After the deluge: The problems of managing the Brisbane floods

 The Queensland floods have caused a mountain of damage and grief for a multitude of Queenslanders throughout the state. The public response of many Queenslanders and their fellows from the other states to help their less fortunate brothers and sisters has been overwhelming and heartening. And the key state leaders demonstrated real leadership in adversity.

The “tsunami” which devastated Toowoomba and the Lockyer seems to be a freak of nature and it is difficult to see how any proactive planning could have made a great deal of difference. But what about the Brisbane – Wivenhoe Flood? Before the good citizens of Brisbane put away their gumboots, they should reflect that the current levels of Wivenhoe are exceptionally high and the provisions for flood management are dangerously inadequate.

While the Brisbane River floods on Wednesday January 12th did reach their highest level since the massive January 1974 floods, there are a few rainfall and flood facts which should cause considerable concern. While BOM monthly rainfall averages vary slightly according to the range (the full data set seems to be available for only 1840 to 1994!), it seems that over the past couple of decades February has been wetter than January, and March has been nearly as wet as January. Since 1840 we see that in SE Queensland January ranks third behind March in flood events and has only about 60% of the flood events of February (based on BOM flood records 1840-2009).

So while the January 12th floods are behind us the averages for the next couple of months are not good. At the same time the Bureau of Meteorology has been warning for some considerable time that a strong La Nina weather pattern has been established over the region. For the foreseeable future heavier rains and possibly cyclones along the Queensland coast are expected. The BOM has warned water managers that they should be conservative in managing their resources. Hydrologist Aron Gingis has warned of the problems of holding too much water in the Wivenhoe given the expectations of an exceptional rainfall season. The Lord Mayor of Brisbane Campbell Newman saw it coming and warned of the risk.

So what have the operators of Wivenhoe Dam been doing over the past couple of months? According to SEQWater Grid chief executive Barry Dennien, dam levels were managed according to the “rules” and exactly according to the “operating manual”. Mr Dennien is comfortable that “everything happened the right way”.

What is clear from the “operating manual” is that the Wivenhoe operator does not seem to distinguish the weather and rainfall outlook of a El Nino (dry – drought) as against a raging La Nina (cyclones, heavy rain, flooding). It is a fact that that Wivenhoe Dam reached 96% of its supply capacity back on 16th March 2010, and has been maintained at close to those levels or higher ever since. Despite the notable strengthening La Nina weather pattern the operator has chosen not to vary the supply capacity one degree. Clearly the “rules” make no allowance for rainfall outlook, El Nino or La Nina, it does not matter.

Since October 2010 the Wivenhoe operator has had three warnings of things to come and chose to sit on the “operating manual” as gospel. On 13th October 2010 the Dam reached 126% of supply capacity, on 21st December 2010 it reached 111%, and on 29th December it reached 123%. In each case the operator chose to reduce the level of the dam only back to 100%.

With the strengthening rainfall outlook, why didn’t the operator reduce the supply capacity more? As former project supervisor on the dam Ian Chalmers observed a Dam operator “would have to have large balls to … reduce the supply capacity in the face of the weather warnings, after 10 years of drought”. Ironically, with the huge investment in securing the region’s water supply with a major desalination plant and significant water recycling facilities, the risk of wasting water as against protecting against a significant flood would seem to be reduced.

But, it gets worse. In the days leading up to the Brisbane – Ipswich flood, the BOM had been warning of an upper level low pressure system dumping a large amount of rain over the SE Queensland region. The warning came in late in the week of the 3d January and was clearly visible on the BOM Interactive Weather and Wave Forecast Maps. Ironically, the system slowed down off the coast for a few days giving operators plenty of time to make any adjustments to capacity levels. But by Friday 7th the gates remained shut on the 100% supply capacity. As the low pressure system began dumping rain over the weekend, the operator did open the gates a little, releasing about 116,000 ML on Friday/Saturday and further releases of about 100,000 ml over the next two days. Despite these small releases the dam level had risen to 148% by Monday the 10th. Now the operator was in the awkward situation that further releases could contribute to flooding downstream. Interestingly the Grid chief executive Barry Dennien seems to be of the view that somehow larger releases at this time would also have caused flooding in “other” parts of the catchment!

But by Tuesday the 11th with the dam sitting at 176% (it seems to have a total capacity of 226%!) the operator decided to let the Wivenhoe gates open and release a phenomenal 645,000 ML. The result was always going to be bad. Significant flooding and tears, all while Mr Dannien expressed his “confidence … that everything happened the right way”.

As a consequence of the massive flood release on Tuesday the flood peaked in Brisbane on Wednesday at 4.46 metres, significantly less than the 1974 peak and only a shadow of the 1883 monster flood (more that 8.4 metres). Thankfully it had stopped raining be Wednesday and the tides were running lower. However, the inflows into the Wivenhoe catchment were still surging, to reach a maximum capacity in the order of 191%. At this point the State Premier admitted that the capacity of the dam was close to a level where the operators nearly lost control. Water was only 90cm from spillway fuse plugs which if triggered would have released a torrent of water to the already flooded system. The results could have been catastrophic.

Subsequently, with the sun shining and the tides being friendly – and we need to remember most other floods in Brisbane at this time of year have been accompanied by a less than friendly cyclone – the Wivenhoe Dam operator has been releasing water dramatically so that the level by Monday 17th is down to a mere 115%.

Now it will be very interesting to see if Mr Dannien and his Wivenhoe Dam operators have “the balls” to stop the releases at 100% supply capacity. Mr Dannien has noted that “it is easy in hindsight to say they (the operators) should have done things differently”. Well now they have hindsight and a whole lot of real information and data which should be telling them to head lower. Much lower. In the light of the release of 645,000 ML on Tuesday the 11th and the fact that Wivenhoe was within 400,000 ML of breaching the spillway fuse plugs, the actual storage capacity needs to be revised downward by somewhere between 400,000 and 650,000 ML. The final level would be formalised in relation to the actual level of releases permissible to keep Brisbane under flood level, and to enable a slightly wider margin of safety on the fuse plugs. Clearly there is room for better hydrology here, but the bottom line is in fact risk management.

This would modify the Wivenhoe Dam supply capacity to between 500,000 ML and 750,000 ML, and should be instigated as a matter of extreme urgency. There is no time for further scientific studies and royal commissions on hydrology. The La Nina weather phenomenon is upon us with a vengeance and more significant rainfall events can only lie ahead of us as conditions for coastal cyclones bloom and two more heavy rainfall months await. The risks of low water availability in the near future now appear to be a pale shadow of the risks of more extended flooding throughout the region, especially in the setting of the massive public investment in desalination and recycling facilities which are now available to provide water supply security to domestic consumers.

That the state government has preferred to mothball the desalination and recycling infrastructure immediately on commissioning is also a controversial matter for further future consideration. It seems that the region’s water managers can’t stay far enough away from integrating the desalination and recycling infrastructure into the region’s water grid and it will be interesting to understand why the Wivenhoe Dam supply level was maintained so high when all the risk signs were indicating that it should be substantially reduced. Is it a case that the political fear of the exorbitant costs of desalination and recycling have blinded the decision makers to risks and costs of managing Wivenhoe for predominantly flood mitigation, the reason it was built, as against a drop of water security?

After the deluge: The problems of managing the Brisbane floods

A. K. Dragun

January 2011


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s