Warragamba Dam Auxiliary Spillway
Planning for extreme floods
Warragamba Dam is the largest concrete dam in Australia. Since completion in 1960, regular reviews have consistently confirmed that the dam as structurally sound.
Warragamba Dam was designed and built to withstand floods with a one chance in 700 (1:700) of happening in any year. In fact, the largest flood on record in the Hawkesbury-Nepean River in 1867 had odds of just one in 200 (1:200).
In 1985, Sydney Water and a panel of international dam safety experts found that Warragamba Dam’s ability to safely handle rare and extreme floods was below modern dam safety standards. If a rare and extreme flood occurred, the volume and power of the floodwaters could have ‘overtopped’ the dam. Sections of the wall could have collapsed and released most of the stored water.
What would happen if the dam collapsed?
A collapse of Warragamba Dam would have critical social, environmental and economic consequences.
Prior to dambreak, the Hawkesbury-Nepean valley, including the population centres of Penrith, Emu Plains, Richmond and Windsor would experience very serious flooding. Dambreak would flood around 4,000 houses and irreparably damage 6,400 more, leaving more than 20,000 people homeless.
Severe water restrictions, unprecedented since the 1934-42 record drought, would be needed while the dam was rebuilt. This process would take around five years, with a further three to five years for the lake to refill.
Dambreak flooding would cause an estimated $4.5 billion damage.
Ensuring Warragamba’s safety
Warragamba Dam must meet current international dam safety standards. It must also satisfy the NSW Dams Safety Committee, the State’s regulator. Safety is a top priority because the dam supplies most of Sydney’s residents with drinking water.
Following the 1985 review of Warragamba Dam’s extreme flood capability, Sydney Water acted quickly to further improve the dam’s safety.
As an urgent first step, the dam wall was raised by five metres and strengthened by installing post-tensioned steel cables, effectively tying the upper part of the wall to its base. These measures cost $29 million and lifted the safety level from 50 per cent to 75 per cent of the required standard.
To better protect the dam, a second spillway was built to divert excess floodwaters around the dam. The spillway will reduce the pressure on the dam wall and protect it in the event of an extreme flood.
Begun in late 1998, the $150 million spillway was completed in June 2002. The auxiliary spillway ensures that Warragamba Dam is 100 percent compliant with required safety standards.
Long-term safety options
A range of further improvements to dam safety were considered before the auxiliary spillway option was selected, including:
- increasing flow through the existing spillway
- allowing a strengthened wall to be overtopped
- temporarily holding the excess flow upstream
- diverting the excess flow around the dam, and
- combinations of the above.
The various options were evaluated against the following criteria:
- effectiveness in reducing the risk of dam failure
- environmental impacts
- cost.
The plan to divert excess floodwaters around the dam through a second spillway, located on the east bank of the Warragamba Gorge, was selected as the preferred option.
The auxiliary spillway ensures that the dam is 100 percent compliant with required safety standards.
Spillway benefits
The Spillway:
- prevents dambreak in extreme floods
- protects Sydney’s major water supply
- reduces risk of dambreak flooding
- reduces potential flood levels at Windsor by up to four metres
- is the most practical and cost-effective way to increase flood handling capacity
- enables the dam to fully comply with modern dam safety standards
- prevents an estimated $4.5 billion in damages if the dam ever failed
- employed 175 construction workers during building
- created additional off-site employment to produce and supply construction materials and equipment.
How the auxiliary spillway works
Warragamba Dam’s existing crest gates and central spillway continue to manage the normal discharge of floodwaters.
In the event of an extreme flood, a series of erodible earth and clay walls or 'fuse plugs' built across the upstream opening of the auxiliary spillway would progressively wash away by the rising floodwaters. The auxiliary spillway would then divert excess floodwater around the dam. The 'flip bucket' at the downstream end of the auxiliary spillway manages the floodwaters entering the river at the point where they meet the flow from the central spillway. This minimises erosion of the riverbed and banks.
The auxiliary spillway operates only in the event of a flood with one chance in 750 of happening in any year. This is more than three and a half times less likely to occur than the record flood of 1867, which had one chance in 200.
Building the auxiliary spillway
The spillway was designed by the NSW Department of Commerce and built by Abigroup Contractors Pty. Ltd., a major Australian civil construction company. The project was managed by Australian Water Technologies Pty. Ltd.
Community and environmental concerns
Like any project of this size, building and operating the auxiliary spillway had some local impacts.
All likely impacts of the construction and operation of the auxiliary spillway, including measures to ease those effects, were comprehensively detailed in an Environmental Impact Statement undertaken by Sydney Water. (The spillway project began under Sydney Water but was transferred to the Sydney Catchment Authority (SCA) along with ownership of all the dams in 1999). All likely impacts on threatened flora and fauna were addressed in a Species Impact Statement, a complementary document to the Environmental Impact Statement.
Following approval from the NSW Minister for Urban Affairs and Planning, Sydney Water decided to build the auxiliary spillway. In response to comments received after the public exhibition of the EIS, some changes were made.
Before work started, a rigorous process was set up to ensure all community and environmental issues were properly dealt with during project planning, construction and restoration stages.
An independently chaired Community Liaison Committee was established to exchange information with the local community and resolve issues affecting residents.
A comprehensive Environmental Management Plan was prepared and an Independent Environmental Management Representative appointed to ensure compliance with all environmental regulations and the NSW Environment Protection Authority requirements.
Construction and rehabilitation
Construction of the auxiliary spillway involved excavating 4.3 million tonnes of rock from around the eastern side of the dam. This created a channel 700 metres long, 190 metres wide at the upstream end, 65 metres wide at the downstream end and with a maximum depth of 50 metres.
The excavated spoil (rock and soil) was removed by large off-road haulage vehicles, transported across the dam and placed on land on the western side of the dam. The disposal site had erosion control measures and was progressively landscaped and revegetated using native species.
The spillway is lined with concrete and features a dam access bridge. A series of earth and clay walls or 'fuse plugs' approximately 14 metres high are installed across the upstream opening of the spillway.
The project also involved some extra works including modifications to the crest gates on the existing central spillway, further strengthening of the existing spillway training walls, and construction of a bulkhead wall around the valve house.
Heritage items
Construction of the spillway involved removing much of the overhead cableway across the Warragamba Gorge - specifically the head tower and lower tail tower, located directly in the path of the spillway and spoil haulage road.
The cableway played a unique and significant role in building Warragamba Dam and a number of initiatives were undertaken to preserve its heritage significance. The heritage significance of all items affected by the project was fully documented, including comprehensive photographic records. A number of these heritage items are now featured in the Warragamba Water for Life exhibition and in interpretive signage around the site.
Picnic areas
Warragamba Dam Grounds are an ideal spot for a picnic. Facilities include picnic shelters, drinking water, toilets, viewing platforms of the lake and dam, and a new visitor centre. Picnic areas are also available nearby in the Warragamba Recreational Reserve, including barbecue facilities and a children's playground. The reserve is managed by Wollondilly Shire Council.
A safe dam - a sure supply
Raising Warragamba Dam by five metres, installing post-tensioned steel cables, and building the auxiliary spillway, collectively ensures the security of Sydney’s major water supply and protects downstream communities from the risk of dambreak flooding.
