A small southeast Queensland town devastated by deadly floodwaters in 2011 would suffer the same consequences if a similar amount of rain was to hit the area again.
That's the view of civil engineer John Macintosh, who was asked to consider what impact Grantham's quarry and railway embankment had on flooding levels and intensity in the town when 12 lives were lost on January 10, 2011.
Dr Macintosh told the Grantham Floods Commission of Inquiry on Monday his models showed the quarry, which residents blame for worsening the flood, actually slightly slowed the torrent heading into the town because water was able to flow in the quarry's pit.
He also said the town's railway embankment channelled water in the town and that a computer model with the embankment removed showed a "significant reduction of intensity in the early stages of the flood".
Dr Macintosh referred to several virtual scenarios from a hydrology report he had authored for the inquiry. The scenarios showed water flows with the railway embankment and quarry either removed completely or changed.
While the flows varied, they weren't enough to significantly impact the severity of the flood.
"The intensities are so high that the consequences would be unaffected I would say," Dr Macintosh said.
He said the unprecedented level of water flowing into the town from nearby Lockyer Creek, which cut off escape routes, was the main cause of the devastation.
Dr Macintosh said if the same weather event occurred, the consequences and flooding characteristics would be the same.
He said the only reason the outcome might be less devastating would be because disaster planning procedures had improved since 2011.
Academic Grantley Smith said the force of the floodwaters was so strong that even champion swimmers would have struggled against the torrent.
Mr Smith, who heads the University of NSW's water research laboratory, said the water was travelling at two to three metres a second (7.2-10.8km/h).
"The best swimmers in the world can swim at about 2m/s (7.8km/h) in an Olympic final," he said.
"The average person would find it very difficult to swim in 2m/s.
"It's obviously a very dangerous situation to be in."
Queensland Rail's representative, lawyer Judy Brien, is likely to question Dr Macintosh's evidence about the railway embankment when he returns to the stand later this week.
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