Historic drought could have been the underlying reason for the collapse of an earthen irrigation dam near Dalby last month.
The dam in the Darling Downs – which held 1.8bnl of water – burst its banks in late December, when considerable rain and floods hit Queensland following almost a decade of drought in the region.
The change from extremely dry conditions to extremely wet ones could be behind the structural failure, said regional consultant Aquatech Consulting managing director Jim Purcell, an irrigation dams specialist.
The ground in the Darling Downs is a heavy reactive clay based soil, meaning it expands and contracts with varying moisture content.
The change from extremely dry conditions to extremely wet ones could be behind the structural failure.
It is structurally strong, compacts well and has a very low permeability, and is thus generally good for earthen dam and embankment construction, said Purcell.
But he said an extended dry period of this length is likely to have caused the soil to dry out to the point of extreme shrinking and cracking, potentially leading to structural weakness.
Purcell said further complications would have arisen if the soil is dispersive – meaning that when moist its clay particles are prone to separating from one other.
If this is the case, dispersion caused by the heavy flooding would have created a muddy solution, which could result in tunnels forming and water flowing through the structure. Water erosion could then have caused the dam to fail.
University of Queensland senior lecturer Alexander Scheuermann said that although it was impossible to determine the cause of the collapse until the floods recede, examples of embankment failures where shrinkage cracks played a major role are well documented, such as in the 2002 Zeyzoun dam collapse in Syria.
If the water level recedes so quickly that the water cannot drain from within the slope, slumping can occur.
Further structural problems could come when the floods recede and the ground dries out, said University of Queensland emeritus professor Colin Apelt.
He warned that dams and embankments could fail due to slump – a slipping of material down the curved surface of a slope which is often associated with heavy rainfall. If the water level recedes so quickly that the water cannot drain from within the slope, slumping can occur.
However, Apelt allayed fears that the conditions of the drought-ravaged ground might impede the recession of the flood waters.
The soil conditions would have already been moist due to the “considerable” rainfall that fell in the weeks prior to the floods, he said.
“I would expect that the moisture content of the ground would have been substantially replenished before the floods occurred,” he said.