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.

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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.

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

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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.

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

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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!”

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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.

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Droughts and flooding rains already more likely as climate change plays havoc with Pacific weather

Scott B. Power, Australian Bureau of Meteorology; Brad Murphy, Australian Bureau of Meteorology; Christine Chung, Australian Bureau of Meteorology; François Delage, Australian Bureau of Meteorology, and Hua Ye, Australian Bureau of Meteorology

Global warming has already increased the risk of major disruptions to Pacific rainfall, according to our research published today in Nature Communications. The risk will continue to rise over coming decades, even if global warming during the 21st century is restricted to 2℃ as agreed by the international community under the Paris Agreement.

In recent times, major disruptions have occurred in 1997-98, when severe drought struck Papua New Guinea, Samoa and the Solomon Islands, and in 2010-11, when rainfall caused widespread flooding in eastern Australia and severe flooding in Samoa, and drought triggered a national emergency in Tuvalu.

These rainfall disruptions are primarily driven by the El Niño/La Niña cycle, a naturally occurring phenomenon centred on the tropical Pacific. This climate variability can profoundly change rainfall patterns and intensity over the Pacific Ocean from year to year.

Rainfall belts can move hundreds and sometimes thousands of kilometres from their normal positions. This has major impacts on safety, health, livelihoods and ecosystems as a result of severe weather, drought and floods.

Recent research concluded that unabated growth in greenhouse gas emissions over the 21st century will increase the frequency of such disruptions to Pacific rainfall.

But our new research shows even the greenhouse cuts we have agreed to may not be enough to stop the risk of rainfall disruption from growing as the century unfolds.

Changing climate

In our study we used a large number of climate models from around the world to compare Pacific rainfall disruptions before the Industrial Revolution, during recent history, and in the future to 2100. We considered different scenarios for the 21st century.

One scenario is based on stringent mitigation in which strong and sustained cuts are made to global greenhouse gas emissions. This includes in some cases the extraction of carbon dioxide from the atmosphere.

In another scenario emissions continue to grow, and remain very high throughout the 21st century. This high-emissions scenario results in global warming of 3.2-5.4℃ by the end of the century (compared with the latter half of the 19th century).

The low-emissions scenario – despite the cuts in emissions – nevertheless results in 0.9-2.3℃ of warming by the end of the century.

Increasing risk

Under the high-emissions scenario, the models project a 90% increase in the number of major Pacific rainfall disruptions by the early 21st century, and a 130% increase during the late 21st century, both relative to pre-industrial times. The latter means that major disruptions will tend to occur every four years on average, instead of every nine.

The increase in the frequency of rainfall disruption in the models arises from an increase in the frequency of El Niño and La Niña events in some models, and an increase in rainfall variability during these events as a result of global warming. This boost occurs even if the character of the sea-surface temperature variability arising from El Niño and La Niña events is unchanged from pre-industrial times.

Although heavy emissions cuts lead to a smaller increase in rainfall disruption, unfortunately even this scenario does not prevent some increase. Under this scenario, the risk of rainfall disruption is projected to be 56% higher during the next three decades, and to remain at least that high for the rest of the 21st century.

The risk has already increased

While changes to the frequency of major changes in Pacific rainfall appear likely in the future, is it possible that humans have already increased the risk of major disruption?

It seems that we have: the frequency of major rainfall disruptions in the climate models had already increased by around 30% relative to pre-industrial times prior to the year 2000.

As the risk of major disruption to Pacific rainfall had already increased by the end of the 20th century, some of the disruption actually witnessed in the real world may have been partially due to the human release of greenhouse gases. The 1982-83 super El Niño event, for example, might have been less severe if global greenhouse emissions had not risen since the Industrial Revolution.

Most small developing island states in the Pacific have a limited capacity to cope with major floods and droughts. Unfortunately, these vulnerable nations could be exposed more often to these events in future, even if global warming is restricted to 2℃.

These impacts will add to the other impacts of climate change, such as rising sea levels, ocean acidification and increasing temperature extremes.

Scott B. Power, Head of Climate Research/International Development Manager, Australian Bureau of Meteorology; Brad Murphy, Manager, Climate Data Services, Australian Bureau of Meteorology; Christine Chung, Research Scientist, Australian Bureau of Meteorology; François Delage, Assistant scientist, Australian Bureau of Meteorology, and Hua Ye, Climate IT Officer, Australian Bureau of Meteorology

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

Antarctic ice shows Australia’s drought and flood risk is worse than thought

Anthony Kiem, University of Newcastle; Carly Tozer, University of Newcastle, and Tessa Vance, University of Tasmania

Australia is systematically underestimating its drought and flood risk because weather records do not capture the full extent of rainfall variability, according to our new research.

Our study, published today in the journal Hydrology and Earth System Sciences, uses Antarctic ice core data to reconstruct rainfall for the past 1,000 years for catchments in eastern Australia.

The results show that instrumental rainfall records – available for the past 100 years at best, depending on location – do not represent the full range of abnormally wet and dry periods that have occurred over the centuries.

In other words, significantly longer and more frequent wet and dry periods were experienced in the pre-instrumental period (that is, before the 20th century) compared with the period over which records have been kept.

Reconstructing prehistoric rainfall

There is no direct indicator of rainfall patterns for Australia before weather observations began. But, strange as it may sound, there is a link between eastern Australian rainfall and the summer deposition of sea salt in Antarctic ice. This allowed us to deduce rainfall levels by studying ice cores drilled from Law Dome, a small coastal ice cap in East Antarctica.

It might sound strange, but there’s a direct link between Antarctic ice and Australia’s rainfall patterns.
Tas van Ommen, Author provided

How can sea salt deposits in an Antarctic ice core possibly be related to rainfall thousands of kilometres away in Australia? It is because the processes associated with rainfall variability in eastern Australia – such as the El Niño/Southern Oscillation (ENSO), as well as other ocean cycles like the Interdecadal Pacific Oscillation (IPO) and the Southern Annular Mode (SAM) – are also responsible for variations in the wind and circulation patterns that cause sea salt to be deposited in East Antarctica (as outlined in our previous research).

By studying an ice record spanning 1,013 years, our results reveal a clear story of wetter wet periods and drier dry periods than is evident in Australia’s much shorter instrumental weather record.

For example, in the Williams River catchment, which provides water for the Newcastle region of New South Wales, our results showed that the longest dry periods lasted up to 12 years. In contrast, the longest dry spell since 1900 lasted just eight years.

Among wet periods, the difference was even more pronounced. The longest unusually wet spell in our ice record lasted 39 years – almost five times longer than the post-1900 maximum of eight years.

Busting myths about drought and flood risk

Although this does not tell us when the next major wet or dry period will happen, it does help us predict how often we can expect such events to occur, and how long they might last. This is critical information for water resource managers and planners, especially when our millennium-long record tells a very different story to the post-1900 instrumental record on which all water infrastructure, planning and policy is based.

Our results challenge the underlying assumptions that govern water resource management and infrastructure planning. These assumptions include:

• that droughts longer than five years are rare;

• that droughts or flood-dominated periods cannot last longer than about 15 years;

• that drought and flood risk does not change over time, so a century of instrumental records is enough to gain a full understanding of the situation.

The fact that these assumptions are probably wrong is a concern. These principles are used to make crucial decisions, not just about predicting the likelihood and severity of droughts and floods themselves, but also about the design of infrastructure such as roads, reservoirs and buildings. Our study suggests that these decisions are being taken on the basis of incomplete information.

Cold case: drilling for ice to reveal long-term weather patterns.
Tessa Vance, Author provided

What’s more, Australia’s increasing population and development will mean that water demands and exposure to droughts and floods are likely to have been different in the past to what they are now (and will be in the future).

Therefore, given that the factors used to quantify risk are most likely wrong, it implies that current hydroclimatic risk assessments are not representative of the true level of risk.

This raises serious questions about water security and the robustness of existing water resource management, infrastructure design and catchment planning across eastern Australia and in other places where hydroclimatic risk is assessed on records that do not capture the full range of possible variation.

Water is a precious resource, meaning that we need the best knowledge about what our rainfall patterns are capable of delivering. Our findings can be used to better characterise and manage existing and future flood and drought risk. Forewarned is forearmed.

Anthony Kiem, Senior Lecturer – Hydroclimatology, University of Newcastle; Carly Tozer, Hydrologist, Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, University of Newcastle, and Tessa Vance, Palaeoclimatologist, Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania

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

Australia: Wagga Wagga – Spiders Trying to Escape Massive Flooding

The link below is to an article reporting on the massive flooding currently impacting eastern Australia. This report is all about the spiders (Wolf and Orb) trying to escape the flood waters.

For more, visit:
http://io9.com/5891091/massive-spiderwebs-engulf-australian-town-as-arachnids-escape-floods