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.




Read more:
What is a 1 in 100 year weather event? And why do they keep happening so often?


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.




Read more:
A rare natural phenomenon brings severe drought to Australia. Climate change is making it more common


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.




Read more:
Not ‘if’, but ‘when’: city planners need to design for flooding. These examples show the way


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.The Conversation

Tom Hubble, Associate Professor, University of Sydney

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

Melting ocean mud helps prevent major earthquakes — and may show where quake risk is highest



Shutterstock

Kate Selway, Macquarie University

The largest and most destructive earthquakes on the planet happen in places where two tectonic plates collide. In our new research, published today in Nature Communications, we have produced new models of where and how rocks melt in these collision zones in the deep Earth.

This improved knowledge about the distribution of melted rock will help us to understand where to expect destructive earthquakes to occur.

What causes earthquakes?

Giant earthquakes, such as the magnitude-9.0 quake in 2011 that caused the Fukushima nuclear disaster, or the magnitude-9.1 event in 2004 that caused the Boxing Day tsunami, occur at the collision zones between two tectonic plates. In these so-called subduction zones, one plate slides beneath the other.




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The sinking plate acts as an enormous conveyor belt, carrying material from the surface down into the deep Earth. Earthquakes occur where the sinking plate gets stuck; strain builds up until it eventually quickly releases. Fluids and molten rocks in the system lubricate the plates, helping them slide past each other and stopping big earthquakes from happening.

When happens when ocean mud ends up inside Earth?

My colleague Michael Förster and I were interested in what happens to sediments when they are carried down into the deep Earth at a subduction zone. These sediments start out as thick layers of mud on the ocean floor but get carried down into the deep Earth as part of the sinking plate.

Michael took a sample of mud collected from the ocean floor and heated it up to the high temperatures and pressures it would experience in a subduction zone. He found the sediments melt and then react with the surrounding rocks, forming the mineral phlogopite and also saline fluids.

A puzzle solved

Geophysical models of subduction zones allow us to map out exactly where the molten rocks and fluids are. These measurements are like x-rays of Earth’s interior, helping us peer into places we cannot otherwise see.

We were particularly interested in models of the electrical conductivity of subduction zones. This is because the fluids and molten rock we were looking at are more electrically conductive than the surrounding rock. Models of subduction zones have long been enigmatic, because they show Earth is very conductive in regions where people did not expect to see a lot of fluids and molten rock.

Melting sediment from the seafloor helps tectonic plates slide over one another without creating major earthquakes.
Selway & Forster, Author provided

I calculated the electrical conductivity of the phlogopite, molten sediments and fluids that were produced in the experiments and found they matched extremely well with the geophysical models. This provides good evidence that what we see in the experiments is happening in the real Earth, and allows us to calculate where the molten rock and fluids are in subduction zones around the world.

Understanding where big earthquakes are likely to occur

Giant earthquakes are not likely to occur in the parts of the subduction zone where the sediments melt. All of the products of the melting — the molten rock itself, the saline fluids, and even the mineral phlogopite — help the two plates slide past each other easily without causing large earthquakes.

We compared our models with locations of earthquakes in subduction zones along the west coast of the United States. We found there were no large earthquakes where sediments were melting, but the movement of fluids from the melted sediments could explain some small, non-destructive earthquakes and very faint signals of tremor where the two plates easily slide past each other.




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Breaking new ground – the rise of plate tectonics


Earthquakes are a tangible reminder that we live on an active planet and that, deep beneath our feet, huge forces are making rocks flow and melt and collide. Accurately predicting earthquakes will be an ongoing goal of geoscientists for decades to come.

It requires intricate detective work to weave together all the tiny threads of information we have about processes that occur so deep in the Earth that we will never be able to see or sample them. Our results are one new thread in this puzzle. We hope it will contribute to one day being able to keep people safe from the risk of earthquakes.




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Underground sounds: why we should listen to earthquakes


The Conversation


Kate Selway, , Macquarie University

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

Rising seas threaten Australia’s major airports – and it may be happening faster than we think



Sydney’s airport is one of the most vulnerable in Australia to sea level rise.
Shutterstock

Thomas Mortlock, Macquarie University; Andrew Gissing, Macquarie University; Ian Goodwin, Macquarie University, and Mingzhu Wang

Most major airports in Australia are located on reclaimed swamps, sitting only a few metres above the present day sea level. And the risk of sea level rise from climate change poses a greater threat to our airports than we’re prepared for.

In fact, some of the top climate scientists now believe global sea-level rise of over two metres by 2100 is likely under our current trajectory of high carbon emissions.




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This makes Cairns (less than 3m above sea level), Sydney and Brisbane (under 4m), and Townsville and Hobart (both around under 5m) airports among the most vulnerable.

Antarctica’s ice sheets could be melting faster than we think.
Tanya Patrick/CSIRO science image, CC BY

In the US, the National Oceanic and Atmospheric Administration (NOAA) has recommended that global mean sea level rise of up to 2.7 metres this century should be considered in planning for coastal infrastructure.

This is two to three times greater than the upper limit of recommended sea level rise projections applied in Australia.

But generally, the amount of sea level rise we can expect over the coming century is deeply uncertain. This is because ice sheet retreat rates from global warming are unpredictable.




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Given the significant disruption cost and deep uncertainty associated with the timing of sea level rise, we must adopt a risk-based approach which considers extreme sea level rise scenarios as part of coastal infrastructure planning.

Are we prepared?

As polar ocean waters warm, they can cause glaciers to melt from beneath, leading to more icebergs breaking off into the ocean and then a rapid rise in global sea level. This has happened multiple times in the Earth’s past and, on some occasions, in a matter of decades.

The Intergovernmental Panel on Climate Change (IPCC) puts sea level rise projections for Australia somewhere between 50 to 90 centimetres by 2090, relative to the average sea level measured between 1986 to 2005. But the emerging science indicates this may now be an underestimate.

Some studies suggest if substantive glacial basins of the West Antarctic Ice Sheet were to collapse, it could contribute at least a further two metres to global sea levels.

Most Australian airports have conducted risk assessments for the IPCC projections.

In fact, there is no state-level policy that considers extreme sea level rise for the most critical infrastructure, even though it is possible sea levels could exceed those recommended by the IPCC within the coming century.




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And for airports, the planning implications are stark when you compare the current projection of less than a metre of sea level rise and the potential of at least a two metre rise later this century.

Taking the most low-lying major airports in Australia as an example, our modelling suggests a collapse of the West Antarctic Ice Sheet would see their near complete inundation – without any adaptation in place.

For more elevated locations, coastal infrastructure may still be inoperable more frequently when the combined effect of storm surges, waves, elevated groundwater or river flooding are considered.

A $200 billion problem

Our airports and other forms of infrastructure near the coastline are critical to the Australian economy. The aviation industry has an estimated annual revenue of over A$43 billion, adding around A$16 billion to the economy in 2017.

While there are many uncertainties around the future cost of sea-level rise, a study by the Climate Council suggests over a metre sea level rise would put more than A$200 billion worth of Australian infrastructure at risk.

It is difficult to assign a probability and time-frame to ice sheet collapse, but scientific estimates are reducing that time frame to a century rather than a millennium.




Read more:
Climate change: sea level rise could displace millions of people within two generations


Uncertainty generally comes with a cost, so proactive planning would make economic sense.

Adapting our most critical coastal assets while sea levels rapidly rise is not an option – mitigation infrastructure could take decades to construct and may be prohibitively expensive.

Given the deep uncertainties associated with the timing of ice-sheet collapse, we suggest airport and other critical coastal infrastructure is subjected to risk analysis for a two to three metre sea level rise.The Conversation

Thomas Mortlock, Senior Risk Scientist, Risk Frontiers, Adjunct Fellow, Macquarie University; Andrew Gissing, General Manager, Risk Frontiers, Adjunct Fellow, Macquarie University; Ian Goodwin, Associate Professor, Macquarie University, and Mingzhu Wang, Senior Geospatial Scientist, Risk Frontiers, Adjunct Fellow, Macquarie University

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

California’s other drought: A major earthquake is overdue


File 20180126 100908 1qg3z3l.jpg?ixlib=rb 1.1
Fires break out across San Francisco after the April 18, 1906 earthquake.
USGS

Richard Aster, Colorado State University

California earthquakes are a geologic inevitability. The state straddles the North American and Pacific tectonic plates and is crisscrossed by the San Andreas and other active fault systems. The magnitude 7.9 earthquake that struck off Alaska’s Kodiak Island on Jan. 23, 2018 was just the latest reminder of major seismic activity along the Pacific Rim.

Tragic quakes that occurred in 2017 near the Iran-Iraq border and in central Mexico, with magnitudes of 7.3 and 7.1, respectively, are well within the range of earthquake sizes that have a high likelihood of occurring in highly populated parts of California during the next few decades.

The earthquake situation in California is actually more dire than people who aren’t seismologists like myself may realize. Although many Californians can recount experiencing an earthquake, most have never personally experienced a strong one. For major events, with magnitudes of 7 or greater, California is actually in an earthquake drought. Multiple segments of the expansive San Andreas Fault system are now sufficiently stressed to produce large and damaging events.

The good news is that earthquake readiness is part of the state’s culture, and earthquake science is advancing – including much improved simulations of large quake effects and development of an early warning system for the Pacific coast.

The last big one

California occupies a central place in the history of seismology. The April 18, 1906 San Francisco earthquake (magnitude 7.8) was pivotal to both earthquake hazard awareness and the development of earthquake science – including the fundamental insight that earthquakes arise from faults that abruptly rupture and slip. The San Andreas Fault slipped by as much as 20 feet (six meters) in this earthquake.

Although ground-shaking damage was severe in many places along the nearly 310-mile (500-kilometer) fault rupture, much of San Francisco was actually destroyed by the subsequent fire, due to the large number of ignition points and a breakdown in emergency services. That scenario continues to haunt earthquake response planners. Consider what might happen if a major earthquake were to strike Los Angeles during fire season.

Collapsed Santa Monica Freeway bridge across La Cienega Boulevard, Los Angeles after the
Northridge earthquake, Jan. 17, 1994.

Robert A. Eplett/FEMA

Seismic science

When a major earthquake occurs anywhere on the planet, modern global seismographic networks and rapid response protocols now enable scientists, emergency responders and the public to assess it quickly – typically, within tens of minutes or less – including location, magnitude, ground motion and estimated casualties and property losses. And by studying the buildup of stresses along mapped faults, past earthquake history, and other data and modeling, we can forecast likelihoods and magnitudes of earthquakes over long time periods in California and elsewhere.

However, the interplay of stresses and faults in the Earth is dauntingly chaotic. And even with continuing advances in basic research and ever-improving data, laboratory and theoretical studies, there are no known reliable and universal precursory phenomena to suggest that the time, location and size of individual large earthquakes can be predicted.

Major earthquakes thus typically occur with no immediate warning whatsoever, and mitigating risks requires sustained readiness and resource commitments. This can pose serious challenges, since cities and nations may thrive for many decades or longer without experiencing major earthquakes.

California’s earthquake drought

The 1906 San Francisco earthquake was the last quake greater than magnitude 7 to occur on the San Andreas Fault system. The inexorable motions of plate tectonics mean that every year, strands of the fault system accumulate stresses that correspond to a seismic slip of millimeters to centimeters. Eventually, these stresses will be released suddenly in earthquakes.

But the central-southern stretch of the San Andreas Fault has not slipped since 1857, and the southernmost segment may not have ruptured since 1680. The highly urbanized Hayward Fault in the East Bay region has not generated a major earthquake since 1868.

Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.

Perspective view of California’s major faults, showing forecast probabilities estimated by the third Uniform California Earthquake Rupture Forecast. The color bar shows the estimated percent likelihood of a magnitude 6.7 or larger earthquake during the next 30 years, as of 2014. Note that nearly the entire San Andreas Fault system is red on the likelihood scale due to the deficit of large earthquakes during and prior to the past century.
USGS

Can California do more?

California’s population has grown more than 20-fold since the 1906 earthquake and currently is close to 40 million. Many residents and all state emergency managers are widely engaged in earthquake readiness and planning. These preparations are among the most advanced in the world.

For the general public, preparations include participating in drills like the Great California Shakeout, held annually since 2008, and preparing for earthquakes and other natural hazards with home and car disaster kits and a family disaster plan.

No California earthquake since the 1933 Long Beach event (6.4) has killed more than 100 people. Quakes in 1971 (San Fernando, 6.7); 1989 (Loma Prieta; 6.9); 1994 (Northridge; 6.7); and 2014 (South Napa; 6.0) each caused more than US$1 billion in property damage, but fatalities in each event were, remarkably, dozens or less. Strong and proactive implementation of seismically informed building codes and other preparations and emergency planning in California saved scores of lives in these medium-sized earthquakes. Any of them could have been disastrous in less-prepared nations.

Remington Elementary School in Santa Ana takes part in the 2015 Great California Shakeout.

Nonetheless, California’s infrastructure, response planning and general preparedness will doubtlessly be tested when the inevitable and long-delayed “big ones” occur along the San Andreas Fault system. Ultimate damage and casualty levels are hard to project, and hinge on the severity of associated hazards such as landslides and fires.

Several nations and regions now have or are developing earthquake early warning systems, which use early detected ground motion near a quake’s origin to alert more distant populations before strong seismic shaking arrives. This permits rapid responses that can reduce infrastructure damage. Such systems provide warning times of up to tens of seconds in the most favorable circumstances, but the notice will likely be shorter than this for many California earthquakes.

Early warning systems are operational now in Japan, Taiwan, Mexico and Romania. Systems in California and the Pacific Northwest are presently under development with early versions in operation. Earthquake early warning is by no means a panacea for saving lives and property, but it represents a significant step toward improving earthquake safety and awareness along the West Coast.

The ConversationManaging earthquake risk requires a resilient system of social awareness, education and communications, coupled with effective short- and long-term responses and implemented within an optimally safe built environment. As California prepares for large earthquakes after a hiatus of more than a century, the clock is ticking.

Richard Aster, Professor of Geophysics, Colorado State University

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

Australia: Warrumbungle National Park – Life Returning


The link below is to an article and photo gallery that reports on the Warrumbungle National Park after the recent major bushfire that swept through the park.

For more visit:
http://www.australiangeographic.com.au/journal/bushfires-ravage-the-warrumbungles.htm

Overfishing: Sharks & Rays


The link below is to an article reporting on how overfishing has become a major threat to sharks and rays around the world.

For more visit:
http://www.redorbit.com/news/science/1112687250/sharks-rays-wcs-iucn-090512/

Blackbutt Reserve


Kevin's Daily Photo, Video, Quote or Link

Since I was unable to visit Gap Creek Falls the other day, I decided I might pop in to have a look at the new animal enclosures at Blackbutt Reserve near Newcastle. I will say straight off the bat that I do have something of a prejudice against Blackbutt Reserve, as I see the place as nothing like a natural bush setting, it being far too ‘corrupted’ by human activity, weeds and the like. Having said that it is a good place for a family or group outing/event. It certainly has its place, but it is not a true nature reserve (in my opinion).

Visitor Centre

ABOVE: Visitor Centre

I do think that some well designed animal and bird enclosures at Blackbutt could lift the value of the reserve dramatically and make it a really great place for families, especially young families. There are opportunities for educational visits for kids, possible environmental…

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Article: How to Buy the Right Bushwalking Boot


Bushwalkers/hikers/trekkers (call it what you will) know that the right boot for an individual walker is absolutely essential out in the wild (or as we might call it Australia, out in the bush or out in the sticks). Many a bushwalk has been ruined or seriously curtailed by having the wrong boot. For me, when I’m doing some serious walking and covering large swathes of territory, blisters become a major problem.

The link below is to an article that provides some tips on what to look for when buying a bushwalking boot.

For more visit:
http://www.australiangeographic.com.au/outdoor/guide-to-buying-the-perfect-hiking-boot.htm.

Article: Sumatran Striped Rabbit


When thinking of endangered species it is difficult to believe that a rabbit could be endangered. Certainly where I work rabbits are a major introduced pest, yet in Indonesia there is a rabbit that is threatened with extinction. The link below is to an article covering the plight of the Sumatran Striped Rabbit.

For more visit:
http://news.mongabay.com/2012/0628-hance-fs-sumatran-striped-rabbit.html.

Earth Hour 2012: Tonight


The link below is to an article on Earth Hour 2012, which is being held tonight. The article below includes a history of the event, which is now a global movement for ‘change.’ However, just how much change is brought about by Earth Hour is still a matter of debate. There seems to be more of an emphasis on going beyond the hour this time round, which is a far better way of drawing awareness to the need of green energy for the future and the major issue of climate change that is facing the planet. If the event is to is bring lasting change, we need to move beyond the hour as just a fun thing to do and actually bring about change to the way we live our lives the world over. There is a long way to go, as can be seen with the great difficulty of reaching any useful agreements on CO2 emission reductions and the like. Hopefully awareness can bring about real change through this event.

For more visit:
http://www.kleenexmums.com.au/sustainability/earth-hour/the-hour-of-no-power/