In a submission to the council, consultant engineering firm Cardno said the culvert blockage policy should be better able to take individual sites into account, instead of a blanket approach across the whole city.

The submission was written by Cardno engineer Anthony Barthelmess, who has co-written several expert papers on culvert blockage.

The policy had been developed after research by Forbes Rigby consultant engineer Edward Rigby and council engineer Pas Silveri, conducted in the wake of the 1998 floods that devastated parts of Wollongong.

Their research confirmed anecdotal evidence that debris had blocked many culverts to varying degrees, inhibiting the floodwaters' escape. Many culverts 4.7 metres and under were completely blocked; many others were not.

But, while the researchers knew site-specific factors would affect blockage, when the council policy was announced, it was inflexible. It dictated that all culverts less than six metres across would be assumed to be 100 per cent blocked when modelling the effects of a significant flood.

In the years that followed, hydrologists and engineers revised and refined their blockage model. By 2009 Mr Rigby, now working with Mr Barthelmess, wrote that not all culverts acted the same way. The prevalence and type of debris, the debris' mobility and the steepness of the creek were all factors in determining the extent of blockage.

They published refined conclusions in the proceedings of the Hydrology and Water Resources Symposium held in Newcastle in 2009, and then in 2011, refined again, in Engineers Australia's respected flood guide, Australian Rainfall and Runoff journal.

Wollongong's culvert blockage policy, however, was not revised in light of the new science, but has remained rigid and prescriptive. It does not differentiate between any culverts as to debris mobility, incline, or prevalence.

This point was made strongly by Mr Barthelmess in the 2009 Cardno submission. It told Wollongong City Council that its Development Control Plan (including the blockage policy) was flawed because it did not allow for merit-based - or site-specific - assessments on flood risk, and produced "suboptimal outcomes" that were inconsistent with planning and zoning.

"We are of the considered view that the DCP is fundamentally inconsistent with the NSW Government ... Flood Prone Land Policy and the Floodplain Development Manual," the submissions said.

"The DCP does not allow for any merit-based decisions relating to development on the floodplain and as such fails to consider the full range of factors described in the Floodplain Development Manual."

The Cardno report also said the DCP was being used as the determining factor in development applications.

"We consider the DCP to be inconsistent with the way other environmental factors are considered in an environmental assessment framework, and the DCP lacks recognition of the societal risk we face every day," it said.

"We consider that the DCP contains unworkable requirements for evacuation and does not consider the potential for improving the existing flood affectation on land by engineering works (such as floodplain re-shaping). We also consider the DCP [is] founded on misleading terminology, describing flood planning control areas as 'risk precincts' when in fact these areas bear no relation to areas of 'risk' on the floodplain."

This is precisely the complaint of residents of Northview Estate at Figtree. It has never flooded but is all designated high risk, as if it were such a "risk precinct".

Wollongong's flood modelling policies would be reviewed if and when more advanced information came to hand, the council’s manager of infrastructure strategy and planning, Mike Dowd, has said.

‘‘There’s a process we’ve had to apply [and] there are assumptions we’ve had to make based on whatever available information we’ve got,’’ Mr Dowd said.

‘‘That’s what we’re bound to in terms of the process and that’s the result.

‘‘We acknowledge there’s changes as a result of that, that changed risk profiles for individuals within those areas.

‘‘We review it as and when new information evolves and new techniques come to hand.

‘‘Until that comes to hand, we have to stand by the procedure we’ve applied.’’

The council’s stormwater and transport services manager, Peter Nunn, said some of the problem areas were developed ‘‘30years ago’’.

‘‘We’ve improved the way we plan for flooding,’’ he said.

‘‘In 1998, I think that woke up engineers in the whole industry in general about blockage, and how you really need to consider it.

‘‘We’ve put structures in place to grab the debris, so the culvert doesn’t block.’’

Wollongong City Council’s senior floodplain engineer, Peter Garland, is the official whose name is familiar to many residents affected by floods. He is the officer who has run many of the meetings with residents concerning the council’s flood policies and answered questions about creeks.

The Mercury sought an interview with Mr Garland but the council declined to make him available. Instead, the Mercury was given a wide-ranging interview with two more senior officers, Mr Dowd and Mr Nunn.

They gave the Mercury as much time as was needed and answered all questions posed.

Some of these responses are not part of today’s story but will be included in related stories in this flooding series.

After the Mercury became aware of the new research that demands a more specific approach, we again asked the council why the policy had not been changed.

‘‘Council’s culvert blockage policy was developed using significant data obtained from the 1998 flood and other floods that have occurred since 1998,’’ Mr Dowd said.

‘‘We reviewed our blockage policy in 2013 and based on the information available at the time, did not amend the policy. This was because the real-life data that we have obtained since 1998 continues to indicate that it is appropriate to apply blockage factors to culverts.

‘‘Recent publications by Engineers Australia discussing blockage are important guidelines, however, the publication recommends the use of local data when it is available.

‘‘As new research and studies become available, we will continue to review our blockage policy and update it when necessary.

‘‘We are currently reviewing our policy again to take into consideration the blockage control structures that we are installing, such as the recently constructed structure in Towradgi Creek [in] Corrimal.’’

Read the next part in the series Going Under in Tuesday’s Mercury.

Related stories: