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.

Flash floods in Wollongong on the evening of 17 August 1998 caused extensive damage.

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:

• 13 October 2002 – Mudgee (Central Tablelands) - 118mm fell in 6 hrs - flash flooding occurred in the Mudgee area.
  
• 16 January 2002 - The Channon (Northern Rivers) - 32mm in 15 mins - The storm also affected Grafton, Lismore and Kingscliff where 7 cm hail was reported and there were over 320 calls to the SES.
• 26 April 2001 - Murwillumbah (Northern Rivers) - 50mm in 30 mins - 3cm hailstones were also reported.
• 25 March 2001 - Lansdowne (Mid-North Coast) - 60mm in 20 mins - very heavy rain caused local flooding from Gloucester to Walcha.
• 5 March 2000 - Nundle (North West Slopes) - 100mm in 60 mins. A set of sheep yards severely damaged by water and the creek levels were reported as being the highest in 50 years.