Reader and geologist Peter Ravenscroft thinks Brisbane’s dams might be leaking as much water as residents are drinking. It’s a speculative view, but one I thought worth publishing to see if anyone knows what the story is.
“Wivenhoe Dam seems to be leaking very seriously.
We may be losing nearly as much water as we are using in South East Queensland. Wivenhoe Dam is dropping far faster than it should be, going by the SEQWater monthly water release figures. In February, they released about 11,000 megalitres from that dam, according to their on-line releases graph.
But the dam dropped, according to their percentage decline rate graph, by 21,000 megalitres in March, as near as I can tell. I do not have the figures to compare within the same month.
What follows may explain the huge amount of missing water, and the discrepancy between the two sets of official figures. It needs to be checked rather carefully, and at every point. I have very imperfect data and am good at making mistakes. But I think the geology is sound and the numbers, I hope, are roughly so.
It seems that about 10,000 megalitres goes missing each month. The official evaporation rate for Wivenhoe is 1.8 metres a year. That should account for roughly 600 megalitres of evaporation a month: which leaves about 9,400 megalitres unaccounted for. A megalitre is not an extra large Coke bottle full, in spite of what your children may claim: it is a million litres. So 9.4 billion litres seems to be wandering off to parts unknown, each month.
That is largely why the government’s scheduling of water projects will fall short and the dams will run dry, if we do not get heavy rain. We are losing almost as much from Wivenhoe as we are using, in all of SEQ. To be more precise, we seem to be losing an extra 85 per cent on top of what we are using.
I cannot be certain, but if the official figures are correct I think the missing water is going around, and perhaps also under, the dam wall at Wivenhoe. That wall is built into Marburg Sandstone, on both sides. That was probably done to leave the Great Moreton Fault System to the east, for safety. On the Queensland Government 1:250,000 geology map, 1973 edition (which is still the best we have) it is called the “Great Moreton System” The word “Fault” was clearly edited out. It exactly labels the fault system that the Brisbane River follows, both to the north and to the south, though not at the wall.
Rivers very often follow faults, so dams often have to be built near them.
Nothing wrong or unusual there.
The Jurassic Marburg Sandstone, into one cliff and one hill of which the wall is set, is very permeable, that is, water leaks happily and easily through it. The proof of its permeability is that the government has drilled it a little way to the southwest, as an aquifer. Local farmers have pumped water from it for decades. Using it to hold back water is better than using a haystack, but not wildly so.
The dam level at present is about the same as that of the farmlands to the southwest. Any bore in that region, a far as Toowoomba, will draw down the water table and so will basically, I think, extract water from Wivenhoe.
Toowoomba is now pumping from below the range, so is effectively using Wivenhoe water already. All the farmers to the southwest with bores, ditto.
Some water will probably first move a short distance east, to the Great Moreton Fault System. Then south along the fault, and then west again, all along the huge surface provided by the fault having Marburg Sandstone on one side. The rest, I believe, will simply move due southwest through the rock on the western side of the dam wall.
If that is correct, some things could be done straight away.
We effectively, if I am right, have two good buckets and one with a huge leak. If water can now be piped to the North Pine Dam from Wivenhoe, that should be done. No water should be released from Somerset to Wivenhoe, except for immediate release to Brisbane and friends. And a pipeline, if one does not exist, should go in, to pump water back from Wivenhoe to Somerset.
As no roads have to be crossed, that could be done with multiple poly lines, diameter as big as can be rolled out, as many as needed.
I am not an engineer, but I suggest the western flank of Wivenhoe, in the section where the rock is Marburg Sandstone, should be clay-lined while the damn is so low. The best rock for that is the Bunya Phyllite, from the D’Aguilars just to the west of the dam. A big quarry can go in up one of the creeks there, and the material can be trucked over the wall. I would guess that the eastern side of the dam, near the wall, is too steep for clay-lining. It may need concrete shot-creting.
A sort of stop press. Linton Brimblecombe, a man with an infectious laugh and also the chair of the Lockyer Water Users Forum, has kindly informed me that they are not getting the missing water. Their aquifers are only partly linked and are now running out. Expect the price of vegies to climb rather steeply when that happens. He says their bores are now down to about 70 to 100 feet. He thinks maybe the Wivenhoe dam is not as big as they thought it was, so the problem is a survey one. If he is right, there may not be any missing water, we may just have a lot less than we thought we had, in reserve, which is still a problem.
Linton is probably right, the farmers may not be directly using Wivenhoe water since they have so little. If however water is being lost to the southwest and into the Great Artesian Basin the farmers are in fact drawing the dam down. It may be that it is decided to allow the use of boreholes to continue because the Lockyer Valley is a major food production area, which is fine. Otherwise they should be stopped and properly compensated. I would suggest Toowoomba be allowed to continue, as they have no alternative water source as yet. Their dams are nearly empty.
If you wish to check the geology, if you can get one, use the old paper map from the Mines Department, now part of the DNR. (1;250,000 Ipswich, SG 56-14, Geology, 1973). You will have to colour it in, as the government ran out of ink: unless a later one is coloured. The new online maps are un-usable gibberish, and did not show the geological cross section AB that is critical the last time I checked.”