Inskip beach collapse: just don’t call it a ‘sinkhole’

Stephen Fityus, University of Newcastle

As was widely reported in the media, at around 10pm last Saturday night, a “sinkhole” opened up at a beachfront campground on the Inskip peninsular.

The thing is, it almost certainly wasn’t a sinkhole.

Unanticipated ground collapses occur around the world from time to time, and these generally get labelled “sinkholes”, for want a more appropriate term. Yet “sinkhole” is poorly defined and often misused, generally referring to some type of geological phenomenon that causes localised ground surface collapse.

In its strict sense, a sinkhole occurs when there is movement of surface soil or rock downward to fill a cavity in the ground below it. Thankfully, open underground cavities are not so common in nature, and are limited to a few characteristic geological settings.

The classic manifestation of sinkholes is in karstic geological environments, such as the Nullabor Plains. These are where the percolation of groundwaters through limestones and dolomites over geological timescales causes them to dissolve, leading to the formation of underground cave systems.

Where the span of the caves becomes too great, or the overlying roof rocks are too thin to support themselves, these may collapse. This produces the stereotypical sinkholes such as those known from Guatemala, Florida, Louisiana, and parts of China.

Sinkholes can also arise from anthropogenic activity, such as mining and engineering works. Poorly backfilled or capped mine shafts may subside if the backfill collapses or is washed to deeper levels in the mine by inflowing water, such as occurred in the case of the Swansea “sinkhole” near Newcastle, New South Wales, in 2014.

Shallow tunnels can also collapse, leading to a hole or depression forming in the ground above. Small sinkholes can also occur above breaks in unpressurised wastewater pipes if soil from around the pipes is able to collapse into the pipe and be carried away with the flowing water.

Sandy straight

So how does any of this explain the Inskip beach “sinkhole”? Well, it doesn’t. And from the photographs and available geological information, it seems like the event at Inskip beach is not a sinkhole at all.

The Inskip beach area is not undermined, and not known for the occurrence of limestones in its bedrock. So its very unlikely that the missing sand has been swallowed into some deep hole in the sea floor.

To understand the likely reasons behind the Inskip event, it is necessary to understand the geological setting of the Inskip peninsular. For millions of years, the coastal river systems of New South Wales have generated vast quantities of clean quartz sand, which have been delivered to the ocean.

Some of this sand is pushed up to create some of the best sandy beaches in the world. Meanwhile, the excess (and there is a lot of it) is swept northward along the coast by ocean currents until it reaches a place where it can be deposited.

Through a complex combination of ocean current, ocean swell, coastal morphology and bathymetric factors, Fraser Island in Queensland – the largest sand island in the world – is the repository for much of this excess sand.

The situation is complicated by the Mary River, which discharges into the ocean at the same place. This means that Fraser Island is separated from the mainland by a channel, which allows the Mary River to discharge to the ocean, mainly northward through Hervey Bay.

The southern end of this channel, the “Great Sandy Straight”, forms an estuary at Tin Can Bay, which accommodates tidal flows inward and outward between the Inskip peninsular and Fraser Island. And this is the site of recent collapse event.

It might look like a sinkhole, but it’s something quite different.
AAP Image/Higgins Storm Chasing

Slippery sand

Tidal channels are dynamic environments, carrying sand backward and forward on a daily basis, depositing sand, and then scouring it out again when the channel becomes constricted. If sand is spilled into a pile, it forms a slope at a characteristic angle, referred to as the angle of repose.

If a slope is made any steeper than this, it is potentially unstable and prone to collapse. Sands deposited to form the submerged banks of the channel are flatter than, or equal to, the angle of repose and exist in a stable condition.

However, if the sandy banks of the channel are steepened through erosion in the bottom of the channel, then the over-steep submerged slope may become unstable, resulting in a submarine landslide. Such a slide, initiated at the toe of the slope, will effectively see the slope unravel, with slices of the slope progressively slumping into the space created by the slumping of the slice below.

This mechanism fits well with the situation at Inskip beach, both in terms of the geomorphological conditions and the reported characteristics of the beach collapse.

Will there be more events like this? At some time in the future, most likely. But when, where and how big are all questions that are difficult to quantify without site specific geotechnical and hydromorphological data. Coastal environments are dynamic, restless environments, and the risks of sudden changes are small, but ever-present.

Stephen Fityus, Professor in Geotechnical Engineering, University of Newcastle

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

Want to see the business case for green energy? Just look at China

John Mathews and Hao Tan, University of Newcastle

The narrative around renewable energy sources is typically framed almost entirely in terms of their contribution to reducing carbon emissions and thereby providing a means to tackle climate change. From this perspective, the drive for renewables is inseparably linked to international negotiations over reducing carbon emissions, which will come to a head at the United Nations summit in Paris towards the end of this year.

But this framing of the story struggles to explain the rise of China as the world’s renewables superpower. It is investing more in renewable energy production and manufacturing of renewable energy devices than any other country. Is China making these huge investments – not to mention launching a national emissions trading scheme – purely to accommodate the world’s desire to see carbon emissions reduced?

This is the question that we address in our new book, China’s Renewable Energy Revolution, published this month.

Our argument is that China is motivated by much more immediate concerns. Because of the dominant role played by coal in its rise as the world’s largest manufacturing economy, China suffers from catastrophic air pollution, particularly the toxic mix of tiny particulate matter that penetrates deep into the lungs of people breathing the air of Beijing, Tianjin, or other major industrial cities. The issue has prompted anger and social agitation, with the public demanding that environmental laws be enforced. (See, for example, the explosive impact of the documentary Under the Dome, by investigative journalist Chai Jing.)

Yet the real problem for China in continuing on a “business as usual” pathway is that it would become increasingly dependent on imports of coal, oil and gas – and therefore grow more vulnerable to price fluctuations and sudden interruptions to supply. More pointedly, as a relative newcomer to the global fossil fuel market, China is forced to locate supplies from more and more unstable parts of the world, putting it at financial risk from war, revolution and terrorism even in distant lands.

These concerns over energy security, and the immediate issue of air pollution, are in our view likely to be weighing more heavily on the Chinese leadership than concerns about climate change.

New paths to growth

Our view is that China is running into the limits of a predominantly fossil-fuelled expansion and now needs to find a new development pathway based on green growth and clean technology. This, we argue, is what lies behind its vast investments in renewables.

In per capita terms, China is of course not yet abreast of the developed countries in its overall energy consumption or its renewable energy use. But this is not evidence that China is going “light” on renewables; on the contrary, as a rising middle-level power, it still has plenty of room to grow its already large renewable energy industries.

Solar energy looks like having a bright future in China.
Reuters/William Hong

Even though per capita use is still modest, the absolute size of the renewables investment in China allows new industries to scale up, which in turn leads to lower costs as efficiencies are captured. Through the principle of circular and cumulative causation, this leads to further market expansion and further cost reductions. The cost reductions then create opportunities for countries in the rest of the world to become involved in renewable energy as well.

China is already on the record in viewing its clean technology sectors as key drivers of future prosperity and export platforms. The 12th Five-Year Plan, which ends this year, set out the comprehensive goals for China’s economic development. It featured seven Strategic Emerging Industries (SEIs) that were earmarked for special promotion, including three industries closely related to the burgeoning energy transition: energy saving and environment protection; new energy sources; and new-energy-powered cars.

The plan laid out a target that production value-added from these seven SEIs should reach 8% of China’s gross domestic product (GDP) by 2015. This target has since been raised to reach 15% of GDP in 2020. There could be no clearer demonstration of how China views the link between building energy security, improving environmental protection and creating the export platforms of tomorrow.

There’s a lot of financial mileage in battery-powered cars.
Reuters/Stringer China

China’s green business case

As we see it, China has a lot more riding on its renewables revolution than (just) climate change concerns. Important as these are, it is a profoundly convenient truth that the more China builds its export platforms around renewables, smart energy grids, and clean transport technologies such as fast rail and electric vehicles, the more it drives down its own carbon emissions and the costs of these clean technologies for everyone else.

The lesson for industrialised countries such as Australia is that renewables do not have to be framed solely as an issue of climate change and its mitigation. In keeping with the new emphasis of the Turnbull government on a 21st-century agenda, with the focus on tackling the challenges and seizing the opportunities created by industries of the future (such as renewables) rather than sticking with those of the past (coal), it is the business case that needs to be made.

China has shown very clearly that renewables make excellent business sense in the 21st century. Now it is up to Australian leaders, such as the new energy minister, Josh Frydenberg, and assistant innovation minister, Wyatt Roy, to act on the same understanding and help build – finally – a great renewables industry in Australia.

John Mathews, Professor of Strategic Management, Macquarie Graduate School of Management and Hao Tan, Senior Lecturer, Faculty of Business and Law, University of Newcastle

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