2011 and History: How big was Brisbane's flood of 13/1/2011

In this post I will be returning again to the size of the Brisbane Flood.  The reason for discussing these issues again is that  I have some new and interesting information, and also because these issues continue to be revisited by people who are trying to drum up a witch hunt against either the dam operators or the state government (or both).



Although this post is going to be about the flood in Brisbane, I must emphasise that there have been much worse floods around the world this January than those in Brisbane (and far more people effected by flooding in Australia outside Brisbane than in).  In particular, the floods in Sril Lanka, South Africa, and Brazil have been far more devestating both in terms of number of people impacted, and in lives lost.  Despite my distress about the misery of our friends overseas, I will continue to blog on the Brisbane flood because I am a local, and have access to local knowledge.  But I do not the other sufferers to be forgotten because of the saturation coverage (in Australia) of the local events.

As many people have observed Brisbane is no stranger to floods.  In fact, in terms of water levels through the city, 2011 represents only the eight largest flood in the history since settlement.  The largest since settlement was the 1841, which was 7 centimeters higher than the famous flood of February 5th of 1893.  The most damaging were the floods of 1893, where in four floods hit Brisbane in two months.  The most damaging of these occurred in February within a week of each other, and ranked as the second and third highest floods since settlement (respectively).  The second major flood was not entirely bad news.  It floated two ships that had been stranded in the Botanical Gardens in the first flood back into the river (though apparently the salvager was still paid his fee).The 1974 flood only ties for fifth spot in the rankings since settlement, while the 2011 flood only comes in at eighth.  All these pale in comparison compared to some of the floods pre settlement, with geological evidence for flood heights up to 11 meters existing in Indooroopilly.

(Brisbane City flood heights unadjusted for the effects of dams, BOM)

Of course, what such comparisons miss is an awareness of the impacts of dams and drainage on flood heights.  It is often stated that the effect of the dams is to remove 2 meters from the flood heights, but that is inaccurate.  For a start, it depends on where the rain falls.  As indicated by Geoffrey Cossins (see chart below), Somerset Dam removed only 1.26 meters from the height of the 1974 flood because much of the rain fell in Brisbane (656 mm over four days) while only a relatively small amount (507 mm) fell over the Stanley river where it could be trapped by the Sommerset Dam.  In contrast, in the first (and largest) flood of 1893, 939 mm fell over the Stanley and only 288 mm over the city.  Had the Sommerset existed in 1893, that flood would have been reduced by 1.56 meters in height.

Estimated heights of historical Brisbane floods assuming, no dams, Somerset, or Somerset and Wivenhoe Dams; from Geoffrey Cossins, "Surface Hydrology: Water Supply and Flooding", in ‘The Brisbane River’ P. Davie et al. 1990, The Australian Littoral Society {and Qld Museum}.  The green line shows the height of the 2011 flood; the blue line the height it would have achieved without the presence of dams.  The red bars show highlight the height some historic floods would have achieved if both Wivenhoe and Sommerset had been constructed.  The yellow bars show the actual height of the two largest 19th century floods, the difference being because of changes in the Brisbane River due to dredging. An umodified reproduction of this graph can be found on the PDF reproduced here {page 7}.

Of course, these figures are just for Somerset and do not include the effect of the Wivenhoe.  The Wivenhoe Dam would have reduced the height of the 1974 flood by another 2.16 meters, and the first 1893 flood by another 2.2 meters.  So the combined effect of the dams is to reduce flood levels by 3.4 to 3.8 meters.  (If you want to use an approximation, it is 2 meters per dam.)


It is, unfortunately, not possible for me to determine the depth the flood of 2011 would have achieved without dams on this information with any precision.   I can, however, compare its actual depth to the probable depths  of historical floods if the two dams has been built at the time of those floods.  Making that comparison we find that the highest floods since settlement would have been, if Wivenhoe and Somerset had both been built at that time, in ascending order:


17/2/1893 at 3.31 meters,
 4/2/1893 at 3.36 meters,
14/1/1841 at 3.43 meters,
27/1/1974 at 3.48 meters, and
13/1/2011 at4.46 meters.

(Note, these figures are all expressed in terms of the Australian Height Datum, approximately mean sea level.  Earlier flood heights have been expressed as above the low tide mark, a one meter difference, which can cause some confusion.)

These figures should not surprise us.  We know that more rain fell in 1974 than fell during the flood of the 17th in 1893.  As 1974 was accompanied by a storm surge as well, it is not surprising that it would have been larger than 1893, or even 1841 without mitigation.  We also know that Wivenhoe and Somerset between them held back around a third of the water that would have flowed through Brisbane, and that even so, more water flowed down the Brisbane River at its peak than flowed down during 1974.

We can look at it another way.  Based on the rule of thumb above, apart from the effects of Wivenhoe, Brisbane would have faced, not 4.5 meters of water, but around 8 meters of water flowing down its river in 2011.  That is probably the most important single fact that should be kept in mind by anyone tempted to seek somebody to blame in the aftermath of the flood.

(Edited 12/3/2011.  New modelling has shown the the height of the 2011 flood without dams would have been seven meters at the port office.  On that basis, it would have been the fourth largest of the historical floods.  Based on flood volume, it is the third largest at 14000 cumecs ignoring the effects of Wivenhoe and Sommerset.  That places it  behind 1893 at 14600 cumecs, 1841 at 14100 cumecs, but ahead of 1974, which would have reached 10,360 cumecs where it not for Sommerset.)

The impact indicated by Cossins has been confirmed by later studies.  A 2003 review of expected flood levels in Brisbane found that the probable effect of the two dams in a Q100 (1 in 100 year) flood was to reduce peak flow levels at the city by 50%, from 12,000 cumecs (cubic meters per second) to 6000 cumecs, with a probable range fro 5000 to 7000 cumecs (Table 4.3).  That translates to a level of 3.3 meters at the city gauge (range of 2.8 to 3.8 meters).  (Comparison with the data above suggests they may need to work on their definition of 1 in 100 years.)  

Indeed, it can be shown to be plausible from publicly accessible data.  In 1974, two thirds of the total volume of water flowing down the river at its peak came from the Upper Brisbane River and Lockyer Creek (see chart below), or approximately 6,300 cumecs of the 9,500 cumec peak flow.  This resulted in a total flow of water past Savages Crossing (below the confluence of the Brisbane River and Lockyer Creek) of 2.394,206 megaliters over the entire month of January 1974.  During the 2011 flood, Wivenhoe held back over 1,000,000 megaliters from the flood.  A similar performance in 1974 would have held back at least 30% of the volume of the flood, and probably more.  The estimate  necessarily underestimates the effects of Wivenhoe because I have to work with the Savages Crossing flows for the entire month, not just the four days of the flood.  If, for example, only half of the flow during that month came during the flood, then Wivenhoe would have reduced the flood height by 55%.

(Contributions to Brisbane 1974 flood peak, from G Cossins, 1990.  S.D. stands for Somerset Dam releases)

So, however surprising it may be to some, in January of 2011, Brisbane faced the largest flood threat it has seen since settlement, ie, in 186 years.  That it came out of it in much better shape than in previous floods is thanks primarily to the Wivenhoe Dam and its operators.