Tuesday, 11 January 2011

The Day it Rained Forever

I'm working on a new novel with the working title, The Day it Rained Forever. Here in Queensland, it is starting to feel a bit like that.

While the floods here are not causing the devastation the recent Pakistan floods caused, because the population is lower and we have good early warning systems, you nonetheless have to start wondering why one in hundred year events are happening so often.

While it’s wrong to label one event as evidence of climate change because climate is a long-term average of weather, one of the predictions of climate theory is that as the energy in the system increases (what is happening now), the hydrological cycle intensifies. That means bigger swings between drought and deluge, and more intense droughts and floods.

If you look at the history of flooding where I live in Brisbane, it looks as if things have actually improved since the big flood of 1893:
However, we need to take into account the completion of the Wivenhoe Dam in 1984, which has had a significant effect in mitigating flooding. We should have had a major flood in 1999, as you can see from the fact that Wivenhoe peaked at about 135%; the sudden drop-off arises from the excess being let out. The way a flood mitigation dam works is 100% is the normal water storage capacity and anything over 100% is the excess that would have gone into a flood. As soon as the possibility of flooding reduces, this excess has to be released. In a tidal river like the Brisbane, water can be released to coincide with low tide. Obviously there is a limit to this: if the dam approaches its true 100% (possibly 200%), there is no option but to release water even if the timing is not ideal.
Here is the overall history of the dam as far back as the official SEQ Water web site goes:
Note the peak in 1999, when the dam first performed its flood mitigation duties. The current peak doesn’t look too dramatic but let’s take a closer look:
Observe the spikes to the right of the curve, all over the 100% line. These are all occasions when the sluices were opened. Let’s see exactly how many spikes there have been to date:
Four so far, and no end to the rains in sight. Contrast that with only one occasion in the past when the dam sluices had to be opened since it was constructed. The latest peak is nearly 150%, much higher than the 1999 peak of 135%, illustrating that you need to look at the numbers rather than eyeball a graph.

Going back to 1893, the pattern then was a series of floods, with one big one that dominated the rest. We won’t see that pattern again in Brisbane because of the role of Wivenhoe, but without the dam, the current flood would be at least as bad as that of 1974, possibly at the 1893 level. Clearly, these “once in a century” events are happening more often than that.

None of this of course is evidence of climate change. It is however a warning. The level of climate change we have seen so far has added about 0.8°C to the pre-industrial global average. James Hansen, in Storms of My Grandchildren, predicts things become hairy with more than another 1°C of warming. He bases this number mainly on the threat of rapid disintegration of ice sheets. However, we should not expect that level of warming to occur without further intensification of the hydrological cycle. Flooding on this scale should happen more often. Exactly how much more often, and how much the variation between wet and dry will intensify, are open questions. Do we do the experiment? People with water above their roofs or even lapping at their floorboards may well say no. I certainly do.

Update: 10:30 am, 11 January 2011

I’ve just heard on ABC local radio, Wivenhoe is at 173% of capacity, despite all the major releases. Further releases that won’t cause flooding are no longer an option.

Update: 8:40 am, 16 January 2011

Since my last update, Wivenhoe was heading to 200%. The dam spills over the top of the wall at 225%, and is not designed to withstand that kind of spillage, so the gates had to be opened to the extent of flooding significant parts of the city. The peak level was 191% on the night of Tuesday 11 January 2011 (though this does not show up on the official dam web site because of the time of day at which the measurement is taken).

Now the flood waters have receded, the flood level was a bit below that of 1974, despite twice the rainfall in Wivenhoe’s catchment, so the dam has had some useful effect. I don’t know how the rainfall over the city itself differed, but that carried on for a long time if not very intensively. At very least that would have contributed to the flood by saturating the ground.

My own home was above the flood level, but homes only a few blocks away were inundated, despite being quite far from the river. Despite being several metres above flood level, the ground was so saturated that a little water seeped up through cracks in my garage floor (the garage is cut into the ground). This is a trivial problem compared with what others have endured, but illustrates how saturated the ground was when the Wivenhoe gates were opened fully.

To help

  • If you would like to contribute financially to helping out with flood victims, I recommend doing so via the state government web site.
  • If you are in Brisbane and want to help, the best thing to do is to walk to your nearest flood site (with waterproof boots, strong gloves and any equipment you can carry).
  • The city council is bussing volunteers around, but their strategy of mustering volunteers at sites far from the rail network means you may be stuck in traffic for a long time getting in, going to your work site, and getting home again. If you aren’t in a location where you can help, look into the council volunteer scheme.
  • Volunteering Qld is also helping out but are currently overwhelmed with offers, so feel free to register with them to help out in the longer term but don’t expect to be used immediately.