Floodplain Manager April 2021


I said last month that the March rainfall and flooding in NSW were not unprecedented as suggested by many media reports, and a special climate report recently published by the Bureau of Meteorology bears that out.  Sydney did have its wettest week since records began in 1900 but elsewhere the daily, weekly and monthly rainfalls had previously been exceeded.  Similarly, the Hastings and Manning rivers reached their highest recorded peaks but rainfall in both catchments had been exceeded in 1929.

Our memories are short and weather patterns follow long cycles.  One of this month’s articles points to research undertaken in the 1970s by former Sydney University Professor Robin Warner who studied rainfall records, flood records and geomorphology and postulated that eastern NSW experiences flood-dominated and drought-dominated cycles of between 30-50 years.  From the early 1950s to 1990 we were in a flood-dominated cycle, and the suggestion in the article is that from last month’s rainfall signals we may be entering the next flood dominated cycle.

Many of the record floods on our rivers along the east coast date from such a cycle which occurred from the 1860s to the mid-1890s before the onset of the Federation Drought, one of the worst recorded.  But there were millennia of such cycles experienced by our first peoples but not recorded in the same way.  We also know that over that time the climate changed and so would have the trends in these cycles.

To try and pick out from our recent experience what are expected variations in these historical cycles and what is due to climate change (and how much of that change is anthropogenic or natural) is just too hard with such limited numerical data.  So, when we report this month that worldwide there were 20% more floods in 2020 than average, we need to keep that in mind.  We also report on research that suggests that cyclones won’t bring more rain to Australia under climate change but they will drop it further south, that improved modelling suggests global sea level rise will not be as great as previously predicted, but other modelling suggests that in parts of WA, sea level rise will be higher than forecast because of local conditions.

It is within this cloud of uncertainty that floodplain planning decisions need to be made now that will have implications for decades, even centuries, because once a decision is made to develop on a floodplain, experience shows it is very hard to “un-develop it”.

I look forward to “seeing” many of you at the FMA annual conference in a few weeks where we can all share ideas on how to improve such decision making

Steven Molino

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