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#1093634 - 20/03/2012 18:27 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Steven]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
I don't believe that the inquiry ever got a satisfactory answer to the question about why the advice re: lowering the dam level was different in late 2010 to what it was after the flood.

That is my main disappointment in the dam management really. Not that they should go to jail for that. I don't think they did any thing different to what the majority of the population expected (i.e. as little as possible).

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#1093714 - 20/03/2012 19:54 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: somebody]
Dave-Wx Offline
Weatherzone Addict

Registered: 27/08/2001
Posts: 4950
Loc: Heritage Park, Brisbane, QLD
Originally Posted By: somebody
Originally Posted By: Dave-Wx
The dam filling tap doesn't open quite so easily. Lets not forget that that sizeable inflow events for Wivenhoe (ie: 20 to 30% - remembering we need 25% to bring it back to normal) happen only a couple of times every decade, and then narrowing it down even further to the period from April and May (I can't see them raising the FSL before April 1) makes the chances of that happening before winter in any year quite miniscule.

Have you been watching the dam inflow this season? I would be very surprised if 25% of FSL won't end up having been let go between 1 Feb and 1 April. And that's in spite of the collapsing La Nina.


Probably not as closely as you and some others, maybe there has been 25% flowing through the gates in the last few months, but again this is the part of the year where they are not going to let it increase from 75 to 100%. They will try to let it increase from 75% AFTER the major 'danger period' of the year has passed...which I imagine is no earlier than April 1 (given that the monsoon can be firing strongly well into late March - as we can see this year).

Also, a moderate to strong La Nina is never a gaurantee of above average rainfall, even in 2011 (post-floods) there wasn't much in the way of significant events that would've helped push the dam up to 100% again.

The bottom line here is that there will always be a significant risk to water security (how many years of water supply is 25% of Wivenhoe again - somewhere between 1-2 years?) while ever they reduce the dam to 75%. This applies unless there are new water sources that are able to provide such water volume that it can significantly increase Wivenhoe. Was Traveston big enough to do this sort of thing with piping over long periods of time? I'm not sure how big it was supposed to be.

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#1094024 - 21/03/2012 00:45 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Dave-Wx]
P Man Offline
Cloud Gazer

Registered: 23/01/2011
Posts: 21
Loc: Cambridge, Waikato NZ
Did the enquiry ever actually quantify its conclusion that the dam water should not have been released during major flooding? In other words,how much lower would the flood have been IF the manual had been followed?
All this talk of court action and jail terms seems so extreme. It all sounds like the entire flood would have been avoided if the manual had been followed.

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#1094063 - 21/03/2012 06:29 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: P Man]
Tarks Offline
Weather Freak

Registered: 30/01/2011
Posts: 389
Loc: Moorooka ( Archerfield )
Originally Posted By: P Man
. It all sounds like the entire flood would have been avoided if the manual had been followed.


Well in my suburb it would have I believe. The rain had backed off the couple of days before peaking , at times the rivers and creeks rose rapidly in heavy rain but they also drained fast back down also. The flooding was caused %90 by Brisbane river breaking it's banks and it broke it's banks because of the extra water coming down from Wivenhoe during release.

The only way it would flood here like in 2011 would be for sustained heavy rain ( 200mm an hour ) for hours on end which would be your 100 year flood scenario.

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#1094286 - 21/03/2012 13:58 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Dave-Wx]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
Originally Posted By: P Man
Did the enquiry ever actually quantify its conclusion that the dam water should not have been released during major flooding? In other words,how much lower would the flood have been IF the manual had been followed?
All this talk of court action and jail terms seems so extreme. It all sounds like the entire flood would have been avoided if the manual had been followed.

I'm not aware it made such a conclusion at all. If anything, I thought the engineers were under the pump for NOT releasing water.

Manual was found to have been breached on two points, one of which is the registration of the engineers. The other is the inclusion of rainfall forecasts. It seems that the engineers interpretation of the manual and the commissions are at odds on this point, but it has been clarified in the new manual. If these had been included, W4 would have been selected before Mt Crosby Weir & Fernvale bridges were closed! One has to presume that this would have resulted in an even larger flood in the Brisbane valley without invoking the "reasonable discretion" clause.

Originally Posted By: Dave-Wx
They will try to let it increase from 75% AFTER the major 'danger period' of the year has passed...which I imagine is no earlier than April 1 (given that the monsoon can be firing strongly well into late March - as we can see this year).

I reiterate: This point is the part I am particularly against. There is a tension between water security and flood mitigation.

TV reports around Feb 2011 when they release 25% of Wivenhoe referred to it as a years supply of water.

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#1094319 - 21/03/2012 15:42 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Tarks]
Brett Guy Offline
Meteorological Motor Mouth

Registered: 05/10/2010
Posts: 5079
Loc: Bently Park, Cairns
Originally Posted By: Tarks
Originally Posted By: P Man
. It all sounds like the entire flood would have been avoided if the manual had been followed.


Well in my suburb it would have I believe. The rain had backed off the couple of days before peaking , at times the rivers and creeks rose rapidly in heavy rain but they also drained fast back down also. The flooding was caused %90 by Brisbane river breaking it's banks and it broke it's banks because of the extra water coming down from Wivenhoe during release.

The only way it would flood here like in 2011 would be for sustained heavy rain ( 200mm an hour ) for hours on end which would be your 100 year flood scenario.


Is that what happened in 74 Tarks?

Because I was under the impression that inflows in 2011 were far greater than 74(more like 1893 in fact), yet flood levels were kept BELOW 74.


Edited by Brett Guy (21/03/2012 15:46)

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#1094327 - 21/03/2012 16:15 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Brett Guy]
Locke Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 4451
Loc: Brisbane
In 1974, the falls around Brisbane were substatially higher. Enoggera recorded over 500mm in a 24 hour period. Other areas like Mt Gravatt and Indooroopilly had in excess of 300mm.

The falls around Brisbane in January 2011 were far lighter.

It has been reported that Wivenhoe would reduce the 1974 flood level by 2m (that would be down to 3.45). The majority of the remaining flood level would be very much attributable to rainfall below the dam.

This is why it is foolish to suggest you can release 2,500-3,000m3 of water from Wivenhoe and confidently predict only $6 million in flood damage.

This is utter BS and completely ignores the limitations of current weather forecasting. Here's an interesting question for you.

On Tuesday 11th January from approximately 6am to 3pm the following stations recorded 340-380mm of rain. Lowood, Mt Glorious, Wivenhoe TW and Savages Crossing. Some of these stations were below the dam, others resulted in inflow into the dam.

We certainly saw the result of this very heavy rainfall in the speed at which the dam level rose on Tuesday.

So what do you think might have been the impact if this rain had all fallen below Wivenhoe and what do you think might be the result if you were releasing 2,700m3 from the dam and rainfall of this magnitude were falling over Brisbane.

There seems to be a double standard here. An expectation that they should plan for the 1 in 100 year fall into the catchment and ignore the possibility of the same type of fall below the catchment. The stupidity of this position is clearly illustrated by the 1974 rainfall distribution.

Why don't you try modelling your early release strategy for the 1974 flood and tell me how it would compare to the strategy used in January 2011 if it were applied in 1974.


Edited by Locke (21/03/2012 16:16)
_________________________
This post and any other post by Locke is NOT an official forecast & should not be used as such. It's just my opinion & may or may not be backed by sound meteorological data. For official information, refer to Australian Bureau of Meteorology products.

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#1094331 - 21/03/2012 16:36 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
Originally Posted By: Locke
In 1974, the falls around Brisbane were substatially higher. Enoggera recorded over 500mm in a 24 hour period. Other areas like Mt Gravatt and Indooroopilly had in excess of 300mm.

This point is true.

Originally Posted By: Locke
Why don't you try modelling your early release strategy for the 1974 flood and tell me how it would compare to the strategy used in January 2011 if it were applied in 1974.

I don't think any of us have access to models which can perform that.

Originally Posted By: Locke
This is why it is foolish to suggest you can release 2,500-3,000m3 of water from Wivenhoe and confidently predict only $6 million in flood damage.

It may be that the prediction is not absolute, but a lot of storm damage Wivenhoe could never do anything about, like the storms in The Gap in 2008. All forecasts of damage with given flows are best guesses. I think it is foolish to think you can be all things to all men. Are you really suggesting that the backwater into the streams like the Oxley Ck, Norman Ck and Breakfast Ck is something that the Wivenhoe engineers should be thinking about? It has a small effect from the tens of centimetres we are talking about. Most of the damage from local flooding would occur anyway. Even tides have a bigger effect than Wivenhoe outflow on these creeks.

I fundamentally disagree with this argument.

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#1095007 - 23/03/2012 09:48 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
Engineer Offline
Cloud Gazer

Registered: 01/02/2011
Posts: 48
Originally Posted By: Locke
In 1974, the falls around Brisbane were substatially higher. Enoggera recorded over 500mm in a 24 hour period. Other areas like Mt Gravatt and Indooroopilly had in excess of 300mm.


The volume from those Brisbane suburban falls was gone from the Brisbane River by the time the water from upstream of Wivenhoe arrived.

Originally Posted By: Locke
It has been reported that Wivenhoe would reduce the 1974 flood level by 2m (that would be down to 3.45). The majority of the remaining flood level would be very much attributable to rainfall below the dam.


Of course. I'm not saying there was anything wrong with them using the forecast flows from the Bremer etc. in their model runs, e.g. Run 17, 2 pm 9th January forecast a peak of 1,200 m3/sec natural flow at Moggill, leaving 2,800 m3/sec available for dam releases. (They continued releasing about 1,400 m3/sec for another 12 hours.) The manual now requires them to close the bridges under the lake level forecast (without further rain) that existed at that time. The manual (Nov 2011) is still biassed against considering forecast rain above the dam while always considering forecast rain below the dam and I'm OK with that. It's just that what happened in January 2011 went way beyond that.

It's interesting to note that that the normal rate of gate opening puts a considerable delay on the time it takes for the release rate to reach the desired level. e.g. even if they had started opening the gates soon after 2 pm on the 9th January, the release rate would not have reached 2,000 m3/sec until soon after 11 pm that night. Of course by that time, the allowed releases would have fallen to 2,000 m3/sec because the forecast natural peak at Moggill had increased to 2,000 m3/sec.

Another thing that illustrates the informality of the dam operation process is the fact that releases began to violate the W3 strategy by 3 pm on the 10th of January (Monday). At that time the release rate was 2,087 m3/sec and the forecast natural peak at Moggill was 2,600 m3/sec, so the operators knew at that time that they were releasing enough to likely exceed 4,000 m3/sec by a significant amount. In spite of this, the releases were increased soon afterwards by another 600 m3/sec, bringing the total expected peak at Moggill to nearly 5,300 m3/sec. And this was purportedly in conformance with W3. (The increase by 600 m3/sec had its genesis in a kerfuffle with Brisbane City Council, but that's another story.) The operators were allowing the expected Moggill peak to exceed 4,000 m3/sec probably because they knew the expected maximum lake level (with forecast rainfall) was over 74 metres. These actions illustrate the informality of the dam operation process and that there is a conflict between what the operators see as the right way to operate the dam and what the manual says is the right way to operate the dam.

Originally Posted By: Locke
This is why it is foolish to suggest you can release 2,500-3,000m3 of water from Wivenhoe and confidently predict only $6 million in flood damage.


That's not what I'm suggesting. I'm suggesting 4,000 m3/sec peak at Moggill with expected rainfall (while in W3). Sometimes that will allow 3,000 m3/sec releases, sometimes not.

Originally Posted By: Locke
On Tuesday 11th January from approximately 6am to 3pm the following stations recorded 340-380mm of rain. Lowood, Mt Glorious, Wivenhoe TW and Savages Crossing. Some of these stations were below the dam, others resulted in inflow into the dam.

We certainly saw the result of this very heavy rainfall in the speed at which the dam level rose on Tuesday.

So what do you think might have been the impact if this rain had all fallen below Wivenhoe and what do you think might be the result if you were releasing 2,700m3 from the dam and rainfall of this magnitude were falling over Brisbane.


What I proposed above wouldn't have made any difference to releases after 9 am Monday since the release rate I proposed was the same as the actual from 9 am Monday. Obviously the dam operators thought the risk from your hypothetical scenario was acceptable after 9 am Monday so you'd have to ask them about that. There is the hypothetical risk that the centre of future rainfall will follow the centre of releases down the river at the same speed as the river but since the centre of future rainfall varies randomly this is very unlikely.

Originally Posted By: Locke
There seems to be a double standard here. An expectation that they should plan for the 1 in 100 year fall into the catchment and ignore the possibility of the same type of fall below the catchment.


No-one's suggesting that.

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#1095040 - 23/03/2012 11:45 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Engineer]
Locke Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 4451
Loc: Brisbane
"Of course. I'm not saying there was anything wrong with them using the forecast flows from the Bremer etc. in their model runs, e.g. Run 17, 2 pm 9th January forecast a peak of 1,200 m3/sec natural flow at Moggill, leaving 2,800 m3/sec available for dam releases."

As you noted they were already releasing 1400m3 (actually it was about 1250 on Sat and Sun) which you feel left them another 1400m3 that they could have released based on 1200m3 they expected coming from the Bremer/Lockyer (which they actually mentiaoned in the 2am Sit Report on Sat 8th January).

This assumes absolutely no impact from rainfall entering the river system below Moggill (eg. widespread 300-400mm 24hr falls over Brisbane itself). During previous discussions you seemed to suggest the impact from this would be minimal. I would suggest the 1974 rainfall pattern illustrates the danger would prove otherwise.

Forecasting the spatial distribution of rainfall to levels that would give a reasonable degree of confidence that the 4,000m3 flow at Moggill is going to cause minimal damage is just not possible given the current state of forecasting. Have a very close look at the GFS forecast through the event. It was forecasting massive rainfall into areas that would have significantly added to the downstream impact of any flow coming from Moggill.

The only time that a release strategy such as the one used in January 2011 becomes an issue are in extreme circumstances that cannot be predicted. Yes you can react on the basis that the extreme conditions might place the rainfall in the dam but in doing so you will be making the wrong decision and causing a significant amount of extra damage if the rain falls below. In past discussions I believe you have underestimated the potential significance of this.

Given these types of events are only likely to occur once in 100 years, I dont see the value in a strategy that only has a 50/50 chance of giving a better result in extreme events but is guaranteed to cause more damage in minor to moderate events.

I think if anything needs to be reconsidered out of this entire affair it is that a 4,000m3 flow at Moggill should be the cutoff point. I would suggest based on what I saw of the impact of 4,000m3 flow combined with minimal rainfall in the Brisbane area that they should consider a cutoff more in the 2500-3000m3 range.

Look at the rainfall that occurred at Maroochydore in the past 24 hours. What would be the impact if rainfall of that nature occurred whilst the lower reaches of the Brisbane River were experiencing the effect of a 4,000m Moggill flow.

Bear in mind most severe weather systems in SE QLD move from North to South. In the majority of cases severe rainfall events are likely to put water in the catchments first then move southwards meaning what ever your releasing is more likely to be adding to the effects of heavy rainfall below the catchment.

That was what was forecast to occur in January 2011 but instead the later rainfall also hit the catchment.
_________________________
This post and any other post by Locke is NOT an official forecast & should not be used as such. It's just my opinion & may or may not be backed by sound meteorological data. For official information, refer to Australian Bureau of Meteorology products.

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#1095109 - 23/03/2012 17:19 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
Are these "GFS forecasts" in the report? I can only see QPF forecasts for the catchment.

The manual only requires considering flow at Moggill. If rain falls below this, then that is not considered in the manual. I'd think it would be unlikely to have a big effect on Ipswich, but might affect CBD/Bulimba/Breakfast Ck flooding. I don't think the catchment for this rainfall is very large.

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#1095302 - 24/03/2012 10:41 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: somebody]
P Man Offline
Cloud Gazer

Registered: 23/01/2011
Posts: 21
Loc: Cambridge, Waikato NZ
There is a lot of numbers flying around here but i am still not seeing an answer to my original question - how much lower would the flood have been IF the manual had been followed? Sorry to repeat but i just think its a really important question.

We keep talking about a million other details yet the end result is surely the one that counts.

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#1095339 - 24/03/2012 12:50 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: P Man]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
Don't ask stupid questions.

If the commissions interpretation of the manual had been followed, W4 would have been in force from Sunday on, and the entire flood would have needed to pass straight through the dam like it wasn't a flood mitigation dam.

Flood would have been much more severe in that scenario.

That's just my previous post in different words.

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#1095361 - 24/03/2012 14:14 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
Engineer Offline
Cloud Gazer

Registered: 01/02/2011
Posts: 48
Originally Posted By: Locke
As you noted they were already releasing 1400m3 (actually it was about 1250 on Sat and Sun)


1,386 m3/sec at 2 pm Sunday increasing slowly to 1,473 m3/sec at 1 am Monday according to the SEQWater Report.

Originally Posted By: Locke
This assumes absolutely no impact from rainfall entering the river system below Moggill (eg. widespread 300-400mm 24hr falls over Brisbane itself). During previous discussions you seemed to suggest the impact from this would be minimal. I would suggest the 1974 rainfall pattern illustrates the danger would prove otherwise.


You tell me, where in the Flood Event Summary at this time, they were concerned about rainfall below Moggill. The strategy statements at this time (2 pm - 7 pm) were:

"Lake levels were starting to rise quickly at both dams, and combined with heavy rain in the Dam catchments during this period, it was decided at the end of the period to no longer consider minimising disruption to downstream rural life and to focus only on protecting urban areas from inundation.

Toward the end of this period, it also became apparent Moggill was likely to experience a second naturally occurring peak on 10 Jan 2011 or later. The Manual required the flow at Moggill to be minimised prior to this peak occurring. This requirement competed with the need to protect urban areas by not allowing the Wivenhoe Dam to reach a level that invoked Strategy W4. It was decided the best course of action was to increase releases as quickly as possible to the limit of non-damaging flows at Moggill. However, before this could occur, Councils needed to be advised, bridges needed to be closed and actions needed to be taken to prepare rural communities for isolation and urban areas below Moggill for river flows approaching 3,500 m3/s."

Absolutely nothing in there about delay being caused by potential rainfall in Brisbane. Plenty of other reasons for delay but nothing about Brisbane rainfall.

Originally Posted By: Locke
Have a very close look at the GFS forecast through the event. It was forecasting massive rainfall into areas that would have significantly added to the downstream impact of any flow coming from Moggill.


I can't find where the engineers gave that any consideration at all but I'll have a look anyway.

Originally Posted By: Locke
The only time that a release strategy such as the one used in January 2011 becomes an issue are in extreme circumstances that cannot be predicted.


The quote I gave above demonstrates that the circumstances requiring increased release rate were predicted. They just took an awful long time to act fully on the prediction.

Originally Posted By: Locke
Given these types of events are only likely to occur once in 100 years, I dont see the value in a strategy that only has a 50/50 chance of giving a better result in extreme events but is guaranteed to cause more damage in minor to moderate events.


Even using your figures, which I dispute, the average rate of saving would be $150 million per hundred years for a cost of $6 million every 10 years. But the engineers themselves are agreeing with what I'm saying about the release strategy, the only issue I had with them is why they took so long to put it into effect.

Originally Posted By: Locke
I would suggest based on what I saw of the impact of 4,000m3 flow combined with minimal rainfall in the Brisbane area that they should consider a cutoff more in the 2500-3000m3 range.


You won't get many people to agree with you on that, not me or the dam engineers or SEQWater.

Originally Posted By: Locke
Bear in mind most severe weather systems in SE QLD move from North to South. In the majority of cases severe rainfall events are likely to put water in the catchments first then move southwards meaning what ever your releasing is more likely to be adding to the effects of heavy rainfall below the catchment.


You seem to have forgotten that the Brisbane River flows mainly from West to East after the dam.

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#1095375 - 24/03/2012 15:18 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: somebody]
Engineer Offline
Cloud Gazer

Registered: 01/02/2011
Posts: 48
Originally Posted By: somebody
If the commissions interpretation of the manual had been followed, W4 would have been in force from Sunday on, and the entire flood would have needed to pass straight through the dam like it wasn't a flood mitigation dam.


I think the clause in W4 that may alter that action is:

"Lower level objectives are still considered when making decisions on water releases."

So the outflow wouldn't necessarily have to always equal inflow while in W4.

Also, any rational action would require full use of the dam's capacity. So equalizing inflow and outflow significantly below full capacity would not be a rational action (if the outflow is significantly damaging).

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#1095382 - 24/03/2012 15:56 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Engineer]
Locke Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 4451
Loc: Brisbane
I can 100% guarantee that if dam engineers released the amounts your suggesting on Saturday or Sunday and the rainfall that fell into the dam from late Sunday through to Tuesday on actually fell over Brisbane instead of over the catchments the dam operators would now be getting slammed for releasing too much water too early and not taking steps to "protect urban areas from inundation".

Armchair experts would no doubt be saying "didn't you see all the rainfall forecast for Brisbane, why the hell didn't you hold the water back when there was stil so much flood mitigation capacity left in the dam".

I wonder what your comments might have been under such circumstances. And where was your criticism of the dam engineers in October or December when they employed exactly the same release strategy as they did in January.

Originally Posted By: Engineer
Even using your figures, which I dispute, the average rate of saving would be $150 million per hundred years for a cost of $6 million every 10 years. But the engineers themselves are agreeing with what I'm saying about the release strategy, the only issue I had with them is why they took so long to put it into effect.


How can that be. If either outcome has the same probability (something unlikely given SE QLD weather patterns), in the scenario where you release 2700m3 early on that then combines with massive falls below the dam, you will have caused several hundred million dollars worth of damage that could have been avoided had you only released 1300m3. Or are you saying a 1300-1400m3 difference in flow wouldn't cause such a significant difference in damage (in which case I would argue the same for last January).

I repeat if 4000m3 is the limit at Moggill for damaging flows, how the hell can you expect that weather that produces such an outcome won't include a very high likelihood of massive rainfall below Moggill that is going to greatly multiply the damage. Thats pretty much completely ignoring normal weather patterns for SE QLD rainfall events. The only time the approach would be reasonable would be when the weather system which generated the flowat Moggill has completely cleared the area. If not you are advocating a very dangerous and foolish approach.
_________________________
This post and any other post by Locke is NOT an official forecast & should not be used as such. It's just my opinion & may or may not be backed by sound meteorological data. For official information, refer to Australian Bureau of Meteorology products.

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#1095429 - 24/03/2012 18:38 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Engineer]
somebody Offline
Weather Freak

Registered: 26/01/2011
Posts: 188
Originally Posted By: Locke
I can 100% guarantee that if dam engineers released the amounts your suggesting on Saturday or Sunday and the rainfall that fell into the dam from late Sunday through to Tuesday on actually fell over Brisbane instead of over the catchments the dam operators would now be getting slammed for releasing too much water too early and not taking steps to "protect urban areas from inundation".

Armchair experts would no doubt be saying "didn't you see all the rainfall forecast for Brisbane, why the hell didn't you hold the water back when there was stil so much flood mitigation capacity left in the dam".

I wonder what your comments might have been under such circumstances. And where was your criticism of the dam engineers in October or December when they employed exactly the same release strategy as they did in January.

Fortunately the manual has now been changed so that their inaction from 2pm Sunday on closing the bridges would be a violation of the manual. This also stops armchair experts from criticising later, or at least reduces it.

As for what strategy they followed in Oct/Dec 2010, I wasn't really paying much attention then.

Originally Posted By: Engineer
Originally Posted By: somebody
If the commissions interpretation of the manual had been followed, W4 would have been in force from Sunday on, and the entire flood would have needed to pass straight through the dam like it wasn't a flood mitigation dam.


I think the clause in W4 that may alter that action is:

"Lower level objectives are still considered when making decisions on water releases."

So the outflow wouldn't necessarily have to always equal inflow while in W4.

Also, any rational action would require full use of the dam's capacity. So equalizing inflow and outflow significantly below full capacity would not be a rational action (if the outflow is significantly damaging).

The rate of release through Monday would have needed to be increasing until the dam level was falling or the forecast peak level fell below 74m. Basically you would have effectively lost 97cm (Wivenhoe level) of flood mitigation, and more because of the forecasts at the end of flood didn't eventuate.

It's awful to contemplate what would have occurred if the engineers had interpreted the manual in the way that the commission did.

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#1095790 - 26/03/2012 00:36 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
Engineer Offline
Cloud Gazer

Registered: 01/02/2011
Posts: 48
Originally Posted By: Locke
I can 100% guarantee that if dam engineers released the amounts your suggesting on Saturday or Sunday and the rainfall that fell into the dam from late Sunday through to Tuesday on actually fell over Brisbane instead of over the catchments the dam operators would now be getting slammed for releasing too much water too early and not taking steps to "protect urban areas from inundation".


So you don't disagree that the engineers didn't record any concern about rainfall below Moggill on ramping up releases beginning Sunday afternoon or that such releases would be expected to give an average net benefit or that rainfall events motivating such releases are not likely to follow the releases in the direction of the river. Your only interest is that it's hypothetically possible that such releases could in some small fraction of cases increase the flood peak even though in the vast majority of cases the flood peak would be reduced. This, of course, is only of academic interest because avoiding those releases is not an expected damage minimization strategy.

However, it's of some interest to consider your hypothetical scenario. The additional releases that should have been made beginning Sunday afternoon would have ramped down from 2 am Sunday and ended by 9 am Monday morning when the actual releases reach the same level. This means their effect at the Port Office would have ramped down from 8 am Tuesday and ended soon after 3 pm Tuesday under the river velocity conditions of the time, so they could only have aggravated a flood peak occurring before about 3 pm Tuesday. If the peak inflow from rain that actually arrived at the dam at 1 pm had instead arrived in Brisbane at the same time then this would have added to the flow in Brisbane at the time which was a bit over 4,000 m3/sec. Considering the catchment area of the short creeks below Moggill (i.e. excluding Oxley creek because of its length), an extreme rainfall event would not have been likely to add more than 1,000 m3/s at the Port Office for a sustained period, if at all. So such an event centred on 1 pm Tuesday might have brought the flow at the Port Office to a few hundred m3/s more than 5,000 m3/sec at that time. Conversely, we expect (at best) that in this scenario 1,000 m3/s is taken off the actual peak which was 9,400 m3/s, thus bringing it down to 8,400 m3/s. So the earlier increase in releases strategy (which would have added about 540 m3/s at most at the dam) would have contributed to a flow totalling less than 6,000 m/s at 3 pm Tuesday, still more than 2,000 m3/s lower than the peak under your alternative rainfall scenario.

The bottom line is that even with the rainfall event centred on Brisbane, an earlier increase in releases would not have increased the peak flow without implausible rainfall intensities over Brisbane.

I very much doubt that SEQWater will be taking the advice of armchair experts advising them to reduce the flow limit at Moggill to 2500-3000m3/s.

Originally Posted By: Locke
I repeat if 4000m3 is the limit at Moggill for damaging flows, how the hell can you expect that weather that produces such an outcome won't include a very high likelihood of massive rainfall below Moggill that is going to greatly multiply the damage. Thats pretty much completely ignoring normal weather patterns for SE QLD rainfall events.


You mean normal weather patterns that "are likely to put water in the catchments first then move southwards" while water flowing down the river moves eastwards?

Originally Posted By: Locke
The only time the approach would be reasonable would be when the weather system which generated the flowat Moggill has completely cleared the area. If not you are advocating a very dangerous and foolish approach.


Yes, just like those dam engineers who increased the releases from 2 am Monday morning while the weather system was still expected to generate enough rain to increase the dam level by nearly 2 metres. You're right that they were foolish to increase the releases at that time.

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#1095824 - 26/03/2012 09:18 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Engineer]
Locke Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 4451
Loc: Brisbane
Sigh.

Your first point completely misses the mark. I mean its nice work on modelling after the event where you know specifically when rainfall hits the Brisbane area so that it doesn't coincide with the impact of any releases from Wivenhoe but for some reason you seem completely oblivious to what might happen if they did coincide. Given the 24-36 hour time frame for flows from the dam to get to the lower reaches this is something you should not ignore.

The point I'm askiung you to ponder which I guess I mistakenly expected someone with an engineering degree (you are an engineer?) to grasp is what they should do when they don't know where and when the rainfall is likely to fall and theres significant inflow into the dam. Its all very well to criticise a decision or come up with a release stategy armed with the knowlege of where and the rain will fall. Try modelling or adopting a strategy without that knowlege.

So listen very carefully. I do not disagree that the strategy employed in January 2011 did not achieve the optimal result in terms of the peak river level in Brisbane. What I assert is that the strategy employed in January 2011 is the one best suited to the type of severe rainfall events that affect SE QLD. You've taken a single event, and with little or no understanding of SE QLD weather, criticised the approach used to deal with it completely ignoring the possibility that it might be an outlier for such severe rainfall events.

I really don't understand your comment "You mean normal weather patterns that "are likely to put water in the catchments first then move southwards" while water flowing down the river moves eastwards?"

The catchment areas for Wivenhoe and Somerset are to the North and North West of Brisbane not to the West last time I checked the map. That the gates for Wivenhoe are WNW is just a red herring. In most cases the event that puts the water into the dams will most likely move generally South and impact Brisbane by the time your starting to release siginificant amounts of water through Wivenhoe. C,mon engineer its not complicated. Look at the map. How familiar are you with significant rainfall events. How many have you closely monitored (outside of this one that you don't really seem to have monitored closely at all). What do you know about the weather patterns that generate such events?

The point I'm making is any release strategy for an event that causes a serious enough inflow into Wivenhoe that you have to release 2,500m3 or more which then leaves absolutely no room for rainfall impacts from the same event in Brisbane is incredibly short sighted and naive. Did they learn absolutely nothing from 1974?

"Yes, just like those dam engineers who increased the releases from 2 am Monday morning while the weather system was still expected to generate enough rain to increase the dam level by nearly 2 metres. You're right that they were foolish to increase the releases at that time."

If you can't see the difference between what was occurring at 2am Monday morning as against what was occurring on Saturday and Sunday then this is, as I've alluded to previously, a pointless discussion. At 2am on Monday 10th January, inflows into the dam were running at 7700m3 with significant rain still falling, for most of the weekend the inflows into the dam were below 2000m3.

I guess I'll have to be more explicit. I don't advocate or support 2500m+ releases at a time when dam levels are stable and current inflows are highly unlikely to push the dam above the 74m level at a time when the weather system that took the dam above 68.5m are still present. I do advocate such releases once the dam is likely to exceed 74m (based on current rainfall and inflows) since at that point the possibility of the NEED for significant releases (which would cause significant damage to Brisbane) more closely matches the likelihood that releasing 2500m3 before the dam hits 74m is going to cause similar levels of damage by coinciding with heavy rainfall to the South of the catchments.

There was absolutely no need to release 2500m3 on Saturday or Sunday based on current rainfall or inflows. The only justification would be to KNOW that further inflows were going to occur to push the dam over 74m. Current forecasting does not allow this justification and the nature of severe rainfall events in SE QLD would not provide that justification.
_________________________
This post and any other post by Locke is NOT an official forecast & should not be used as such. It's just my opinion & may or may not be backed by sound meteorological data. For official information, refer to Australian Bureau of Meteorology products.

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#1095854 - 26/03/2012 11:47 Re: SE QLD Flood Crisis January 2011 -- The Washup [Re: Locke]
Locke Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 4451
Loc: Brisbane
The 1996 floods put an interesting context on this subject.

During this event Wivenhoe Dam never exceeded 100% FSL. There was some rainfall in the catchment but the heaviest rainfall was South of the catchments surrounding the Brisbane CBD.

There were heavy rainfall totals approaching 600mm but spaced over 4 days (from 2nd to 5th May 1996) with the average totals per day in the 100-150mm range.

The result of this was a 2.1m peak at the city gauge with most if not all of this arising from falls below the catchment. (I cant see any indication of dam releases at all).

Now 2.1m at the city gauge does not seem like too much of a cause for concern in the context of last years events. However, that 2.1m was the result of 400-600mm over 4 days. What would be the result of 300-400mm in the space of 8 hours (this is what occurred in the vicinity of the dam on Tuesday 12th January) keeping in mind it was rainfall of this nature that lead to massive releases in Wivenhoe on that Tuesday.

What would be the result if you were releasing 2500m3 from the dam and 24-36 hours later you had 300-400mm of rain in 8hrs in Brisbane 24 hrs later. Tell me it wouldn't be just as devastating as what occurred last January and I'll say your dreaming. And don't rely on current weather forecasting skill to tell you exactly when and where such rain will fall. It wasn't able to do so in January 2011 and it still can't now!

So again what is your justification for releasing 2500m3 on Saturday and Sunday whilst the dam level was stable and inflows were below 2000m3.

Severe weather events in SE QLD have the capacity to produce massive rainfall totals with potentially devastating consequences. Dam or no dam. Falls of up to 400mm in less than 12 hours will always produce major flooding in the Brisbane River whether below or above the catchment. Release stategies should never presume otherwise.

The only exception to this is possibly having Wivenhoe permanently at 50%. I'm fine with that as long as they can find a safe way to secure our water supply without the additional storage capacity.
_________________________
This post and any other post by Locke is NOT an official forecast & should not be used as such. It's just my opinion & may or may not be backed by sound meteorological data. For official information, refer to Australian Bureau of Meteorology products.

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