Managing floods at Hume Dam

Information for downstream floodplain communities

Hume Dam, located just upstream of Albury/Wodonga, is the main river regulating structure on the River Murray. The dam forms Hume Reservoir and plays a crucial role in managing flows and securing water supplies along the entire River Murray system, including to Adelaide.

As well as fulfilling its primary water conservation role, Hume Reservoir provides additional benefits including flood mitigation, recreation, tourism and hydro power generation.

The River Murray operating assets (including Hume Dam) are controlled by a joint venture comprising the Australian Government and the states of NSW, Victoria and South Australia. These four asset-controlling governments exercise responsibilities for high level decision making in relation to River Murray System operations. The Murray–Darling Basin Authority directs the operation of these structures as agent of the joint venture. The information below describes the operating approach used at Hume Dam during floods and gives a summary of communications provided and the role of the various agencies involved when flooding takes place. It represents contemporary practice at the date of publication (September 2016).

Hume Dam spilling 40,000 ML/day. Photo by Tony Crawford.

Flood management priorities

Hume Dam’s primary purpose is water conservation with water storage being maximised during wetter periods as insurance against future drier times. The four asset controlling governments require the MDBA to manage floods to meet the following objectives (in priority order):

1. protect the structural integrity and safety of the dam; then
2. maximise water availability (i.e. fill the storage to at least 99% of capacity prior to any ensuing drawdown to meet downstream needs); and then
3. limit flood damage to downstream communities and increase benefits to the environment and public amenity

Storing floods

When the storage level is low, Hume Dam provides a high degree of flood protection to downstream communities as it can store large floods flowing in from upstream. Since the commissioning of Dartmouth Dam in 1979, about 70% of flood events in the upstream catchment have been completely stored in Hume Reservoir, with no flooding immediately downstream.

Passing floods downstream

In wetter years, operations are managed to increase Hume Reservoir towards Full Supply Level with the aim of filling the storage to 99% by the time downstream demand exceeds inflow. However, once the storage is close to or at Full Supply Level, it has very limited capacity to mitigate flood events. Unlike some other dams, Hume Dam has practically no surcharge capacity meaning it cannot store water above the Full Supply Level. Floodwaters entering from upstream must therefore be released with only limited reduction in flood peak height possible. (See Figure 1).

Figure 1: Example of how 2 large floods were captured in Hume Reservoir when the storage level was low before a moderate flood was passed downstream with limited peak mitigation when it was near full. Once at full supply level, dam releases must be closely matched to inflows to prevent the reservoir rising further (Sep-Dec 2010).

Snowy Hydro inflows

During upper Murray flood events, Snowy Hydro Ltd continues to release water via Murray 1 Power Station into the Swampy Plains River, downstream of Khancoban Dam. The releases are necessary to pass inflows into the Snowy Scheme from upper Murray catchments. Whenever the flow in the Swampy Plains River downstream of Khancoban Dam exceeds channel capacity (approximately 9,750 ML/d) the release is limited to the calculated ‘natural’ flow that would have occurred from the same rain event prior to the development of the Snowy Scheme.

Further downstream inflows from upstream of Tooma Dam only reach the River Murray once Tooma Dam spills. In most smaller inflow events, inflows from upstream of Tooma Dam are diverted to the Murrumbidgee River via Tumut Pond Dam.

As a consequence of these operating arrangements the Snowy Hydro Scheme releases do not exceed ‘natural’ inflows during upper Murray flood events.

Airspace management

Airspace is the difference between the actual volume of water in storage and the volume when full. The airspace is used to capture a proportion of the peak inflow during a flood event; with the degree of downstream flood reduction dependent on the volume of airspace prior to the event and the volume of the event. Active airspace management prior to a major rain event can assist in providing some flood mitigation, even if the storage is close to full.

Airspace targets at Hume Reservoir are determined by MDBA dam operators to provide a measure of flood protection without jeopardizing the ability to fill the reservoir. The airspace target at Hume Reservoir takes account of the storage level, catchment conditions, rainfall and inflow forecasts, and likely water demands during the weeks and months ahead. The airspace target is regularly reviewed by operators as conditions and forecasts change. A key factor in determining the airspace target is that there must be a very high chance that the reservoir will be full when downstream demand recommences, even if conditions suddenly turn very dry.

Key goals of airspace management are:

• preserve airspace during small inflow events to help reduce the impact of future larger floods
• use airspace carefully, taking account of Bureau of Meteorology short and long range forecasts
• recover any used airspace as soon as reasonably practicable (aiming for about 1 week)
• ensure that the reservoir is full by the time downstream demand exceeds inflow.

Passing smaller events to maintain airspace

When storage levels are high, the airspace target is constantly reviewed and smaller inflow events will be passed, with only limited mitigation, to preserve airspace. If further rain is forecast in the short term, releases may be increased to match inflows even if downstream channel capacity is exceeded. This approach means that airspace is maintained to provide some ability to reduce any future larger flood event that may quickly follow. An extra benefit of passing smaller floods through the reservoir is a more natural pattern of flow downstream to improve river health and floodplain environments.

Improvements to airspace management

In recent years, airspace and release management has been enhanced by ongoing improvements to Bureau of Meteorology rain forecasting. More accurate, detailed and accessible rainfall forecasts have improved the ability of dam operators to assess future inflows.

Dam operators can now anticipate the need to vary releases earlier with greater confidence than was previously possible. However, uncertainty in rainfall forecasts cannot be eliminated.

Large downstream floods will occur again

Communities should not be complacent about floods downstream of Hume Dam. When the storage is near full, large flood inflows must be passed downstream as they arrive. This means that moderate to large floods below the dam will occur from time to time in the future, although less frequently than they did prior to the dam’s construction.

The largest flood recorded downstream of Hume Reservoir since dam completion in 1936 was in October 1975. Major flooding occurred when the river height reached 5.69 m at the Albury gauge. It is important to note that this flood was only 20 cm below the highest recorded flood level from before the dam’s completion of 5.89 m in 1870. From time to time, damaging floods have occurred and will occur again, with floods even larger than the 1975 and 1870 events certain to occur at some future date

Communications and roles during flood operations at Hume Dam

Key operational and communication roles during times of flood and high releases are outlined below.

Bureau of Meteorology

• provides flood watches and flood warnings.
• provides rainfall and river level forecasts and information.

Murray–Darling Basin Authority

• directs dam operations including flood operations and release decisions.
• liaises with relevant agencies to provide advice on releases, planned changes and other matters.

WaterNSW

• operates Hume Dam acting on direction from MDBA to make necessary water releases.
• Manages an early warning network (EWN) notification system for the public

NSW SES

• responsible for the emergency management of floods in NSW including the coordination of evacuations and flood rescues
• plans for floods and educates people about how to minimise the risks to themselves and their property
• provides flood information, safety advice and can arrange for the delivery of essential supplies to communities isolated by floodwater

Victoria SES

• responsible for the emergency management of flooding in Victoria.
• in conjunction with Victoria Police, undertakes evacuations, searches and rescue from land and water.
• plans for floods and educates the community about how to be prepared

NSW Police

• assists in distributing emergency warnings and provides a coordinator for larger flood events to ensure all agencies work together and arrange resources as required.
• in conjunction with NSW SES, responsible for evacuations, searches, rescue from land and water and registration of evacuees.

Victoria Police

• assists in distributing emergency warnings and provides a coordinator to ensure all agencies work together and arrange resources as required
• in conjunction with Victoria SES, responsible for evacuations, searches, rescue from land and water and registration of evacuees

Check your local radio station for flood warning updates.

• ABC Local: FM 106.5 or AM 675
• 2AY: AM 1494
• HitFM: FM 104.9
• Triple M: FM 105.7
• Edge FM: FM 102.5

If evacuation becomes necessary, the SES and other emergency services will door knock to advise residents and businesses and provide information on what to do.