Grantham: once-in-400-years flood ‘beyond imagination’

Volumes of floodwater flowing to Grantham in 2011 were far higher than anything previously measured.

Extraordinary volumes of floodwater flowing to Grantham after extreme rainfall in 2011 were far higher, and more intense, than anything previously measured — and quickly overwhelmed a relativel­y small quarry pit and its surrounding earthen embankment, an expert study reveals.

The study by hydrologist John Macintosh, an engineering expert­, was released by the Grantham floods inquiry yesterday after The Australianrevealed yesterday its central finding that a quarry owned by the Wagner family did not have any adverse impact on the town.

The quarry’s spare capacity was 200 megalitres on top of the water already in the pit.

Dr Macintosh, who has spent three months running computer simulations of the disaster that killed 12 people in the town west of Brisbane, found that the “magnitude and rapid rise of the flood produced inundation characteristics never before experienced or imagined by the residents”.

The estimated flow rate of the floodwater measured at Helidon, a short distance upstream from Grantham, was four million litres a second or 4000 cumecs (cubic metres a second), compared with the previous highest flow rate of 108 cumecs.

DOWNLOAD: Report on expert hydrology report – circumstances and contributing factors

Dr Macintosh said the flood at the Helidon gauge on Lockyer Creek was estimated to have a probability of once in 400 years. It inundated Grantham differently from previous surges because of its rare scale and intensity.

He described how a 2m-high railway embankment, built in the 1800s, was the structure most significant in the flood as it was “stopping all flow to the north” where it could have dispersed.

“My assessment has concluded that the railway embankment increased peak flood flow intens­ities through western Grantham and central Grantham to the west of Sandy Creek,’’ he said.

Dr Macintosh said “the railway embankment has clearly had an effect on the concentration and direction of flood flows”.

But, although the railway embank­ment worsened the flood for the town, Dr Macintosh exonerated the quarry and its embankment, owned and operated by Denis Wagner and his family.

Grantham residents who have blamed the quarry are facing the prospect of inquiry head Walter Sofronoff QC ruling that the quarry did not have an adverse impact.

Mr Wagner, who has always insisted his quarry slowed the flood and lessened its intensity, said he was saying nothing for now but would have a lot to say when the inquiry concluded next month.

Dr Macintosh said: “I consider that it is most likely that the prim­ary effect of the presence of the quarry has been to delay the time of inundation of the Grantham area (by one to three minutes).

“I consider that the primary reason for this delay is due to the time it takes to fill the quarry pit by floodwater inflows.”

Dr Macintosh concluded that the quarry pit had a free storage capacity of about 200ML in addition­ to its existing lake level before the onset of floodwater.

He said failure of the quarry levee “would have most likely had the greatest impact in the local vicinity of the quarry with largely diminished impacts on flooding in ­Grantham”.

He said he had investigated the likely level of the water in the quarry­ pit immediately prior to its inund­ation by the flood as this would have a bearing on the quarry’s influence. “The significance of available capacity is not only that this volume is effectively removed from the flood, but also that outflows from the pit area back into the downstream Lockyer Creek waterway will only commence once the pit has filled,’’ he said.

Dr Macintosh modelled “worst case” scenarios that sped up the inund­ation time by up to three minutes. However, he cautioned that this was “unrealistic because of the underlying assumptions”.

He found there were multiple breaches of sections of the quarry’s embankment and these totalled 330m in length, leading to flood flows into the pit that retained water and delayed the flood.

He said locals who gave witness accounts in their evidence provided “vital information with which to better define the critical time of arrival of the flooding of Grantham”. Computer simu­l­ations of the flooding would rarely match reality in all respects, but would give good approximations.

In addition to the report, Dr Macintosh produced video files to depict “complete simulation outputs for the extent of inundation, flow intensity, velocity direction and velocity magnitude”. However, he found the quarry “accelerated the rate of rise of floodwater” at one nearby address but it had “minimal impact on the resulting flow depths and intensity”.

For local Marty Warburton, the final outcome is not nearly as important as having a chance to be heard. He said yesterday he was prepared to accept the findings, regardle­ss of whether he agreed with them: “I’m not an expert but I have the utmost faith in the commissioner and, when he hands his findings, the important thing will be that we ­finally have answers.”

He said recurring nightmares and unexpected triggers such as the smell of wet mud still took him back to the day he stood on the roof of his service station for five hours and watched as his communit­y was destroyed.