Someone who has been closely involved with floods of the past is Woolamia’s John Downey, who was Shoalhaven City Council’s flood mitigation engineer from 1966 until 2002.
While Mr Downey said last week’s rain event was on the less serious end of the scale, there was potential for a repeat of the type of flooding Shoalhaven experienced in the 1970s.
He remembers that when the 1974 flood hit, Tallowa Dam was still being built, and parts of the wooden form work at the construction site were washed away.
He said the flood that followed in 1978 was actually worse than the 1974 flood. And to put that into perspective, the great flood of 1870 that basically destroyed Terara and forced development into Nowra, was one metre higher than the ’78 flood.
“A flood like the 1870 flood would be classed as greater than a ‘one in one hundred year’ event, as a measure of the likelihood of it ever happening again,” Mr Downey said.
Council flood reports characterise the 1870 flood as one in 150 years, the 1974 flood as one in 12 years, and the 1978 flood as one in 20 years.
Mr Downey said the Tallowa Dam was completed by the flood of ’78, but the river levels still came up higher than in 1974.
“People got a shock in 1974 because it was so long since we’d had a flood,” Mr Downey said.
“It was big enough in both the events of the ’70s to flood Terara village, but the other interesting flood was in 1959.
“The water only got to the top of the banks so it didn’t flood Terara, but because of the nature of the overall river pattern, this flood created the highest levels ever recorded in the Worrigee Swamp.”
He said during his tenure a great deal of flood mitigation work was done to help the flood basins to drain.
“We commenced mitigation in 1964, and it was basically a drainage scheme. But if you look back in history even Alexander Berry had put in a drainage system.
“The concept was a rural scheme to help get the water away more quickly, and to restore the natural levy.”
The work was ongoing, he said, due to erosion of the banks.
He agreed with residents living along the river that many people were complacent about the potential for the river to flood, because we have such big gaps between big events.
He also agreed with council’s current natural resources and floodplain manager Isabelle Ghetti, that it was not a good idea for residents to try and manually open the river mouth at Shoalhaven Heads.
“Opening of the heads only affects flood levels up to about Broughton Creek anyway,” Mr Downey said.
He said last week’s heavy rain had potentially set the scene for a more serious event.
“The fact that the ground is saturated and the rivers and creeks are up – there is a reasonable risk of it getting more serious if we received very heavy rain in the Kangaroo Valley catchment.”