Warragamba Dam sits on the Warragamba River which is a major tributary to the Hawkesbury-Nepean river system. The following information has been prepared as part of the Hawkesbury-Nepean FloodSafe project.
Where is Warragamba Dam? Is it on the Hawkesbury-Nepean River?
Warragamba Dam is a concrete gravity structure and is about 65km west of Sydney in a narrow gorge on the Warragamba River. It's Sydney's main water storage dam, and one of the largest domestic water supply dams in the world. It is not on the Hawkesbury-Nepean River, but the Warragamba River, which is a major tributary of the Hawkesbury-Nepean.
Why doesn't the dam fill when Sydney's had a lot of rain?
When it's raining in the Sydney metropolitan area, it isn't necessarily raining over the Warragamba Dam catchment. The dam impounds Lake Burragorang and is filled by run-off from rain falling in a catchment area of more than 9,000 square kilometres. The Warragamba catchment extends from south of Goulburn, north to Lithgow, east to Werombi and Mittagong, and west to part of the Crookwell local government area. The main rivers that flow into the Warragamba Dam are the Wollondilly, Coxs, Kowmung, Wingecarribee, and Nattai.
Does Warragamba Dam reduce downstream flood levels?
The dam was built for water-supply purposes and was not designed as a flood-mitigation dam, but it can actually do so to a limited extent. If it is partially empty before the floods, it initally mitigates floods by storing the incoming flood waters. When full, it slows the release of flood waters into downstream areas. For example, in 1961, the worst flood since Warragamba was built, the dam reduced peak flood levels at Penrith and Windsor by between 0.8m and 1.2m. This was despite 52 percent of the rainfall causing flooding in the Hawkesbury-Nepean valley falling outside the Warragamba catchment. The contribution of the Warragamba River to the flood was reduced by a quarter and was delayed by several hours.
What happens when the Warragamba Dam is full?
When heavy rain falls over the catchment area, the water level in the dam can rise above the full storage level. "Full storage level" is the level of the top of the drum gate when it's in its closed position. The drum gate is in the centre of the spillway and is the first to open when the dam is full. If the water level in the dam rises and reaches full storage level, water begins to flow over the top of the closed drum gate. When the water reaches 80mm above the full storage level, the drum gate automatically begins to open gradually to allow more water to pass over it. If the water level continues rising, the four radial gates, two on each side of the drum gate, start opening at 230mm above full storage level. The five gates help control the flow of water. As the lake's water level drops, the gates progressively close, so that the dam is at full supply level at the end of a flood.
Why was an auxiliary spillway built?
The new auxiliary spillway will come into play during rare and extreme floods to supplement the existing spillway. It will divert excess floodwaters safely around the dam to prevent overtopping and possible dam failure. This means it will protect the dam in extreme floods, and areas downstream from the devastating effects of a dam break, and safeguard Sydney's water supply.
When will the auxiliary spillway be finished?
The auxiliary spillway was completed in 2003.
Will you tell us when you open the gates?
The spillway gates at Warragamba normally work automatically depending on the dam's water level. Warragamba's gates open automatically when the lake level is about 80mm above full storage level. The gates are routinely partially opened for maintenance. The SCA provides the State Emergency Service (SES) with flood information.
Where does the excess water from the dam go?
Water released through the gates at the top of the dam races down the spillway into the stilling pond more than 100 metres below. This acts as an energy dissipator, slowing the water before it is released down the Warragamba River to make its way out to sea via the Hawkesbury-Nepean system.
How can we find out what the level of Warragamba Dam is during a flood?
It's probably not a good idea to ring Warragamba Dam in flood times as SCA staff will be working around the clock to manage the dam's operation to feed information to the SES and manage the water supply. The SES will coordinate information on the flood and can answer your questions about flooding. For information on the dam level, contact the SCA on 02 4725 2100.
When Warragamba Dam is full and water is spilling over it, how long will it take to reach Richmond?
Usually, once the flood has peaked at Warragamba, Richmond residents can expect flood levels to continue rising for the next 12 to 16 hours. However, these times can vary significantly depending on the size of the flood and the relative sizes of the contributions from the various tributaries. The tributaries in the Richmond-Windsor area are the Grose River, South/Eastern Creek, Cattai Creek, Little Cattai Creek, Rickabys Creek, Redbank Creek, Howes Creek and the massive Colo River, which disgorges at the downsteram end of the floodplain.
Can we watch the water spilling over Warragamba Dam from the viewing platform? How much water will you spill out?
The viewing platform is far enough away to be safe, but access may be cut for other reasons. In flood times, the main access road to Warragamba (Silverdale Road) is often cut at Wallacia by flood waters coming down the Nepean River.
The volume of water flowing out depends on the water level within the dam, and this determines the gate openings.
If the Richmond area is already flooded, why can't the Sydney Catchment Authority keep the Warragamba Dam gates shut?
The gates only start opening when the dam is full. The radial gates must be opened before water flows over the top of them or they may be damaged. Under extreme floods, the SCA needs to open the gates to prevent the dam from failling. Attempting to keep the gates shut also increases the risk of damaging the dam as the excess flood waters may overtop the dam wall.
Does the SCA use the dam water to make electricity?
A hydro-electric power station, operated by Eraring Energy (formerly Pacific Power), starts working once the dam is less than a metre from full storage level and still rising. The plant discharges 5,000 megalitres of water per day and puts 50 megawatts into the State electricity grid.
Will the area below Warragamba Dam flood first?
Not necessarily. The actual sequence of flooding varies from event to event depending on where it rains. SCA's dams in south and southwest Sydney, such as Nepean, Cataract, Cordeaux and Avon, generally flood before Warragamba. Flooding can also occur due to the impact of flood waters from the Grose River and South Creek.
We're planning a bushwalking trip and will have to cross the Kowmung/Coxs/Wollondilly rivers. Do you think they will be safe to cross in flood times?
It's not advisable to be planning bushwalking in the catchment in times of potential floods, as there are many creek crossings and it could be unsafe.
If the dam is about to spill, should I still go to work/school?
For information on what to do in a specific flood, phone your local SES unit on 132 500.
What is the risk of the 1867 flood -- the worst on record -- happening today?
Keep in mind that there is no regular pattern to the size or frequency of floods in the valley. Records show, however, that floods and droughts have alternated over the last 200 years. Analysis has shown that the 1867 flood at Windsor has about a one-in-280 chance of occurring in any given year.
What's the worst possible flood that could happen in the Hawkesbury-Nepean valley?
A Probable Maximum Flood (PMF) can happen, but is very rare. Very deep flooding can happen because a 16,000-square-kilometre catchment drains in to a floodplain at Windsor where the river bed is less than a metre above sea level. The valley is something like a big bathtub that sees water banking up from the tiny "plug hole" at Sackville, east of Richmond. This means floods can rise by up to 25 metres even if the dam doesn't fail. In fact, there have been floods of up to 10 metres at Windsor even when Warragamba Dam isn't full. This happened in August 1986. Flood modelling has shown that flood levels at Windsor can reach more than 12 metres without any spill from Warragamba Dam. Backwater flooding produces Windsor's severe flooding. This can be aggravated by rainfall in the Colo and Macdonald catchments (that is, outside the Windsor catchment).
A PMF would occur if an average of 770mm of rain fell within three days over the entire Warragamba catchment of more than 9,000 square kilometres. It could happen on average about once every 100,000 years, but it could occur at any time. The peak outflow of the PMF would be about 2.5 times the largest flood recorded -- the flood of 1867.
According to the most up-to-date projections, if a PMF occurred, the flood level at Windsor would reach 19 metres above the bridge.
If the dam spills, how will that affect our drinking water downstream?
In times of flood, much debris and silt is washed into the lake. The raw water quality deteriorates, but Sydney Water treats all drinking water before distribution into homes. During floods, the Sydney Catchment Authority and Sydney Water will continue to supply the best quality water available. This means the SCA will find the best water in the lake profile and take it from there to Sydney Water to treat before it reaches homes.
For example, the flood-affected raw water at North Richmond Water Filtration Plant, which Sydney Water operates, will be murky. This is because it comes from the Nepean-Hawkesbury River system. This will happen because it has runoff water from the catchment even before Warragamba Dam spills. Once the dam spills, this is not likely to create extra problems for the plant, which is designed to treat "turbid" or murky water.
In most cases, the water spilling over Warragamba Dam won't be murky as the run-off from the catchment takes considerable time to reach the dam wall, and in this time the silt in the water will settle.
Can you predict when the dam will spill?
The Sydney Catchment Authority can predict when the dam will spill because of our instrumentation on the four major rivers that flow into the dam. It depeneds on the rainfall over the whole catchment -- usually we can only give a prediction based on an assumption that if the rain would continue at this level, we would expect the dam to be full by a certain time
Further information on the Warragamba Dam can be obtained from the Sydney Catchment Authority .
Related pages: Hawkesbury-Nepean FloodSafe .