The bushfires are horrendous, but expect cyclones, floods and heatwaves too



Bushfires are not the only weather and climate events set to ravage Australia in coming months.
Dave Hunt/AAP

Neville Nicholls, Monash University

Public attention on the disastrous bushfire crisis in Australia will rightly continue for weeks to come. But as we direct resources to coping and recovery, we should not forget other weather and climate challenges looming this summer.

The peak time for heatwaves in southern Australia has not yet arrived. Many parts of Australia can expect heavy rains and flooding. And northern Australia’s cyclone season is just gearing up.

The events will stretch the ability of emergency services and the broader community to cope. The best way to prepare for these events is to keep an eye on Bureau of Meteorology forecasts.

Fires and other extreme events will test emergency services this summer.
Darren Pateman/AAP

Let it rain

2019 was Australia’s driest year on record. Since early winter the Bureau of Meteorology has correctly predicted the development of these widespread dry conditions.

But relief may be coming. The latest bureau outlooks suggest more normal summer conditions from February to April. If it eventuates, this would include more rain.




Read more:
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The arrival of drought-breaking rains is notoriously hard to predict – in the past, they have come any time between January and May. Global warming is also complicating seasonal climate predictions.

We all hope the rain arrives sooner rather than later, and eases the fire situation. But rain will bring other risks.

Continental-scale droughts such as that experienced over the past few years are often broken by widespread heavy rains, leading to an increased risk of flooding including potentially lethal flash floods. The decade-long Millenium drought that ended in 2009 was followed by two extremely wet years with serious flooding.

A similar situation was seen in Indonesia in recent days when very heavy rains after a prolonged drought produced disastrous floods and landslides.

Indonesian rescuers searching for missing people after a landslide in West Java, Indonesia, triggered by heavy rain.
EPA

The flood risk is exacerbated by the bare soil and lack of vegetation caused by drought, and by bushfires that destroy forest and grassland.

Australia’s north may be particularly hard hit. The onset of the tropical wet season has been very much delayed, as the bureau predicted. Over the last three months, some parts of the Australian tropics had their lowest ever October-December rainfall. But there are some suggestions widespread rain may be on its way.

Further south, drought-breaking rains can also be heavy and widespread, leading to increased flood risk. So even when the drought breaks and rains quell the fires, there will likely still be bouts of extreme weather, and high demand for emergency services.

Cyclones and heatwaves

The tropical cyclone season has been much delayed, as predicted by the bureau, although there are now signs of cyclonic activity in the near future.

Cyclones often bring welcome rains to drought-affected communities. But we should not overlook the serious damage these systems may bring such as coastal flooding and wind damage – again requiring intervention from emergency services.




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And we are still a month away from the riskiest time for heatwaves in southern Australia. We’ve already had some severe heatwaves this summer. However they usually peak in the middle and end of summer, so the worst may be yet to come.

Lives have undoubtedly been saved this summer by improved forecasting of high temperatures and better dissemination of heatwave information by state and local governments. But after an already devastating early summer of fires and heat, warning fatigue may set in amongst both warning providers and the public. We must ensure heatwave warnings continue to be disseminated to populations at risk, and are acted on.

Shop staff clean up storm waters after Cyclone Debbie hit iQueensland in 2017.
AAP

Be thankful for weather forecasters

The recent experience of farmers, fire fighters, water resource managers and communities illustrate the value of the service provided by the Bureau of Meteorology. Greatly improved weather and climate forecasting developed over the past few decades means communities can plan for and deal with our highly variable weather and climate far better than in the past.




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It’s only October, so what’s with all these bushfires? New research explains it


Recent drought, fires and heatwaves – exacerbated by global warming – have been devastating. But imagine if we only had the limited weather forecast capabilities of even a few decades ago, without today’s high-speed computers to run weather forecast models, and satellites to feed in enormous amounts of data. How much worse would the impacts have been?

These forecasts have allowed heat alerts to be disseminated to vulnerable communities. Detailed information on weather conducive to fire spread has helped fire agencies provide more targeted warnings and direct resources appropriately.

An air tanker makes a pass to drop fire retardant on a bushfire in North Nowra, NSW, as fires spread rapidly.
Mick Tsikas/AAP

Never before have weather forecasts been so readily available to the public. Here are ways you can use them to reduce risks to life and property during an extreme event:

  • Listen to ABC local radio for emergency updates and detailed Bureau of Meteorology forecasts
  • load your state fire service emergency app onto your phone and check it regularly. Or check out the information online, such as at the NSW Rural Fire Service’s Fires Near Me website
  • check the bureau’s website for climate and weather forecasts
  • download a short-range rainfall forecast app such as Rain Parrot onto your phone. These apps use the bureau’s radar data to make short-range forecasts of rainfall for your location, and notify you if rain is coming.

Global warming is already lengthening the fire season and making heatwaves more intense, more frequent, and longer. It is also increasing the likelihood of heavy rains, and making droughts worse.

We must keep adapting to these changing threats, and further improve our ability to forecast them. And the community must stay aware of the many weather and climate extremes that threaten lives and property.The Conversation

Neville Nicholls, Professor emeritus, School of Earth, Atmosphere and Environment, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate explained: why coastal floods are becoming more frequent as seas rise



As sea levels rise, it becomes easier for ocean waves to spill further onto land.
from http://www.shutterstock.com, CC BY-ND

James Renwick, Victoria University of Wellington


CC BY-ND

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz

I saw an article claiming that “king tides” will increase in frequency as sea level rises. I am sceptical. What is the physics behind such a claim and how is it related to climate change? My understanding is that a king tide is a purely tidal effect, related to Moon, Sun and Earth axis tilt, and is quite different from a storm surge.

This is a good question, and you are right about the tides themselves. The twice-daily tides are caused by the gravitational forces of the Moon and the Sun, and the rotation of the Earth, none of which is changing.

A “king” tide occurs around the time when the Moon is at its closest to the Earth and Earth is at its closest to the Sun, and the combined gravitational effects are strongest. They are the highest of the high tides we experience.

But the article you refer to was not really talking about king tides. It was discussing coastal inundation events.




Read more:
King tides and rising seas are predictable, and we’re not doing enough about it


When tides, storms and sea levels combine

During a king tide, houses and roads close to the coast can be flooded. The article referred to the effects of coastal flooding generally, using “king tide” as a shorthand expression. We know that king tides are not increasing in frequency, but we also know that coastal flooding and coastal erosion events are happening more frequently.

As sea levels rise, it becomes easier for ocean waves to penetrate on to the shore. The biggest problem arises when storms combine with a high tide, and ride on top of higher sea levels.

The low air pressure near the centre of a storm pulls up the sea surface below. Then, onshore winds can pile water up against the coast, allowing waves to run further inshore. Add a high or king tide and the waves can come yet further inshore. Add a bit of sea level rise and the waves penetrate even further.

The background sea level rise has been only 20cm around New Zealand’s coasts so far, but even that makes a noticeable difference. An apparently small rise in overall sea level allows waves generated by a storm to come on shore much more easily. Coastal engineers use the rule of thumb that every 10cm of sea level rise increases the frequency of a given coastal flood by a factor of three.

This means that 10cm of sea level rise will turn a one-in-100-year coastal flood into a one-in-33-year event. With another 10cm of sea level rise, it becomes a one-in-11-year event, and so on.

Retreating from the coast

The occurrence rates change so quickly because in most places, beaches are fairly flat. A 10cm rise in sea levels might translate to 30 or 40 metres of inland movement of the high tide line, depending on the slope of the beach. So when the tide is high and the waves are rolling in, the sea can come inland tens of metres further than it used to, unless something like a coastal cliff or a sea wall blocks its way.

The worry is that beaches are likely to remain fairly flat, so anything within 40 metres of the current high tide mark is likely to be eroded away as storms occur and we experience another 10cm of sea level rise. If a road or a house is on an erodible coast (such as a line of sand dunes), it is not the height above sea level that matters but the distance from the high tide mark.

Another 30cm of sea level rise is already “baked in”, guaranteed over the next 40 years, regardless of what happens with greenhouse gas emissions and action on climate change. By the end of the century, at least another 20cm on top of that is virtually certain.




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The 30cm rise multiplies the chances of coastal flooding by a factor of around 27 (3x3x3) and 50cm by the end of the century increases coastal flooding frequency by a factor of around 250. That would make the one-in-100-year coastal flood likely every few months, and roads, properties and all kinds of built infrastructure within 200 metres of the current coastline would be vulnerable to inundation and damage.

These are round numbers, and local changes depend on coastal shape and composition, but they give the sense of how quickly things can change. Already, key roads in Auckland (such as Tamaki Drive) are inundated when storms combine with high tides. Such events are set to become much more common as sea levels continue to rise, to the point where they will become part of the background state of the coastal zone.

To ensure cities such as Auckland (and others around the world) are resilient to such challenges, we’ll need to retreat from the coast where possible (move dwellings and roads inland) and to build coastal defences where that makes sense. The coast is coming inland, and we need to move with it.The Conversation

James Renwick, Professor, Physical Geography (climate science), Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Catastrophic Queensland floods killed 600,000 cattle and devastated native species


Gabriel Crowley, James Cook University and Noel D Preece, James Cook University

In February, about 600,000 cattle were killed by catastrophic flooding across north Queensland’s Carpentaria Gulf plains.

The flood waters rose suddenly, forming a wall of water up to 70km wide. Record depths were reached along 500km of the Flinders River, submerging 25,000 square kilometres of country. Cattle were stranded. Many drowned.




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Even though cattleman Harry Batt lost 70% of his herd, he was more concerned about the wildlife. He said, “all the kangaroos, and bloody little marsupial mice and birds, they couldn’t handle it”.

Harry was right to be concerned. As our research, published today in Austral Ecology, reveals, floods sweeping Australia’s plains have disrupted native species for millions of years. Now, as climate change drives more intense flooding, we will see this effect intensify.

Flooding causes major disruptions to gene flow

February’s flood came ten years to the day after a far bigger flood on the adjoining river systems that submerged an area larger than Ireland. It was this flood that first drew our attention to the plight of native species.

Noel was asked by Northern Gulf Resource Management Group to survey wildlife in areas affected by the 2009 flood. Over the following four years, he found almost no ground-dwelling reptiles, despite them occurring elsewhere in the region. They appeared to have been washed away or drowned.

Biologists have long known that many species’ ranges are interrupted by the Gulf Plains. Hence, these floodplains are considered one of Australia’s most important biogeographic barriers: the Carpentarian Gap.

Many closely related species with a common ancestor are separated by this Gap, including the Golden-shouldered Parrot of Cape York Peninsula and the Hooded Parrot of the Northern Territory. They are thought to have separated around 7 million years ago.




Read more:
South-East Queensland is droughtier and floodier than we thought


The Gap also separates many other species, including birds, mammals, reptiles and butterflies, at the subspecies or genetic level. Even more species found on either side are just absent from the Gulf Plains.

Huge flooding across the Gulf Plains, including the Norman and Flinders Rivers, in February 2009.
NASA Worldview, CC BY-SA

Flood impacts are immense and under-appreciated

When biologists first tried to find a reason for these patterns, they only considered aridity. They proposed Australia’s arid zone expanded to the Gulf of Carpentaria during ice ages.

There is no evidence for this, but the misunderstanding is completely understandable.

Any dry-season visitor to the Gulf Plains will find a dry, inhospitable environment with few trees or shrubs for shade, cracked clay soils, and lots of flies. European explorers described the region as “God-forsaken”.

But it can be quite a different place in the wet season.

Rains in the Gulf are caused by the summer monsoonal troughs or cyclones. About once a decade, these generate massive downpours. Historical records show at least 14 major floods since 1870.

So, to us, it seemed floods rather than aridity could be the cause of the odd distributions of plants and animals.

We set out to see whether Noel’s findings could have been caused by flooding or whether other factors such as soil, vegetation or climate were more important.

We also wanted to know what other effects floods might have on the region’s ecosystem. Could floods, by eliminating trees and shrubs, be responsible for the hostile appearance of the region? Could ground-dwelling reptiles and birds be underrepresented, not just at Noel’s sites, but on floodplains across the area?

To find out, we divided the area into floodplains and higher-altitude land, and generated 10,000 random sites across the Gulf Plains. We extracted soil, vegetation and rainfall data from national information sources, and examined the patterns.

We found trees and shrubs were significantly less common on floodplains than on land above the flood zone, regardless of soil or rainfall, and tree cover was further reduced on cracking clays. We concluded the plain’s open, hostile appearance is caused by a combination of soils and flooding.

We then examined all gecko, skink and bird records from the Atlas of Living Australia.

We found ground-living reptiles and birds were much less common on the floodplains, regardless of vegetation or soil. As expected, reptiles were more sensitive to flooding than birds, which can fly to safety during floods.

Finally, we found the sites affected by the 2009 flood had significantly fewer geckos and skinks than other sites across the Gulf Plains.

Increased flooding from climate change could have major consequences

Our findings have evolutionary significance that extends into the future. Repeated disruption of species across their distributions affects gene flow and ultimately produces new species. If floods become more frequent, as expected under climate change, so might the rates at which new species form.

They also have serious land management implications. Climate change planning emphasises conserving river corridors as safe refuges from arid conditions. However, periodic scouring of many of the nation’s floodplains – expected to increase under climate change – means that this approach needs rethinking.




Read more:
Townsville floods show cities that don’t adapt to risks face disaster


We conclude that on the most arid occupied continent on Earth, unpredictable floods may cause the most disruption to the Australian plant and animal life.The Conversation

Gabriel Crowley, Adjunct Principal Research Fellow, James Cook University and Noel D Preece, Adjunct Asssociate Professor, James Cook University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Heatwaves and flash floods: yes, this is Britain’s ‘new normal’


Hayley J. Fowler, Newcastle University

“It’s hard to believe, isn’t it, that we had a heatwave just last week?”

Those words were spoken by a BBC news presenter, in front of graphic images of fire service rescues, as heavy rain caused floods and landslides which closed many roads and railway lines. In recent days there have dramatic floods across the north of England, particularly around Manchester, the Peak District and Yorkshire.

For me, this is personal, as I am from the worst affected area. I went to high school where people spent the night in their Civic Hall. Three miles away from where I grew up, a dam holding back Toddbrook Reservoir has been at risk of collapse and the town of Whaley Bridge was evacuated. But I’m not surprised that we are seeing flash flooding and I expect it to get worse in the future.

I am a professor at Newcastle University, where I lead a large research group focused on understanding changes to intense rainfall events and flash floods. Over the past eight years we’ve been working closely with colleagues at the UK Met Office to develop new very high-resolution climate models that can simulate these very intense summer storms and therefore predict what might happen in a warming climate.

Our models tell us that by 2080 summers in the UK will be much hotter and drier. Heatwaves will be more common. In fact a report released by the Met Office on the same day as the latest flash floods tells us that heatwaves are already happening more often. When Cambridge recently hit 38.7℃, the UK became one of 12 countries to break its national temperature record this year.

The world is warming. But although UK average summer rainfall is predicted to decrease, our models tell us that when it does rain it will be more intense than has been the case. Flash flooding in the UK is generally caused by intense rainstorms, where more than 30mm falls in an hour. Climate models predict these will happen five times more often by 2080.

Part of the reason for this is the simple fact that warmer air can hold more moisture. But that’s too simple: the availability of moisture also increases in areas close to warm oceans – warmer sea surface temperatures cause more moisture to be evaporated into the atmosphere, providing additional fuel for these intense storms. And here’s the scary bit: the Atlantic Ocean provides a vast source of moisture for storms in the UK.

But that’s not the whole story. Heavy, short rain storms are intensifying more rapidly than would be expected with global warming (what we call the Clausius-Clapeyron relationship). Research also suggests that more intense storms can themselves grow bigger, and with both the intensity of the rainfall and the spatial footprint of the storm increasing, the total rainfall in an “event” could double.

What’s more, the larger storms seem to have an ability to draw in more moisture from the surrounding area and become even more intense: the additional energy (heating) fuelling the uplift of air within the storm’s core draws in even more moisture from the surface, allowing them to grow even larger, with more potential for flooding. These also provide the perfect ingredients for large hail storms.

So, it is entirely consistent that we might expect both more heatwaves and more intense summer thunderstorms in a warmer climate. We also know which areas of the country are already susceptible to these flash floods from our analysis of historical records of flooding. Newspapers have reported on the dramatic impacts of these floods for centuries and this has allowed my team to reconstruct a flash-flooding history of the UK.

Certain parts of the country are highly vulnerable as their rivers respond quickly to rainstorms. These rivers tend to be found in steep, upland catchments underlain by non-permeable rocks, mainly in the north and west of the UK. High-risk catchments also include urban areas where the ground is also non-permeable, for entirely different reasons.

Many of the towns reported to have suffered “biblical” flooding recently have suffered repeated flooding through history, but perhaps not within living memory. For example, Whaley Bridge is mentioned twice in the flood chronologies for events in June 1872 and July 1881:

On 19th [June 1872] the Goyt was 12 to 14 feet above its normal level. At Whaley Bridge houses near the river were completely flooded and people were taken into the chapel and inns … in Macclesfield a woman and child were drowned when the river Bollin overflowed. Two reservoirs burst in the vicinity.

This rich archive of knowledge, including the prevalence of flooding in certain towns, even specific roads, is something we should draw upon in planning both the emergency response to these flash floods and for reducing their future impact. We can learn a lot from the past in how to manage the greater risks of flooding the future will bring.The Conversation

Hayley J. Fowler, Professor of Climate Change Impacts, Newcastle University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Townsville floods show cities that don’t adapt to risks face disaster


Cecilia Bischeri, Griffith University

A flood-ravaged Townsville has captured public attention, highlighting the vulnerability of many of our cities to flooding. The extraordinary amount of rain is just one aspect of the disaster in Queensland’s third-biggest city. The flooding, increasing urban density, the management of the Ross River Dam, and the difficulties of dealing with byzantine insurance regulations have left the community with many questions about their future.

These questions won’t be resolved until we enhance the resilience of cities and communities against flooding. Adaptation needs to become an integral part of living with the extremes of the Australian environment. I discuss how to design and create resilient urban landscapes later in this article.




Read more:
Queensland’s floods are so huge the only way to track them is from space


Flood risk and insurance

Another issue that affects many households and businesses is the relationship between insurance claims and 1-in-100-year flood event overlay maps. Projected rises in flood risks under climate change have led to concerns that parts of Townsville and other cities will become “uninsurable” should the costs of cover become prohibitive for property owners.

Council flood data used for urban planning and land-use strategies is also used by insurers to assess the flood risk to individual properties. Insurers then price the risk accordingly.




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Lessons in resilience: what city planners can learn from Hobart’s floods


However, in extraordinary circumstances, when the flooded land is actually larger than the area marked by the flood overlay map, complications emerge. In fact, that part of the community living outside the map’s boundaries is considered flood-free. Thus, those pockets of the community may have chosen not to have flood insurance and not have emergency plans, which leaves them even worse off after floods. This is happening in Townsville.

Yet this is nothing new. Many people experienced very similar circumstances in 2011. Flood waters covered as much land as Germany and France combined. Several communities were left on their knees.

Notwithstanding the prompt and vast response of the federal government and Queensland’s state authorities, a few years later Townsville is going through something alarmingly similar.

Adaptation to create resilient cities

To find a solution, we need to rethink how to implement the Queensland Emergency Risk Management Framework. That is no easy task. However, it starts with shifting the perspective on what is considered a risk – in this case, a flooding event.

Floods, per se, are not a natural disaster. Floods are part of the natural context of Queensland as can be seen below, for instance, in the Channel Country.

Floods are part of the Australian landscape. Here trees mark the seasonal riverbeds in the Queensland outback between Cloncurry and Mount Isa.
Cecilia Bischeri, Author provided

The concept of adaptation as a built-in requirement of living in this environment then becomes pivotal. In designing and developing future-ready cities, we must aim to build resilient communities.

This is the ambitious project I am working on. It involves different figures and expertise with a shared vision and the support of government administrations that are willing to invest in a future beyond their elected term of office.

Ideas for Gold Coast Resilientscape

I live and work in the City of Gold Coast. Water is a fundamental part of the city’s character and beauty. In addition to the ocean, a complex system of waterways shapes a unique urban environment. However, this also exposes the city to a series of challenges, including flooding.

Last September, an updated flood overlay map was made available to the community. The map takes into account the projections of a 0.8 metre increase in the sea level and 10% increases in storm tide intensity and rainfall intensity.

These factors are reflected in the 1-in-100-year flood overlay. It shows undoubtedly that the boundaries between land and water are changeable.

Building walls between the city and water as the primary flood protection strategy is not a solution. A rigid border can actually intensify the catastrophe. New Orleans and the levee failures during the passage of Hurricane Katrina in 2005 provide a stark illustration of this.

Instead, what would happen and what would our cities look like if we designed green and public infrastructures that embody flooding as part of the natural context of our cities and territory?




Read more:
Design for flooding: how cities can make room for water


The current project, titled RESILIENTSCAPE: A Landscape for Gold Coast Urban Resilience, considers the role of architecture in enhancing the resilience of cities and communities against flooding. The proposal, in a nutshell, explores the possibilities that urban landscape design and implementation provide for resilience.

RESILIENTSCAPE focuses on the Nerang River catchment and the Gold Coast Regional Botanic Gardens, in the suburb of Benowa. The river and gardens were adopted as a case study for a broader strategy that aims to promote architectural solutions for a resilient City of Gold Coast. The project investigates the possibility of using existing green pockets along the Nerang River to store and retain excess water during floods.

Gold Coast Regional Botanic Gardens is one of the green areas along the Nerang River that could be used to store and retain flood water.
Batsv/Wikimedia Commons, CC BY-SA

These green spaces, however, will not just serve as “water tanks”. If mindfully planned, the green spaces can double up as public parks and facilities. This would enrich the community’s social realm and maximise their use and return on investment.

The design of a landscape responsive to flooding can, by improving local urban resilience, dramatically change the impact of these events.

The goal of creating urban areas that are adaptive to an impermanent water landscape is the main driver of the project. New Orleans after Hurricane Katrina and New York after Sandy are investing heavily in this direction and promoting international design competitions and community participation to mould a more resilient future. Queensland, what are we waiting for?




Read more:
Floods don’t occur randomly, so why do we still plan as if they do?



This article has been updated to clarify the use of flood data by insurers in assessing risk and the cost of cover.The Conversation

Cecilia Bischeri, Lecturer in Architecture, Griffith University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Floods don’t occur randomly, so why do we still plan as if they do?


Anthony Kiem, University of Newcastle

Most major floods in South East Queensland arrive in five-year bursts, once every 40 years or so, according to our new research.

Yet flood estimation, protection and management approaches are still designed on the basis that flood risk stays the same all the time – despite clear evidence that it doesn’t.

We analysed historical flooding data from ten major catchments in South East Queensland. As we report in the Australasian Journal of Water Resources, 80% of significant floods arrived during five-year windows, with 35-year gaps of relative dryness between.




Read more:
Old floods show Brisbane’s next big wet might be closer than we think


The early 1970s brought a succession of severe floods to South East Queensland. This was followed in the 1980s by a raft of floodplain development projects, together with extensive research on floodplains and flooding risk, carried out by a group of researchers who described themselves as the “Roadshow” because of their frequent visits to flood-prone regions.

Throughout the 1980s, some Roadshow members noticed that large floods in South East Queensland seemed to follow a 40-year cycle, with five-year periods of high flood risk separated by 35 years of lower flood risk. They speculated that the next “1974 flood” (a reference to a devastating flood that hit Brisbane and South East Queensland that year) would arrive some time around 2013 .

Sure enough, South East Queensland was once again hit by large floods in January 2011 and January 2013.

Evidently, large floods in South East Queensland are not random. This is a problem, given that development policies and engineering practice, by and large, still assume that they are.

History repeating

In 1931, the Queensland meteorologist and farmer Inigo Jones linked the Brisbane River’s floods to the Bruckner Cycle of solar activity, which he determined to be 35 years long, but which has since been found to vary from 35 to 45 years.

In 1972, flood engineer John Ward argued that flood frequency distributions differ in space and time because higher flows originate from a variety of different rainfall mechanisms. At the time, minimal insight was available into what those different rainfall mechanisms were.

In the 1990s, drought research in Queensland by, among others, researchers Roger Stone and Ken Brook and John Carter identified cyclical variations in Queensland rainfall associated with the Southern Oscillation Index (SOI), supporting the idea of non-random occurrence of floods.

In 1999, Australian hydrologist Robert French also noticed that irregular clustering of flood events was associated with the SOI, and pointed out that flood planning needed to take into account more than just seasonal or year to year variability.

More recently, flood incidence has been strongly linked to large-scale ocean processes such as the El Niño/Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). These phenomena seem to have a marked effect on eastern Australian rainfall variability, and therefore on the risk of both floods and drought.

Is the 40-year cycle real?

We compiled records of major floods in South East Queensland between 1890 and 2014. As the table below shows, roughly 80% of large historical floods happened within a series of five-year flood-prone periods, despite these periods together representing only 16% of the study period.

The South East Queensland study area (approximately indicated by the orange box) and the 10 catchments analysed in this study.

Timing of the largest flood events within the 40-year cycles. Superscripts next to each flood event indicate the ranking of that flood event in that catchment (that is, the largest flood in each catchment is ranked 1).

On average, the number of large floods per year was 4.9 times higher within the five-year flood-prone periods.

Not only were floods more frequent, they were also more severe, with flood heights 41% higher during the five-year flood-prone periods than at other times.

Even though a few large floods occurred outside the five-year flood-prone periods, the 40-year cycle of flooding in South East Queensland appears to be a genuine phenomenon.

What drives the cycle?

The most likely physical explanation for cyclic or non-random flooding is the IPO, which is rather like the ENSO cycle except on longer time scales. The IPO influences eastern Australia’s climate indirectly, by affecting both the magnitude and frequency of ENSO impacts.

Recent “negative phases” of the IPO – meaning warmer than average Pacific Ocean temperatures north and south of the tropics – happened roughly during 1870–95, 1945-76, and 1999–present.

If we compare these with the five-year flood-prone periods in the table above, we can see that with the exception of 1930–34, all five-year flood-prone periods happened during these negative IPO events. Interestingly, the large floods in the 1950s and 1960s happened outside the five-year flood-prone periods identified by the 1980s Roadshow, but do align with IPO negative conditions.




Read more:
Planning for a rainy day: there’s still lots to learn about Australia’s flood patterns


In spite of all this evidence, most engineers and flood planners still assume that floods occur randomly and that flood risk is the same all the time. Phrases like “one in 100-year event” or “1% annual exceedance probability” are routinely used to describe floods, despite the fact that for some years and decades the risk is significantly higher. This gives a false sense of security during times when major floods are much more likely.

If this approach continues, then every few decades our flood defences will not be as reliable as we thought – a fact to which many Queenslanders can now attest.

We need new approaches to deal with the reality that large flood events do not occur randomly. It would arguably be more sensible to separate flood records into two (or more) categories – one for times when flood risk is “normal” and another for periods where the risk is higher – and then reevaluate flood frequency distributions and flood risks for each category. Decision makers then get a more realistic estimate of the true risk of flooding which leads to more informed and more resilient flood planning and defences.

This new approach might also help plan for the changes to flood risk expected in the future, whether from climate change, land use change, or whatever else the oceans and skies throw at us.


The ConversationThis article was coauthored by Greg McMahon, a Brisbane-based independent consultant on flood risks and Academic Chair at Rhodes Group Australia.

Anthony Kiem, Associate Professor – Hydroclimatology, University of Newcastle

This article was originally published on The Conversation. Read the original article.