Barrages and Murray Mouth Mirages

There is a vigorous campaign being waged by Jennifer Marohasy and her Australian Environment Foundation to remove the barrages at Lake Alexandrina and somehow “restore” what is claimed to be the area’s inevitable destiny as a barrier estuary. As if the Coorong was not such a barrier estuary already. And for some reason known only to themselves, the AEF have decided that any future mixing of fresh water that takes place inside where the barrages are located is more ecologically noble than current mixing that takes place outside the barrages.  Do the fish give a tinkers cuss?

This campaign has had a great deal of publicity recently due to some very ham fisted effort from ABC Media Watch to paint the AEF as some sort of dangerous foreign funded right wingers under the bed. This has turned Ms Marohasy into a ‘cause célèbre’ in some conservative circles but it has also drawn attention away from the very serious flaws in this proposal. And worse, it is also distracting attention from the real and present flaws in the science behind the MDB Plan. There are more than enough of these already without adding imaginary ones steeped in anecdote and vaudeville.

The AEF have borrowed from the standard green spin book by claiming that the benefits of removing the barrages and the assumed replacement of fresh water evaporation with tidal sea water evaporation are, a disturbingly vague, “up to 1 million megalitres a year”.  And it has been implied that this ‘saving’ will somehow accrue to the tax payer in general and upstream irrigators in particular. But no detail on how this figure has been calculated have been provided and the only semblance of disclosure has been on a blog site with reference to current net annual evaporation for both lakes. And just like the climate mafia MO, legitimate critique is dismissed as ‘junk science’ and the work of ill-informed opinionators with an axe to grind.

The reality is quite different. For a start, the “1 million megalitre” figure can only come from subtracting the mean annual rainfall figure from the raw pan evaporation total for that region. Ms Marohasy either didn’t understand, or chose to ignore, the fact that actual evaporation from a large body of water is generally about 20% less than the pan evaporation total that is obtained under experimental conditions.  Local pan evaporation is about 1565mm so the actual is only 1252mm.  So when we deduct the 468mm mean annual rainfall as recorded at the Meningie weather station we get a net figure of 784mm which, when applied to the 75,350 hectare surface area when the lakes are at AHD,  amounts to only 590,600ML. Even when the 1st decile (1 in 10 year drought) rainfall total is used the net evaporation rate only rises to 910mm and a total volume of 685,500ML.

Interestingly, there is a wide range of conversion rates from pan evaporation in the literature. The Coorong, Lower Lakes and Murray Mouth Long Term Plan, Appendix 3 appears to have adjusted Pan to Actual evaporation by a factor of 0.92 while The MDBA’s modeling for end of system flows (Heneker 2010) uses 0.85.

But note that net evaporation would only produce a significant reduction in lake volume if there was zero flow in the Murray River over a full 12 months interval.  And it would only be at the end of that 12 months that the cumulative 685,500ML shortfall allowed the same volume of sea water to enter the open lake. We can assume that the CLLMM Long Term Plan has done a better job of estimating actual lake volumes at various lake levels.  It estimates the volume of the lakes when the surface has dropped to AHD at 1,288,000ML so the lakes would still be a notional 47% fresh after 12 months of no flow. And only in the second year of a combined no-flow and a second consecutive 1st decile annual rainfall (a 1 in 50+ year event) would the volume of fresh water evaporation drop below 322,000ML.

At this point the AEF/Marohasy logic starts to get even fuzzier. They suggest that the ebb and flow of the tides will produce a much greater mixing of the two water bodies so that much more of the evaporation comes from sea water. But they fail to appreciate that perfectly good fresh water that mixes with sea water and flows out to sea is just as ‘wasted’ as fresh water that is evaporated. And none of either type will amount to any benefit to up-stream irrigators.  They have also failed to explain how this mixing might take place through the narrow entrance to Lake Albert when the lake levels, sans Barrages, will have dropped by half a metre to AHD.  Achieving greater mixing through a shallower cross section is more than just a little bit problematic.

It should not be forgotten that this “up to 1 million megalitres” claim implies that it will be saved each year.  But when we deduct the mean annual rainfall figure from the actual evaporation it should also be obvious to most that 590,660ML average evaporation can be completely negated by a river flow of the same modest volume.  As long as the river flow is greater than the evaporation shortfall there is minimal scope for a permanent transfer of sea water into the lakes because the fresh water continues to push the sea water back.  But the fuzzy logic of the AEF appears to have a problem with the fact that the sea water that comes in on the few  high tides each month that exceed 70cm amplitude is the same stuff that goes back out six hours later, leaving the fresh water exactly where it was earlier.

When I explained that there can be no significant transfers of sea water while river flows were anything more than very modest, Ms Marohasy then asserted that significant intrusions could still take place during the low-flow half of the year. This actually sounds plausible at first reading but Ms Marohasy either did not know, or chose not to disclose, the fact that the lowest flows are in Autumn and Winter when evaporation is also lowest and, more importantly, when rainfall is highest.

From May to September in an average year local rain falling on the lakes themselves actually exceeds evaporation from them to produce a net outflow independent of interstate river flows.  This period also coincides with maximum run-off from local creeks that drain into the lakes. The combined outcome is substantially diminished capacity to accommodate tidal inflow at the very time when Ms Marohasy claims such inflows will be greatest.

At no stage does the AEF appear to have made any attempt to quantify the likely volume of tidal intrusion based on the atypical (to the point of yet more SA weirdness) tidal behavior in that location. There is generally only one tide each day, and that is only during the 50% of the lunar cycle when they even take place at all. And, aside from storm surges, they only exhibit a tidal range greater than 70cm on 4 or 5 days each month. There has also been no mention of the fact that in 9 years in 10 there will be a continuous flow of warmer, less dense river water that will occupy the surface of the lake while the colder, denser sea water forms a salt wedge underneath, where it obviously cannot evaporate.

This was not the case before the major storages were built. The annual melt water previously flowed quickly to the mouth and was colder than the sea water and produced much greater mixing with that denser sea water.

One must conclude that the above mentioned substantially reduced estimate of fresh water evaporation savings will be reduced a great deal further due to the occurrence of the salt wedge effect. It is highly unlikely that actual savings would amount to even 10% of the AEF claim and even then it would be a one in ten year event or longer.  And this would bring the actual annual average savings of fresh water evaporation back to only 1% of the AEF claim. They want farmers to get into a lather over just 10,000 megalitres of annual water savings.

So next time you hear those seductive, almost plausible, claims to “restore the estuary” and remove the barrages you would do well to keep in mind that the estuary only functioned properly with the help of every drop of water that is currently in storage upstream.  Without that average extra 12 million megalitres of river flow each year the Murray mouth would be almost continually closed.  The overwhelming geophysical dynamic in that location is the deposition of beach sand inside the mouth from the differing velocities of inflowing and outflowing tidal flows.  The AEF have consistently refused to accept the fundamental scientific reality that the capacity of flowing water to shift sediment is determined by the square of the flow velocity.  At most times the inflowing tides take 8 hours while the outflow takes 16 hours and that means the inflowing tides, in the absence of river flows, deposit four times more sediment than the outflow can remove.

An equilibrium of sediment movement is only achieved when river flows are sufficient to double the volume of outflows so the velocities of both flows are matched.  Prior to irrigation allocations this was usually the case but since then it has become a less frequent norm.  The barrages have been a very useful response to this by substantially reducing the size of the tidal prism so both tidal inflow volume and velocity have also declined and sand deposition has declined with it.

Nowdays, when storm surges which would normally dump tens of thousands of cubic metres of sand inside the mouth in a single event, the smaller tidal capacity produced by the barrages reduces both the volume and velocity of the inflow and the volume of sand it brings with it.   Enlarge that tidal prism by a blunt, unmeasured removal of the barrages and the scale, speed and frequency of mouth closure is increased. And that will only produce even more strident demands for yet more irrigator’s precious fresh water in a futile attempt to get the Murray mouth open again.

Normal tidal intrusion through a small mouth opening would be far too little, and travel far too slowly, to ever get much beyond where the barrages are today.  So the long term prospects for replacement of fresh water evaporation are dim.

More importantly, mouth closure would pose a very serious problem for every farmer in the MDB. An open Murray mouth and a healthy, tidal Coorong is a key ecological performance indicator. Farmers have enough trouble hanging onto the water they still have without further degrading that indicator and strengthening the arm of those who want even more water.

Accordingly, Ms Marohasy and the AEF must provide detailed estimates of the actual volume of fresh water evaporation they think barrage removal will deliver over the climatic cycle.  And she could then explain, in a context of continued outflows from the Murray mouth, how these “savings” will actually accrue to the taxpayer in general and farmers in particular.  Without such detailed substantiation of these claims the MDBA has a professional duty of care to ignore submissions that do not stand up to informed scrutiny.  Detailed critique of real, rather than imaginary, problems with the science behind the MDBA Plan is an absolute imperative.  It is the only way we can get real, rather than imaginary, alternate solutions.

Ian Mott,  First published as a formal submission to the MDBA on the Basin Plan.


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