Worst case scenarios 4: Droughts and floods

Another in my series of worst case scenarios. A bit more locally oriented this time.

h4. Droughts

There have been important changes in the way we think about droughts, especially in relation to public policy. In the past, droughts were viewed as natural disasters, requiring a public policy response, which was typically triggered when an area was declared drought-affected. This happened so often that some parts of Queensland were drought-declared one year in three. But a climate event that happens as regularly as this, can’t be regarded as an unpredictable disasters. It is simply part of the natural cycle, which farmers, water supply enterprises and others must take into account in their normal management. This is the central feature of the National Drought Strategy adopted in the 1990s, which faced its first real test in the recent large-scale drought. Although the drought was one of the worst in Australian history, most observers agree that it was handled more successfully than previous droughts that were, in purely climatic terms, less severe.

But recognition of droughts as part of the natural cycle does not provide a complete answer. No matter how good your management, it is not economically feasible to prepare against the worst-case droughts. A common criterion used in water supply is to prepare plans based on a 1 in 100-year drought, that is, one that would be expected to occur once in every hundred years on average. Implicitly, though, this means that if the drought is worse than the 1 in 100 years, normal planning will fail, and some emergency response will be needed.
A further twist to the worst-case scenario is added by the process of climate change. Estimates of the 1 in 100 year drought level are based on historical observations of the climate. But climate change will make some areas drier and others wetter. In areas where average rainfall declines, severe droughts will become more frequent.

h4. Floods

In economic terms, droughts are normally a bigger problem for Australia than floods. But the worst-case scenario is considerably worse for flood than for drought. During a 1 in 10,000-year flood event, the Wivenhoe Dam could fail catastrophically, putting between 50 000 and 250 000 lives at risk. This possibility might seem so remote as to be negligible, but for a dam with a life of 50 years, the chance of a 1 in 10000 year event occurring at some point becomes 1 in 200, enough to be a threat worth considering

Fortunately, SEQWater is currently engaged in an upgrade which should protect against this remote possibility by raising the height of the spillway and dam wall to a level that should protect against a 1 in 100 000 year event.

It might be asked how it’s possible to make any sort of rational decision about whether to spend money on risks like this. The easy answer is that anything that reduces risk is worth doing, but since it is never possible to eliminate risk completely, it’s not possible to dodge the necessity for hard choices. The best way is to work out the average number of lives saved and compared the amount that would be needed to save the same number of lives in some other way, for example, through improvements in medical care. A typical estimate is that a treatment that is just on the margin of acceptability will cost around 100 000 for each additional year of life gained, which translates to about $5 million for each life saved from a disaster such as a flood.

4 thoughts on “Worst case scenarios 4: Droughts and floods

  1. My dad, bless him, was a water engineer who once had the misfortune to be on the losing side of a 1 in 100 year bet. IN his case it was against the rapid growth of Canberra in the 1960’s and the timing of the construction of the Corin Dam. Canberra had a 1 in 100 year drought and he got blamed for the water restrictions. BUt like he said, you can’t tell the difference between a 1 in 100 or a 1 in 1000 year event half the time, and in a country with short actual records like Australia it’s virtually impossible.

    While the National Drought Strategy worked succesfully this time it was mainly because of the development of timely indicators and plans for destocking early and ensuring cross state cooperation with fiancial compensation and agistment.

    This is all well and good but an effective second step would involve farm diversification and the diversification of water supplies etc. YOur Wavenhoe Dam example is actually a good example of something that might equally well fail. Whose to know whether a 1 in 100,000 year might occur. THe current level of CO2 is higher than it’s been in 100,000 years.
    An alternative solution would be to
    lower the spillway and develop early evacuation plans for those downstream.

    This sort of diversification has a name – permaculture – and it works.

  2. one of the things I can take a bit of credit for is Commonwealth drought policy, which I have contributed to in various ways. John is actually being a bit kind to Queensland – in the 20 years leading up to the changes to national drought policy there were some parts of Queensland that were drought declared more than 50% of the time: so much for unanticipated natural disasters! My own view, and I know it may be regarded as a bit extreme, is that drought is a notion that simply does not apply to Australia – so much of the continent is dry for so much of the time that European notions of “drought” that were brought over here are just not relevant. Core samples taken from coral reefs – which provide one of the few reliable sources of data over long periods of history – show that there have been times in the past when Queensland was very much drier than ever recorded in the history of European settlement. (what coral samples do is look at sediment from inland river runoff – which of course is related to rainfall).

    However, what I am led to from this item is the argument, that I think is entirely supportable by almost all the evidence, that climate change will lead to greater climate variability. Floods, cyclones, whatever: we will get more of them. This means that the direct costs to the community of climate change are bound to increase. What worries me is that we are already seeing this in action, and yet governments in both the US and Australia are ignoring the cost in their determination to stay out of Kyoto.

  3. Stephen, I’ve heard a simple explanation given by a climatologist as to why global warming leads to greater variability in rainfall.

    Warmer air can hold more water, hence it rains less. But when there is a triggering mechanism, such as an upper atmosphere low or a front, then the is more water in the air to come down, hence more rain.

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