Council & Business
9 August, 2024
Independent assessment finds Carisbrook Flood Levee not fit for purpose
In its current state, the Carisbrook levee will not protect the town from a significant flood event, according to a new report — which has found numerous modelling and design flaws in the structure.
An independent review of the levee, conducted by New South Wales engineer Fred Spain, has raised multiple issues that could impact Carisbrook’s flood-safety.
Among the 17 concerns highlighted are the lack of an internal flood study, selection of a tight safety buffer, and absence of flap valves in certain stormwater pipes.
The levee was originally constructed in response to the 2010/2011 flooding that led to the evacuation of the Carisbrook township and left over 250 properties inundated.
Mr Spain’s findings follow extensive lobbying from residents, who for the past decade have questioned the effectiveness of the levee.
Earlier this year, in response to the push for answers, the Central Goldfields Shire Council, partnering with a group of locals, appointed Mr Spain to assess the levee.
His report now “vindicates” the community, after their concerns were “dismissed” on several occasions.
When Mr Spain began the assessment, he was charged with reviewing “the history of the project” while trying to determine what was agreed to at the public meetings before the levee was built, if anything was altered during construction and if so, why the change occurred.
However, the purpose of the report soon evolved to include a review of the adequacy of the modelling, both from the 2013 Flood Study and 2019 revised Flood Study, design of the works, and the levee’s construction standard.
“This led to a number of issues of concern with both the Flood Modelling and Design being found which need addressing...”, the report said.
These problems include:
No Internal Flood Study has been undertaken, which means even though the town is protected from external flooding, it will be adversely impacted by “all rain falling inside the levee banks”.
The Light Detection and Ranging (LiDAR) and flood model were “assumed to be ‘perfect’ when in fact there are (always) inherent uncertainties”.
Selection of the nominal 300 millimetre levee freeboard — added for additional safety — is “operating on a very tight tolerance/slim margin for error”.
Stormwater pipes under the levee south of the Pyrenees Highway serve no purpose and are only a source of a failure risk.
Vegetation clearing is required to be undertaken in the creek “to ensure that it meets the modelling intent of preventing flooding of Carisbrook”.
While some of these issues were expected the report also packed in big surprises, Carisbrook resident, Ian Boucher said.
“There’s more concerns than we ever thought of,” he said.
“I think it just vindicates us. It shows that we knew what we were talking about — we knew the design wasn’t right.
“This review has brought more to light and it is very significant [because] council told the community that this levee will protect Carisbrook from a future flood and that’s proven to not be the case.”
Underscoring all concerns, according to Mr Boucher, is ignoring the advice of people with boots on the ground.
“When we spoke about our doubts, we were told that experts had designed the levee — this has proven the experts didn’t know [everything],” he said.
“They should have taken notice of people who are impacted and that’s the locals.”
Although Mr Boucher isn’t assigning blame, he believes with the release of the report, the Central Goldfields Shire Council needs to take a more proactive approach.
“It’s been a combination of people not listening to us,” he said.
“Council has probably taken the view that they’ve had expert advice, not being experts themselves.
“I don’t want to point fingers, even though our concerns should’ve been seriously considered — but now that the recommendations have been made to rectify the levee, if they aren’t followed up on, then council is a hundred percent to blame.”
Part of that follow up, fellow resident Greg Corcoran said, involves acknowledgment from council of problems in the development of the levee.
“Some of the things the report talks about should’ve been picked up earlier by council,” he said.
“The review only happened because the community kept pushing for it — our concerns were dismissed by council.
“There needs to be recognition of what went wrong. If that happens, people would respect them more and have confidence that the levee would be fixed.”
Council CEO Lucy Roffey’s takeaway from Mr Spain’s report did not match that of the residents’ — she said the review reiterated that the modelling undertaken to construct the levee “was as accurate as it can be”.
She is referring to a section in the report which reads: “It is to be acknowledged that the modellers seem to have done the best they could with the limited information they had available to create a workable flood model”.
Ms Roffey said this new information should “allay” the community’s concerns.
“It should be noted that one of the main concerns raised by the community over many years was in regard to the flood modelling accuracy and in particular they didn’t think that local information was used to inform the modelling,” she said.
“The independent report clearly states that the flood modelling did use local information, captured all relevant inputs and water flows across the catchment and used the most sophisticated modelling tools available.
“These findings confirm that the modelling used as a basis for the levee design was as accurate as it can be, while noting that no modelling is perfect given the variables involved in storm events.”
At this stage, beyond the modelling software’s accuracy, the community wants council to amend the issues identified in the report by adopting all of Mr Spain’s suggestions.
“Many of the recommendations in the report relate to ongoing maintenance of the levee and these recommendations will be included in the Levee Maintenance Manual that is currently being developed,” Ms Roffey said.
“A number of recommendations can be implemented using unspent funds from the levee project, and some recommendations will be funded from our regular operations budget.
“The recommendations relating to further flood studies or modelling can be included in the scope for the recently announced $300,000 that council is to receive from the State Government to update flood modelling — we are currently finalising this funding agreement with the state.”
She added that if further funding is required to make any other modifications to the levee, council could consider these in the mid-year budget review.
“The reviewer’s report did not provide cost estimates for any of the recommendations, and some further investigations may need to be undertaken to ascertain the extent and costs of these works,” Ms Roffey said.
For Carisbrook residents, while the report has validated some of their suspicions, it has not given them peace of mind.
“A January 2011 flood event would most likely still flood Carisbrook,” the report states.
And if council delays necessary work on the levee, there could be a costly price to pay.
“It’s the human cost,” Carisbrook local Leonie Van Rooyen said.
“We’ll be in a constant state of fight or flight until something is done.
“Everything the report has recommended needs to be implemented — we just want to live in our houses without the ongoing fear that we might not have a house tomorrow.”
The Central Goldfields Shire Council will formally receive the independent reviewers report at the September council meeting, before the caretaker period commences ahead of the elections.
“[It will] ensure implementation of recommendations is not delayed by the election,” Ms Roffey said.