Managing floods at Dartmouth Dam

Information for downstream floodplain communities

The operations and assets of the River Murray system, including Dartmouth Dam, are controlled by a joint venture made up of the Australian, New South Wales, Victorian and South Australian governments. These 4 governments are responsible for high-level decisions and have appointed the Murray–Darling Basin Authority to direct daily operations and manage infrastructure.

The information below describes the operating approach used at Dartmouth Dam during floods and gives a summary of communications provided and the role of the various agencies involved when flooding takes place.

Dartmouth Dam. Photo by Michael Bell.

Flood management priorities

Under the Murray–Darling Basin Agreement,  the River Murray system, is managed primarily to conserve water and ensure supply. Dartmouth Dam, because of its size, configuration and location on the Mitta Mitta River, also plays an important role in the reduction of floods downstream, even though it was not specifically designed to provide a flood management function.

The 4 governments require that we manage floods to meet the following objectives (in priority order):

1. protect the structural integrity and safety of the dam
2. maximise water availability (ie. fill the storage to at least 99% of capacity if inflows are sufficient to do so)
3. limit flood damage to downstream communities and increase benefits to the environment and public amenity.

Reservoir and flood characteristics

The capacity of Dartmouth Dam is large relative to its catchment size. Storage levels therefore respond slowly to rain and inflow in comparison to most other dams. This means longer lead times for planning and managing releases.

There have been droughts when the storage level has remained low for extended periods. However, during wetter periods the storage can stay high for years at a time. Flood management at Dartmouth Dam typically occurs for a few months each decade, within wetter periods when the storage level is high.

There have been only 4 years when water has flowed over the Dartmouth spillway since its completion in the late 1970s. 

The vast majority of flood inflows are contained within the reservoir, or managed through regulated (non-spillway) releases.

Since its construction, Dartmouth Dam has provided significant downstream flood protection. The largest flood recorded in the Mitta Mitta River downstream of Dartmouth Reservoir since dam completion was in October 1996. Moderate flooding occurred when the river height reached 5.12m at the Tallandoon gauge. Before the dam was completed (and since reliable measurements began) there were at least 20 occasions when higher flood levels were reached, with peaks in excess of six metres recorded in 1934 and 1955.

Coordinating operations with Hume Dam

Any water released from Dartmouth Dam directly affects Hume Reservoir. It takes about 1 day for water released (or spilled) from Dartmouth Dam to flow along the lower Mitta Mitta River and into Hume Reservoir. The close proximity and connection between the 2 dams means a high degree of coordination is required, particularly when volumes are at or near capacity.

Dartmouth and Hume Dams are operated based on the overall chance of spill and to share flood protection downstream of both dams. Releases from Dartmouth Dam, when water storages are high, are called harmony transfers.  Harmony transfers normally occur before the flood season to manage the relative airspace in the 2 reservoirs. (Airspace is the difference between the volume of water in storage and the volume when full and is important in the management of floods).

For example, keeping levels in Dartmouth Reservoir high, when the level in Hume Reservoir is low, unnecessarily increases flood risks in the lower Mitta Mitta valley. Therefore, we would release water (as a harmony transfer) to Hume Reservoir to increase airspace in Dartmouth Reservoir.

Harmony transfer decisions are guided by hydrological computer modelling and must be anticipated and carefully planned, as the rate of transfer is limited by the channel capacity in the lower Mitta Mitta River. These transfers are generally made in a flow pattern that benefits the riverine environment and normally take place when the modelled risk of a future spill at Dartmouth Dam is greater than at Hume Dam.

Active airspace management

Once the water level in Dartmouth Reservoir is almost at full supply level and forecast inflows indicate it would exceed 99% capacity, the reservoir is considered to be ‘effectively spilling’ and actively managing the airspace becomes a priority focus.

While effectively spilling, releases continue to pass through the power station and the normal outlet works, and are carefully managed to prevent flow rates in the lower Mitta Mitta River from exceeding downstream channel capacity whenever possible. As inflows vary, releases are adjusted to meet airspace targets.

Release and airspace decisions are also informed by Bureau of Meteorology rainfall forecasts, inflows downstream of the dam, catchment wetness and expected demand for water.

Active airspace management reduces the probability of downstream flooding, maintains operating flexibility for hydro-electric power generation, and prevents spillway erosion; while ensuring that water storage will be maximised as conditions dry.

Using the spillway

If high inflows or upstream flooding occur, or are forecast, the airspace priority is usually at Hume Reservoir and flood operations would generally be underway at both dams.  Releases from Dartmouth Dam may be decreased to delay inflows into Hume Reservoir, and the Dartmouth Reservoir level may begin rising.

If Full Supply Level (FSL), 100% capacity, is reached at Dartmouth Reservoir, water will begin flowing over the spillway and the reservoir will be ‘physically spilling’. However, because the spillway’s outflow capacity is restricted by its size and shape, some flood inflows are temporarily stored – or ‘surcharged’ – while the reservoir level rises above FSL.

The surcharged water passes over the spillway crest at a much lower peak rate than it entered the reservoir, but for a longer period of time (see Figure 1). As a result, flood peaks downstream of Dartmouth Dam can be decreased by around 40% or more (see Figure 2).

Figure 1: Schematic representation of Dartmouth Reservoir 'surcharging' water during a physical spill. The maximum flow rate of water passing over the spillway is considerably lower than the peak inflow rate, as water is temporarily stored above the full supply level

Passing water over the spillway benefits the lower Mitta Mitta River valley by decreasing the downstream flood peak, but also assists flood management at Hume Reservoir – where surcharging capacity is very limited – by decreasing the peak inflow and delaying the arrival of flood water from the Mitta Mitta River.

Figure 2: A graph of Dartmouth Reservoir inflow, outflow and storage volume during the 1996 flood event, showing the process of downstream flood peak reduction during storage surcharge.

Downstream floods will occur again

Although the construction and operation of Dartmouth Dam has reduced the frequency and peak heights of floods in the lower Mitta Mitta River, communities should not be complacent about future floods.   Downstream flooding will continue to occur when a large flood occurs upstream and the reservoir is full

Dartmouth Dam design

• Reservoir capacity – 3,856 GL
• Full supply level – 486 m AHD
• Spillway crest height – 486 m AHD
• Top of embankment height – 494 m AHD

Tallandoon (downstream gauge) flood levels

Communications and roles during floods at Dartmouth Dam

The operational and communication roles that occur during times of flood and high releases are:

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 water release decisions and the co-ordination of flood operations with Hume Dam
• determines the flow target for the lower Mitta Mitta River (measured at Colemans gauge) when the dam is not physically spilling water
• liaises with relevant agencies to provide advice on releases, planned operational changes and other matters.

Goulburn–Murray Water

• operates Dartmouth Dam acting on direction from the MDBA, to make the necessary water releases
• instructs AGL Hydro to set the required flow into the lower Mitta Mitta River (measured at Colemans gauge).

AGL Hydro

• operates Dartmouth Dam power station, the Banimboola Regulating Pondage and the Banimboola power station, ensuring that the flow of water into the lower Mitta Mitta River meets the required flow.

Victoria State Emergency Service (SES)

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

Victoria Police

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

Check your local radio station for flood warning updates

• ABC Local – FM 106.5/AM 720
• 2AY – AM 1494
• Star FM – FM 104.9
• The River – FM 105.7
• Edge FM – FM 102.1.