The recent disastrous floods on the Tweed and Richmond rivers and moderate flood events in the Clarence Valley have prompted discussions among residents about the level of protection afforded by the levees that protect Grafton and South Grafton.
Clarence Valley Council’s (CVC) local emergency management officer, Kieran McAndrew, says those discussions have “helped illustrate a lack of understanding of flood protection in and around Grafton”.
“Many people are under a misunderstanding the levees provide one in 100-year protection – they don’t,” Mr McAndrew said in a media release.
He said that, while the levees were initially designed to provide one in 100-year protection, “better modelling in the 50 years since they were constructed shows they only ever provided one in 25-year protection.
The South Grafton levee system would protect at least against a one in 30-year flood, and up to a one in 50-year flood. Maclean’s levee offers one in 50-year protection, and Ulmarra is less than one in 20-years, according to council estimates.
“We have been incredibly fortunate since the levees were built that, apart from a very brief overtopping event in early 2013, the levees haven’t been breached,” Mr McAndrew said.
“That good fortune won’t last forever, so we need people to be aware of the level of protection there is.”
Mr McAndrew warned that in Grafton a prolonged overtopping of the levees would result in a “much more serious” event than the flood that devastated Lismore.
“In Lismore, there are hills people can reach from the CBD; we don’t have that luxury in Grafton,” he said.
“And because of the volume of water in the Clarence, flood heights fall much more slowly, [which] means the city would be inundated for much longer.”
Meanwhile, the council has made a fourth funding application to the NSW Government to determine the floor heights of all properties in flood-prone areas around Grafton.
The council’s works and civil director, Troy Anderson, said the government advised CVC that “its 2014/15 application was unsuccessful but was ranked highly and placed on a reserves list”.
“In 2015/16 we were told that we did not rank highly enough,” he said in an emailed response to the Independent’s enquiry.
“In 2016/17 we were told this project [surveying floor levels] should form part of a broader flood study update.”
As far as raising heights of the levees was concerned, Mr Anderson said that “gathering floor level information about the properties located in the floodplain, behind the levees and upstream and downstream of levees” was the first step of the process.
“Without floor level information, it is impossible to undertake a cost/benefit analysis,” he said.
However, he added, “floor level information does not guarantee levee augmentations will occur”.
“Any levee augmentations in Grafton are complicated, and would be decided on costs, benefits, feasibility, aesthetic issues and community acceptance,” he said.
“As an example, to increase the Grafton levee to a one in 100-year level (using the current flood model and with no freeboard) would require raising the levee between about 200mm and 500mm and that would require a total rebuild of most of the levees.”
Mr Anderson said that CVC would face “challenges” because “levees are usually on private property, some with easements some without”.
“Changing the levees in Grafton may have unintended consequences to places downstream of Grafton,” he said, “therefore no changes are made without significant efforts to model proposed changes; [and the] costs of any augmentations would be significant and would rely heavily or solely on government grants.”
Mr Anderson said that the media release was “prompted [by] the confusion that was evident after the Lismore levee overtopping”.
“Council staff has presented this information at numerous community forums,” he said. “This information has also been on council’s website for several years.”