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December 15th 2011 print

Ian Mott

Has it worked?

There is no better time to judge the quality of the outcomes that the Murray Darling Basin Authority claims will be achieved by delivering additional environmental water. 


So much Murray River flow and so little to show for it


Some time around the 10th December 2011 the 14 millionth megalitre of 2011’s Murray River flows crossed the border into South Australia. This is on top of what was needed to fill all the upstream storages and deliver some serious water to Red Gum Forests and wetlands along the way. After South Australia has taken its full 2 million ML allocation for Adelaide water and irrigation this leaves 12 million ML (12km3), or 240% of the 5 million ML long term average discharges, to flow into the sea. The market price of this 12 million ML, if there was sufficient storage to capture it, is around $2.4 billion so there is no better time to judge the quality of the outcomes that the Murray Darling Basin Authority claims will be achieved by delivering additional environmental water.

You may recall that the primary purpose of the irrigation water buy-backs is to deliver additional water to “the environment”. But contrary to the impression given by far too many media reporters, the environment already gets quite a bit, except in drought of course when nobody but city dwellers and townies in the Basin gets any at all. In reality the water that is used by Adelaide and other towns of SA, and SA irrigators, is classified as an “extraction” and is assumed to deliver no ecological benefits, despite the fact that it flows 95% of the entire river length, delivering substantial ecological benefits to every river reach it flows through, even in the kind of droughts that once left the river bed bone dry.

And this is on top of the above mentioned 5 million ML of long term average annual discharge into the sea, which obviously flows 100% of the river length, delivering ecological benefits to every mile it flows through. The same applies to all the irrigation water used at Mildura, Leeton, Griffith, Yarrawonga and every other extraction point in the basin, which has delivered ecological services to all the upstream catchment before it is diverted along channels, where it also maintains new wetland habitats, before it actually becomes an “extraction”.

So it is in this context that the greens and academics on the MDBA drip are claiming that the environment needs another 4 million ML each year to avoid ecological collapse. And the clearly stated end use for that extra water is to keep the Murray mouth open for nine years in ten and to eliminate hyper-salinity in the Coorong.

So this is why 2011 is the ideal year for examining whether these buy-backs, at a cost of close to $9 billion to the taxpayer, will actually achieve what their proponents claim they will do.

The Murray mouth is now open. The small dredge that kept the mouth open through the worst drought in a century at an annual cost of only $2.6 million stopped work as soon as the January floods became apparent. But few people understand that it is not silt from upstream farms that normally restricts the river flow. Rather, it is sand from the beach, deposited by the inbound tides and storm surges, that can undo all the corrective work of a million ML of river outflow in a single storm event.

And it is all to do with the asymmetrical tides in that part of the world. For two weeks in each lunar cycle the tide takes about eight hours to come in and sixteen hours to flow back out again. The capacity of water to shift sediment is determined by the square of the velocity. So when the inflow is twice the speed of the outflow it means the inflow brings four times more sediment in than the outflow takes away.  And it is to correct this imbalance that the SA government and the MDBA wants the extra four million ML of upstream farmer’s valuable water allocation.

So after 12 million ML (12KM3) of river outflow in a single year we can say that, yes, it did its job. But was that $2.4 billion worth of water the best option for achieving it? It is likely that the work of quite a few of those cubic kilometers was undone by storm surge tidal inflows in as little as a week later. And this then required another valuable cubic kilometer of fresh water to push it back out again.

Furthermore, the other two weeks in every four have a very modest tidal range and this produces much less sand deposition to be corrected. So half of each cubic kilometer that flowed out to sea in 2011 was wasted because there was no sand deposition to be counteracted while that outflow took place.

The underperformance of 12 million ML is even worse in respect of reducing salinity in the Coorong. The generally accepted belief has been that large volumes of fresh water are needed to ensure that enough of it will “slosh” (yes, that’s a scientific term in SA) into the lower Coorong to correct the hypersalinty that was as high as five times that of sea water during the drought.

The total volume of the South Lagoon is only 140,000ML so only 1.166% of the 12 million ML outflow would need to “slosh” into the South Lagoon to completely replace the hypersaline water. But for the record, the salinity reading at Parnka Point on the 8th December 2011 was still EC 80,019 (1.6 times sea water) and the reading at Cattle Island was still EC 110,761 (2.1 times sea water). From this we can conclude that only about half the hypersaline volume of the South Lagoon (about 70,000ML) was actually replaced.

This 12 million ML of valuable fresh water was used at a delivery efficiency rate of 0.58 of 1%. For each megalitre that made it into the South Lagoon, another 171 was wasted. They basically “spent” $34,200 to deliver each $200 megalitre to the Coorong. Is there any wonder that informed upstream farmers might get a little angry when it is they who are accused of using water inefficiently?

Don’t forget that this is the level of service delivery that has come from a one in twenty year flood. The actual achievements of the MDBA’s preferred average flow level will be a great deal less and at a much lower rate of delivery efficiency. And it is all because they have consistently refused to look at any other alternatives. At an opportunity cost of $34,200 per effective megalitre they could build a pipeline and pump it all the way from the Kimberley and still have plenty of change. In fact, they could back load bulk ore carriers with water from the mouth of the Yangtze and still do a cheaper job.

The alternatives to fresh water for both tasks are far more effective and come at a fraction of the cost. Just one large, unidirectional, 1km pipe under the dunes, of the same 6.4m diameter as the 22km Tumut-Eucumbene tunnel that was cut through solid rock, would passively on each high tide, deliver more than a million ML of fully oxygenated sea water a year to the South Lagoon. This would flow north to the Murray mouth, completely flushing the entire Coorong four times a year and returning that entire system to habitat values and water quality that it would only have experienced briefly in the past 10 millennia.

Two more pipes on either side of the Murray mouth, with ocean intakes above the sea floor, will ensure that half of each tidal inflow gets inside the mouth without bringing any sand with it. And by blocking the pipes on the outflow the total volume will then flow out the mouth, taking the sand with it. The higher the storm surge, the faster the water will flow through the pipes and the greater the volume of water that does not bring sand with it. Unlike river flows that can take months to flow from Dam to river mouth, the “plumbing of the Coorong” will deliver the right volume of water for each circumstance and at the exact time that it is needed.

Local Federal MP, Patrick Secker (Barker), has estimated the cost of such pipes at no more than $30 million each, for an annual cost in the order of $2 million, delivering abundant, free and reliable sea water at a cost of only $2 per megalitre. The MDBA has been informed of this option but to date has proceeded with their grossly inefficient, blatantly ineffective, and outrageously expensive, fresh water monomania in breach of their statutory obligations to consider all relevant alternatives. And we all need to ask, why?