result in a large inflows and vice-versa. Significant drawdown of storage levels can only be made prior to a rainfall event once an accurate rainfall forecast is available which is generally 3-4 days in advance. This limits the volume of water which can be drawn down at short notice.

The primary role of a water storage is to harvest and store customers' water entitlements, GMW operates the dams to provide flood mitigation benefits where possible. Generally large water storages are not designed or operated specifically for flood mitigation

The amount of flood mitigation a water storage can provide depends on a number of factors - including the operating rules of the storage, the size of the flood event, the level of water in the dam at the beginning of the event and the release capacity of the spillway.

Water storages may provide significant mitigation for small floods and may significantly reduce downstream flooding. The amount of mitigation generally reduces as the size of the flood increases, so there may be little mitigation benefit for large floods.

There are four ways water can be released from Eildon.

Most releases from Eildon are made through the hydro power station operated by AGL. Water is taken in through the base of the outlet tower and passed through a large conduit under the embankment to the power station. Releases up to about 20,000 ML/d are made this way. You won’t see much evidence of the releases other than some turbulent flows downstream of the dam.

The other ways water is released is through a large ‘low level’ valve adjacent to the hydro station, large ‘irrigation’ valves in the spillway or through the spillway flood gates. Generally the low level and irrigation valves are only operated as a back-up when releases can not be made through the hydro station. Releases will generally only be made through the spillway gates during flood events when the volume of outflow exceeds the release capacity of the hydro station.

A spillway is a feature of a dam which is designed to pass excess flood water from the storage. Spillways are generally large concrete structures constructed beside or within the embankment. When the dams are full and inflows enter the dam the excess water is released either through the operational outlet works (for normal small flows) or over the spillway for larger flood flows.