http://www.floodcommission.qld.gov.au/__data/assets/file/0004/7195/Barton_Maher__Annex_BM4a.pdf

In case anyone is interested, I've come across this report from 2009. It seems that had they not changed the manual less than 2 years before the 2011 flood, even less water would have been held in Somerset which would not have been good.

Old Operating Target line ran from 72m/102.25m to 80m/107.46m rather than the new one which is from 70m/104.45m to 80m/109.7m

2008 Somerset target level @ 75m Wivenhoe = 105.1m
2010 Somerset target level @ 75m Wivenhoe = 104.2m

I think they should find a way to allow releases from Somerset at levels below 100.45m so long as Wivenhoe is below the target line's change point (currently 70m).

One of the limitations with Jan 2011 was that the first gate opening @Wivenhoe was delayed until it reached about 68m (7/1 15:00) then had to be operated continuously (hourly) for 23 increments. I can't imagine that if Somerset was to be discharging water that they would leave Wivenhoe gates closed so long.

I think set targets are dangerous in that they may not take into account differing scenarios.

Take the 1974 floods. A significant proportion of the 1974 flooding was generated by massive rainfall below the dams. When they say Wivenhoe would have reduced the 1974 floods by 2m (down to 3.45m at city gauge) it really only tells half the story. Most of the remaining 3.45m would not come from dam releases but rather from the rainfall below the dam.

The question is do you want to not use Wivenhoe for flood mitigation for minor to moderate flood events. This is what those advocating large releases when the dam is in the 68.5-74m range are really arguing for. And are you prepared to cut off rural areas each time the dam gets above 75% FSL.

Dam operators were clearly aware of the potential for siginificant rainfall below the dam at a reasonably early stage in last January's event. The Saturday sit report that mentions a possible 1200m3sec flow in the Bremer is an indication of this (also an indication that the operators were thinking about the risk of urban inundation - a W3 factor if the dam is above the 68.5m mark). Add this expectation to a stable dam level and minimal rainfall in the catchment over Saturday and into Sunday morning and I can't possibly see any validation for larger releases prior to late on Sunday.

Because the of the inability for rainfall forecasts to achieve the level of detail to predict whether rainfall will fall above or below the dam catchments nor to accurately predict what amount of rain will fall, actual release startegies must be based on river flows and previous and current rainfall. None of these factors on Sat 8th and sunday 9th January 2011 indicated the need for higher releases.

You cannot just have a strategy that works only for 1 in 100 year events (and then only for 50% of such events).

Agree broadly, but I disagree with this:

How are you going to predict where the Q100 level actually is without reasonably well defined behaviours through the flood event? The 2011 manual revision defined behaviours in W4 but not at the lower levels, where there is a far bigger question mark about whether the appropriate actions were taken IMO.



I think moderate releases when the dam is over 70m or in the 68.5-70m range with strong probably of significant inflow coming are a reasonable expectation. The problem is that it is not defined what is the target or minimum flow through Moggill, only a maximum is defined. Which leaves the engineers under pressure to not release water when it would be prudent to do so. EDIT: And the W2 strategy is still valid up to 74m in the 2011 manual. I think this is an error. /EDIT


Agree to some extent, but I think once the rain was falling and inflows rising well above outflows from 15:00 Sun that it was pretty obvious addition outflow was prudent. Just not actioned until 2am. I think closing the 2 remaining bridges should have occurred much more swiftly than this, but perhaps I have unreasonable expectations.

John Tibaldi's evidence re: Sat 8th is correct though. I just wasn't justified to increase outflow with the dam level falling in both dams. And he didn't point out that doing so would have reasonably quickly dropped the level below 68.5m and into the W1 strategy. Seems pretty pointless to me.


Pretty sure I've already agreed with this.

I can only hope that the 3rd round of the inquiry leads to further manual revisions for the future, but I'm not sure it will.

Seems like were in broad agreement on most points Somebody.

In terms of the W3 strategy though it should be remembered that the primary consideration under this strategy is to protect urban areas from inundation. Any time you release 2500m3 or more from the dams (instead of the 1250m3 being released on 8th and 9th) this is almost certainly going to be impossible to achieve.

In the majority of possible scenarios there would be a significant likelihood that the weather system putting the dam level above 68.5m would also be generating significant flows below the dam pushing the Moggill flow well above 3000m3. (I totally agree that the impact of flows in the 3,000 to 4000m3 range at Moggill need to be better understood).

Having said that, I have video footage shot on Coronation Drive on Friday 7th January showing water nearly up to the Drift restaurant in Toowong. The council had put up barricades on the bike path and had council workers stopping people. I remember telling the council worker at the time that the water would be into the Drift Restaurant within the next couple of days. At this stage the impact of releases from Wivenhoe was minimal but abnormally high tides were combining with what was already being released to start to impact urban areas. At this time the only way to reduce the immediate threat of urban inundation was to use the flood mitigation storage capacity of the dam adn increasing the releases could not possibly have been construed as meeting the W3 strategy.

I saw a similar result in terms of water levels in October 2010 when they were again releasing about 1250m3 sec with the dam at about 69.5m. Had releases been upped to 2500m for that event I'm absolutely certain a number of areas in Brisbane would have flooded.

I'd question about areas which go under with approx 2500m3 being released (say add 1000m3 for Lockyer & Bremer flows combined). I don't really think these areas should be developed. Or if the council does approve such developments it should be with the full knowledge of council and the property owner that they are to be flooded with moderate inflows into Wivenhoe rather than irregular larger floods.

What was your view of the 2010 recommendation against lowering Wivenhoe to 75% in readiness for the flood season?

I think the problem is that many of these areas have already been developed and a "buy back" for those areas would be so horrendously expensive as to not be practical.

Not a big fan of the 75% FSL. I don't believe it provides sufficient time to react should we go more than 5 years without a signficant rainfall event which happened on multiple occasions last century.

Whilst we would no doubt adapt, the consequences might well be far more costly than last years floods. At the very least no action should have been taken until those risks were better understood.

5 years from now we could again be looking at a dam level sitting at 15-20%.

I think that scenario is unlikely even if we don't get significant inflows. We have the water project and the Hinze Dam stage 3 so the water which would go over the spillway there can be pumped to Wivenhoe if necessary.

I think the 64m interim FSL should have been adopted but only until Feb. That gives enough of the rainy season left to fill up a significant portion of the top 25%.

And if no rainfall events occur outside of Feb. Your starting point for the next "drought" is only 75% FSL where as in 2001 it was 100% FSL.

Now we would probably not drain the dam as quickly as we did last decade. My understanding is we currently lose only 8-9% supply each year which means it would take 6-7 years to get to critical level (20% or less).

Last century we had 3 occasions where we went more than 10 years with out a significant inflow event (irrespective of whether the dam was present). Plus a few more in the 6-9 year range. The average time between inflow events last century was 3.5 years.

Seriously, the thought of my water supply being so dependent on rust prone desalination plants and government works that are yet to be completed is far more scary than a 1 in 100 year flood.

My understanding was the two way Hinze Dam connector was up and running.

Mmmmm....Locke, you clearly live on a hill. I'm not so sure that the good folk of North Booval or Goodna or Jindalee or Rocklea or Yeronga would share your views at this stage. As I said in a previous discussion we had on this matter the issue of security of water supply can be tackled in other ways. Have you bought your tank yet?

http://forum.weatherzone.com.au/ubbthrea...venh#Post983867

By the way, I think the drop to 75% in mid Feb 2011 and again in Spring 2011 were both ludicrous decisions, clearly political rather than scientific. Mind you, following the recent rain event over the week of Australia day I'd have been asking my panel of experts to consider whether it was appropriate to let the dam levels approach 100% with the full month of February and a very wet catchment in play. The point I'm trying to make, and it is reinforced by your discussion of how infrequently these sort of wet season conditions will prevail, is that the release of water from the dams to increase their flood mitigation capabilities should be considered every couple of weeks every summer. If the people on the panel have half a brain and aren’t politicians then most of the meetings will be cancelled before they are scheduled, and the real impact on our overall water supply strategy will be quite limited.

Keep safe
Neil

As always you make excellent points Neil. My only questions would be whether residents of SE QLD can actually exercise the self discipline needed to reduce their water usage to a point where a 75% FSL is safe. Although people did respond somewhat when dam levels got low last decade it nearly wasnt enough.

As far as water tanks go, I've heard evidence that when water tanks were more widespread last century that we had greater issues with mosquitos. Maybe a scare campaign by a government wanting us reliant on the water from their grid. I don't really know.

Nevertheless, last decade our dams nearly ran dry from a spell of drier weather that didnt come close to matching the longest dry spell of the previous century. Once everyone has full water tanks in their back yards then yes go ahead and drop the FSL but not until safeguards are in place. Incidentally would the cost of putting a water tank in every home be less than the cost of raising the storage capacity at Wivenhoe? A quick search shows that 2x 22,000 litre tank costs about $4,000 and not all properties would have the room to fit such tanks. And given I live in an apartment complex that has 70 units, unless we utilize our swimming pool, I doubt we could possibly fit enough tanks in the complex to satisfy everyones water needs.

Incidentally I read a very interesting article on the weekend on water usage in the International Space Station. Whilst the average person uses 50 litres a day to shower, occupants of the space station use only 4 litres to bathe.

Slightly off topic but is the water grid only for treated water? Is any water spilling over a dam still effectively lost? Is there capacity to divert this water, or is the water at the Hinze Dam treated and put into the grid for the rest of the SE corner to use.

Greg

It would be diverted before spilling over the dam. Perhaps I could have worded that better.

This should have been the other way around:

Hope that was clear from the text.

I'm surprised that there hasn't been more coverage in the media that there have been proposals to raise Wivenhoe's flood mitigation capacity and keep the FSL at 100%.

This would appear to address both the long term water needs of SEQ and improve the flood mitigation requirement.

I can't recall any cost estimates for this - perhaps it was considered too expensive. At the time only the fuse plugs got built - the rest of the money went into the water grid as it looked like we were running out of water then.

With the benefit of hindsight we didn't need the water grid in it's current form and could have had a full Wivenhoe's dam, no major flooding of Brisbane, and no flood inquiry.
Now there's a thought!

Bit hard when you have alarmist "experts" stating the drought is permanent and we'll never get enough rain to fill the dams again.

Build the dam properly, and send Tim Tam Flannery the bill

Its hard not to see the water needs within SE QLD increasing in the next few decades. Given the economic uncertainties of our times surely we need to look at the most cost effective ways of managing the areas water needs.

Yes, this is a discussion which should be occurring within the water authorities. Is Wivenhoe the best place for it? I'd say that we should be raising the dam wall then. Raising the dam wall at Hinze seems like it was a good project.

There are other possibilities like connecting & raising Borumba.

Surely we shouldn't be managing now according to the demand which might exist in two decades.

Big infrastructure projects can take up to a decade to be realised only to find out by the time they're complete the demands have again crept ahead of you.

Maybe 2 decades is a long way out but don't forget it is now nearly 30 years since Wivenhoe was built. At the time it was built I'm sure it was large enough to fulfil both flood mitigation and water supply. Nearly 30 years on, this is no longer the case.

The problem with governements today is that they no longer look beyond the timeframes of the next election.