Just to be a pedant Jennifer, in engineering a barrage is not military term. It refers to a specific type of dam.
http://en.wikipedia.org/wiki/Barrage_(dam)

And a Dyke is a levee, which is also as specific type of earthworks.
http://en.wikipedia.org/wiki/Levee

Just so you know the 'correct' terms.
I believe the dams referred to would classify as 'barrages', not dykes.

Peter,
I thought you were the spokesperson for Lock Zero, not one mention about it. I was of the belief that you wanted to construct Lock Zero below Tailem Bend and create a new pool upstream of the new lock and below D/S Lock 1 at Blanchetown, which would protect the pump stations at Tailem Bend,Jervois,Murray Bridge,Mannum and Swan Reach which supply potable water to Adelaide and other areas in S.A.. Remember we must learn to crawl first, that is establish this new pool first then look at the next stage i.e.automating the barrage gates.

Now you are descending to high farce, Jennifer. A barrier that holds fresh water a full 50 to 70cm above AHD is clearly NOT holding back the ocean as you claim. And during this most recent (1 in 100 year) drought the period of zero flow only lasted long enough for the lake level to drop to just below AHD. And that can only mean that 5/6ths of the original fresh water remained in place. If the barrages had not been there then the missing 1/6th could only flow in when the upper half of a tide was greater than AHD +0.5 metres.

That condition only applies for just one hour on the 3 or 4 days each month when tidal range is on that scale. For the rest of the days in each lunar cycle the upper half of the tidal range is much less than AHD +0.5 metres. So for all but 4 hours in each 672 hour monthly tide cycle there can only be minimal intrusion of sea water.

To suggest that these piddly little tides could completely exchange the remaining 5/6ths of the lake's fresh water with sea water in such a limited window is pathetic. Even storm surges will simply push the fresh water to the north before it reverts to its normal state.

The way you have been [Deleted for abuse] less informed members of the rural community with your snake oil is a disgrace. What next, Jubilation T Cornpone?

Spus42,
Peter is 56flood and I am the spokesperson for Lock Zero and I have not mentioned it for a reason as first we must make people realise that the opening or removing the Barrages would be a disaster in the Lower River Murray. A total investigation/study into Lock Zero is vital and we are still seeking that investigation and also the total up-g4rtading of the Barrages.

Lance
Peter R. Smith OAM is 56flood.
From one of my sources Ian Mott who I believe puts it perfectly in perspective: -
I think the impact of tidal intrusion has been overstated because the local tidal regime makes serious intrusions the exception rather than the rule. Given that half of each monthly lunar cycle involves very minimal tidal variation, and 16 hours of each remaining daily cycle also involves either minimal variation or outflow, then high tides can only occur for 1/6th of the year (ie 1/3rd of 50%).
The historical references to continuous fresh water then become a function of pure probability. The lake, due to its shallow nature, was no place for small boats during storm events as the chop would be very dangerous. And when these events coincided with high tides they were also effectively “off the radar”. And that means that any local anecdotal observations of the composition of lake water took place during the more than 5/6th (84%) of the time when no tidal intrusion was present. If peak tidal flows were also likely to produce dangerous flow rates near the populated centres then the interval in which saline intrusions would not be detected would increase to well over 90% of the time.
The strongest conclusion is that historical references to fresh water conditions in the lake are likely to be roughly representative of actual conditions but the sequence of anecdotal observations was unlikely to have picked up the character and scale of anomalous conditions. These anomalous sea water intrusions were unlikely to have lasted longer than 8 hours at a time. And given the distances involved in each transit, could not have extended far enough, under all but 9th decile circumstances (drought and storm surges), to impact on northern lakeshore ecosystems or agriculture.
And that means that the fears of total barrage removal are overstated but the impacts on river mouth closure of a larger tidal prism are such that the barrages need to remain in place until a better solution is shown to be working

Yes, 56Flood, the last sentence of that quote is the critical point. The barrages have played a major role in slowing down the closure of the Murray mouth. Mouth closure is not caused by sediment from upstream because that material would build up in the north of the lake and on the upstream side of the barrages. Mouth closure is caused by sand being swept in from the beach, mostly when storm surges coincide with the inflow phase of king tides.

SA has only one tide each day and they are asymetrical. That is, they have a rapid inflow over 8 hours and a slow outflow over 16 hours. And as the capacity of water to transport sediment is determined by the square of the velocity, the inflowing water moving twice as fast as the outflow, transports four times more sand in than the outflow removes. So the mouth under natural conditions will close up whenever river flows are insufficient to balance the equation.

The larger the tidal prism (the area inside the mouth x tide height)the larger the potential for sand deposition. So the barrages have played a vital role in keeping the mouth open. Removing them without an adequate alternative mechanism in place (see http://www.mdba.gov.au/files/submissions/Ian%20Mott%20-%20Landholders%20Institute.pdf ) will prompt demands for even more buy-backs to keep the mouth open. And that is not in the interests of farmers in SA or anywhere else.