You may have heard the ‘moon wobble’ will intensify coastal floods. Well, here’s what that means for Australia

Mark Gibbs, Australian Institute of Marine ScienceExtreme floods this month have been crippling cities worldwide. This week in China’s Henan province, a year’s worth of rain fell in just three days. Last week, catastrophic floods swept across western Germany and parts of Belgium. And at home, rain fell in Perth for 17 days straight, making it the city’s wettest July in 20 years.

But torrential rain isn’t the only cause of floods. Many coastal towns and cities in Australia would already be familiar with what are known as “nuisance” floods, which occur during some high tides.

A recent study from NASA and the University of Hawaii suggests even nuisance floods are set to get worse in the mid-2030s as the moon’s orbit begins another phase, combined with rising sea levels from climate change.

The study was conducted in the US. But what do its findings mean for the vast lengths of coastlines in Australia and the people who live there?

A triple whammy

We know average sea levels are rising from climate change, and we know small rises in average sea levels amplify flooding during storms. From the perspective of coastal communities, it’s not if a major flood will occur, it’s when the next one will arrive, and the next one after that.

But we know from historical and paleontological records of flooding events that in many, if not most, cases the coastal flooding we’ve directly experienced in our lifetimes are simply the entrée in terms of what will occur in future.

Flooding is especially severe when a storm coincides with a high tide. And this is where NASA and the University of Hawaii’s new research identified a further threat.

Researchers looked at the amplification phase of the natural 18.6-year cycle of the “wobble” in the moon’s orbit, first identified in 1728.

The orbit of the moon around the sun is not quite on a flat plane (planar); the actual orbit oscillates up and down a bit. Think of a spinning plate on a stick — the plate spins, but also wobbles up and down.

When the moon is at particular parts of its wobbling orbit, it pulls on the water in the oceans a bit more. This means for some years during the 18.6-year cycle, some high tides are higher than they would have otherwise been.

This results in increases to daily tidal rises, and this, in turn, will exacerbate coastal flooding, whether it be nuisance flooding in vulnerable areas, or magnified flooding during a storm.

View of Earth from the Moon — The moon’s orbit isn’t on a flat plane. It oscillates up and down, like a plate would when it spins on a stick.
Shutterstock

A major wobble amplification phase will occur in the mid-2030s, when climate change will make the problem become severe in some cases.

The triple whammy of the wobble in the moon’s orbit, ongoing upwards creep in sea levels from ocean warming, and more intense storms associated with climate change, will bring the impacts of sea-level rise earlier than previously expected — in many locations around the world. This includes in Australia.

So what will happen in Australia?

The locations in Australia where tides have the largest range, and will be most impacted by the wobble, aren’t close to the major population centres. Australia’s largest tides are close to Broad Sound, near Hay Point in central Queensland, and Derby in the Kimberley region of Western Australia.

However, many Australian cities host suburbs that routinely flood during larger high tides. Near my home in Meanjin (Brisbane), the ocean regularly backs up through the storm water drainage system during large high tides. At times, even getting from the front door to the street can be challenging.

Derby, WA, has one of the biggest tidal ranges in Australia.
Shutterstock

Some bayside suburbs in Melbourne are also already exposed to nuisance flooding. But a number of others that are not presently exposed may also become more vulnerable from the combined influence of the moon wobble and climate change — even when the weather is calm. High tide during this lunar phase, occurring during a major rainfall event, will result in even greater risk.

In high-income nations like Australia, sea-level rise means increasing unaffordability of insurance for coastal homes, followed by an inability to seek insurance cover at all and, ultimately, reductions in asset values for those unable or unwilling to adapt.

The prognosis for lower-income coastal communities that aren’t able to adapt to sea-level rise is clear: increasingly frequent and intense flooding will make many aspects of daily life difficult to sustain. In particular, movement around the community will be challenging, homes will often be inundated, unhealthy and untenable, and the provision of basic services problematic.

What do we do about it?

While our hearts and minds continue to be occupied by the pandemic, threats from climate change to our ongoing standard of living, or even future viability on this planet, haven’t slowed. We can pretend to ignore what is happening and what is increasingly unstoppable, or we can proactively manage the increasing threat.

Some coastal communities, such as in Melbourne’s bayside suburbs, may experience flooding, even if they never have before.
Shutterstock

Thankfully, approaches to adapting the built and natural environment to sea-level rise are increasingly being applied around the world. Many major cities have already embarked on major coastal adaptation programs – think London, New York, Rotterdam, and our own Gold Coast.

However, the uptake continues to lag behind the threat. And one of the big challenges is to incentivise coastal adaptation without overly impacting private property rights.

Perhaps the best approach to learning to live with water is led by the Netherlands. Rather than relocating entire communities or constructing large barriers like sea walls, this nation is finding ways to reduce the overall impact of flooding. This includes more resilient building design or reducing urban development in specific flood retention basins. This means flooding can occur without damaging infrastructure.

There are lessons here. Australia’s adaptation discussions have often focused on finding the least worst choice between constructing large seawalls or moving entire communities — neither of which are often palatable. This leads to inaction, as both options aren’t often politically acceptable.

The seas are inexorably creeping higher and higher. Once thought to be a problem for our grandchildren, it is becoming increasingly evident this is a challenge for the here and now. The recently released research confirms this conclusion.

Mark Gibbs, Principal Engineer: Reef Restoration, Australian Institute of Marine Science

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

Spiders are cloaking Gippsland with stunning webs after the floods. An expert explains why

Lizzy Lowe, Macquarie UniversityStunning photographs of vast, ghostly spider webs blanketing the flood-affected region of Gippsland in Victoria have gone viral online, prompting many to muse on the wonder of nature.

But what’s going on here? Why do spiders do this after floods and does it happen everywhere?

The answer is: these webs have nothing to do with spiders trying to catch food. Spiders often use silk to move around and in this case are using long strands of web to escape from waterlogged soil.

This may seem unusual, but these are just native animals doing their thing. It’s crucial you don’t get out the insecticide and spray them. These spiders do important work managing pests, so by killing them off you would be increasing the risk that pests such as cockroaches and mosquitoes will get out of control.

Using silk to move around

What you’re seeing online, or in person if you live locally, is an amazing natural phenomena but it’s not really very complicated.

We are constantly surrounded by spiders, but we don’t usually see them. They are hiding in the leaf litter and in the soil.

Spider webs blanket the ground in Gippsland — When floods happen, spiders use silk to evacuate quickly.
Darren Carney

When these flood events happen, they need evacuate quickly up out of holes they live in underground. They come out en masse and use their silk to help them do that.

You’ll often see juvenile spiders let out a long strand of silk which is caught by the wind and lifted up. The web catches onto another object such as a tree and allows the spider to climb up.

That’s how baby spiders (spiderlings!) disperse when they emerge from their egg sacs — it’s called ballooning. They have to disperse as quickly as possible because they are highly cannibalistic so they need to move away from each other swiftly and find their own sites to hunt or build their webs.

Small spiders have been seen on a post in Gippsland after floods.
AAP Image/JEFF HOBBS

That said, I doubt these webs are from baby spiders. It is more likely to be a huge number of adult spiders, of all different types, sizes and species. They’re all just trying to escape the flood waters. These are definitely spiders you don’t usually see above ground so they are out of their comfort zone, too.

This mass evacuation of spiders, and associated blankets of silk, is not a localised thing. It is seen in other parts of Australia and around the world after flooding.

It just goes to show how versatile spider silk can be. It’s not just used for catching food, it’s also used for locomotion and is even used by some spiders to lay a trail so they don’t get lost.

Don’t spray them!

The most important thing I need readers to know is that this is not anything to be worried about. The worst thing you could do is get out the insecticide and spray them.

These spiders are making a huge contribution to pest control and you would have major pest problems if you get rid of all the spiders. The spiders will disperse on their own very quickly. In general, spiders don’t like being in close proximity to each other (or humans!) and they want to get back to their homes underground.

If you live in Gippsland, you probably don’t even need to clear the webs away with a broom. There’s no danger in doing so if you wish, but I am almost certain these webs will disperse on their own within days.

Until then, enjoy this natural spectacle. I wish I could come down to see them with my own eyes!

Lizzy Lowe, Researcher, Macquarie University

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

Sydney’s disastrous flood wasn’t unprecedented: we’re about to enter a 50-year period of frequent, major floods

Tom Hubble, University of SydneyLast month’s flood in the Hawkesbury-Nepean River region of western Sydney peaked at a staggering 12.9 metres, with water engulfing road signs and reaching the tops of many houses.

There hasn’t been a major flood on the Hawkesbury-Nepean for more than 30 years, with the last comparable one occurring in 1990. Long-term Sydneysiders, however, will remember that 12 major floods occurred during the 40 years before 1990. Five of these were larger than last month’s flood.

So what’s going on? The long-term rainfall pattern in the region and corresponding river flow is cyclic in nature. This means 40 to 50 years of dry weather with infrequent small floods are followed by 40 to 50 years of wet weather with frequent major floods.

As river and floodplain residents take stock of the recent damage to their homes and plan necessary repairs, it’s vital they recognise more floods are on the way. Large, frequent floods can be expected to occur again within 10 or 20 years if — as expected — the historical pattern of rainfall and flooding repeats itself.

Living in a bathtub

Many of the 18,000 people who were evacuated live in and around a region known as the “Sackville Bathtub”. As the name suggests, this flat, low-lying section of the floodplain region was spectacularly affected.

The flooded Hawkesbury-Nepean River last month. Brown floodwater is evident between Penrith (right) and the Pacific Ocean (top left). The Sackville Bathtub is located left of centre.
Digital Earth Australia Map, Geoscience Australia, Tom Hubble

The Sackville Bathtub is located between Richmond and Sackville. It’s part of the Cumberland Plain area of Western Sydney and formed very slowly over 100 million years due to plate tectonic processes. The bathtub’s mudstone rock layers are folded into a broad, shallow, basin-shaped depression, which is surrounded by steep terrain.

Downstream of Sackville, the Hawkesbury-Nepean River flows through sandstone gorges and narrows in width. This creates a pinch-point that partially blocks the river channel.

Just as a bath plug sitting half-way over a plughole slows an emptying bath, the Sackville pinch-point causes the bathtub to fill during floods.

How the bathtub effect in the Hawkesbury-Nepean Valley causes floodwaters to back up and lead to deep and dangerous flooding.

Will raising the dam wall work?

The NSW state government is planning to raise the wall of the Warragamba Dam to help mitigate catastrophic floods in the region. But this may not be an effective solution.

Typically, somewhere between 40% and 60% of the floodwater that fills up the Sackville Bathtub comes from unimpeded, non-Warragamba sources. So, when the Hawkesbury-Nepean River floods, the bathtub is already quite full and causing significant problems before Warragamba begins to spill. The Warragamba water then raises the flood level, but often by only a couple of metres.

Raising Warragamba Dam’s wall as a mitigation measure will only control about half the floodwater, and won’t prevent major floods delivered by the Nepean and Grose rivers, which also feed into the region. This represents a small potential benefit for a very large cost.

The timing of observed flood peaks during the August 1986 Hawkesbury-Nepean flood, in relation to the time when Warragamba Dam began to spill. The arrival of Warragamba water in the Sackville Bathtub increased the flood depth only by about a metre above the floodwaters delivered earlier during the flood from the Grose and Nepean rivers.
Tom Hubble – Redrawn from data presented in Appendix One of the Hawkesbury-Nepean Flood Study; Infrastructure NSW 2019.

A long flooding period is on our doorstep

The idea of drought-dominated and flood-dominated periods for the Hawkesbury-Nepean River system was proposed in the mid-1970s by the University of Sydney’s Robin Warner. Since the late 1990’s, it hasn’t been the focus of much research.

He showed a century-long cycle of alternating periods of dry weather and small floods followed by wet weather and big floods is normal for Sydney. This means the March flood may not have come as a surprise to older residents of the Sackville Bathtub, who have a lived experience of the whole 40-50 year flooding cycle.

As a rough average, one major flood occurred every four years during the last wet-weather period between 1950 and 1990. The largest of this period occurred in November 1961. It filled the Sackville Bathtub to a depth of 15 metres and — like the June 1964 (14.6 metres) and March 1978 (14.5 metres) events — caused more widespread flooding than this year’s flood.

A photo of a flood that occured in Maitland in September 1950.
Sam Hood/NSW State Library/Flickr, CC BY

We’re currently 30 years into a dry period, which may be about to end. Conditions might stay dry for another 10 or 20 years.

These cycles are likely caused by natural, long-term “climate drivers” — long-term climatic fluctuations such as El Niño and La Niña, the Pacific Decadal Oscillation and the Indian Ocean Dipole, which are driven by oceanic current circulations. These global phenomena bring both benevolent weather and destructive weather to Australia.

Eastern Australia experiences decades-long periods of wetter weather when these climate drivers sync up with each other. When they’re out of sync, we get dry weather periods.

These long-term cycles are natural and have been operating for thousands of years, but climate change is amplifying and accelerating them. Dry periods are getting drier, wet periods are getting wetter.

The good news and bad news

The bad news is that 12-plus metre floods at Hawkesbury River (Windsor Bridge) are not all that unusual. There have been 24, 12-plus metre floods at Windsor Bridge since 1799.

The good news is meteorological forecasters are excellent at predicting when the storms that generate moderate, large and catastrophic floods are coming. We can expect several days’ to a week’s notice of the next big flood.

We can also prepare our individual and communal responses for more large and frequent floods on the Hawkesbury-Nepean. Residents of the area need to think about how they might live near the river as individuals. Decide what is precious and what you will fit into a car and trailer. Practice evacuating.

As a community, we must ensure the transport infrastructure and evacuation protocols minimise disruption to river and floodplain residents while maximising their safety. It’s particularly important we set up inclusive infrastructure to ensure disadvantaged people, who are disproportionately affected by disasters, also have a fighting chance to evacuate and survive.

Upgrading the escape routes that enable people to evacuate efficiently is absolutely vital. As is rethinking whether we should continue urban expansion in the Sackville Bathtub.

So remember, the next major flood is going to occur sooner than we would like. If you live in this region, you must start preparing. Or as a wise elder once said, “Live on a floodplain, own a boat!”

This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. Read the rest of the stories here.

Tom Hubble, Associate Professor, University of Sydney

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

Floodplains aren’t separate to a river — they’re an extension of it. It’s time to change how we connect with them

Melissa Parsons, University of New England and Martin Thoms, University of New EnglandDramatic scenes of flood damage to homes, infrastructure and livelihoods have been with us on the nightly news in recent weeks. Many will be feeling the pain for years to come, as they contend with property damage, financial catastrophe and trauma.

But what if, for a moment, we removed the humans and their structures from these tragic images — what would we see?

We would see a natural process of river expansion and contraction, of rivers doing exactly what they’re supposed to do from time to time. We’d see them exceeding what we humans have deemed to be their boundaries and depositing sediment across their floodplains. We’d see reproductive opportunities for fish, frogs, birds and trees. The floods would also enrich the soils. Floods can be catastrophic for humans, but they are a natural part of an ecosystem from which we benefit.

These scenes clearly depict the intersection of humans and nature, and it’s not working out well for either side.

We must envision a new way of interacting with floodplains – these brilliant social-ecological systems that are not separate to rivers but rather part of the riverine landscape.

Humans can live on and with floodplains — but the way we do that has to change.

What is a floodplain?

Floodplains are relatively flat stretches of land located next to rivers. It helps to think of them as an extension of the river; it is natural and normal for a river to flood their adjacent plains.

Floodplains are composed of sediment the river has transported and then deposited, which makes them incredibly fertile. Flow and sediment regimes interacting over decades — or millennia — determine the physical and ecological character of floodplains, and the way they flood.

There are more than 15 generic floodplain types in Australia. Each harbours a unique set of evolutionary properties, physical features and ecosystems.

These influence the way floodwaters traverse floodplains, how long water remains on a floodplain, the velocity, turbulence and depth of floodwaters, and ecosystem responses to flooding. Floodplains are complex and highly variable.

Floodplains are also dynamic and ever-changing — and we should expect them to change even more in the coming years. Australian rivers have experienced regular periods of increased flood activity in the past 100 years.

And climate change is predicted to increase flood activity.

Humans benefit from floodplains

Floodplains are among the most productive ecosystems on the planet – they are biodiversity hotspots.

That’s in large part due to periodic flooding between different parts of a river-floodplain system; flooding is crucial to the function of floodplains. Without floods, these floodplains wouldn’t “work” — they would not be able to deliver the ecosystem services we benefit from. Those benefits include, but are not limited to:

food grown in these fertile soils
regulation of a balanced ecosystem
cultural heritage
transportation (as floodplains are easy to build roads on)
the supply of good quality drinking water
recreation.

The economic value of floodplain ecosystem services exceed US$25,681 per hectare. Roughly 25% of global terrestrial ecosystem services come from floodplains.

Humans are drawn to live on floodplains because of their productivity. In Australia, the floodplains of the Murray Darling Basin, heavily developed for agriculture, yield more than A$10 billion annually. These floodplain ecosystems provide an estimated A$187 billion per annum from their various ecosystem services.

However, the more we interrupt floodplain processes with development, the more we diminish the supply of ecosystem services.

The perils of living on floodplains

Putting the people back into the news footage reveals a social picture that is costly, traumatic and disruptive. The events of the past weeks have now brought into focus the perils of living on floodplains.

Humans have come up with ways to contend with this peril. Dams and levees. Land use planning. Building codes. Engineered floodscapes. Insurance. Emergency preparation systems and community engagement.

But if floodplains are a social-ecological system, where society gains great benefits but is also periodically placed at risk, which side should get the greatest policy attention? The humans or the ecosystem?

The answer is: both. But they also need to be better integrated.

Balancing the social with the ecological

Balancing the social and ecological aspects of floodplains requires a mindset change. We must combine community participation with research, resilience and adaptation to make long-term decisions about the future of these complex social-ecological systems.

If society wants to continue to derive the billions of dollars of benefits from floodplains, we need to ensure that flooding continues to occur on floodplains, and adapt to risk in imaginative and innovative ways that also protect the benefits.

Business as usual is not an option. The limitations of technocratic controls such as dams and levees should now be obvious. Time and time again, these have increased flood risk and failed to flood-proof the floodplain.

Rarely do such linear solutions solve complex problems in social-ecological systems. Linear solutions often exacerbate a problem or simply move it on to other parts of the system, creating social inequality, environmental decline and future risk.

The Australian government’s 2018 National Disaster Risk Reduction Framework sets the challenge to join up the built, social, economic and natural environments to address disaster risk in Australia.

Accepting the challenge requires a broader focus on balancing the social-ecological sides of Australia’s vast floodplains. Complexity, not linear thinking, must be embedded in the way we reimagine policy about floodplains and floods.

This requires transformative collaborations between government departments, researchers, business, and community stakeholders.

If this article has raised issues for you, or if you’re concerned about someone
you know, call Lifeline on 13 11 14. This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. You can read the rest of the stories here.

Melissa Parsons, Senior Lecturer, Geography and Planning, University of New England and Martin Thoms, Professor of Physical Geography, University of New England

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

After the floods, stand by for spiders, slugs and millipedes – but think twice before reaching for the bug spray

Caitlyn Forster, University of Sydney; Dieter Hochuli, University of Sydney, and Eliza Middleton, University of SydneyRecord-breaking rain has destroyed properties across New South Wales, forcing thousands of people to evacuate and leaving hundreds homeless.

Humans aren’t the only ones in trouble. Many of the animals that live with and around us are also heading for higher ground as the floodwaters rise.

Often small creatures — especially invertebrates like spiders, cockroaches and millipedes — will seek refuge in the relatively dry and safe environments of people’s houses. While this can be a problem for the human inhabitants of the house, it’s important to make sure we don’t add to the ecological impact of the flood with an overzealous response to these uninvited guests.

Warragamba Dam in southwestern Sydney has been spilling a Sydney Harbour’s worth of water each day during the rains.
Eliza Middleton, Author provided

What floods do to ecosystems

Floods can have a huge impact on ecosystems, triggering landslides, increasing erosion, and introducing pollutants and soil into waterways. One immediate effect is to force burrowing animals out of their homes, as they retreat to safer and drier locations. Insects and other invertebrates living in grass or leaf litter around our homes are also displaced.

Burrowing invertebrates come to the surface during floods, providing food for opportunistic birds.
Dieter Hochuli, Author provided

Snakes have reportedly been “invading” homes in the wake of the current floods. Spiders too have fled the rising waters. Heavy rain can flood the burrows of the Australian funnelweb, one of the world’s most venomous spiders.

Some invertebrates will boom; others may plummet

Rain increases greenery, which can support breeding booms of animals such as mosquitoes, locusts, and snails.

Even species that don’t thrive after floods are likely to become more visible as they flock to our houses for refuge. But an apparent short-term increase in numbers may conceal a longer story of decline.

After periods of flooding, the abundance of invertebrates can fall by more than 90% and the number of different species in an area significantly drops. This has important implications for the recovery of an ecosystem, as many of the ground dwelling invertebrates displaced by floods are needed for soil cycling and decomposition.

So before you reach for the bug spray, consider the important role these animals play in our ecosystem.

What to do with the extra house guests?

If your house has been flooded, uninvited creatures taking shelter in your house are probably one of the smaller issues you are facing.

Once the rain subsides, cleaning in and around your property will help reduce unwanted visitors. Inside your house, you may see an increase in cockroaches, which flourish in humid environments. Ventilating the house to dry out any wet surfaces can help get rid of cockroach infestations, and filling crevices can also deter unwanted visitors.

In the garden, you may see an increase in flies in the coming weeks and months as they lay eggs in rotting plants. Consider removing any fruit and vegetables in the garden that may rot.

Mosquitoes are also one to watch as they lay eggs in standing water. Some species pose a risk of diseases such as Ross River virus. To prevent unwanted mozzies, make sure to empty things that have filled with rainwater, such as buckets and birdbaths.

If you do encounter one of our more dangerous animals in your home, such as venomous snakes and spiders, do not handle them yourself. If you find an injured or distressed snake, or are concerned about snakes in your house, call your local wildlife group who will be able to relocate them for you.

Just like the floods, which will subside as the water moves on, the uninvited gathering of animals is a temporary event. Most visitors will quickly disperse back to more appropriate habitat when the weather dries, and their usual homes are available again.

You may see an increase in slugs in your local area after rainy conditions.
Eliza Middleton @smiley_lize

Don’t sweat the small stuff

While many of the impacts of floods are our own making, through poor planning and development in flood-prone areas, effective design of cities and backyards can mitigate the risks of floods. Vegetation acts as a “sponge” for stormwater, and appropriate drainage allows water to flow through more effectively. Increasing backyard vegetation also provides extra habitat for important invertebrate species, including pollinators and decomposers.

With severe weather events on the rise, it is important to understand how ecosystems respond to, and recover from natural disasters. If invertebrates are unable to perform vital ecosystem functions, such as soil cycling, decomposition, and pollination, ecosystems may struggle to return to their pre-flood state. If the ecosystems don’t recover, we may see prolonged booms of nuisance pests such as mosquitoes.

A few temporary visitors are are a minor inconvenience in comparison to the impacts floods have on the environment, infrastructure and the health and well-being of people impacted. So while it may seem like a bit of a creepy inconvenience, maybe we should let our house guests stay until the flood waters go down.

Caitlyn Forster, PhD Candidate, School of Life and Environmental Sciences, University of Sydney; Dieter Hochuli, Professor, School of Life and Environmental Sciences, University of Sydney, and Eliza Middleton, Laboratory Manager, School of Life and Environmental Sciences, University of Sydney

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

Managed retreat of settlements remains a tough call even as homes flood and coasts erode

Tayanah O’Donnell, Australian National UniversityIt is no joke that New South Wales residents are in the midst of their fourth “one in 100 year” event since January 2020. Much of the Australian east coast continues to experience heavy rainfall, strong winds and abnormally high tides. All will make the current floods worse.

As climate tipping points are reached and the Earth’s systems begin to buckle under the strain, the need for considered adaptation strategies is overwhelmingly clear. One of these strategies is for human settlements to retreat from areas most at risk, whether from floods or bushfires. While something needs to be done to ensure future generations do not suffer catastrophic consequences, managed retreat is a complex tool.

These strategic decisions in the next five to ten years will be challenging. And these decisions really matter: where and how do we build residential areas that can cope with a climate-changed world?

What is managed retreat?

Managed retreat can be defined as “purposeful, co-ordinated movement of people and assets out of harm’s way”. Managed retreat more often refers to the retreat of existing development out of harm’s way. Planned retreat is usually the preferred phrasing for new development that is planned for possible future relocation.

Both planned and managed retreat are focused on the permanent relocation of people and assets, as opposed to the evacuations we are seeing now.

Managed retreat is experiencing a resurgence in scientific literature as the impacts of climate change become increasingly frequent, severe and more obvious. These impacts bring with them a recognition of the need for adaptation even as we urgently reduce greenhouse gas emissions.

Of course, relocating away from high-risk locations is not an entirely new concept. However, managed retreat in response to a changing climate is not only complex, but also has a lot of political baggage. The complexity spans legal, financial, cultural and logistical factors among others: the political baggage seemingly associated with effective climate action in Australia often hinders governments’ abilities to respond properly.

Societies around the world need to grapple with the reality that managed retreat will become a suite of tools to respond to crisis. Insurers will not always be available, and the costs to governments (and therefore to you, the taxpayer) of responding to increasing rates of disasters, irrespective of insurance, will continue to grow exponentially.

Responding to events after the fact is an unsustainable model of adaptation. There is, too, a need to acknowledge settlement needs and historical built environment legacies that have put significant state infrastructure in harm’s way.

Managing difficult trade-offs

We know trade-offs need to be made between what we protect and what we let go in suburban floodplain areas.

Legal machanisms to force people and assets to move can and must be thoughtful. The implementation of managed retreat in urbanised areas faces multiple hurdles. These include:

people’s values, attachments to their homes and desires to live near waterways
property rights to protect those attachments
over-reliance on market mechanisms to enable adaptation policy.

It is wrong to see managed retreat as the panacea for climate risk and development in vulnerable locations. In many cases, once development is in place, it can be more appealing to some to protect an at-risk area rather than work towards managed retreat. Even where managed retreat has been successful, as in the case of the flood-prone township of Grantham, it was not without pain.

There are also other, more basic needs, such as having land available where people can relocate.

Working out highest and best use of land

There are ways that land can be used for its highest and best use at a point in time. For example, tools like easements can enable vulnerable land to be used, subject to event-based or time-based trigger-point thresholds. Once these thresholds are reached, the land is put to some other use. The advantage of these machanisms, especially for new development, is that owners are clear about the risks from the start.

This still leaves us with hard decisions about responding to at-risk current developments. Putting off these hard decisions and leaving them for future decision-makers will result in a huge injustice, because there will be catastrophe as Earth’s tipping points are passed. Development decisions made now will determine the impacts on our children and grandchildren.

Urban development decisions for both new and existing development in this coming decade demand courage and leadership. If we accept that Australian cities will continue to expand and increase in density, then we have some serious questions to ask ourselves. What kind of future do we want?

Some areas should simply not be developed.

There is a risk that an over-reliance on managed retreat will over-simplify the challenge of working out what to do about development in at-risk locations. There is a clear need to separate out what to do about current and past developments, and how to approach new developments.

The latter is easy – do not rebuild residential homes in at-risk areas. Governments should repurpose these zones for uses that permit nature-based solutions to the need to adapt to climate change.

Current development is much more complex. In some cases, managed retreat – done thoughtfully and considerately – will be the only option.

Tayanah O’Donnell, Honorary Senior Lecturer, Australian National University

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

Yes, Australia is a land of flooding rains. But climate change could be making it worse

Etching of the 1867 flood in the Hawkesbury-Nepean Valley, depicting the Eather family.
illustrated Sydney News/author provided

Joelle Gergis, Australian National UniversityOver the past three years, I’ve been working on the forthcoming report by the United Nations’ Intergovernmental Panel on Climate Change. I’m a climate scientist who contributed to the chapter on global water cycle changes. It’s concerning to think some theoretical impacts described in this report may be coming to life – yet again – in Australia.

The recent flooding in New South Wales is consistent with what we might expect as climate change continues.

Australia’s natural rainfall patterns are highly variable. This means the influence climate change has on any single weather event is difficult to determine; the signal is buried in the background of a lot of climatic “noise”.

But as our planet warms, the water-holding capacity of the lower atmosphere increases by around 7% for every 1℃ of warming. This can cause heavier rainfall, which in turn increases flood risk.

The oceans are also warming, especially at the surface. This drives up both evaporation rates and the transport of moisture into weather systems. This makes wet seasons and wet events wetter than usual.

So while Australia has always experienced floods, disasters like the one unfolding in NSW are likely to become more frequent and intense as climate change continues.

People watch swollen river — Flooding is likely to become more severe as the planet warms.
AAP

Understanding the basics

To understand how a warming world is influencing the water cycle, it’s helpful to return to the theory.

From year to year, Australia’s climate is subject to natural variability generated by the surrounding Pacific, Indian and Southern oceans. The dominant drivers for a given year set up the background climate conditions that influence rainfall and temperature.

It is a combination of these natural climate drivers that makes Australia the land of drought and flooding rains.

However, Australia’s climate variability is no longer influenced by natural factors alone. Australia’s climate has warmed by 1.4℃ since national records began in 1910, with most of the warming occurring since 1970. Human-caused greenhouse emissions have influenced Australian temperatures in our region since 1950.

This warming trend influences the background conditions under which both extremes of the rainfall cycle will operate as the planet continues to warm. A warmer atmosphere can hold more moisture (higher water vapour content), which can lead to more extreme rainfall events.

A warmer atmosphere can hold more moisture which can lead to more extreme rainfall events.
Climate Council

Since the winter of 2020, Australia has been influenced by the La Niña phase of the El Niño–Southern Oscillation (ENSO). Historically, sustained La Niña conditions, sometimes with the help of a warmer than average Indian Ocean, have set the scene for severe flooding in eastern Australia.

During these events, easterly winds intensify and oceans around Australia warm. This is associated with the Walker Circulation – a giant seesaw of atmospheric pressure that influences the distribution of warm ocean waters across the Pacific Ocean.

The last La Niña occurred in 2010–2012. It led to widespread flooding across eastern Australia, with particularly devastating effects in Queensland. The event caused the wettest two-year period in the Australian rainfall record, ending the 1997–2009 Millennium Drought.

Oceanographers from UNSW studied the exceptional event. They demonstrated how a warmer ocean increased the likelihood of extreme rain during that event, primarily through increased transport of moist air along the coast.

Their analysis highlighted how long‐term ocean warming can modify rain-producing systems, increasing the probability of extreme rainfall during La Niña events.

It is important to point out that changes in large-scale atmospheric circulation patterns are still not as well understood as fundamental changes in thermodynamics. However, because regional rainfall changes will be influenced by both factors, it will take researchers time to tease everything out.

So what about climate change?

The theoretical changes to the global water cycle are well understood. However, determining the contribution of natural and human influences on climate variability and extremes – known as “attribution” – is still an emerging science.

More studies are needed to distinguish natural or “background” rainfall variability from recent human-caused changes to the water cycle. This is particularly the case in a country like Australia, which has very high yearly rainfall variability. This contrasts with some regions of the Northern Hemisphere with less variable rainfall, where a clear climate change signal has already emerged.

Right now, La Niña conditions are decaying in the Pacific Ocean. As expected, the 2020–2021 La Niña has brought above-average rainfall to much of eastern Australia. This helped ease the severe drought conditions across eastern Australia since 2017, particularly in NSW.

NSW rainfall total, week ending March 22, 2021 — NSW rainfall totals for the week ending March 22, 2021.
Bureau of Meteorology

What’s interesting about the 2020–2021 La Niña is that it was weak compared with historical events. The relationship between La Niña and rainfall is generally weaker in coastal NSW than further inland. However, it’s concerning that this weak La Niña caused flooding comparable to the iconic floods of the 1950s and 1970s.

The rainfall totals for the current floods are yet to be analysed. However, early figures reveal the enormity of the downpours. For example, over the week to March 23, the town of Comboyne, southwest of Port Macquarie, recorded an extraordinary 935mm of rainfall. This included three successive days with more than 200mm.

The NSW coast is no stranger to extreme rainfall – there have been five events in the past decade with daily totals exceeding 400mm. However, the current event is unusual because of its duration and geographic extent.

It’s also worth noting the current extreme rainfall in NSW was associated with a coastal trough, not an East Coast Low. Many of the region’s torrential rainfall events in the past have resulted from East Coast Lows, although their rainfall is normally more localised than has been the case in this widespread event.

Remember that as the air warms, its water-holding capacity increases, particularly over the oceans. Current ocean temperatures around eastern and northern Australia are about 1℃ warmer than the long-term average, and closer to 1.5℃ warmer than average off the NSW coast. These warmer conditions are likely to be fuelling the systems driving the extreme rainfall and associated flooding in NSW.

Sea surface temperature anomalies along the NSW coast.
Bureau of Meteorology

A nation exposed

Weather and climate are not the only influences on extreme flood events. Others factors include the shape and size of water catchments, the presence of hard surfaces in urban areas (which cant’t absorb water), and the density of human settlement in flood-prone areas.

The Hawkesbury–Nepean region in Western Sydney, currently experiencing major flooding, is a prime example. Five major tributaries, including the Warragamba and Nepean Rivers, flow into this extensively urbanised valley.

Improving our understanding of historical weather data may help improve future climate change risk assessment. For example, past floods in the Hawkesbury–Nepean have been a lot worse than the current disaster. In 1867, the Hawkesbury River at Windsor reached 19.7 metres above normal, and in 1961 peaked at 14.5 metres. This is worse than the 13.12 metres above normal recorded at Freemans Reach on March 23.

It’s sobering to think the Hawkesbury River once peaked 6 metres higher than what we’re seeing right now. Imagine the potential future flooding caused by an East Coast Low during strong La Niña conditions.

It will take time before scientists can provide a detailed analysis of the 2020–2021 La Niña event. But it’s crystal clear that Australia is very exposed to damage caused by extreme rainfall. Our theoretical understanding of water cycle changes tells us these events will only become more intense as our planet continues to warm.

Joelle Gergis, Senior Lecturer in Climate Science, Australian National University

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

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

A triple whammy

So what will happen in Australia?

What do we do about it?

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Using silk to move around

Don’t spray them!

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Living in a bathtub

Will raising the dam wall work?

A long flooding period is on our doorstep

The good news and bad news

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What is a floodplain?

Humans benefit from floodplains

The perils of living on floodplains

Balancing the social with the ecological

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What floods do to ecosystems

Some invertebrates will boom; others may plummet

What to do with the extra house guests?

Don’t sweat the small stuff

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What is managed retreat?

Managing difficult trade-offs

Working out highest and best use of land

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Understanding the basics

So what about climate change?

A nation exposed

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Let it rain

Cyclones and heatwaves

Be thankful for weather forecasters

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When tides, storms and sea levels combine

Retreating from the coast

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Flooding causes major disruptions to gene flow

Flood impacts are immense and under-appreciated

Increased flooding from climate change could have major consequences

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