Severe thunderstorms and flash floods in Victoria

Flash floods occur when soil absorption, runoff or drainage cannot adequately disperse intense rainfall. The most frequent cause of flash flooding is from slow-moving thunderstorms. These systems can deposit extraordinary amounts of water over a small area in a very short time. Flash floods are extremely dangerous weather events as water in creeks, drains and natural watercourses can rise very rapidly.

The radar animation above is from the Melbourne Flash Flood that occurred 3rd December 2003 (see recent examples below)

The strong updrafts of air within thunderstorms can suspend huge amounts of rain before releasing a deluge onto the ground. Such rain can reach intensities of more than 100 mm per hour, provided the environment is humid enough to feed sufficient moisture to the storm. Often topography acts to focus thunderstorm development over a particular location, further accentuating rainfall accumulation.

Recent examples of flash flooding in Victoria include:

• 03 December 2003: Severe thunderstorms formed near Craigieburn to the north of Melbourne around midnight. Exceptionally heavy rainfall was produced as they grew in size and moved slowly to the south and east. Widespread flash flooding resulted from these storms. Most of the rainfall occurred within a one to two hour period centred around 2am. The hourly rainfall rates are consistent with a 1-in-100 year event. Flash flooding caused damage to houses, cars, schools, shops, stock and contents. Flood waters were up to 2 metres deep in some areas and 10 motorists who were trapped on the Eastern Freeway had to be rescued by boat.

• 26 Feb 2003: Days after the Alpine Fires, Flash flooding resulted from a severe thunderstorm in the Buckland Valley near Mt Hotham. A woman was killed when the vehicle in which she was traveling was washed over the road.