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July 2, 2022

Engineering ‘solutions’ won’t get us out of this mess

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Magellan Street Lismore with rising floodwaters, 30 March 2022, the second time in a month that the Lismore levee was topped by floodwater. Photo David Lowe.

Engineering works have been proposed to mitigate the extreme flooding experienced in Lismore in February and March 2022. Here are the suggested options:

Build a higher levee

The cost of raising the levee is as yet unknown but could be expected to be many millions, if not billions. The money is not yet sourced or assured. 

Enormous disruption would be caused by the removal of existing infrastructure and houses, by the extensive excavations required for the site, and by the transport of earth and rock from other areas to provide fill for the levee. The construction would devastate efforts to rehabilitate the current levee area.

The gross inequity involved in attempting to protect only the CBD and flood-prone parts of East Lismore is unacceptable.  A higher levee would:

(a) signal the abandonment of South and North Lismore, and significantly intensify their flood hazards and problems.

(b) speed up and magnify the flood effects on downstream farmers and townships such as Coraki.

Now that climate change and its damaging consequences are beyond all doubt we have to ask how high the next major flood will be. It could be 18m or 20m or 30m. There is no known limit – all we know is that more frequent and bigger floods are assured. Spending huge amounts of money in an attempt to alleviate, but possibly worsen, a threat of unknown size is the height of poor planning.

We need to admit that the existing levee was an expensive failure, which merely protected parts of Lismore from small and medium floods. It has made major floods much more dangerous and destructive for both the ‘protected’ areas and the remainder of Lismore.  We must not compound this blunder by repeating it and subjecting more people to greater peril and heartbreak.

The proposed Dunoon Dam (the DuD), below the existing Rocky Creek Dam, has been advocated by some, although not Rous County Council (RCC), as the prime candidate for supposed flood mitigation in Lismore.

Apart from the intractable problems of destruction of (a) nationally significant Indigenous heritage and (b) internationally significant rainforest, this dam would provide no flood mitigation and would, in fact, increase risk to downstream residents.

Map of Lismore catchment printed with permission from Dr Rob Garbutt.

Ineffectiveness of upstream containment dams

As the map above shows, the catchment of Rocky Creek, on which the DuD would be built, contributes very little to the flood waters that inundate Lismore. Figures provided by RCC show that Rocky Creek catchment above the DuD is 50km², or 3.57 per cent of the 1,400km² catchment above Lismore.

Even in the extremely unlikely event that Rocky Creek Dam and the DuD were empty at the beginning of the rain event, they could not intercept more than 3.57 per cent of the flood water heading for Lismore. A second flood within a month, as happened in March 2022, would be even less able to be contained as there would be no possibility of emptying the DuD into an already flooded landscape.

This would be true of any other dams constructed on other tributaries with the aim of holding back flood waters. They could not provide mitigation, their cost would be astronomical, and their construction would entail large-scale loss of farmland and native vegetation.

Increase in downstream risk

The downstream risk from a large dam is explained in the 2012 Environmental Flows Assessment (EFA), commissioned by RCC. It states that the most extreme floods may be exacerbated in the 3km immediately below the dam wall.

‘Modelling shows that the magnitude of the largest floods may be increased, with the largest flows in the natural regime of 17,280 ML per day shifting to 20,456 ML per day with Dunoon Dam operating’.  This is an increase of 18.3 per cent on the largest flood.

The additional impact is explained by the surface area of a large dam. ‘When full, the dam acts to increase catchment runoff efficiency, with all rain that falls directly onto the water surface delivered directly into downstream flow when the dam is spilling. This phenomenon results in increased peak magnitude of the modelled flood events when these conditions are met.’

This phenomenon on Rocky Creek Dam already increases the flood peak downstream. On February 28 the following text was sent to all residents below Rocky Creek Dam, including The Channon village and parts of Keerrong:

‘NSWSES DAM FAILURE EVACUATION ORDER ROCKY CREEK DAM. People at your home address BELOW ROCKY CREEK DAM should evacuate now to higher ground’

What warning would the SES have to send if there were another dam with an even larger surface area located only 3km above The Channon village? And at 3.5 times the volume of Rocky Creek Dam, what would happen if that dam did indeed fail?

Modify rivers by straightening bends, cutting diversion channels or deepening river beds

Any modification which hastens the flow of flood water past a town can only compound the problems for downstream people, towns, farms and land. It is a moral issue. Prioritising one community for protection over another is intolerable.

Dredging the river, as done in the past to keep channels open for shipping, would have no effect on floods. The river at Lismore is tidally influenced so a deeper channel will always be full of water and will have no capacity to hold flood water.

In conclusion, it is clear that Lismore will continue to flood in ever-escalating cycles in the years ahead. Whatever the solutions are, attempting to control with engineering remedies the colossal natural forces at work is at best childish and at worst criminally negligent.

♦ Nan Nicholson is a founding member of WATER Northern Rivers.


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18 COMMENTS

  1. Whilst in full agreement with most of your assessments of the issues, especially the redundancy of dams in flood mitigation, raising the levee wall is likely not the only engineered option.
    It would be a shame to condemn all options that can be proposed by engineers, especially as they work to job specifications and are generally not at liberty to provide options unless instructed. The Netherlands is a very successful example of a country that has managed floods and water invasion from the sea very successfully through engineering innovations.

  2. The problem occurring everywhere is the tidal flow that backs up the flooding water in the catchments so to solve this , flood barriers have to be built at the mouth of every river so that the fresh water can be drained out to sea

  3. Nan – the theoretical maximum flood height is 16.5 metres – at that height there is no hindrance to the water moving towards Wyrallah / Coraki. Suggesting that a flood could be 20 or 30 metres is incorrect. And yes, nothing will ameliorate or stop another major flood. We need to move off the flood plain (and maybe plant lots of rainforest back into the flood plains).

  4. Easy for people to say, who don’t own business or homes directly affected by the floods, what should they do with their lives? just cop it?

    There are no real solutions provided in this post, just negativity towards engineered solutions, seemingly for ideological reasons.

    The Rocky Creek Dam did not fail, the design of that dam cannot fail like a typical wall built dam….the warning was saying that there were uncontrolled flows over the causeway which could fluctuate therefore could be sudden water surges down the valley. Stick to the facts.

    Nan I am a huge fan of yours and what you did in Terania, but this article is highly biased and not scientifically correct in many aspects, and I encourage we explore all options for the long-term future of Lismore and surrounding flood affected areas.

    • Mick it is not possible for Nan to have been “negative” towards “engineering solutions”, because there is no engineering solution to this problem. Especially not a levee.

      Can you stick to the facts too?

  5. Many Humans continue to believe they are “smarter” and can control/conquer Nature, even though the evidence is overwhelmingly to the contrary. Time is to pay due respect and learn to live in awe & harmony instead….. if we can. Let’s hope we have time……

  6. Mick, our house, business and farm all went under in that flood and we are still struggling to recover, financially and psychologically. I know exactly what it is like.

    That is why it is important that we don’t try to fool people into believing that they can be protected from larger future floods by engineering. More people will die if we persist with this delusion

    Unfortunately, as you say, I don’t have the answers, apart from govt stepping up and buying out the affected people in Lismore or helping them to adapt if they choose to stay. They must not be abandoned.

    Many people below the dam received that text from SES saying “Dam Failure Evacuation Order”. I knew it probably hadn’t failed but it was nonetheless very alarming and confusing. I have taken this up with Rous, asking them to please be more accurate so that in the event of a real emergency they would be believed.

    I agree that Rocky Creek Dam probably can’t fail, being earth wall construction, unlike a wall construction as you point out.

    BTW, the proposed Dunoon Dam is a wall construction, holding 3.5 times the volume of Rocky Creek Dam, and only 1 km above the Channon village.

  7. Government should never consider rebuilding. If it were to do so and then at some stage in the future Lismore suffers another flood, they would be liable for the cost – human and property. This is made worse by the now apparent effects of climate change. Settlement in the Lismore basin would never be considered if it was a fresh site!!!

    This article must be hard to digest for locals caught up in this flood dilema. Long term, the only solution is to move. Making that decision is a difficult one, which is why it is taking so much time. There is only one door out of this situation and Nan has highlighted that. Knocking on the others is a waste of time.

  8. Nicholas Reese states that the probable maximum flood is 16.5m, but that is based on pre-climate change data. For every one degree Celsius rise in temperature the atmosphere holds 7% more moisture. In Eastern Australia that global rise seems to have translated into about 15% increase in event intensity. Of course flood height is not directly related to rainfall event as the floodplain gets wider the deeper the water gets. However, when engineers calculate flow they have two components, one is what soaks in and the other is what runs off. With a bigger event, all of the extra runs off. Prudence would therefore say that PMF could be 16.5 + 15% = 19m. Prudence would also say that we have had about a one degree global rise in land temperature and it is quite likely that will double and that would mean PMF could be 16.5 + 30% = 21.5m.
    I have outlined some possible numbers above, but no-one really knows except that Nan is almost certainly correct in general that floods will probably get bigger.

    • My understanding is that the 16-metre figure is the technical maximum that the catchment could hold in the Lismore area.

  9. If we dont engineer for flood mitigation and deem parts of Lismore uninhabitable we will be giving up alot of valuable land and if large parts of lismore is deemed uninhabiable what does that mean for the rest of the towns and land susceptable to flooding along the east coast of Australia, if we dont engineer a solution the only place to go is up, will people start eyeing off the he high ground there’s lots of flood free land up around whian whain state forest and the border range maybe we need to open the national parks and state forest up for future flood free farming and housing developments.

  10. 1. Anthony Albanese has the money as he is spending it on other things. Raise the Levee no matter the cost as NOT raising the levee has brought the cost forward to be astronomical in 2022 from 2017. The next flood is due in February, 2023.
    2. The government is removing the rail line for 131 kilometres so don’t worry about the infrastructure removal and houses that needs to be removed because of floods as it cost a lot of money to remove 131 kilometres of track from Casino to Murwillumbah. The Government has the money.
    3. Nan Nicholson seems to be putting down excuses for non-action when non-action in the past has caused this disruptionand devastation. It is oK Nan is from the country she like non-action. Get things done and don’t take any notice of Nan Nicholson and her inaction.
    4. Climate Change will now speed up, so there wil lbe more intrense floods than we have just expereinced and more bushfires. Just do what needs to be done to save yourself.
    5. The new Dunoon Dam was to be put in by the National Party for growth and for political reasons. Evereone in Australia now knows that there are floods here so there will be little or no new growth,. The new dam is not needed.
    5. Read Nan Nicholson’s Conclusion. It means that no one is safe.

  11. I see nan !! What has the green council of lismore
    Done this pat 10 years to mitigate for the next
    Flood ? Mid 1800’s a major flood hit lismore
    That went over 2022 by two meters ..!!
    Pre- industrialisation no hydrocarbon’s
    No fossil fuels… Nan was was that you were
    Implying that climate change was a contributing factor to the resent Floods ?

  12. Open the flood gates and let it flow.
    House up stilts and all available land hillside going…
    Town sice…little Venice….canal boat…cafe…

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