If Wollongong City Council wants to use the best and most advanced research to help it model flood risks across the city, a team of expert engineers has just completed the very guidelines it needs.
Engineers Australia recently released the latest edition of its Australian Rainfall & Runoff, which publishes for the first time the most advanced guidelines on how to calculate the blockage of drains, culverts and bridges during flooding.
The respected AR&R series is described by hydraulic engineers as "the bible" of their industry.
The latest guidelines, completed last month, are part of a project developed in the wake of the 1998 floods in Wollongong, and largely based on this region's geography.
The last revision, the culmination of an almost six-year project, has refined the way to model how water flows in flood events, when large drains and culverts are at risk of being blocked by debris.
But while Wollongong City Council's review is yet to begin, managers appear to have already ruled out fully adopting the AR&R guidelines.
The council's infrastructure strategy and planning manager Mike Dowd said the council would not commit to enacting these guidelines.
"Council is of the opinion that we should have our own blockage policy and that we will not be adopting AR&R Project 11 as our blockage policy," he said.
"The proposed review will inform any potential change to council's policy."
Months after the review was announced, council is not yet at the stage where it can award the work to an external group.
Wollongong engineer Anthony Barthelmess, who worked on refining the AR&R guideline over many years, said he had previously developed a new blockage policy for the council in 2009, based on Wollongong data, and offered it for free, but was knocked back.
"This work involved thousands of hours of research and development and was peer reviewed by the top hydrologists in the country," he told the Mercury.
"It had acceptance by the local engineering community. It wasn't a blanket policy across the LGA. What was council's response? Ignore it."
Mr Barthelmess said the council should accept the new national guidelines.
"It is disappointing for me that WCC is forging ahead down a path that will likely see the same outcomes we've had over the last 14 years," he said.
"We must not allow another version of the policy to not deliver on outcomes. This will be difficult if WCC continues to opt away from compliance with a national guideline."
Mr Dowd said the scope of the review was still being worked out, after which submissions would be taken from independent groups interested in doing the work.
"Council is currently preparing the scope and specification for the review, including for community input," Mr Dowd said.
"We are currently identifying appropriate industry-recognised, experienced and independent bodies from whom we will seek submissions to carry out the review."
The new AR&R guideline says residents should be consulted and photographic evidence used to establish the base information for flood mapping. Calculations should be based on evidence and historical data, wherever possible, so a culvert's history of blockage helps determine what is expected in the future.
This point directly contradicts the methodology used by the council in calculating blockage.
The council's policy holds that every structure less than six metres in diameter is assumed to be 100 per cent blocked, regardless of its history or the debris conditions nearby.
It makes no distinction between culverts that have a history of blockage, or a lot of debris nearby, and those which do not. Nor does it distinguish in any meaningful way between severe and rare flood events, and those which are much more frequent.
This has the effect of turning the railway line and freeway, that both cut through Wollongong, into a large dam wall, when flood maps are created.
The AR&R project is clear in its demand that local evidence, history, and inspections be used to set the flood modelling at a particular bridge or culvert.
"All available history should be sought from relevant local stakeholders, including residents, in assessing the reasonableness of blockage levels and mechanisms produced by this guideline," it says.
"In particular, if there has been no long-term history of blockage at a particular structure and similar drainage structures in the catchment have not demonstrated blockage problems, blockage may not need to be considered, or a nominal allowance only may be appropriate in design."
To read the Engineers Australia published guidelines on blockage calculation, visit the website here.