Pearls and Irritations

Not even the Premier of New South Wales understands the meaning of the term the ‘one-in-100-years’ flood. Nor does the Prime Minister, who this week repeated the Premier’s misguided words on it.

It has been refreshing to see the pair together and showing some solidarity in the face of the recent floods-some of them quite disastrous in their impacts-which have ravaged areas from the Illawarra to the Hunter over the past two weeks. But it has been alarming to see how tortuous is their understanding of this much over-used and badly misunderstood and dangerous term.

What Dominic Perrottet said on Four Corners was, more or less, that we’d seen the 100 – year flood and perhaps even the 1000-year flood but that in reality this flooding had come down virtually to being the one-year flood. Anthony Albanese in effect repeated Perrottet’s words. It would have been helpful, in the interests of community comprehension, had the Premier’s advisors informed him of the meaning of the concept and the need for him to be careful in his retailing of it. The misunderstanding does not need to be confused even further.

Let’s unpack the misunderstanding. Perrottet seems to be labouring under the misapprehension that the term applies to the state as a whole. In fact it applies to every individual piece of it–for convenience, let’s say to every town considered separately (that is, on its own). So there is a one-in-100-years flood for each of the-for Lismore, for Maitland, for Windsor and any of the other hundreds of locations in New South Wales which are prone to flooding. It makes no sense to think of the state as a whole in this matter.

For each location there is a range of floods, and we can think of floods with average return periods of one, five, ten, 20, 50, 100, 200, 500, 1000, 100,000 and 1,000,000 years-or indeed any other number we care to choose, for each one of them. Note, especially, the use of the word ‘average’. It must be used to ensure the concept can be properly understood.

The idea that what we used to think of as the one-in-100-years event is suddenly being experienced every year, which Perrottet seemed to be saying, is ludicrous.

And anyway, the term long ago lost any power of clarity. Increasingly it is used in public discourse to mean ‘a big flood’. The idea has also been got across in the community that if you’ve just seen a one-in-100-years flood you don’t need to worry about having to put up with another one as big-because you won’t live long enough. That is simply untrue.

It is one of the many myths that have grown up around floods – like the myth that a town-protecting levee has been constructed to keep out all floods, even huge ones. That myth is particularly dangerous, because it creates the complacency that encourages councils to promote further development in the areas behind the levee. Then the levee is overtopped and there is more damage than would have been incurred had no levee been built at all and no complacency about it developed over the often very long period over which it had kept floods out.

Note, here, the number of towns whose levees have been overtopped inn recent times. Since 1990 in New South Wales, this has happened in Nyngan (1990), Kempsey (2001), North Wagga Wagga (2012) and Lismore and Murwillumbah (2017). This year’s flood at Lismore overtopped the levee again, and astonishing by more than three metres.

The many people who have lost so much in the Hawkesbury of late may not be cheered to hear that they have not lived through a one-in-100-years’ flood, let alone three of them since March 2021. What they have experienced is three floods which peaked at the Windsor gauge at between 12.8 and 13.9 metres. In all likelihood, these floods have been something between the one-in-15-years and one-in-30-year events. These numbers should be seen as indicative rather than precise.

The estimated 100-year flood, as it happens, is thought likely to peak at 17.3 metres at the Windsor gauge.

Bigger floods than those that have recently occurred on the Hawkesbury have been experienced in living memory. In 1978 a flood reached more than 14 metres at Windsor and in 1961 one peaked at more than 15 metres there. Neither of these was as big as the estimated 100-year event.

Beyond all those cited, the flood of 1867 reached a staggering 19.3 metres, vastly higher than anybody now alive in the area has experienced. That one might have been of the order of the 200-500-year event: it is difficult to be precise because we have little data on such massive floods on the Hawkesbury. It is the only one we know of that was of such a magnitude, though bigger ones must have occurred long ago because the soils of Windsor are of alluvial (water-deposited) origin. There have been bigger floods than 1867s.

In appreciating what their predecessors saw, present residents should think about what a flood more than five metres higher than what has just occurred would look like. It would see thousands of houses in the reference area of the Windsor gauge with water completely submerging them – not over their floors, but over their roofs.

They might also contemplate the Probable Maximum Flood, the highest believed possible at Windsor. It might reach 26 metres there and would leave the whole town completely submerged. Such a flood, or something approaching it, might occur when the floodplain is completely saturated (as it currently is) and an absolutely massive rain event, dropping vastly more rain than has fallen there in the past fortnight) is experienced over the entire catchment. Incredible rainfall intensities do occur, if very rarely in individual locations.

Of course, we should ideally not use the ‘one-in-X-years’ terminology, but it is difficult to avoid. The one-in-100-years flood is best referred to as a flood big enough that it has only a 1% chance of occurring or being exceeded in peak height at a location (say Windsor, in this case) in any given year. That means there is a 1% chance of seeing such a flood in 2022, 2023, 2024 and every other year thereafter.

As we have just seen, three floods whose average exceedence probabilities (for an individual year) were probably between roughly 7% (reaching 12.8 metres at the gauge) and 3% (reaching 13.9 metres as it did last week) have been seen and felt at Windsor in the past 16 months. These figures too are indicative, not precise.

The fact that three such large floods have occurred at Windsor in the past 16 months does not mean that the figures lie or are flawed, or that people are being deliberately misled by experts who don’t know what they are talking about or are malicious in intent. Conspiracy theories to such effects abound. Repeated severe flooding merely indicates the madness of the ‘once-in-X-years’ rubric. These three floods in quick succession have occurred after a period of more than 30 years without a single flood at Windsor which was of the scale of these three.

The last flood which could be called ‘big’, which occurred in 1990, peaked at 13.3 metres at Windsor. That means the Windsor area has experienced four big floods in just over 30 years. Clearly, they were not evenly space in time. Temporal unevenness is the way of big events in nature be they floods, tropical cyclones, other severe storms, earthquakes, landslides, droughts or pandemics. Several can occur in a location in quick succession with none being felt thereafter for decades or centuries.

It may be that Windsor will not experience another very large flood for a very long time. Or it may get another one later this month, beyond the current horizon of forecasting accuracy. Nobody can tell.

The same goes for Lismore, Kempsey, Maitland, Moree, Narrabri, Wagga Wagga and all the other riverside communities in the state, and for the suburbs in the valley of the Georges River in Sydney which have just experienced a not-very-severe flood. The only thing that we can be sure of is that they will all experience very big floods at some stage in the future.

And a final sad word: with climate change almost certainly exacerbating the flood problem by creating more intense rainfalls from a warmer atmosphere (as many scientists believe it is, and the atmosphere has definitely warmed over the past century), the level reached by the 1% flood as now estimated will be even higher than is currently thought likely. In due course, that flood might become the 2% one. That is, the probability of its occurrence will increase, which is to say there will be more of them.

This is not a comforting thought, but it is one which should encourage us to recognise we should do more to minimise the climate change we are promoting.

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