An estimated 2,000 residents were made homeless, council assets suffered more of AUD 350M in damages, and more of AUD 400M of production was lost across the towns economy.
The event raised critical questions about flood prevention and planning, and underscored the urgent need for more resilient town and city planning approaches that anticipate extreme events and protect communities and economic activities more effectively.
MP: There's a saying in my industry that no two floods are the same. And the 2022 floods, two floods in the space of two months or one month demonstrates this.
MP: The only real way to eliminate the risk is to remove the population from the floodplain. But what are the costs of that and what are the implications of that?
Welcome to Engineering Matters. I’m Tim Sheahan, and I’m Alex Conacher. In this episode, part of a mini-series produced in partnership with Egis, we will explore how engineering solutions can be used to protect communities from the impacts of flooding.
Floods can threaten lives and bring widespread damage to homes, businesses and critical infrastructure.
When assessing flood risk of a town or city, engineers must carefully weigh this things against the financial and logistical costs of implementing mitigation measures.
Floods don’t spread evenly across landscapes. The relative risks to different areas need to be estimated and built into a broad response plan should the unthinkable happen.
In Lismore, Mark and his team had to think on their feet as the threats they were assessing actually happened.
MP: On Sunday the 27th of February, the flood height was initially forecast to peak at 10.6 metres
MP: So the forecasting drastically changed through the late afternoon and evening, and the predicted flood level was then estimated to be 11.5 evacuations were ordered to Lismore residents within the floodplain. The actual event ended up peaking at 14.4 on the Monday at around 2 o'clock. And that event across those 24 hours from 2 o'clock on Sunday to 2 o'clock on Monday, there was a rate of rise of approximately eight metres over that 24-hour period. So it just shows the sheer volume of water coming through Lismore, which actually equates to nearly three times the volume of water in the Sydney Harbour.
MP: So we originally engaged in 2019 to prepare a flood study and flood risk management study. So the flood study includes developing a flood model which defines flood behaviour for a range of events to then inform the flood risk management study.
MP: Yeah, so a town like Lismore, and this applies to a number of towns where there is a flood risk, it does require a combination of options. And the way that we look at it in the flood risk management industry is we define them as, or categorise them as, property modification measures, flood behaviour measures, which are your engineering solutions, and response modification measures, which are generally your emergency management.
MP: So the flood study and the flood plan risk management study were the first part, and we actually part way through the flood risk management study when this event occurred.
Floods like those that hit Lismore are the hardest to plan for
MP: So flash floods are very difficult because you don't necessarily have the warning time, although there are forecasting and response measures that you can put in place.
MP: But this includes flood forecasting, warning, evacuation planning. So how long have you got for certain events to be able to evacuate, as well as emergency response recovery. And then once the flood's over, the water's receded, how do you recover?
MP: And then that feeds into a flood risk management plan, which really is the recommendation based on all the evaluation that's been undertaken. And then that also includes an implementation strategy as well in terms of what funding is required.
When the 2022 floods hit, so did the risk assessment.
The government-funded science agency, CSIRO, eventually took the lead and implemented a broader examination of the wider floodplain while Engeny focused on solving for the local management issues.
MP: the focus changed from identifying flood protection measures for events up to a 1 in 100-year flood, there became a greater opportunity to look at events that could provide protection or risk reduction for larger events.
MP: And we then focused on, okay, what are the immediate changes that we can make. And that included defining flood risk precincts within the town. So the planning scheme changed whereby development controls were implemented based on flood risk, which went beyond the one in a hundred-year flood events. So really trying to control the development within the town. And then we also provided significant input into the identification of properties eligible for house raising, relocation, and also retrofitting as well to try and remove or provide better protections for some of those vulnerable properties in the floodplain.
MP: The CSIRO are currently working through mitigation options. Once that is complete, will look at that with council and identify how they relate to council specifically and how they relate to Lismore specifically and go from there. And I dare say there may be an update to the flood risk management plan once we know the outcomes from the CSIRO.
Identifying remedies is a key part of flood risk assessments, but they all come at a cost.
MP: Yeah, unfortunately for Lismore and a lot of other towns there's no silver bullet or one single solution. Often a combination of measures are required and engaging with stakeholders to get their input is an important part of the process, including the community as well.
MP: You'll find a number of flood risk management plans, such as the one that we developed, provide recommendations for structural measures or engineered solutions, and they don't ever get implemented because they're not affordable or cost effective. What is cost effective and what you can implement almost immediately are those planning measures, which relates to defining, for example, precincts which prohibit development or where there's a control on what development can occur and the type of development, setting your flood planning levels, also raising homes, relocating homes, those sorts of things can in some cases be more affordable than your large-scale structural mitigation measures such as Lismore
MP: If you've got 10 properties in a one in 10 year, it might be more significant than 20 or 30 properties flooded in a 100-year flood because of the frequency of that event as well.
MP: This is a good example where there was roughly a 1 in 500-year flood in February followed by roughly a 1 in 10 to 1 in 20 year flood a month later.
MP: And so getting away from that thinking, we know that any flood can happen at any stage.
Engeny scored a local win by enabling the reversal of insurers’ reluctance to pay out to some affected policyholders
MP: A lot of the policyholders in Lismore were not covered for flood, which relates to creek and river, but were covered for stormwater.
MP: So we actually simulated the February 2022 rainfall. So we incorporated that rainfall into the model, ran the model and looked at where the stormwater inundation levels were. And based on that, we estimated a depth within the properties that we did assessments for. And that information then led to the assessment of their claim, where in many instances, stormwater inundation was found to be the initial cause. And so they were successful in that situation. So for us, a significant outcome was to be able to help the community that was devastated beyond measure, in just a small way, to try and get them back on their feet.
Mark says the Lismore experience shows there is a need to rethink flood risk planning and also to look again at the feasibility of building on flood plains.
MP : Typically the 1 in 100 historically has been used as that benchmark for that's what we need to try and provide protection for. And that is something that this event exposes as a flaw.
MP: Floods can be highly dynamic and rainfall events can change from hour to hour. And you saw that with the flood forecasting and warnings through the night, going from not predicting the levy to over top to significant overtopping of the levy.
MP: I think there definitely needs to be a greater focus on that as well.
MP: I think some of those hard questions need to be had. And I think Lismore emphasises that point that there are communities at risk that really need to think about what the implications of them being located within a floodplain provide. And if they're willing to take that risk, if their tolerance is such that they accept that, well, then what can they do to at least try and mitigate the risk as best they can?
