Far too much water left in the dam

The public response to the travails has been overwhelming. But before the citizens of Brisbane put away their gumboots, they should reflect that the current levels of Wivenhoe Dam are exceptionally high and the provisions for flood management are dangerously inadequate.

The Brisbane floods were the highest since the January 1974 floods, but there are some rainfall and flood facts which should cause concern.

While Bureau of Meteorology monthly rainfall averages vary over the past 20 years, February and March have been as wet as or wetter than January. Since 1840, January ranks behind March in flood events and has about 60 per cent of the flood events of February (BOM 1840-2009).

Unfortunately, the outlook for the next couple of months is not good. The BOM warns that a strong La Nina weather pattern exists. Heavier rains and possibly cyclones are expected. Hydrologist Aron Gingis has warned of the problems of holding too much water in Wivenhoe and Brisbane Lord Mayor Campbell Newman saw it coming.

What have the Wivenhoe Dam operators been doing for the past couple of months? According to SEQ Water Grid chief Barry Dennien, dam levels were managed according to the rules and strictly by the operating manual. Dennien is comfortable that "everything happened the right way".

It seems the manual and the operator do not differentiate between the weather outlook of an El Nino (dry drought) and a La Nina (rain, flooding). After the drought, Wivenhoe reached 96 per cent of its supply capacity on March 16, 2010, and has been maintained at that level or higher since.

Despite the La Nina weather, the operator has chosen not to vary the supply capacity one degree. The rules make no allowance for rainfall outlook, El Nino or La Nina: it does not matter.

Since last October the Wivenhoe operator has had three warnings. On October 13 the dam reached 126 per cent of capacity, on October 21 it reached 111 per cent, and on December 29 it reached 123 per cent. In each case the operator reduced the water level only to 100 per cent.

With the rainfall outlook, why didn't the operator reduce the capacity further? As former dam project supervisor 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 desalination plant and water recycling facilities, the risk of wasting water would be small as against protecting for a serious flood.

In the days before the flood, the BOM warned of an upper level low pressure system dumping a large amount of rain over southeast Queensland. The warning came late in the first week of January and was visible on the BOM interactive weather and wave forecast maps. Ironically, the system slowed off the coast for a few days, giving operators plenty of time to make any adjustments to capacity levels. But the gates remained shut on the 100 per cent capacity.

As the low system dumped rain, the operator opened the gates, releasing about 116,000 megalitres on Friday-Saturday, with releases of 100,000ML over the next two days. Despite these releases the dam level rose to 148 per cent by Monday last week.

By Tuesday, with the dam at 176 per cent, the operator released a phenomenal 645,000ML. The result was bound to be bad. Significant flooding and tears, all while Dannien expressed his "confidence . . . that everything happened the right way".

Consequently, the flood peaked in Brisbane on Wednesday at 4.46m. However, the inflows from the catchment were surging, and the dam reached a capacity of 191 per cent.

Anna Bligh admitted that the operators nearly lost control, with water only 90cm from spillway fuse plugs. If triggered, the plugs would have released a torrent of water to the system. The results could have been catastrophic.

Subsequently, with the sun shining and the tides friendly, the operator has been releasing water so that the level by this Monday was down to 115.2 per cent.

It will be interesting to see if the dam operators have "the balls" to stop the releases at 100 per cent supply capacity. Dannien noted that "it is easy in hindsight to say they should have done things differently". Now they have hindsight and data that should be telling them to head lower.

Following the release of 645,000ML and the fact that Wivenhoe was within 400,000ML of breaching the spillway, it seems that the storage capacity needs to be cut by between 400,000 and 645,000ML. There is room for better hydrology here, but the bottom line is risk management.

This would modify the Wivenhoe dam supply capacity to between 500,000ML and 750,000ML, and should be instigated as a matter of extreme urgency. The risks of low water availability in the near future appear to be a shadow of the risks of more extended flooding, especially with the massive public investment in desalination and recycling facilities, which provide water supply security

That desalination and recycling infrastructure has been mothballed immediately on commissioning is also a matter for further consideration. The region's water managers seem loath to integrate such facilities into the water grid and it will be interesting to understand why the Wivenhoe Dam supply level was maintained so high when all the risk signs indicated it should be reduced.

Is it the case that the fear of exorbitant costs of desalination and recycling have blinded the decision-makers to risks of managing Wivenhoe for predominantly flood mitigation, the reason it was built, as against a little water security?

Andrew Dragun is an adjunct professor in economics at the Australian Rivers Institute, Griffith University. He is editor of the International Journal of Water.