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Sunday, March 20, 2011

The Insurance Council of Australia Reports on South East Queensland Flooding

Belatedly, I have come across the reports prepared by the Insurance Council of Australia by WorleyParsons, Water Matters International, and Water and Environment.  There are three reports, one dealing with theToowoomba flood, one dealing with the local effects of the Brisbane flood, and one dealing with the background and causes of the floods in the Lockyer Valley and Brisbane River.  The later two are in fact part 2 and part 1 (respectively) of a two part report, and of those two parts, it is the first part (and third linked) that interests me most.  That is because it is that report which contains the information relevant to the issue of the Wivenhoe Dam's operations and strategies that are continuing to interest me.

I approached the report with some trepidation.  Mick O'Brien and Hedley Thomas has relied upon it in their his attacks on the dam operators, and it has been widely reported as the basis for a potential law suite against SEQWater.  Contrary to my expectations, however, it is a fair and balanced document, and the second most comprehensive source of information on events leading up to the Brisbane 2011 floods available, with only the SEQWater report being better.  Its only methodological flaw is the lack of detailed information about Wivenhoe releases, which were not reported by SEQWater until sixteen days after the completion of this report.
(Update, 22/3/2011:  Mick O'Brien has denied relying on the ICA report, see 2nd comment.  As he undoubtedly knows his own mind, I therefore withdraw my claim made above.)

Allowing for this, what then is the conclusion of this report which is without conclusion independent of SEQWater and the Dam operators?

This report does not quantify the relative contributions of releases from Wivenhoe Dam and floodwaters from Lockyer Creek and the Bremer River to flows in the Brisbane River or along the lower reaches of these two tributaries.   Notwithstanding this fact, on the basis of recorded water level behaviour, it is concluded that releases from Wivenhoe Dam was the principal immediate cause of flooding along the reach of the Brisbane River downstream of the Dam, along the lower reaches of Lockyer Creek and the Bremer River, and along tributaries further downstream.
However, performance of Wivenhoe Dam during the January 2011 flood event is more complex than simply assigning flood causation to „dam releases‟.  The dam sits astride the Brisbane River; all upstream runoff must flow into the dam and become „dam release‟.  In January 2011, Wivenhoe Dam was called upon to accommodate massive inflows made up of surface runoff from the upstream catchment area, releases from Somerset Dam, and direct rainfall.  All of this water – generated by natural meteorological processes – had to pass through the dam.  The greater the volume and peak discharge of an incoming flood, the less effective are dams at mitigating flood flows, and the more constrained „management options‟ (releases) become for dam operators.  Pertinent questions in relation to the performance of Wivenhoe Dam during the 
January 2011 flood event are:
 Would the flooding have been worse in the absence of Somerset and Wivenhoe Dams?  In other words, did the dams mitigate the incoming flood?  If the peak inflow discharge occurred when water levels in the dam were rising (ie when inflow was greater than outflow), the peak flood discharge will have been reduced.  If the peak flood inflow occurred when water levels in the dam reached their maximum, the dam outflow at this time will have been equal the peak inflow (no mitigation).  In the former case, downstream communities will have suffered less flooding than in the absence of the Flooding in the Brisbane River Catchment, January 2011 ICA Hydrology Panel dams.  In the latter case, downstream communities will have experienced (more-or-less) the same „natural‟ flood that would have occurred in the absence of the dams.
 Were releases from the dam unnecessarily high?  Or did releases reflect prudent operations of the dam during the passage of a major flood event.  

 Would a different pattern of releases reduced downstream flooding to a significant extent?  Or was the volume of water in the flood so great as to eliminate options for effective mitigation.
It is beyond the scope of this Report to address the above questions.  However, this information is needed to provide a full interpretation of the primary cause of flooding in January 2011.  Thus, whilst correct, it is simplistic to assign the primary cause of flooding to „releases from Wivenhoe Dam‟ (dam release flooding according to the terminology of this Report).  As noted above, all inflows to the dam ultimately become „releases‟, and it may be more appropriate to assign flood causation to „succeeding storm events over the catchment area of the dam, each characterized by exceptionally heavy rainfalls and massive surface runoff volumes‟.

All highlighting is mine.  What I have noticed is that news items on this report, including by Headley Thomas, heavily focus on the underlined section above, but in doing so, they ignore the bolded section above.  In scholarly circles that is called "quoting out of context", or less  politely, "quote mining" and is considered a sign of intellectual dishonesty and bankruptcy.  In journalism it seems to be just standard practice.  In this case it leads directly to an interpretation of events which the Report calls "simplistic".

So what is meant by the underlined text?

Above are flood heights at the Brisbane Port Office as modeled by SEQWater.  You should focus particularly on Case 2 (purple) and Case 3 (red), which are respectively the modeled height of the flood if there had been no releases from Wivenhoe, and the modeled flood height if there had only been releases from Wivenhoe.  Clearly, without the Wivenhoe releases the flood would have been a moderate flood, and caused little damage.  The Wivenhoe releases added approximately 2 meters to the flood level, increasing a moderate flood to the most damaging (to property) flood in Brisbane's history.  On the other hand, without the other flood waters, the Wivenhoe releases would only have caused a minor flood in Brisbane.

One way of expressing that is to say that both the other flood waters and the Wivenhoe releases where necessary for the flood to be as damaging as it was, but neither were sufficient.  Of course, no human was in control of the other flood waters, but some humans where in control of the Wivenhoe releases.  That makes a crucial ethical and legal difference.  We can say that by saing, given the prevailing circumstances, the Wivenhoe releases were necessary and sufficient to cause the level of flooding in Brisbane.  The Wivenhoe operators were responsible for the critical difference in flood levels.

That is all the underlined section is saying.  It is not saying, as it has been interpreted, that the majority of the flood waters at the peak flow came from Wivenhoe.  Claims that Wivenhoe contributed 80% of the flood water at its peak, for example, are just absurd.

Of course, having said that the Wivenhoe releases made the critical difference, we then have to ask the crucial question, where the Wivenhoe operators mistaken or unreasonable to make the releases they did?  Note, we have to ask the question.  The facts that require us to ask it do not by themselves automatically answer it.  For that answer we must consider carefully the circumstances they were in, and what they could reasonably be expected to do in those circumstances.  The authors of the ICA Report are aware of this, and carefully delineate the type of facts that need to be considered, including suggesting a very likely answer to the question, that " may be more appropriate to assign flood causation to succeeding storm events over the catchment area of the dam, each characterized by exceptionally heavy rainfalls and massive surface runoff volumes".

They also point out that, "The greater the volume and peak discharge of an incoming flood, the less effective are dams at mitigating flood flows, and the more constrained 'management options' (releases) become for dam operators. "

"The greater the volume and peak discharge ..."

I will discuss in detail in a later posting the three questions that the ICA Report raise (although I have discussed them before, so you may well know my opinion).  Here I will content myself by noting that the answer to the first question is that the dam did indeed mitigate the flood.  In fact, peak outflows were approximately 40% less than peak inflows, which reduced flood levels at the Brisbane Port Office by 2.5 meters compared to what they would have been with no dam; and flow rates by 4,500 cubic meters per second (32%).  So the operation of the dam, without question, mitigated the flood.


  1. So simple.

    Peak outflows were less than peak inflows.

    Says it all.

  2. Tom I have made some additional comments under your blog called “Mick O’Brien and the Brisbane Flood 2011”.
    While I was very pleased to see the Insurance Council Report on the flooding I could not, as you suggested, “rely” on anything in it as it came out after my own report was substantially completed. However I was very pleased to see that it clearly supported my main conclusions.
    My report only suggests that releases from Wivenhoe were responsible for between 50% and 60% of the flow past the Brisbane City Gauge. I believe that the differences between Case 1 and Case 2 shown in your plot copied from the SEQWater report substantially support this conclusion and even potentially suggest a much higher contribution.

    Mick O’Brien

  3. Mick O'Brien, I stand corrected regarding your reliance on that ICA report.

    Regarding relative contributions to the flood peak, and referring to your section 7.1 and 7.2, I notice that you claim that between 866,000 and 949,000 mega-liters of water flowed past the City Gauge. Allowing for the typical time for water to travel from Wivenhoe to the City Gauge, you then calculate that 518,000 mega-liters was released from Wivenhoe over the corresponding period but 23 hours earlier. From this you calculate your 55% to 60% figure for the peak of the flood.

    However, the plot from the SEQWater report shows a peak level for Wivenhoe only flows of about 2.2 meters. Assuming incorrectly that that level was maintained for the entire 32.15 hours of major flooding, and using your flood rating curve, that represents a flow of 465,000 mega-liters over the period, or just 49 to 54% of the total. Thus, that plot does not support your contention, but significantly undermines it.

    Indeed, my calculation significantly overestimates Wivenhoe's contribution to the flood peak. During most of the period of major flooding, Wivenhoe only levels were significantly under 2 meters, indicating the total Wivenhoe contribution to the flood was probably between 42 and 47% or less at that time.

    It appears evident that your calculation is methodologically flawed, assuming as it does that water released from Wivenhoe will travel downstream as a mass, instead of spreading out and dispersing as water actually does. The very fact that the Brisbane River peak contributed substantially to flooding in the lower Lockyer and lower Bremmer shows much of the water from Wivenhoe was delayed in reaching the City Gauge, resulting in a peak of greater duration than the peak releases, but with lower flow rates.