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59 responses to “Wivenhoe dam management”

  1. Hal9000

    Brian, a major factor in the ’74 flood was that the water coming down the river (again, much of it from the Bremer and Lockyer catchments) peaked at the same time as the Brisbane suburban creeks were peaking. A lot of the flooding around Brisbane was from the creeks backing up – e.g. Moggill Ck at Kenmore, Norman Ck at East Brisbane and Kedron Brook at Nundah. On this occasion, the Wivenhoe engineers were able to hold the upper Brisbane and Stanley peaks back for enough time to allow the peaks from Lockyer, Bremer and suburban creek flash flooding to subside before opening the sluices. There is no doubt in my mind that this substantially reduced the flooding in metropolitan Brisbane. Moreover, in 2010 most of the flooding in metro Brisbane occurred after the rain had subsided, making relief and rescue efforts more effective and less dangerous. Again, in ’74 the main river inundation occurred as the rain bucketed down.

    IMHO the Wivenhoe engineers deserve medals, but in all likelihood the OO campaign will mean they’ll get nothing.

  2. jane

    So, all the “experts” coming out of the woodwork don’t need to wait until all the information is in, collated and the inquiry held, Brian?

    And all are no doubt experienced flood mitigaters and could have imparted this 20/20 hindsight at the height of the floods? I’m astonished they didn’t come forward with all this sage advice when it was so desperately needed.

  3. Grace

    Is, perhaps, Professor Hubert Chanson the “expert hydrologist” you refer to?
    http://www.abc.net.au/news/stories/2011/01/21/3118379.htm?

  4. John D

    Brian: Good one. Of course there is nothing like hard facts to stuff up arguments.
    To my mind the idea of Brisbane running out of water is more frightening than floods – Where the hell would the Brisbane refugees go to, particularly given that the drought would not be restricted to the Brisbane area.
    Have you any feel for how long the various stages would last under current consumptions and the various stage restrictions.
    My feeling would be that we should kick in desal and recycling much sooner than the current politically timid policy allows for. In addition the trigger point for a sudden boost in desal construction should be sooner than the policy suggests.

  5. sg

    I sense the OO’s basic agenda here is to discredit the BOM and anything to do with climate mitigation planning. If they can pin the blame for the floods on any form of planning process even vaguely related to the BOM and climate-related mitigation, they’ll be reminding us of how this is all “too difficult to trust to government” for years to come, as a fatalistic and cynical argument against AGW mitigation/prevention.

  6. Jacques Chester

    I’d be more than happy to cut them some slack, pending the inquiry.

    Me too. But the tricky part, as you know, is that any scenario will be a counterfactual. We’ll never know what might have been done.

    Interestingly, decision makers under extreme pressure use a highly instinctive decision making process that does not resemble the process taught in decision theory. For a full (and very interesting) discussion, see Sources of Power: How People Make Decisions by Gary Klein.

  7. Polyquats

    The arm chair critics are really giving me the irits. People should wait for the evidence. I also think the guys from SEQWater deserve medals. Not only did they manage the dam, but they kept the water supply on to most of SEQ, with only small areas having boil water alerts.
    One of the impressive factors of the crisis was the way assets developed to protect us from drought were deployed to save us in a flood – desal water into the drinking water supply, and recycled water used for the clean-up all took pressure off a struggling, but still functioning, Mt Crosby water treatment plant.
    Of course, the role of certain elements of the MSM will be beating this up for all it’s worth. But I wish the ABC would behave better.

  8. sg

    Jacques, in this case the counter-factuals are clear – we have exact information on what was happening at the dam and what would have happened if things were done differently. The media just don’t want to look at that information because a) scary numbers omfg and b) pack of fucking arseholes.

    It’s not exactly “what would have happened if hitler was admitted to art school,” is it?

  9. Craig Mc

    I’m astonished they didn’t come forward with all this sage advice when it was so desperately needed.

    I wouldn’t be so quick to assume it wasn’t offered in a timely fashion.

  10. Kate

    Just as a heads up on storage, there had been talk for a while about damming the top-most reaches of Clarence in northern NSW, just 50km south of the border. That would have secured SEQ’s water supply, specifically the Gold Coast’s way better than the Wivenhoe could, therefore allowing the Wivenhoe to divert even more storage to flood mitigation. (Not that it would mattered much in this case)

    From what I’ve heard the NSW Govt apparently got the shits even thou QLD would have paid for both the Dam & the water, specifically because NSW wouldn’t get much benefit out of it. Long story short, the discussion was canned.

    When you think about it, it makes you wonder why some issues aren’t taken directly out of the hands of State politicians & handled directly by Canberra.

  11. John D

    Brian @6: What I really wanted to know was how much time is left after T3 is reached?

  12. Wozza's ghost

    sg @10 (“the counter-factuals are clear”)and Brian @7(“[the Australian] are evil cynical bastards.”)

    I thought we were waiting for the inquiry and the evidence? You can’t have it both ways. Unless of course you really mean “people who disagree with my view need to wait for the inquiry and the evidence”.

  13. John Quiggin

    To be clear, my post didn’t criticise the decisions made by the dam managers, but made suggestions for the future based on the experience of the flood. So, some responses:

    1. Taking all your data as given, I don’t find your analysis convincing. As you show early on, the fact that the dam reached 191 per cent necessitated a qualitative change in the management strategy, which greatly increased the required outflow from the dam. Given an extra 25 per cent buffer, there would have significantly more flexibility. Without having access to the operating rules I can’t say how much, but neither, I assume can you.

    2. You nowhere give any basis for your repeated assertion of “a few centimetres”, or even any range to cover this. Linear interpolation would suggest substantially more than a few.

    3. Flood damage is not a linear function of peak river height. A 4.45 metres flood submerged houses that would be safely above water level in a king tide of 2.6meters

    4. The extra cost of using recycling/desal more of the time is trivial in relation to either the cost of flooding or the social cost of water restrictions – rough guess would be around $100/household/year, or maybe $100 million a year, as opposed to many billions in damage from the Brisbane floods. On any plausible rate of return, this is a mistake reflecting an excessive focus on the internal accounts of the water business.

    5. An obvious modification to the proposal would be to adjust the target seasonally, aiming to go into the wet season with less than 75 per cent capacity and end the wet season with more. History suggests that flood risk is greatest in January, although of course climate change implies that we should not rely too closely on history.

  14. Alexander

    Argh! Argh! You cause me pain and I cannot finish this article!

    2.6 million ML, not 2.6 million MLs. You can’t pluralise these metric abbreviations! (That said, it should be 2.6 TL, just because the prefix “tera-” isn’t in the popular mind yet, and it is appropriate here.)

  15. Shaun

    Excellent post Brian. Wonderful job of pulling all this information together.

    Mott does seem wrong about one thing. He seems to insinuate that that there was no date for the weekend before flood and that SEQ took the weekend off. Of course he wrote his post on the 13th but today’s article in The Australian reveals emails that show the situation was being monitored over that weekend.

    In the end I have no issue with the management of Wivenhoe Dam being examined but it does seem to have strangely become an idealogical battle.

  16. Matt C

    Great post, lots of great information.

    Note that while no dam sites available Wivenhoe’s dam wall could be increased by 4m at the cost of about $138m. That would deliver around 481,000 ML of extra contingency

    See here http://www.qwc.qld.gov.au/planning/pdf/support-docs/provision-of-contingency-storage-in-wivenhoe-and-somerset-da.pdf

    On a separate note, why does almost every post on this site have to attack the Oz?

    I see nothing wrong in their articles, it’s not turning into a political game, no one is blaming Anna Bligh. But investigative journalism should ask these questions so they start the debate.

    You probably would not have written this article without theirs.

  17. Razor

    The Royal Commission hearings on this will be worth reading.

  18. marks

    JQ, your point 2.

    Given the high non-linearity of dam capacity with stage (height), you should not try to infer what you did using linear interpolation.

  19. Fran Barlow

    JQ said:

    An obvious modification to the proposal would be to adjust the target seasonally, aiming to go into the wet season with less than 75 per cent capacity and end the wet season with more. History suggests that flood risk is greatest in January, although of course climate change implies that we should not rely too closely on history.

    I find this an appealing idea. The link I provided in the open floods thread allows us to to look at flood events in QLD going back a fair way (I started browsing from the 1940s). There was a very definite cluster between October and April and within that, between November and February.

    As I’ve said elsewhere, the inflows to Wivenhoe were so large that it’s hard to see how even a bone dry Wivenhoe would have foreclosed flooding by very much this time around. The event itself was huge and of course it came on top of previously good rains. With Climate Change, we will probably get more such events more frequently, which does suggest a change in built development patterns in low(ish) areas near the rivers.

  20. Tia

    Brian @ 28, it’s Professor Neal Ashkanasy. He’s cited in this article by Tony Koch in The Australian:
    http://www.theaustralian.com.au/national-affairs/brisbane-saved-from-ruin-by-dam/story-fn59niix-1225992598096

  21. John D

    The link @30 has some very quotable quotes from Prof Neal Ashkanasy, a water resources engineer and psychologist who now works at the University of Queensland and a former national president of the Hydrology and Water Resources Institution of Engineers.:

    “Wivenhoe accounts for only 40 per cent of Brisbane’s run-off water, with a major effect on flooding in the city being Lockyer Creek and the Bremer River, which empty into the Brisbane River below the dam”
    “In my estimation, the Wivenhoe Dam kept 1.5m off the top of the 2011 flood, so its contribution to mitigation was significant”
    “Bear in mind that the really big floods in 1893 and 1941 were over 8m on the gauge, and this one was only 4.5m. When John Oxley discovered Brisbane 180 years ago, the local Aboriginal people were very agitated about flooding, and they showed him high-water marks that would have been 12m.”
    “The rules about the release of water are… mandated and cannot be played with, so it is my view the operators last week had little alternative other than to do what they did.”
    “1974 the then premier, Joh Bjelke-Petersen, argued strongly for politicians to have the say on when water should be released, but the then water commissioner, Peter Bevan “put his job on the line” by refusing to allow it.”
    “I have read where it is suggested Wivenhoe should have been kept at 75 per cent capacity. Imagine if that had been the case in the recent drought, when it went from 100 per cent to 17 per cent. What if the starting point then had been 75 per cent and not 100 per cent?”
    “Brisbane would never have needed water restrictions in recent times if the Wolfdene dam, which had been planned south of the city, had gone ahead instead of being scrapped by the Goss government after enormous protests by the local communities.

    The real danger with this inquiry is that the focus will be on flood mitigation. It is crucial that the inquiry is an integrated one that looks at both floods and water shortages as well as making the city less vulnerable to the effects of both.
    One thing is for sure. Brisbane will have to face worse droughts and worse floods than we have experienced recently at some time in the future. We need to be ready to deal with both.

  22. John D

    Brian @22: The diagram says that construction of drought resistant supplies would start at T2 which looks as though it would be around 30%. I am not sure that 30 months would be enough if construction of a desal plant was to start from scratch. There should be a very clear plan with conservative construction times and possibly stages that would allow the job to be stopped if the dam level rose.
    It is worth keeping in mind that the city would be in crisis once business has to start shutting down.

  23. Fran Barlow

    When John Oxley discovered Brisbane 180 years ago, the local Aboriginal people were very agitated about flooding, and they showed him high-water marks that would have been 12m.”

    I always love sentences like this. It’s up there with Captain Cook discovering Australia and Blaxland, Lawson & Wentworth discovering the route over the Blue Mountains.

    Discover in this sentence calls a whole paradigm.

  24. Roger Brown

    There is movement underway ! See -

    http://www.wivenhoedamclassaction.com

  25. Merv R

    After lotsa searching I’m unable to find the first instance of 225% being quoted. I’d always had it in the back of my mind that the original design parameters were to be one third for water supply & two thirds for flood mitigation.

    The reason I’m searching is that before the drought broke a friend of mine was working for Tyco Water on the water grid. He mentioned an international dam expert had been called in because the dam wall was moving downstream at a rate of 10-20mm pa iirc. They were investigating ways of adding more anchorages into the rock either side & downstream to arrest this drift.

    How many mm below overflow did it peak? Could it be they’ve reduced the original Safe Working Level of the dam wall as an increased safety factor?

  26. Colette

    Whose fault is it that Wolffdene dam was never built? A case of the Labor Party machine putting their electoral interests over the common good. If Wolffdene had been built I suspect we wouldn’t have had the water crisis of 2007, and SEQW would be under less pressure to maintain Wivenhoe’s “supply water” capacity.

  27. moz

    Fran, I can’t help reading that as “when … discovered the thriving community at …” Much as Oprah recently discovered Australia and one day Brian might discover the metric system.

  28. John D

    Fran: I have been married to a language teacher whose current specialty is teaching ESL and writing books for very very low level students. she has been trying to fix up my language skills since before we got married – So I think you are wasting your time.

  29. Fran Barlow

    Moz said:

    Fran, I can’t help reading that as “when … discovered the thriving community at …”

    Too charitable. Oxley must have assumed when he went to the land on which Brisbane now stands that there would be indigenes there. The first community he met (“discovered”) would not have seen themselves as living in the Brisbane area and would have been only one of quite a few whom Oxley had yet to “discover”. Discovery of a community here was revelation to Europeans and their incorporation into what came, through European policy, to be Brisbane. In short, Oxley “discovered” (i.e revealed) a piece of land that Europeans could usefully appropriate, sought data from the local indigenes that might be useful to the project and defined objective reality in terms that suited him. As the European narrative advanced, the indigenes were made a footnote and ultimately disappeared altogether, as they were when Blaxland et al. crossed the Great Dividing Range in 1813.

  30. Fran Barlow

    I wasn’t having a go at your syntax John D. I was exploring the cultural specificity of discovered.

  31. terangeree

    Until Oxley was directed by a shipwrecked timbergetter to the location of a broad, long river which enters Moreton Bay, the wider world did not know of that river’s existence. So one could say that Oxley did “discover” the Brisbane River (although I believe that credit should go to Richard Parsons, Thomas Pamphlett and John Finnegan).

    Yes, the river was there for a long time prior to Oxley. And yes, there were people living there for a long time before Oxley was born.

    But to the world in general, that river was unknown to and undiscovered by the wider world until.

    And commenting on to whom the credit is due is rather a distraction from the subject, isn’t it?

  32. Chris Grealy

    Thanks for an excellent article. However I don’t think it will make any difference to the LNP astroturf mob who are denigrating the great work by the people who manage the dams. And let’s not forget that in October 2010, the LNP wanted Wivenhoe to be filled up completely in case of another drought, despite the fact that this would have made it useless for flood mitigation. Appallingly bad judgement, and they got smacked down fast; but that won’t stop them from going after the government now – they have no shame at all. The line “anyone could have seen it was going to rain well ahead of time” is in full flight.

  33. yank been 2 Toowoomba

    There are many people who seem to think SEQ Water should have released drinking water storage, the 1.15 million ML component, to a lower level, or to have prevented the dam from reaching 100% by releasing enough water to hold it at a lower percentage as a “La Nina” strategy.

    Looking at the Wivenhoe dam, it does not have an obvious capability to do such a thing in a rainy season. Does anybody know the dam’s actually release capability form 100% full, the drinking water component (1.15 million ML, to a significantly lower level during the rainy season? There appear to be no large feeder tubes to a lower level hydro station. There is a single mini-turbine below one of the floodgate spillways. Versus 1.15 million ML in a rainy season, that’s an enlarged prostate. It has water supply tube(s). Again, these are probably small for the job of keeping the dam at a low level when water from a saturated basin is continually flowing into the dam.

    I think is likely the lower portion of the dam cannot be quickly evacuated. Does anybody know the actual numbers on it.

  34. Chris Grealy

    I love being fair. What I don’t love is when the LNP come up with these wonderful ideas which are totally bereft of thought and detail. Like the Rabbit’s latest idea – let’s build lots more dams! Um….. where Tony? But the clear implication is that the LNP would have prevented the flood, and their faithful will spread the meme far and wide.

  35. Jesterette

    IMO, I think the Wivenhoe was managed extremely well with people under great pressure to make proper, and very difficult decisions. But for the most part, the Wivenhoe couldn’t have prevented the tidal wave through the lower Brisbane valley and the Lockyer, which directly contributed to the floods. They aren’t part of any dam catchment system – and no proposed dam’s catchment to date includes these areas.

    I braved the trip to Esk last weekend to visit an elderly relative who was affected by the flooding on the same day as the Toowoomba flooding, and was astounded at the absolute destruction that has barely been reported on, due to the larger stories of Toowoomba, Brisbane, Ipswich and the upper Lockyer. I don’t think they’ve been intentionally overlooked, I just think the scale of the disaster through the mountain ranges out from Toowomba and the valleys is largely unreported and difficult to appreciate.

  36. Nick

    Yankb2T @ 44,I believe most of the volume of Wivenhoe is held by the gates,so releases down to the lip of the main spillway can be quite fast if needed. That takes you down to 40%. Below that things slow down with narrow pipe outlets via the small hydro unit and another outlet.

    Once again we have the ghost of Wolffdene dam summoned to remind us of the path we could have taken. Everybody knows that the ghost talks politics,but does it have a file or three so we can see the bloody specifics? No,never.

    Recently ,I have seen people claim that Wolffdene would have been as big as/bigger than Wivenhoe,and implicitly with a yield to match.They’ve even claimed it was meant to be Brisbane’s primary water source,never mind Wivenhoe.

    Simply getting to know the Albert River catchment completely destroys the technical claims. Wolffdene would have had a catchment one tenth the size of Wivanhoe’s,so getting anywhere near the sustainable yield of the latter is impossible. As broad and shallow as Wivenhoe,Wollfdene would have been worse. Fillable only in a flood,it would have had massive evaporative losses,and destroyed a lot of valuable land. The idea was crap,and the real estate story commented on by Brian is very much what I have heard as well.

    The actual idea developed for the Albert was abandoned ,too.Glendower was the site,upstream past the biggest horse studs,and east of Beaudesert,but still shallow. An 86GL dam with a yield after environmental flows of around 20GL/annum,similar to Wyaralong,with which it was to be paired as part of the south-eastern water strategy. Chicken feed.

  37. Roger Jones

    Brian,

    excellent post. As comprehensive as one can get without digging into the technical side of things.

  38. yank been 2 Toowoomba

    Nick – I read that it is “law” that any water above the lip of the floodgate spillways has to be released within 7 days. I’m certain there are situational exceptions: flooding downstream, but it appears to me their intention is to never store water above the lip of the floodgate spillways.

    The dam fills to 100%, and no further storage is allowed. That component is left empty for flood mitigation. I cannot imagine a PM would interfere with that. Here in the states it would take a legislative act. In any seven-day period, all flood water is gone if downstream conditions allow the release.

    At issue, is the cry that they should have lowered the dam as a La Nina strategy. I do not think the dam has that sort of capability in a rainy season. They would have to have outlets in the lower portion of the dam, that part below the lip of the floodgate spillways, that could release water much faster than rainwater flowing in from the basin. It’s an earthen dam. Best not to tease it with lots of big openings. Earthen dams seep water. More than a little Dutch boy’s thumbs can stop.

  39. jane

    @12, I feel pretty confident that wouldn’t be the case, or these “experts” would have been trumpeting “I told you so”, instead of “If they’d just asked me!”

  40. Nick

    Yank@ 49,the dam can lower quickly below 100%,policy aside,so the dam does have the capability you wonder about in your third para.

    This is the breakdown of levels: the foot of dam at riverbed EL23m to concrete bottom lip of spillway at EL57m is the first 40% of storage.Removing water below EL57 IS slow. The gates,16m high,sit on the lip and take the elevation to 73m [57+16].The next 60% of storage is EL57 to EL67,making up the 100%/1165Gl of full water supply component. This leaves 6m from water line at EL67 to the top of the gate at EL73 as the flood buffer,which rises to around EL75 in extremes. IOW in attaining the highest possible surface elevations[above EL73],the gates must be partially open,with inflows being sufficient to balance or exceed outflows and push up elevation.

    So the intention is to retain water at 100%,which means to never store water above the 67m line,6m below the top of the fully lowered gates. So,if there is mechanical failure and gates are compromised,at least there are 6 vertical metres [about 750GL] to fill while desperate repairs are made,and evacuation warnings sent!

    In flood mode the intention is to never store/impede water above 75m,because the fuse plugged secondary spillway soon comes into play after that,and that means uncontrolled release,and may threaten structural integrity. So the gates are opened further to keep all release directed through the main spillway. At full stretch the spillway can pass 13400cusecs,which is an almighty flood.

    This event saw a peak release of 7465cusecs from the gates for a short period.Downstream,this translates to a considerably lower peak flow,given the lower gradient,higher drag,broader river profile than the spillway.I’m not sure how much,but I think well more than 1000 cusecs lower peak at the next gauge,excluding tributaries.

    Unfortunately at the time of the peak releases,Lockyer Creek,entering just below the dam, was in major flood.I believe operators were trying to avoid a coincidence of peaks[this is operational policy],but the rapidly developing situation forced their hands.Holding on any longer meant perhaps reaching secondary spillway activation. However ,even with the addition of the major Lockyer flood,peak flow below the dam was still 500 cusecs below that peak at the dam gate.

    30% of peak period volume was from the Lockyer,and another unconsidered factor is the contribution of 500km2 of unregulated side catchments[such as England Creek and Lake Manchester] between the dam wall and the Bremer River junction. This 500km2 was absolutely deluged in the last 36 hours of the rain event.

    By the time the peak was approaching the city,I’d estimate the non-Wivenhoe[non-controllable] component at around 40% of volume,possibly more,but I’m an amateur.

  41. kika

    re our recent qld floods, there are many articles mentioning “greedy developers” and “inept local councils”, etc. 
    however, the real causes of the over-development of our cities (s.e. qld. in particular) are two-fold:

    1. the current tax system.  it rewards property speculation through negative gearing, reduced capital gains tax (thanks john howard), and generous allowances for expenses.  property development, therefore, is a great way to get rich, and is encouraged by our banks, tax laws, and governments.  we have too many buildings now, and the so-called ‘shortage’ of housing is highly debatable.  the oz. bureau of statistics shows that we currently have an excess of residential housing.
    the community (us) pays for the infrastructure and the services which add value to land.  a private individual can buy this land cheaply, leave it vacant until the community builds up the area, then sell it at huge profit – the latter mostly goes back into the seller’s pocket even though the community has paid for the increases in land value.  big developers are notorious for what is called ‘land banking’.  (This is a very incomplete summary.)

    it could be very different with a reformed, more efficient and much less expensive tax system which did not reward property speculation.  http://www.prosper.org.au

    2.  overpopulation.  australia’s birth rate is higher than most of the rest of asia, and the highest of any of the OECD countries.  we still give out baby bonuses.  we then bring in hundreds of thousands of immigrants, all of whom require more resources, services, and produce more waste and pollution – and lots more buildings – which are often built on flood-prone land. http://www.population.org.au

    get the picture?

  42. Electrical Engineer

    Brian said:

    On the basis that a cubic metre of water is 1,000 litres, I did some calculations on how much water would flow past a given point in 24 hours. You have a one-off saving of 287,000 ML from your 25% of dam storage.

    Between 7:30 am Tuesday the 11th and 1:00 am Thursday the 13th, the flow at Savages Crossing (below Lockyer Creek) exceeded 3,500 m3/s, the limit allowed by the manual for non-damaging floods at Lowood. The volume of water in excess of 3,500 m3/s flowing past Savages Crossing during that period was 280 GL.

    Compare this floodwater volume of 280 GL with 25% of Wivenhoe FSL, 290 GL.

    Quite clearly, the volume of floodwater above 3,500 m3/s could have been absorbed by 25% of Wivenhoe. There would have been no major flood in Brisbane if the flow at Savages Crossing had been limited to 3,500 m3/s.

    Also, I should point out that Wivenhoe started at 115% on the morning of Sunday the 9th because the manual requires the dam to hold back 15% if necessary to keep Fernvale and Mt Crosby bridges open. That 15% caused a lot of damage in Brisbane, far more than the cost of two high-level bridges.

  43. Electrical Engineer

    Brian, you asked:

    You cite a flow rate for Savages Crossing for a time when I think the flood was peaking around the CBD. It was certainly in retreat by Friday 14. If Ian Mott is right it takes 36 hours for the water to flow from the Wivenhoe to the CBD. I think the Toowoomba/Lockyer deluge happened on Monday 10, didn’t it?

    The Lockyer Creek gauge at Rifle Range Road peaked sometime between 10 am Tuesday morning and 10 am Wednesday morning. We don’t know exactly when because the gauge stopped working in that period. At a guess I’d say between 9 pm and 10 pm Tuesday night which, given that it takes a few hours to flow from Rifle Range Road to Savages Crossing, more or less coincided with the maximum release rate from Wivenhoe. It was certainly in the period when they were forced to release most of the inflow.

    The basic bit of information I provided is that we were told that

    the Brisbane River would have been flooding at the rate of 13,000 cubic metres per second, if Wivenhoe did not exist, compared to 9,500 in 1974.

    I’m a bit dubious that there was 13,000 cubic metres per second inflow to the dam for any significant period of time. The reason is that the fastest rate of rise of Wivenhoe was 0.2 metres/hour between midday and 1 pm on Monday. This was about 28 GL or 7,800 cubic metres per second. It was also rising about 1/6 metre per hour for a much longer period than one hour which is equivalent to 6,500 cubic metres/hour. I’m going by the gauge on the dam and the gauge at Savages Crossing for my earlier figures. You may have encountered a case of “don’t believe everything you read in the papers”.

  44. Electrical Engineer

    Brian also said:

    We were also told that at one stage water was entering the dam at the rate of 2.6 million ML per day, but I don’t know for how many hours. (Actually I query that figure. I heard on the radio it was two Sydney Harbours, or 1 million ML pd for a time.)

    This flood was actually two floods, one occurring centred on Sunday night which the dam absorbed and the second occurring centred on Tuesday night which they had to let through because they had a full dam.

    The dam rose from 68.71 m at 5:40 pm Sunday to 72.93 m at about 6 pm on Monday, which is around 550,000 ML. During that time they released 170,000 ML past Savages Crossing which includes Lockyer Creek water so the inflow to the dam in that time was about 720,000 ML minus the Lockyer flow, i.e. nothing like 2.6 million ML per day.

    I haven’t been through the figures in detail for Tuesday night but the figure I checked suggests a similar inflow rate on Tuesday evening as on Sunday night/Monday morning. So talk about a rate of 2.6 million ML per day just looks like exaggeration to me.

    They certainly took the front off the flood by absorbing the flood on Sunday evening/Monday morning but as for how much they took off the Tuesday night flood, I’m not sure they took much off it because they already had a full dam so were forced to more or less let the whole flow through.

    You might have noticed I mentioned that they let 170 GL past Savages Crossing from 6 pm Sunday to 6 pm Monday during which the dam was rising rapidly. The dam operators were allowed by the manual to release 300 GL/day past Savages Crossing so they released 130 GL less than allowed just during that single 24 hour period. They could have had a lot less water in the dam on Tuesday when the second flood arrived but their response was very slow until the dam fuses were in danger.

    I think it’s time to wait for the true narrative which should come out during the royal commission.

    There’s not much information easily available at the moment. I hope this issue get the factual publicity it deserves because I don’t know why they didn’t do a lot better on Sunday and Monday.

  45. Robbie Laurence

    Sorry Brian, you have confused the levels used to forecast tides with AHD levels when you quote tide levels at Brisbane City. This puts your tidal data at about a metre too high (at 2.6 metres AHD, the river is already flooding). Unfortunately, this means your assessment of the impact of reducing the level of Wivenhoe to 75% is based on wrong assumptions. Further, your assessment of the statistical likelihood of floods occurring in the river is so broad that it is meaningless to use them in your discussion. I know you are just ‘shooting the breeze’, and why not, but I thought I should let you and the readers of your ‘blog’ know.

    Robbie