New flood maps due to be released by the Council next month are already causing confusion and distress among some Logan residents – and experts are not surprised.

They say that flood maps are confusing, and in order for people to understand them they need to be presented with plenty of context and explanation.

Environmental historian Dr Margaret Cook told *PS Logan: “What's missing from the discussion is that these maps are really, really hard to read and I think it's really important that local governments work with the community to explain it all.”

As we have previously reported, some locals who have asked for advance information about the maps say they will make their properties all but worthless.

The maps, which will be available online, will mark some land as at risk of major flooding despite it never having been considered as being in danger in the past.

This means some residents are suspicious about the accuracy of the new maps, which are based on modelling, and want more clarity about how the information has been compiled.

Cook, who has researched the waterways of South-East Queensland extensively, said: “These residents’ concerns are absolutely, utterly genuine. They need answers, it’s only fair.”

The author added that the complexity of flood mapping requires a “level of math literacy that many of us don’t have.”

Part of the difficulty in understanding them comes from the huge number of variables that lead to flooding, along with how much rain falls, where it falls, and how long it falls for.

Debris in tree by Logan River shows how high the water reached earlier this year. Photo: Christine Schindler

Cook explained: “First thing any hydrographer tells you is that no two floods are the same.

“If you get 100 millimetres over a day, it's OK. But if you get an hour, it's terrible. If it falls above the dam, we're OK. If it falls downstream of the dam, we're not. And so it's never the same.”

Thomas MacPherson*, a flood engineer who works with local councils in developing flood maps, told *PS Logan that they are built around a number of different factors.

He said that while historical flood levels and chance of rainfall in a given area and within a given time are part of the modelling, new technology and engineering industry standards also play a role.

MacPherson added: “The council might also have new information, it might have a sensor on a particular stream that they want to make sure that their model aligns with.

"A lot of it is updating standards as new data becomes available, and just constantly modernising that.”

The City of Logan, whose last flood maps were released in 2012, is built around a flood plain.

History records an ongoing fight between the whims of Logan River and the roads, buildings and ferries that have existed near it over the years, as our Tales From The River article has illustrated.

Flooding can also be both exacerbated and changed due to development, new houses, land clearing, and even debris from trolleys, bikes, branches, fill, mud, and rubbish washed downstream.

Even the slightest change in one section can have an impact further downstream.

Environmental historian Dr Cook, whose book A River with a City Problem: A History of Brisbane Floods was published in 2019, explained: “It’s really important that the flood maps get revised after every single flood, because every flood gives us more information.

Clean-up operations in Beenleigh after this year's flooding. Photo: Logan City Council

“And also, we're doing things every day that change flood profiles. Every hard surface we build or every new housing estate changes the flow rate and changes where floods will go.

“One advantage of having done this mapping is we get a much better idea of where the areas of highest risk are. It's not a case of a blanket rule of what works here will also work there.”

Industry standards for developers do include flood mitigation – for example, shopping centres with large paved areas are required to store rainfall that would have normally soaked into the soil, rather than letting it run off into drains and waterways.

Flood expert MacPherson said these kinds of mitigations can really help, but they have to be considered right from the earliest stages of the design.

He said: “If a flood engineer comes along in the later part and says, “Oh, no, you can't do that”, they just get ignored. It's needed from the very start in the planning where they build on a parcel of land.”

While councils and developers are aware of the importance of flood maps, MacPherson said he is less confident in the ability of real estate agents to advise potential buyers or sellers about risks based on their non-expert understanding of the maps.

He explained: “It does need a lot of context, knowing what it is and what it isn't (in the flood zone). It's not saying that if your building is outside of it, that it will never flood. It's saying that based on their modelling, with whatever caveats come with that.”

Watch *PS Logan reporter Christine Schindler's video on the flood map issue

“I think the sharpest edge of it all is the communication aspect, communicating what it is and isn't going to flood.”

Meanwhile, the term “one in a 100-year flood”, which is often used when discussing flood risks, is also often misunderstood – and for this reason there is a push by experts to change the terminology.

Cook said: “What it actually means is that there's a one per cent chance of a flood of that height or higher in any given year – it doesn't mean we won't get another one for 100 years.”

She continued: “We've also got the really real reality of climate change. What is happening is it's actually getting harder to model and predict rainfall so these maps can get out of date quite quickly.

“We need to recognise that our weather patterns are changing and we need to be more proactive to respond to that.”

The historian warned that all locals should have a clear idea of what they will do in a flood emergency.

She said: “If you live on a flood plain, you should have a flood plan.”

Poles at Tully Memorial Park are marked with the levels of past floods. Photo: Christine Schindler

And she believes this sort of advance preparation also needs to be done by local and state governments, rather than concentrating on emergency responses.

Cook said: “We keep waiting until we get flooded. We spend 97 per cent of our money on recovery. Even 20 per cent of that spending could be used on public education.”

According to flood engineer MacPherson, a good example of simple, effective education are the signs in the centres of Warwick and Stanthorpe which indicate past flood levels.

While Logan has historical flood marker posts in Tully Memorial Park in North Maclean, MacPherson says that putting these sorts of posts in more prominent places, where they can be seen regularly, would help educate people.

He said: “As you're driving around, you can actually start to notice these things. So if there's a warning coming out saying that, ‘On this creek, we're expected to get to level blue’ (you can) go look out on the post and see how high it's going to get.”

Keeping waterways and river systems clean, or even planting them with new vegetation, can also make a difference to the water flow rates, as well as building resilient structures.

Cook explained: “In between the ‘don't build’ and ‘go straight ahead and build’ areas, there's a whole lot of incremental things that you can do.”

And she said of Logan Council’s flood zone management and its new mapping: “It has real consequences for everybody on that map.

“The whole character of the place has changed and so has the population. So again, I'm coming back to the need to work very closely with the community so that they're better informed.

“I think that sometimes a conduit between the mapmakers, and the people on the ground is missing. And that's the critical step that we need to really address.”

* The flood engineer asked *PS Logan not to use his real name.

  • Logan City Council provides information on flood emergencies and preparedness in this downloadable brochure.


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