Facts on Flash Floods in NSW
Flash floods occur when soil absorption, runoff or drainage cannot adequately disperse intense rainfall. The most frequent cause of flash flooding is slow-moving thunderstorms. These systems can deposit extraordinary amounts of water over a small area in a very short time.
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.
One such event occurred at Coffs Harbour on the evening of 23 November 1996. Flash flooding from heavy rainfall saw water rising to a height of around 1.5 metres in the centre of the town and up to 300 people had to be moved to higher ground. Tragically a woman drowned when she was swept away by the rising flash flood. The insurance payout was around $30 million.
Flash floods are extremely dangerous weather events as water in creeks, drains and natural watercourses can rise very rapidly. On the evening of 26 January 1971 in Canberra, seven people died as flash flood waters from a nearby thunderstorm flooded roadways near a drainage channel. It was estimated that around 95mm of rain fell in one hour during this event.
In another event in Sydney on 7th November 1984, 127 mm fell in one hour leading to damage of around $128 million (in July 1996 terms).
Recent examples of flash flooding in New South Wales include: