Flood caused by 1-in-100-year rain event

Published: 13 Jun 2018

Flash-flooding that occurred in Redlynch Valley in March was the result of a one-in-100-year rainfall event, a forensic review of the event has revealed.

An independent review into the severe weather event that occurred on 26 March was today presented to Cairns Regional Council.

The study, undertaken by BMT WBM Pty Ltd, found that rainfall intensity during the event, which was associated with ex-Tropical Cyclone Nora, had an average recurrence interval (ARI) of greater than 100 years.

“The amount of rain that fell in the valley that night was immense,” Mayor Bob Manning said. “The Copperlode Dam gauge recorded rainfall intensity of up to 97mm an hour, which is the equivalent of a one-in-115-year event.”

A separate independent report into the Emergency Action Plan for Copperlode Dam, completed by Ironside Risk Partners, confirmed that the overflow from the dam was not the primary cause of flash flooding of the valley, as the floodwaters had started to recede by the time the spillway peaked. (Read more about Ironside report)

It stated that the floodwaters had begun to recede by the time the spillway peaked, and that water from the spillway would take between 45 and 60 minutes to reach the valley. Therefore, the peak flow from the spillway would have arrived at the Crystal Cascades Caravan Park about an hour after floodwaters had started to drop.

The BMT WBM review states that flooding in the Rocks Road area was primarily due to rainfall in Freshwater Creek Valley, below Copperlode Dam, with Currunda Creek providing about a quarter of the total flood flow.

Large amounts of debris and sediment from landslips caused a blockage on Redlynch Intake Road at the Currunda Creek crossing  and this contributed to flooding in the immediate area. However, the report stated there was no evidence of dam or levee collapses in Currunda Creek.

The report took into account 23 submissions that were provided by residents who documented the flooding with photos and other material.

Cr Manning said the data contained in this report and supplementary modelling being undertaken as stage 2 of the review would provide new insight into the characteristics of the catchment in a major rain event.

“Until now, all of the modelling that had been done around a 100-year ARI flood in that area was hypothetical,” Cr Manning said. “We didn’t have data sourced from actual flood events – everything was based on mathematical calculations and assumptions.

“With this flood event, we have now been able to study the flow of the water, where it went and why. We actually found that we could confirm the accuracy of a lot of the modelling that we already had and now we can add further detailed data to this.”

The report did not make any recommendations for Council to action.

The Stage 2 report, detailing the results of the hydraulic modelling, is expected to be provided to Council later this month.

View the Redlynch Valley flooding report

View the Copperlode Dam Emergency Action Plan report

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Last updated: 14 June 2018