COMMENT by ERWIN CHLANDA
The “Alice Springs Flood Mitigation Dam EIS Volume 1” of October 1990 could not make it clearer: Only a dam at Junction Waterhole, upstream from the Telegraph Station, can save the town in a Todd River flood with a one-in-100-years probability, a Q100.
There seems to be no need then for the new advisory committee to re-invent the wheel. There is every need for urgent government action. It’s noteworthy the Chief Minister has given no deadline for the committee to report.
The 120 page report, of which Alice Springs News Online has a copy, was commissioned by the NT Government’s Power and Water Authority (as it was then called).
It says on page 48: “The dam will achieve the objective of protecting the town … from Todd River floods of an intensity up to 1% (100 year) probability” except for a small area in The Gap of eight houses and Giles House, now Aranda House.
It is widely assumed that a flood of the magnitude described is now likely to happen more frequently as a result of climate change – making effective flood mitigation the more pressing.
The report says: “Flood counter-disaster warning times will be substantially increased by the dam for all floods, and people frequenting the river environs will achieve considerable protection from drowning.
“It is quite probable that no lives would have been lost in the 1988 flood if the dam had been in place.”
Three people were killed in that flood.
The report describes alternatives to a dam, including levees, improvements to the town’s drainage and removal of obstacles, as “limited in the extent to which they reduce, rather than prevent, damage from general river flooding”.
Flood mitigation by the alternatives put forward is “not sufficient”.
Yet these inadequate other measures are two to eight times more expensive and cause damage to the environment in a variety of ways, says the report.
The 1990 document – of course referring to a smaller town – describes consequences of a Q100 flood with the “development levels” in place at that time: It would flood 1063 residential properties and almost all of the Central Business District, directly affecting 572 commercial properties.
“Three lives were lost in each of the last two major floods in the past decade, as well as millions of dollars of property damage caused,” says the report.
Friday, April 19, 2024
The freedom of the press still furnishes that check upon government which no constitution has ever been able to provide – Chicago Tribune.
I would be very wary of anything produced by the CLP Government of those days.
This is the fox guarding the henhouse.
Engineers want to make things, that’s what they do.
“There’s an election coming up, I feel a dam coming on,” is what politicians do.
By the way, there have been several dam proposals, which one is this?
Is it the “dry dam” that died because the then Government would not commit to leaving it dry?
We really are spitting in the face of logic when the great unwashed of the extreme left tell us we can’t drink the water to sustain us, because we will kill freaking frog!
We urgently need a dam, Adam! Hopefully before the Todd floods again. Katherine and Darwin got help. What about in your own electorate Adam? I see Northside is at risk too!
This report is available for viewing in the Alice Springs collection of the town library.
The report recommends the full (wet) dam option that would permanently flood Tnyere-Akerte. It would be 35 meters high with a spillway 20 m above the riverbed. The report also discusses the empty (dry) dam option starting on page 66 of volume 1.
Both options mitigate the 100 year flood and were roughly equal in cost (~$20M in 1990 dollars).
The empty dam would have a 6.5 meter diameter channel in the bottom to drain at a rate that would not cause the Todd to flood. Both options would trap sediment above them that would have to be moved downstream to keep the riverbed refreshed.
The empty dam option required some extra engineering at an uncertain cost to manage high flow rate (50km/hr) in the 6.5 m diameter channel. The full dam option could be used as a recreation lake or to provide some supplemental drinking water, but there is no further discussion of a recreation lake.
As Erwin noted in his water plan article the “Pan evaporation” is 3 m per year. This means the proposed recreation lake level would decrease an average of 8mm every day there is no runoff. When there is runoff in the 360 km^2 dam catchment the 1.2 km^2 lake would rise 300mm for every mm of runoff.
My recommendation is that the empty dam option be reconsidered. A recreation lake with highly variable lake levels does not seem to me to be much of an advantage.