The Insurance Council of Australia’s third hydrology report, examining flooding in the Ipswich local government area (LGA), has concluded that most of the region’s flooding was unlikely to have been caused by flash flooding.

The report, which was produced by hydrologists from Worley Parsons, WRM Water and Environment and Water Matters International, found there were four rainfall periods in the region with rain which fell from 6am to 3pm on January 11 being the most significant.

It concludes that as the rain subsequently stopped, only flooding that occurred before 6pm on January 11 could be classified as overland flow inundation.

However, the report concludes that the rainfall was not severe enough to have caused widespread flash flooding.

“Overland flow inundation is generally associated with intense rainfalls of short duration,” the report says. “In general, such rainfalls did not occur across the built-up areas of Ipswich City LGA in January 2011. Therefore, it is unlikely that overland flow inundation will have occurred at many locations in Ipswich City LGA.”

River levels in the area did not peak until the afternoon of January 12, and the report says the majority of the region’s floods were caused by “waterflow inundation from the Brisbane River and its tributary creeks” which, in turn, was mainly caused by releases from Wivenhoe Dam.

The hydrologists also found that low-lying urban areas away from the Brisbane River or its tributaries most likely suffered backflow flooding.

“It is likely that high tailwater levels in the receiving waterways caused backflow up the piped drainage system, leading to inundation in the low lying areas,” the report says.

Most insurers do not cover backflow or waterflow inundation.

However, the hydrologists say that “uncertainties” do exist as to whether some properties, especially those further from waterways, may have initially been flooded by overland flows. It suggests more in-depth examinations of local conditions such as topography and drainage patterns would be needed to clarify these cases.