Warragamba: A dam full of myths
Some common myths about Warragamba Dam
- It protects the Hawkesbury-Nepean Valley from flooding
- Floodgates are opened frequently and at will
- It fills whenever it rains over Sydney
Some believe that Warragamba Dam, located about 65 kilometres south-west of Sydney, protects the Hawkesbury-Nepean Valley from flooding. In fact, Warragamba was never designed as a flood mitigation dam. As Sydney's main water storage dam - and one of the world's largest domestic supply dams - it can only mitigate floods to a limited extent. If partly empty before any flood, it can store the incoming floodwaters. When full, it slows the release of floodwaters into downstream areas. In the 1961 flood - the worst since the dam was built - water coming from the Warragamba River was reduced by a quarter and delayed by several hours.
Another myth is that the dam gates are opened frequently and deliberately. This does not happen. The central drum gate and four radial gates open automatically when the dam reaches and passes full storage level by about 80 millimetres. They are rarely opened manually.
Some people also think the dam is on the Hawkesbury-Nepean River, when it is actually on the Warragamba River - a major tributary of the Hawkesbury-Nepean.
The dam forms Lake Burragorang and is filled by run-off from rain falling in a catchment area of more than 9,051 square kilometres. The Warragamba Catchment extends from south of Goulburn, north to Lithgow, east to Wollondilly 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.
Heavy rain falling in the catchment area can overfill the dam. The dam gates begin to open gradually when the water reaches 80 millimetres above the full storage level. The central drum gate gradually opens automatically at this point, and if water continues rising, the four radial gates start opening at 23 centimetres above full storage level. The progressive gates help control the flow of water.
The auxiliary spillway was completed in June 2002, and will only operate during extreme floods.
