Fortunately recent flooding was not as severe as our 2007 Flood, when two commercial properties were inundated as water from the Town Creek catchment confronted back water from the Williams River in what approximated to “a 1 in 100 year flood” event.
This commonly used term does not mean such a flood would not occur for another100 years. It is officially called the “100 year ARI Flood” (Average Recurrence Interval) or more aptly, the 1% AEP (Annual Exceedance Probability) indicating the probability of a certain flow occurring in a river.
While it has a 1% or 1 in 100 chance of occurring in any one year, it has a 10% (1 in 10) chance of occurring in a 10 year period, a 25% (1 in 4) chance in 30 years and a 50% (1 in 2) chance over 70 years. Compare this with the 1 in 45 million chance of winning Lotto!
The default flood planning level for NSW is the 100 year ARI plus half a metre freeboard, which is a compromise between minimizing loss of life and building costs. This works well in most locations, but not necessarily on a floodplain.
A “floodplain” is defined as land that is likely to be inundated by the most extreme flood that could conceivably occur in a particular catchment i.e. the Probable Maximum Flood (PMF). The NSW Government has co funded numerous catchment specific flood studies that determine flood levels for rainfall events up to and including the PMF.
The Clarence Town Flood Study 2014 considers flooding from local rainfall in the Town Creek catchment and of course what happens in the Williams River with comprehensive maps showing the extent of the flooding: www.dungog.nsw.gov.au/Plan-Develop/Land-Environmentand-Heritage/Flood-Plain-Management
As few catchments if any have experienced a PMF since 1788, our experience with Australian weather is merely a small sample of its climate and early settlers learned the hard way about our droughts and flooding rains.
Australia’s deadliest flood was at Gundagai in June 1852 when at least 89 people died. The town was built on the banks of the Murrumbidgee River against the advice of the Wiradjuri people. A 1:50 ARI rainfall event left only three buildings standing. About 90 people were rescued from trees by two Wiradjuri men using bark canoes. A similar flood occurred there in 1853. The town was rebuilt on higher ground, which in hindsight should have provided a lesson for so many other towns that were also established on floodplains.
The recent flooding in Dungog Shire was declared a Natural Disaster with joint Commonwealth and State funding available to restore substantial damage caused to our infrastructure.
Your local B Ward Councillors:
James Campbell 0484 599 709
james.campbell@dungog.nsw.gov.au
Steve low 4996 4022
steve.low49@gmail.com
