The Insurance Council of Australia’s (ICA) final hydrologist report on the Lockyer Valley floods has classified the event as a flash flood.

The report, produced by hydrologists from Worley Parsons, WRM Water and Environment and Water Matters International, says there were three distinct flooding episodes in the Lockyer Valley local government area from January 9 to 13.

It says the first, which occurred overnight on January 9 until the early hours of the following day, was minor and didn’t cause any significant flooding.

However, the second episode manifested as a flash flood on the afternoon of January 10, generated by intense rainfall in the upper tributaries of the Lockyer Creek.

The report says this caused an “abrupt spike”, characterised by water level rises, high flood depths and destructive water velocities.

By the time the third episode happened on January 11, the velocity of the floodwaters had returned to a “normal” pattern.

The report says rainfall severity was equal to or higher than 100-year records for events of 30-minutes to three hours.

“The flash flooding caused hazardous conditions close to the waterways it travelled down,” the report says. “Buildings adjacent to waterways were swept away in Murphys Creek township, Postman’s Ridge and other locations.

“Conditions were especially hazardous at Grantham because of the number of dwellings on the floodplain.”

The hydrologists say nearly every house on the floodplain suffered major damage from inundation and water velocity, with some washed off their stumps and others totally destroyed.

The report has determined that flash flooding was experienced in the areas of Spring Bluff, Murphy’s Creek township, Withcott, Postman’s Ridge, Helidon, Carpendale, Grantham, Rocky Creek and Monkey Waterholes Creek.

Other areas affected by the event have been considered to be “normal” flooding.