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It’s a Barrier Estuary: Duh! by Jennifer Marohasy

This discussion has been cross posted at the request of Jennifer Marohasy from her blog site. It follows on from the previous discussion - Talking Turkey, But Not About the Barrages.

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IN South Australia the Coorong fishermen say that before irrigation, before the weirs, locks, levees and barrages (sea dykes), the Murray River would flog down from September until maybe Christmas, filling the lagoon, then out the mouth. By Christmas, flow had usually slowed and water levels dropped right down. Then when the South Westerly wind picked up the sea would pour in through the mouth and work its way across the lake.

So Lake Alexandrina was fresh in spring and summer, but salty by autumn.

What the old fishermen describe is an estuary: a transition zone. The Murray River had a barrier estuary with a central lagoon, Lake Alexandrina, and a sand barrier, the Younghusband Peninsula. A single, narrow and shallow inlet that often closes over is also a characteristic of barrier estuaries.

There are many barrier estuaries along the southern Australian coastline including Lake Illawarra just south of Wollongong. According to the Lake Illawarra Authority’s management guide, freshwater flows into the lake from the escarpment and salty water from the ocean tides and therefore it is an estuary. There are 70, of these Intermittently Closed and Open Lakes and Lagoons, known by the acronym ICOLL, from Sydney to the Victorian border. A significant issue is management of the risk of flooding when their ‘mouths’ close over. NSW State government policy doesn’t support the artificial opening of ICOLLs.

The South Australian government insists the Murray mouth, which is the inlet to Lake Alexandrina, be kept open. In fact since European settlement there have been many schemes devised to change the Murray’s mouth to make it deeper and wider, including through blasting and dredging and more recently through water reform.

There is this invented narrative that the Murray’s mouth closes over because greedy upstream irrigators have stolen all the water, but the reality is Barrier Estuary’s close over naturally. The long-term solution is to remove the Murray Mouth barrages – the sea dykes – that have interrupted the evolution of this system and so let it develop to a fully mature state. Mature barrier estuaries tend to be fully tidal.

The sea dykes dammed the estuary making it totally dependent on river flows. Stopped the tide. Limiting natural scouring of the sea mouth in spring by the river flow and in autumn by the Southern Ocean. Not surprisingly the hydrology and geomorphology of the Murray’s mouth has changed with sand that used to shoal behind the mouth consolidating into Bird Island that continues to grow and may one day permanently plug the Murray’s mouth.

Indeed Federal Water Minister Tony Burke can buy back all the water from all the irrigators across the entire Murray Darling, but this will have very little real impact on the Murray’s mouth.

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To learn more about barrier estuaries and why Lake Alexandrina was once part of a wave-dominated barrier estuary and which sea dyke should be removed first read my latest technical paper ‘Plugging the Murray River’s Mouth: The Interrupted Evolution of a Barrier Estuary’ at http://jennifermarohasy.com/publications/

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This discussion was first published [HERE]

Follow the link & scroll down the page to read comment made on Jennifer's blog site.

Tags: Basin, Darling, ICOLLs, Murray, Plan, barrages, closed, estuaries, intermittently, laggoons, More…lakes, open

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Replies to This Discussion

Permalink Reply by Dale Stiller on February 14, 2012 at 2:37pm

A media release from Barnaby Joyce

Murray-Darling Plan must be based on best available science not out of date data

The Murray-Darling Basin Authority is relying on out of date data for its recommendations to take 2,750 gigalitres from productive use in the Murray-Darling Basin, said Senator Barnaby Joyce today.

“We are borrowing billions from overseas to buy water, which could put towns out of production, in the middle of a flood period. There must be something slightly incongruous about this, which at the very least requires the updating of models with easily accessible current information.

“It was revealed at Senate estimates today that the MDBA has calculated the average inflows into the Murray-Darling by taking a simple average from 114 years of data, from 1895 to 2009.

“There are lies, damn lies and statistics, or so said Disraeli, but it seems convenient that the dataset used to calibrate water in the Murray-Darling Basin begins and ends with Australia’s two biggest recorded droughts. The federation drought conveniently becomes one bookend of the statistics, and the recent millennium drought becomes the other. The other side of these two periods are some of the wettest times in Australia’s history, the one we are living through now and the one that included the 1891 floods.

“At least the figures should be current and include the most recent figures, as I continually hear this discussion about current changes to climate. The data must surely include the current rain events that have flooded my local town three times in the last two years, which we have all no doubt seen on the news.

“We are going to spend over $100 million just developing the Basin Plan which is not even based on up to date information.

“The government must update the figures.”

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Permalink Reply by Ian Mott on February 14, 2012 at 3:21pm

Lets be clear. This is not a continuation of the previous post "Talking Turkey etc" because it is yet another massive sidestep of the key issue of tidal imbalances that Jennifer continually fails to address. Jennifer would have us believe that there is, or was, a "natural scouring" by tidal inflows in those times of the year when river flows were low. But for this to be anywhere near the truth there would have to be a fundamental reversal in the shape of the tidal curves so that tide water flowed in slowly (without depositing sand) and flowed out quickly (and removed sand). But the shape of the tidal curve is determined by the moon (of course), latitude and broadscale landform, none of which are seasonal.

So whether Jennifer likes it or not, her key assumption, that of balancing tidal and river flow seasonal scouring, is bull$#it. Once again she has failed to consider the implications of a larger tidal prism, especially during storm surges, on sand deposition. Instead we get this almost religious belief that the removal of mans work will automatically restore "the" natural balance. But in this case it is, "not without an extra 10 million megalitres of irrigation water it won't".

I am still waiting for her to explain, in detail, why opening the Mundoo Channel will not simply enhance the capacity of storm surges to shift the sand of Bird Island right into the Channel and block it up permanently. And I am still waiting for her to explain how all this tidal interchange will take place if the mouth is already closed by storm surges flowing into a larger tidal chamber. Any time Jennifer would like to give her ideas a proper testing in a public debate she can always organise a venue in Brisbane where I will be happy to oblige.

And how come this "Susan M" is allowed to operate without her full name? I thought this was a condition of entry here.

But I should thank her for providing one of the most preposterous bits of casuist crap I have ever read. The MDBA wants to waste $9 billion on buy-backs, and cause another $25 billion in real estate losses alone, in the basin. And she has the gall to suggest that a couple of buried pipes costing less than $90 million (ie just 1,000th of the cost) is, wait for it, "solution overkill". And then she has the gall to imply that she speaks for the local Land Council on a proposal that could fully restore 10,000 hectares of totally dead habitat in their South Lagoon. It ain't your call, sunshine.

And for what? A naieve belief in an untested experiment and the vane hope that the greens will accept a partially tidal Lake Alexandrina as a substitute for their pathalogical hatred of farmers? Give us a break. The facts are that the two deeply entrenched key indicators of ecological health in that region are;

1. The extent and duration of the mouth being open, and

2. The absence of hypersalinity in the Coorong.

Tinkering with the barrages will only deliver a minor benefit in serious drought (ie 1 year in 10). At all other times it will seriously increase the extent and duration of mouth closure, possibly terminally so. And it will do absolutely nothing to improve conditions in the Coorong. The greens key excuses for predatory anti-farmer policies will not just remain intact, they will be reinforced.

And readers should also take a look at Susan M's last link at the "Talking Turkey" thread. She is clearly of the belief that destroying existing dams is a wonderful idea. So what next, Susan, Dartmouth? The Hume?

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Permalink Reply by Dale Stiller on February 14, 2012 at 3:34pm

Susan, I sent you a message about this some time ago. To access your messages when signed in look at the top of the right hand column where you will see your name.

Below is Inbox and if you have a message it will be there in brackets (1).

Click on this and on the new page click on the message title.

If you have any questions there is a Reply link just above the message.

Ian Mott said:

And how come this "Susan M" is allowed to operate without her full name? I thought this was a condition of entry here.

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Permalink Reply by Jennifer Marohasy on February 14, 2012 at 9:57pm

Ian,

I think you are generally missing the point. You see I don't accept any of your key assumptions including about tidal balance. Your questions are, as far as I'm concerned, red herrings.

It's sort of like I think we are dealing with a horse, and you keep describing a cow... for want of a better analogy.

If we are to make progress we first need to define the type of estuary we are dealing with? Do we, or don't we, have a wave dominated barrier estuary?

In order to define the beast, lets start with some of its characteristics:

Before construction of the barrages was Lake Alexandrina the central basin and the Younghusband Peninsula the sand barrier?

If you don't accept the Geoscience Australia classification, how would you classify the estuary.

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Permalink Reply by Rob Moore on February 15, 2012 at 7:21am

Dale - I don't know about you but it annoys me that we have to post stuff from the Nats or Barnaby when Jenny Swan has about 5 staff over there in St George who have Barnabys password(and get paid to do it!). Unfortunately for them they have forgotten this site exists.

Dale Stiller said:

A media release from Barnaby Joyce

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Permalink Reply by Dale Stiller on February 15, 2012 at 8:12am

Moderator comment

Ian,

Susan has now complied with this requirement.

There are also conditions about being civil to other people. Ian, I welcome you to take up your case to the views that Jennifer has put forward in her article. Debate on the science of any endeavour is essential for good outcomes. But mate, please tone down the aggressive language; I know you are a troll slayer from way back but I ask you to adjust you methods for those who enjoy a far more genteel discourse.

Ian Mott said:

And how come this "Susan M" is allowed to operate without her full name? I thought this was a condition of entry here.

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Permalink Reply by Ian Mott on February 15, 2012 at 12:04pm

Jennifer would have us believe that tidal nuances are mere assumptions, not fully verifiable facts. Just as she would have us believe that a concrete barrier that is behind a kilometer of sand dune and 2km of sheltered water (9 to 13km by stream bed) is a "sea dyke". One can either be in the facts business or the bull$#it business, you can't be in both.

So I enclose links to a sequence of sites around the coast from Brunswick Heads NSW to Goolwa Beach SA showing the tide graphs for this week ending 19th Feb so readers can see for yourselves how atypical the tides actually are in SA and how assumptions drawn from the study of NSW barrier estuaries do not apply to the Coorong and lakes.

And keep in mind that this is the time of year when Goolwa tides are least abberant to the East Coast. In Spring the contrast is even greater with the seriously asymetrical flows I have referred to already.

http://tides.willyweather.com.au/nsw/far-north-coast/brunswick-head...

http://tides.willyweather.com.au/nsw/illawarra/kinghorne.html

http://tides.willyweather.com.au/nsw/south-coast/eden.html

http://tides.willyweather.com.au/vic/east-gippsland/lakes-entrance-...

http://tides.willyweather.com.au/vic/barwon/lorne.html

http://tides.willyweather.com.au/sa/limestone-coast/port-macdonnell...

http://tides.willyweather.com.au/sa/fleurieu-peninsula/goolwa-beach...

The first thing to note (for Feb 19) is that Brunswick Heads gets two complete tides each day while Goolwa gets only one. The variation at Brunswick Hds is 1.4 metres from 0.2m to 1.6m and this remains consistent down to Lakes Entrance Vic. but with a smaller second tide (0.6m to 1.7m to 0.3m to 1.3m). But at Goolwa the variation is about half, at 70 to 80cm. (1.0m to 0.3m to 1.1m).

So when we apply the FACT (not an assumption) of waters capacity to move sediment being a function of the square of the waters velocity we can see why the lessons Jen thinks she has learned by the study of southern NSW barrier estuaries do not apply at the Murray mouth.

Not only is the volume of Goolwa tidal variation only half of an equivalent area on the east coast, the FACT of there being only one tide each day means the time it has to flow in and out is double. And unless Jennifer has discovered some new form of basic maths, that can only mean that the velocity through a similar sized entrance as the murray mouth must be FOUR times faster. And that can only mean that the scouring capacity of an equivalent tidal movement in NSW would be 16 TIMES greater than what takes place at the Coorong entrance.

And as I have said before ad nauseum, the only exceptions to this core geophysical reality is when storm surges coincide with an inflowing tide to produce a rapid flow which deposits more sand than usual. And as storm surges usually last longer than 12 hours there is no corresponding outflow velocity to take that sand back again.

The other exception is when a hideous amount of valuable fresh water is allowed flow out to sea. But as I mentioned in my MDBA submission, http://www.mdba.gov.au/files/submissions/Ian%20Mott%20-%20Landholde... more than half of every flood volume flows out the mouth at times of the tide cycle when it cannot make any meaningful contribution to sediment discharge. Yes, more than 7 million megalitres of the 14 million ML that flowed into SA in 2011 made zero contribution to sediment discharge.

Meanwhile, back at the (tee hee) "sea dykes", does Jennifer really expect us to believe a very sluggish two foot tide is going to get all the way up the Goolwa Channel, past Clayton, to get anywhere near Lake Alexandrina proper?

And does she seriously expect us to believe that the forces of darkness will accept this minor tinkering as the price for ending their predatory anti-farmer monomanias? [Exit flying pigs] Give us a break.

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Permalink Reply by Peter Richard Smith OAM on February 15, 2012 at 12:57pm

Thank you Ian but it seems impossible to get Ms. Marohasy to either understand or accept other points of view. Ian I have misplaced/lost your email adress could you please send it to me?

Ian Mott said:

Jennifer would have us believe that tidal nuances are mere assumptions, not fully verifiable facts. Just as she would have us believe that a concrete barrier that is behind a kilometer of sand dune and 2km of sheltered water (9 to 13km by stream bed) is a "sea dyke".

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Permalink Reply by Jennifer Marohasy on February 15, 2012 at 1:00pm

Ian

You still haven't told us what type of estuary we are dealing with? How would you classify the estuary?

Can we agree that there are two different basic types of estuaries in Australia: tide dominated (along much of the northern Australian coastline) and wave dominated (along much of the southern Australian coastline).

And can we agree that the Murray River's estuary is wave dominated?

As regards storms, along this coastline, given this type of estuary they tend to remove sand, not add it as you suggest.

Consider this comment from a South Australian historian:

“The position of the channel at the mouth is governed principally by the ocean... During the great 1956 flood, the highest ever recorded on the lower Murray, the river outlet, although wider and deeper than normal, was situated in the easterly section of the overall movement pattern and was in a similar position as the situation of the mouth during the dry year of 1914.

However in April 1938, during a violent storm the mouth doubled its width in a few days and a great deal of sand at the western extremity was washed away. Within two months the channel had narrowed and when surveys were carried out 12 months later the position of the outlet was in almost the same situation as before the storm. During this period there was no great fluctuation in the volume of fresh water coming down the river.”

You don't appear to have even a basic understanding of estuaries and coastal processes in Southern Australia.

If the dykes were removed, the estuary could be restored. Until this happens the South Australians are going to continue to demand more water from upstream to maintain their 750 square kilometre shallow artificial freshwater reservoir created from the damming of an estuary.

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Permalink Reply by Ian Mott on February 15, 2012 at 2:48pm

You have repeatedly implied, Jennifer, that I was questioning the obvious fact that the Coorong and its path to the sea is a barrier estuary. But once more for the record, it is a barrier estuary where wave action is more significant than tide action. But my links to the tide graphs above make it absolutely clear that lumping all estuaries into just two categories is both simplistic and misleading. The factors at play in east coast barrier estuaries are an entire order of magnitude greater than what takes place in SA. And the notion that tidal estuaries are not found in the south is absurd. The mouth of the Parramatta River (a.k.a Sydney Harbour) is a very good example.

Your suggestion that the inevitable outcome of what you call a functioning estuary is the opening of the dune system flies in the face of all the science. Instead, you troll through the historical records for a particular circumstance that suits your purposes and ignore the work of Webster (2005) who found that the maximum potential for deposition of sand in the mouth is when sea level is highest, and that "transport is particularly intensive under conditions of high waves and spring tides".

This is not doubting that the event you described did take place, but we also know that it was a very rare event indeed. You have given no background data on this event. And for anyone to simply extrapolate a conclusion from this one anomalous event, in the face of a huge body of contradictory evidence is highly unprofessional. For the record, Chappell (1991) recorded 3,849 significant storm events between 1940 and 1990, an average of 77 each year.

I have already mentioned Webster's report "in a two week period between 14-28 May 2002, which included a storm having offshore significant wave heights of ~4m and spring tides, approximately 46,000M3 of sand was deposited in the shoals and channels inside the mouth".

The underlying oceanographic trend, in the absence of very large river flows at the expense of irrigators, is for closure of the dunes to produce a standard arid zone terminal lake. The times when these sort of closure trends have been thwarted have been either by serious sea level change at interglacials, or by changes in the supply or flow of sand itself. A large breakwater on the southern side of the mouth, for example, would produce a build up on that side with serious erosion on the other side, possibly leading to a break through outside the Goolwa barrage. This is what happened on nearly every river entrance in NSW that got breakwaters in the 1960's.

In the absence of these factors the overwhelming geophysical trend is for closure, not the opening that you and your sad followers would like to believe. There is a very good example of a number of sequential dune closures on the coastal plain near Lenox Head NSW. In each case a barrier dune closed and a new one formed in front of it.

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Permalink Reply by Jennifer Marohasy on February 15, 2012 at 3:10pm

The Coorong and its path to the sea is not obviously a barrier estuary. There is no obvious central basin (eg. Lake Alexandrina) or freshwater inflow (Murray River).

The Coorong, and saving it, is an all together different issue from that of restoring the Murray River's estuary. And as I have explained I have no problems with your plans to put pipes through the sand dunes of the Coorong but this will do nothing to relieve the problems of the Lower Lakes and their need for freshwater during drought.

Your pipes into the Coorong will not solve the issue of the Murray River's estuary or stop Lake Alexandrina drying out next drought. The Murray River's estuary once comprised Lake Alexandrina and the Younghusband Peninsula but was crippled by the barrages.

I have provided specific examples including anecdote, and general concepts, you reject both. And continue to argue at a tangent to the real issue. I think I've done my best to explain my position honestly to you, Ian. Clearly we are not even on the same page if you think my technical report was about restoring the Coorong. I care about the Coorong, but my report and all the information I have tried to honestly provide was about the Murray River's estuary proper.

I suggest you find someone else to argue with.

Kind regards,