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Wednesday, February 2, 2011

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.

20 comments:

  1. Tom,DERM figures say 2394 GL went past Savages Xing for all January 1974?

    I kept figures for this flood from 6/1/2011 to 17/1/2011,and they total 2426 GL. I missed the 18/1,but estimate at least 120GL,as this was the tail off of the manual-prescribed buffer discharge.

    Given the first few days of the month carried discharges from the late December floods,and the dam dropped 10% then,if we add the last two weeks of available data totalling 177.4GL, the monthly total for 1/2011 will be over 2700 GL.

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  2. Wow.

    The scans from the Cossins book are exactly how I envisioned a real historic flood graph should look.

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  3. Further rummaging through my notes gives and estimate for Lockyer Creek in January 2011.I have DERM figures from 9/1 to 17/1 and from 19/1 to end month. Estimating the missing period,we get around 500GL for the month's volume at Rifle Range Road gauge... so the dam component comes to around 2350 GL for the month.

    Carving up the flood numbers gives the Lockyer input at about 400GL,and Wivenhoe about 2100-2200...so closer to your estimate from the last post.

    If we bump in 300 flood gigs for the Bremer,and another 100 for the rest of the lower catchment,we get 3000GL,of which about 1000 was held out of the peak by the buffer.

    Lowering the highest three-day volume by up to 1000GL is pretty good work.

    And I reckon at the peak three days of the flood in Brisbane,non-regulated contributions kicked in at least 700GL to Wivenhoes 900-1000GL,which puts Hedley Thomas' 80% figure in the bin.

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  4. Sorry,2250,not 2350,in first para,second comment.

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  5. JCH, I've been sitting on that chart for a while now. My wife used it in a university assignment on the 1893 floods, but did not provide an explicit reference. I then came across the PDF version linked in the article, again with out an explicit reference. However, only one book was provided as a general reference in both my wife's assignment and the PDF. And then by happenstance, I found the book itself in my local library, and promptly copied the article.

    The stand out quote:

    "The greenhouse effect is expected to bring increased rainfall, including larger storms, to the Brisbane Valley as well as raising the sea level. ... The increased storm activity ... will bring more flood so, in spite of existing flood mitigation exams, average flood losses will increase. Larger floods amy threaten existing dams with damage and destuction by overtopping so that it may be necessary to spend tens of millions of dollars to improve the safety of the dams."

    So as far back as 1990, experts were warning that global warming would increase flood severity. Despite this, deniers continue to argue Queensland's supposed lack of preparedness was a consequence of accepting expert advice from climate scientists. For these people, the facts clearly do not matter.

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  6. Nick, you may find these comments from the Age interesting:

    "Wivenhoe was spilling three Olympic swimming pools of water a second for several hours on the Tuesday evening, and some have blamed this rapid release for Thursday's flood, when peak flows past the Brisbane River gauge approached four Olympic swimming pools a second.

    Professor Hubert Chanson, a flood expert at the University of Queensland, rejects this.

    ''To blame the flooding in Brisbane solely on the Wivenhoe release would be naive, simplistic and foolish,'' he says.

    The Brisbane River's flood peak lasted much longer than the period of peak release, causing flooding that lasted 30 hours and requiring the evacuation of thousands of properties.

    Water from the Lockyer Creek and Bremer River played a much larger role in the flooding than some commentators have allowed.

    Chanson points out that three large peaks of water arrived in 24 hours from the normally docile Lockyer Creek, at the same time the Bremer River was devastating Ipswich.

    The question, then, becomes whether the flow from Wivenhoe, which was already spilling on Sunday, could have been increased earlier to reduce the flood peak.

    But when?

    Some have argued that Wivenhoe's operators should have pre-empted the rain by releasing much more water on Saturday and Sunday, increasing the amount of spare storage.

    But a manual of operational procedures for flood mitigation at Wivenhoe Dam, made public on Thursday by the Queensland government, shows that South-East Queensland Water Grid follows a pre-determined flow chart for releases from the dam, based on expected dam levels.

    According to the flow chart, the releases made by SEQ Water Grid's engineers from Saturday to Monday indicate they did not initially expect the dam to reach levels that would trigger dangerous releases from the emergency spillway during the flood event.

    As a result, they were still obliged to do what they could to minimise disruption to rural areas below the dam.

    The manual also compels operators to maintain a certain level for drinking water purposes, stating: "There should be no reason why the dams should not be full following a flood event."

    Imagine the fury in a city recovering from a decade of drought if SEQ Water Grid had emptied half of its drinking water at the weekend, only to find there was less rain than forecast.

    In any case, SEQ Water Grid did respond on Monday, increasing releases to more than one Olympic swimming pool a second that evening, inundating several river crossings, and ensuring evacuations would be required in Brisbane early Wednesday.

    From this point, SEQ Water Grid was in a tough spot. Open the floodgates too wide, and more homes would be needlessly inundated, with little more than a day's warning. Open them too little and it would be forced to make heavier releases later to protect the dam, thereby worsening the peak of the flood.

    To balance this perfectly, when rainfall and river flows were still highly uncertain, would have required clairvoyance."

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  7. This is what I posted on Climate Etc. on January 18. I had commented several times before that to comments that were claiming dam mismanagement. A commenter named Michael had tried to reason with the mob. I thought what he was saying sounded reasonable. Not many any others did, but Michael got me digging for more information.

    Anyway, my prediction:

    JCH | January 18, 2011 at 2:04 pm | Reply
    It holds 2.61 million ML. It fills to 1.16 and then has to release anything above that in 7 days. When it is at 1.16 million ML, it can hold an additional 1.45 million ML to provide flood mitigation. I would assume those numbers were deemed sufficient to preclude a 1974-level flood. The dam reached, depending on the press report, around 190% of the capacity – 60 cm from topping the spillway. At that point they aggressively released water. What they feared was a water intake faster than they could release in order to avoid have (always embarrassing - ing) the water top the dam, which is earthen. As I said before, as a college student I was close by to where that happened, and it killed more people than have died in this disaster.

    What tilted the pinball machine is likely an extreme rain event in this dam’s intake network: one in excess of any seen in recorded history. That is my speculation and I think an inquiry will confirm it. ...


    Before this comment I made another comment about how downstream conditions could preclude releasing flood water in 7 days.

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  8. Thanks,Tom. Chanson's article is another straightforward appeal for some rationality.

    Hindsight reveals that slightly larger releases were physically possible on one or two early days of the event,and that could have helped clip a bit more off the peak when it arrived,but that is_just_hindsight. Such a finding would be a perfectly reasonable conclusion if an authoritative re-construction arrives at it,but it would not be the crux of any reasonable inquiry,which is really to reconstruct the weather and hydrology, the operating performance in real time,and calculate the no-dam and with-dam flood curves,not to point fingers.

    Yet too many people seem to think that hindsight is the useful equivalent of a real inquiry,and have passed judgement. I don't seem to remember any media /engineering voices at the time suggesting releases be continued below 100%,or increased on any of the days including the Saturday or Sunday. I may as well accuse these voices of failing to provide their expert advice to the operators. To Chanson's "Naive,simplistic and foolish" ,I'd add "presumptuous" to describe these armchair experts.

    Roger Pielke,Jr has added his fatuous voice,saying that ,"If it is in fact the case that management of Wivenhoe is made without regard to ENSO then this would be a case of decision making under wilful ignorance." Useless rhetoric. If you get a quick 300mm in the catchment under any phase of ENSO including neutral [and that has happened],you deal with it the same way. ENSO phases are not three or seven-day weather forecasts.

    Operating the dam in live flood circumstances is a pretty rare experience,and when a big flood tests capacity constraints a perfect result is very unlikely.It is a mitigation,not prevention,facility.

    No matter how many times this is pointed out this fact is ignored,as it seems with some respondents at Judy Curry's turgid blog,courtesy of JCH.

    As for the damage downstream,there can be no doubt that generations of poor planning decisions,made against some of the original advice of the Grigg report of 1977, is the real story.

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  9. Roger Pielke's current position appears to be that Wivenhoe is incapable of providing effective mitigation to large floods, so comparison of flood records in the 19th and 20/21st centuries without adjustment for the effect of dams is appropriate; but that Brisbane only flooded because the dam operators were incompetent and failed to mitigate the flood (that he does not believe they could have mitigated). Anyone who ties himself in such logical knots deserves only contempt. Clearly he is not interested in the coherence of his argument, only their rhetorical effect (a tendency I have noted in many of his posts).

    Judith Curry is almost as bad. As shewonk has observed, Curry feeds the denialists chum, apparently in attempt to gain popularity. She would rather be a big fish in a scummy pond than a small fish in a clean one. I tried swimming amongst the denialist feeding frenzy she calls "comments", but frankly, I could not take it. I prefer my disputants to be rational.

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  10. JCH, I read some of your discussion on Climate Etc. It's good to see at least two rational voices amongst that see of ignorance. Two minor points, though. The top of the dam (EL 80) was never threatened by the flood. The risk was that the first fuse plug on the auxiliary dam (EL 75.7 meters) would be triggered. In that event, any water over the full supply capacity would automatically drain over the auxiliary spillway anytime the water was over EL 67 meters, until the fuse plug could be replaced sometime in the dry season. That, of course, would make managing dam levels much more difficult, though not impossible.

    In the event, the water level eventually rose to 74.85 meters (according to the Australian), or 0.85 meters below the level of the fuse plug.

    The dam operation manual says:

    "This strategy normally comes into effect when the water level in Wivenhoe Dam reaches 74.0 m
    AHD. However the Senior Flood Operations Engineer may seek to invoke the discretionary
    powers of Section 2.8 if earlier commencement is able to prevent triggering of a fuse plug.

    Under Strategy W4 the release rate is increased as the safety of the dam becomes the priority.
    Opening of the gates is to occur generally in accordance with the requirements of Section 8.6,
    until the storage level of Wivenhoe Dam begins to fall.


    There are no restrictions on gate opening increments or gate operating frequency once the storage level exceeds 74.0 AHD, as the safety of the dam is of primary concern at these storage levels. However the impact of rapidly increasing discharge from Wivenhoe Dam on downstream reaches should be considered when determining gate opening sequences."
    (My emphasis)

    In fact, the dam operator would have been required to implement strategy W4B, the most urgent strategy whose key priority was saving the dam structure, if possible.

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  11. An addition to my previous comment, Peter Allen lists the Wivenhoe gate operation rules that existed just prior to the last revision (2009). Under those rules, if water exceded 74 meters, the gates were to be opened until the levels began to fall; and no gate opening restrictions applied (Table 2).

    Any notion that the operator did not follow the manual is just nonsense.

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  12. TC - I will admit many of my posts were shooting from the hip, and some of my hunches as to what happened were off. In those first days a lot of the information was not on the web - like details on the auxiliary spillway. I found it mentioned: built in 2005, but no real explanation. It said fuseplug. I imagined some sort of explosives! There were news stories and logic, and not a lot of information, versus this wild-eyed narrative of brainwashed dam operators practicing bad management that killed people. I think I did okay for a Texas cowboy. You provided the first links to real information, and it took me a couple of days to read through them. You're obviously very proficient at science and math. Nick too. Diggers for accurate details, and I admire that greatly. That's not really me. That's what was needed (which is why, once I saw you were interested in the flood, I kept coming in here to pester you.) Details like the Cossins scans. The BOM should have those style graphs on their website. The graph they have is dangerous in the hands of fools, and I can name them.

    Speaking of digging, I cannot find a map that shows where the Wolfdene Dam was to be located. An article says South of Brisbane. Can that be right? It would be interesting to discover how the Wolfdene Dam would have changed things. Most news stories emphasize its enhancement of drinking water supply for SEQ.

    I've found a bunch of quotes from Cossins. He wanted the Wolfdene Dam, but all he talks about is water supply. I'm certain he also considered flood mitigation, but he did not mention it in that article. It probably had a flood-mitigation component. How would it have worked in 2011? Hopefully the inquiry will include such an analysis.

    I read the manual the night you linked to it. If I remember correctly there is an emergency clause. I would presume activating it would have required a specific January forecast from BOM indicating an extreme weather event threatened to exceed their flood mitigation capacities. I can't see that they got one.

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  13. JCH, use Google Map, and search for Eco Park, Luscombe Queensland. Wolfdene is just the other side of the river. If you zoom out a little, and click on Satellite view you will get a fair idea of the layout of the proposed dam.

    The proposed dam was south of Brisbane on the Albert River, a tribuatery of the Logan River which runs through the city of Logan (big surpise, that one). Like most of the rivers in the region, the Albert is quite short. It's tendency is to have fairly frequent but relatively poor rainfall due to a good but small catchment. In contrast, the Brisbane Valley which contains the Upper Brisbane river is in a rain shadow. Consequently, although the Brisbane River's catchment is very large compared to other rivers in the area, it often has poor rainfall. The result is not so frequent, but larger floods.

    Because of this difference, I believe that the proposed Wolfdene dam was probably primarily a storage dam like the North Pine Dam, which is on the North Pine River (we Australians are very original namers). Consequently, while it may have had some flood mitigation, it would have been minimal. For what its worth, flooding on the Albert River and Logan River was minimal this summer.

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  14. I've often looked for a plan and detailed assessment of Wolffdene,and not found one.

    However,simply considering the site and catchment size,it would have never been the magic bullet that various political opportunists have painted it as.

    The Albert catchment is one tenth the size of Wivenhoe's,and while the Darlington Range is a pretty moist boundary for the east of the catchment,the sustainable yield of a largish dam would probably be in the 40-60 GL/annum area.Useful but not necessarily worth the inundation of high value agricultural/rural residential land.The dam would have been shallow with high evaporative losses like Wivenhoe. Increasing local land values was one of the reason the dam was killed,but I think its advocates always exaggerated its potential,particularly to attack Rudd in his post-state career.

    The only dam proposal on the Albert to reach assessment stage was Glendower,which involved a longish earth wall across the river well upstream from Tamborine Village and upstream from the Canungra Creek confluence,which provides half the rivers total volume. The site is somewhere near the bridge on the Albert that gets you over to Bidadabba and Boyland from the Tamborine-Beaudesert Road. This dam was envisaged to be coupled with Wyaralong,but failed cost/benefit analysis.I think the design was for about 80-90GL capacity and a yield of around 20/GL/a.

    It made better sense to raise Hinze in its very reliable catchment,build the cheaper Wyaralong as a sop to the big engineering fans,and to integrate de-sal and efficiency gains in the water grid.

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  15. So the notion out there that dam cancellations caused the 2011 Brisbane flood could be in some jeopardy? If true, the analysis that Pielke Jr. dislikes sinks him entirely. No wonder he doesn't like it.

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  16. Nick, like you I have very little information on Wolfdene, and most that I do have I do not trust because of its obviously partisan nature. I know it was not included in the recent review of various dam options in SE Queensland, but that may be for political reasons. (It may also be because there was not point pursuing it because it was canned for environmental reasons that have not changed).

    Now that Traveston is firmly ruled out, it seems to me the best option is raising Wivenhoe by 8 meters at the cost of a quarter of a billion dollars. Of course there are difficult issues with that, most notably that at least 50 houses upstream would then lie in the flood compartment. Raising Wivenhoe doubles full supply storage.

    The next best option would be the Cedar Grove dam, which yields 300 gigaliters storage for 0.8 billion dollars. It would be prefferable to my mind, in that it is on a different catchment, except that Wivenhoe provides nearly four times the storage capacity at a third of the cost.

    I don't like the option of a second desalination plant to meet SE Qld's supply needs as it would be very expensive, and more importantly, have a high carbon cost to operate. Consequently, I do think we need a new dam, or a much enhanced existing dam. But that's a supply issue, not a mitigation issue.

    JCH, the Wolfdene argument is that had it existed, Wivenhoe could have been run at below full supply capacity, thus enhancing the flood mitigation of Wivenhoe.

    That is possibly true. However, as existing modelling showed that from a full capacity, Wivenhoe could deal with a 1 in 500 year flood event without needing to release more than 4,500 cumecs at peak release; and without the water level coming within 1.5 meters of the first fuse plug, it is doubtful that anyone would have suggested it, or that any review of the proposal would have thought it warranted.

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  17. TC and Nick:

    http://www.newsawards.com.au/files/pdf/06/features-02_3.pdf?download=1&filename=features-02_3.pdf

    "In 1977, Trevor Grigg, a young engineer and now Professor and Deputy Vice-Chancellor (International and Development) at the University of Queensland, was commissioned to evaluate the economic, social and environmental effects of the future Wivenhoe Dam for the state co-ordinator general’s department. His comprehensive 159-page evaluation was one of the state’s first environmental impact-style studies.
    It is, in short, a remarkably prescient document. In it, Grigg writes in his introductory summary: “The water from the Dam will be needed in 1981/82 and the Dam should meet the demand for water until about 1995.” Later, he questions the efficiency of Brisbane water resource management in light of the expanding “number of authorities” involved.
    Eerily, Grigg foresaw our current dilemmas: “While some local supply problems could occur this century, some time early next century, provided the regional population continues to grow and after all the possible dam sites listed earlier are developed, severe supply problems of a regional scale will occur. Measures to reduce demand will be necessary or else alternative supply sources will have to be developed.”

    Doesn't sound like there would have been much room to jettison any drinking water supply components in 2011.

    Unless, of course, climate change indicated flood-mitigation conditions at the Wivenhoe were changing: incidents of more rainfall in fewer hours jeopardizing the dam wall.

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  18. Your absolutely right. There was no room to reduce water supply, and I am unaware of anyone broaching the idea prior to 2011. And it's a silly idea even now. It is based on the assumption that because we have just had a flood, therefore we are unlikely to have a repeat of the 2001 - 2008 drought.

    You may be interested to know that I have found out a little more about the key pusher of the idea. His recent commentary (prior to the flood) has been to the effect that Australia was not likely to suffer unusual dryness, and that the Murray Darling Basin Scheme (a project to protect inland rivers from drought) was entirely unnecessary. I strongly suspect that his commentary is politically motivated.

    Also interested to see in your linked article a bit more about Cossins'background. It appears that I could not have picked a more credentialed source for my post above.

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  19. Tom.

    At one point in the few days following the start of the 2011 flooding one of the managers of the Wivenhoe dam mentioned on ABC TV 24 the volume of water that had fallen in the dam's catchment, and compared it to the volume that fell prior to the 1974 flood.

    At the time I was agog watching the footage of the flooding, and I didn't click that the numbers where being given until after they were mentioned. Do you know what these broadcast figures were?

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  20. Counting, don't know the figure quoted on the ABC, but in the recent report by SEQwater, it was reported that the total combined flow from the upper Brisbane and Stanley rivers was 190% greater than in 1974.

    In the same report, it was revealed that without Wivenhoe and Sommerset, peak flow at the City Gauge would probably have been 14,000 cubic meters per second, significantly greater than in 1974. For comparison, the 1841 flood (the highest recorded since settlement) had an estimated peak flow of 14,100 cubic meters per second; and the 1893 floods had an estimated peak flow of 14,600 (the highest since settlement).

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