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Friday, January 14, 2011


The Brisbane flood peak is over, and the peak was below that predicted on Tuesday, though one meter higher than that predicted when I made my last post on the floods.  Unfortunately the flooding in the capital has drawn focus away from victims who have suffered far more from the floods: those west of the range whose towns have been flooded, two, three and in one case five times in less than a month (and who in many cases were also flooded in March); and above all, those in Toowoomba and the Lockyer Valley who faced the devastation of the "inland instant tsunamis".

It has also allowed AGW deniers to start playing there predictable, but disappointing game of "blame the victims".  The floods, we are told, was only so bad because because of inadequate mitigation, poorly managed.  And only a problem at all because fools built houses on flood plains.   Oh, and above all else, the flood did not exceed what commonly occurs due to natural variability.

Well, let's consider that last point first:

(Brisbane City Flood Heights, from the Bureau of Meteorology)

Today the Brisbane River peaked at 4.46 meters, 1.04 meters less than the peak predicted at one stage, and 0.99 meters less than the 1974 peak (as measured at the city gauge).  Yesterday, the Bremer River peaked in Ipswich at 19.40 meters, 2.1 meters below the predicted peak, and 0.6 meters less than the 1974 peak.  That makes this years flood the seventh deepest in Brisbane since settlement in 1824, and the third deepest at Ipswich.

These figures appear to support the claim that the 2011 flood (so far) is nothing out of the ordinary, but that assumption ignores certain key facts.  The major difference between 1974 and the floods of the 19th century was the existence of the Somerset Dam on the Stanley River (one of the Brisbane Rivers major tributaries).  According to the BOM, 25% of flood waters in the Brisbane River system normally pass that site, and the dam is designed to hold back 524 Gigaliters of flood storage on top of its normal capacity, a volume of water just larger than that in Sydney Harbour.  Famously, Clem Jones, then Mayor of Brisbane demanded that city engineers not release water from the dam until the flood peak had passed, thus using the entirety of that extra capacity (and placing the dam at risk).  His gamble together with the Brisbane Rivers deeper channel due to dredging prevented the '74 floods from being a near rerun of the 1893 floods.

The influence of the Somerset Dam can clearly be seen on the chart above, with the absence of even moderate flooding in Brisbane, with now just two exceptions.  (The influence of dredging can be seen already in the 1900 to 1950 record.)  However, sobered by 1974, Clem Jones ordered the refurbishment of Brisbane's flood drainage system, and the Bjelke Petersen government constructed the Wivenhoe Dam.  The Wivenhoe Dam  is located on the Brisbane River below the confluence of the Stanley River and also Lockyer Creek, but above the confluence of the Lockyer Creek and the Bremer River.  It has a flood mitigation capacity of 1,450 Gigaliters, or just short of three Sydney Harbours (or three times that of the Somerset Dam).  The improved drainage and the presence of Wivenhoe more than compensates for the cessation of dredging so that Brisbane residents could enjoy a river in their midst rather than a sewer. 

Given these massive flood mitigation measures, Brisbane has been confident that repeats of '74 style floods will be at best rare.  Indeed, it is difficult to see how Brisbane could improve its flood mitigation.  The top of the Wivenhoe dam lies level with the valley floor that surrounds it, so that its capacity could only be significantly improved by building a 60 odd kilometer wall across the entire Brisbane Valley.   No other suitable sites for a large capacity dam exist in the Brisbane Valley.

So, how did the Somerset and Wivenhoe dams fair in their latest test?

On Tuesday the 4th, the Wivenhoe dam ceased drawing down its level from storage that prevented, according to government announcements, a near '74 flood in Brisbane last week.  At that time, its storage was 2.1% over normal capacity.  Over the following days it increased its storage until it peaked at 88.5% over normal capacity on Wednesday the 12th.  That represents an increase in storage of equal to the volume of two Sydney Harbours.

During the same period, the Somerset Dam increased its storage from 2.9% over normal capacity to 89.7% over normal capacity.  That represents in increase in storage equal to two thirds of the capacity of Sydney Harbour.  Between them that represents a stored water volume approximately equal to 90% of the entire flood water volume that passed through Brisbane during the peak period of the 1974 flood.  Without the flood mitigation from those two dams, the 2011 floods would have significantly exceeded in level those of 1841 and 1893.  As it happens, even those extraordinary measures would not have saved Brisbane from a '74 style flood were it not for the far improved drainage in Brisbane, and the fact that the rain stopped twelve hours earlier than expected, and (unlike in 1974 and 1893) there was no accompanying storm surge.

The telling difference between 2011 and 1974 is in the sheer volume of rainfall involved.  Reports indicate that over the last week, twice as much rain has fallen in the Brisbane River catchment as fell over the equivalent period in 1974.  And tellingly, unlike in 1974 (and 1893) this has not been because of the influence of a tropical cyclone.  The general weather pattern has been one which might well have brought about moderate flooding (in the absence of mitigation), but has lacked the normal signature cause of extreme flooding in Brisbane, the influence of a tropical cyclone.

This has been reflected in the experience in Dalby, in Emerald, in Condamine and in dozens of other places around Queensland (and now New South Wales, Victoria and Tasmania).  A weather pattern that in the past would have brought rain, and an occasional flood is bring floods everywhere, and record breaking floods in many locations.  (I should probably say the three weather patterns, because the weather system bringing floods to NSW and Queensland is not the one bringing floods to Victoria, Tasmania and the Northern Territory.  The one that brought floods to Western Australia last week was different again.)

This is most graphically indicated by the experience of Toowoomba and the Lockyer Valley.  The one storm system produced events not seen before at either location since settlement in around 1850.  There has simply been no comparable experience in either area.  And though one storm produced both events, the water that fell on Toowoomba flowed west, and did not contribute to the tragedies in the Lockyer valley (nor to the Brisbane floods).

The recent flooding in South East Queensland is, beyond question, unprecedented in the last 150 years of history.  That does not mean it proves global warming (and nor have I seen any suggestions that it does).  Rather, it is weak evidence for global warming, but certainly not outside the range of natural variability.

But let's not have the silly, and offensive suggestions that Brisbane was unprepared for floods due to inadequate mitigation; or that Toowoomba (which is on the crest of a range) is some how on a flood plain.  And let's also avoid the idiotic evasions of evidence that suggest that looking at a bar graph of flood heights without consideration of the effects of changes in flood mitigation can in any way be informative.

Further reading:
Bureau of Meteorology report on the 1974 Flood
Recent river height data
Brisbane and Ipswich flood history


  1. TC - do you know if during 2010 anybody was advising the Wivenhoe Dam be lowered below its drinking water component of 1.16 million ML?

    I see press reports where people were demanding the Wivenhoe should store even more than 1.16 million megalitres, but I can find no sage who was demanding the dam be drained in advance of the summer rains.

    Now, after the storm, the internet is full of genius, and they all seem to think this sort of move by the dam managers should have been obvious.

    I would quantify the amount of rainfall above the Wivenhoe required to support such a move to likely be in excess of any rainfall seen in recorded history, or at least very close to it.

  2. No, I am not aware of any such comment before the recent floods. About three months ago, Brisbane's Lord Mayor, Campbell Newman warned that Brisbane might experience a 1974 style flood this year, if we were hit by a cyclone (a qualifier that seems to have been lost to the armchair critics). And in 2000, a report recommended that the spillway capacity of Wivenhoe be increased to avoid the risk of excess rainfall causing the dam to overflow, which, which because the dam wall is a concrete faced earth embankmant, would destroy the wall. But this is the first week I have seen recomendations to increase Wivenhoe's capacity (which I don't think is practical) or in one case, to reduce the storage capacity to allow more room for flood mitigation.

  3. When I was in college I worked a summer job that was close by a small earthen dam that broke during an extraordinary deluge. There was a tremendous loss of life and the dam giving way played a big part in that, so I can fully appreciate the need for adequate spillway capacity. It sounds like they added a spillway in 2005.