Melbourne’s naturally high-quality water supply is largely unfiltered. While this helps keep down the cost of drinking water, it may under certain circumstances present other problems. Fires, for example, can significantly impact on water quality. As can the potential impact from debris flows, which become more likely after fires.
A debris flow is triggered by rainfall and is an avalanche of water, soil and rock flowing down a gully. The rainfall does not have to be heavy to cause a debris flow, which usually occurs on steep slopes covered in dry eucalypt forests, rather than in the wetter, higher forests.
Debris flows can deliver many thousands of tonnes of sediment to streams or dams, resulting in poor water quality that will last for months. The probability of a debris flow increases after bushfire and the risk remains high until vegetation has re-established. This means that although most debris flows occur in the 12 months after a fire, the risk may last for years after a fire affects an area.
Researchers from the University of Melbourne are using a combination of geographic information system (GIS) topography data, observed debris flows from areas of Victoria outside Melbourne’s catchment, and field experiments to map areas at risk of debris flows within Melbourne’s catchment, as well as the probability of these flows taking place. Melbourne Water is utilising this information to focus firefighting efforts and identification of high-risk areas for rehabilitation after fires in water catchments.