Floodplain Manager February 2019


The Townsville floods dominate this edition of Floodplain Manager and rightly so. Our thoughts go out to all of those who have been impacted by the event. It has been interesting, however, to read how the popular media has liberally used the term “unprecedented” to describe the rainfall event and the flooding. I would be more measured and say that they are rare but even that is a loaded term. Yes, Townsville got a year’s worth of rain in 10 days but that only broke an 80 year record and, given that it has been subjected to similar climatic conditions for thousands of years, February’s rain has probably been exceeded many times. And that is just in Townsville. Yes, there were rainfall totals in parts of the catchment that had an estimated 1 in 2,000 AEP but I have seen similar frequencies quoted for some of the rainfalls and floods in SE Queensland in 2011. These are very low probability events but they do happen and, when you look across a big enough area, they happen quite often. As Drew Bewsher once said to me, ”Rare events are not that rare” and I tend to agree.

Some of the impacts I would also note as being unusual but not uncommon. Two fatalities in what might have been a police pursuit after looting, another two in the Ross River after the flood peak had passed and another two from water borne bacteria. These deaths are tragic and avoidable. Less avoidable was the loss of 300,000 head of cattle or the flooding of hundreds of homes. Many would like to think that if the Ross River dam were operated differently these losses too could have been avoided. In hindsight, some of them may have been but the dam was reportedly operated to its operating rules which are set to minimise losses in the 1% AEP event. The trade-off is that it increases losses in a 0.2% AEP event. Mitigation dams mitigate flooding, not eliminate it, and the water has to go somewhere. If the dam was operated such that houses below the 1% AEP level flooded without the levels getting much higher than that then I am sure the dam operating rules would have been called into question. In designing the operating rules the operators had to make a choice between less damage more often vs more damage less often. The choice they made is understandable.

Similarly, Council is being criticised for approving development which flooded. I am sure that there were many old parts of Townsville which were developed without consideration of flood risk and today we would avoid developing there but it seems that new suburbs which were above the 1% AEP flood level also flooded. That should hardly be a surprise when we set the 1% AEP flood level as our designated flood event for town planning. While I don’t believe we should be using the same probability event for all catchments, I do believe that we have to pick an event for each catchment and accept that there will be consequences if that event is exceeded. Of course that is easy to say when it is not my house or business that is flooding. But it is also our experience that people will accept almost any chance of flooding when it’s not flooding and almost no chance of flooding when it is. Check out the results of an online survey we did several years ago in this regard.

Steven Molino


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