Cyclone Debbie: we can design cities to withstand these natural disasters


Rob Roggema, University of Technology Sydney

What happens after Cyclone Debbie is a familiar process. It has been repeated many times in cities around the world. The reason is that our cities are not designed for these types of events. The Conversation

So we know what comes next. Queenslanders affected by Debbie will complain about the damage, the costs and the need for insurers to act now to compensate their losses. The state and federal governments will extensively discuss who is to blame.

The shambles will be cleared and life will eventually get back to normal. Billions of dollars will be spent on relocating people and on repairing the damage and public works. A state-level levy may even be necessary to pay for all the extra costs. Two storms, Katrina and Sandy, cost the United States more than US$200 billion between them.

Yet we know what cyclones do. They bring, for a relatively short time, huge gusty winds. These are inconvenient but have proven not too damaging.

The greatest risk comes from storm surge and rainfall. Both bring a huge amount of water. And all this water has to find a way to get out of our living environment.

Despite knowing, approximately, where cyclones tend to occur, we never thought about adjusting our cities to their effects. It would make a huge financial difference if we did.

So, what can we do to build our cities differently to ensure the impacts of cyclones – and the accompanying rainfall and storm surges – do not disrupt urban life? The answer to all of this is design.

The usual design of current cities and towns brought us problems in the first place. We need to fundamentally rethink the design of our built-up areas.

Rethinking coastal and urban design

It starts with coastal design. We are used to building dams and coastal protection against storm surges happening once in 100 years. For comparison, the protection standards in the low-lying Netherlands are designed to protect the country against a once-in-10,000-years flood. But nature has proven to be stronger than our artificial constructs can handle.

An alternative design approach is to rely on the natural coastal processes of land forming – such as reefs, islands, mangroves, beaches and dunes. Humans can help the formation of these natural protectors by providing the triggers for them to emerge.

As an example, when we put sand in front of the coast, the currents and waves will transport the sand towards the coast and build up new and larger beaches. This example is realised in front of the Dutch coast and is known as the sand engine. But nature will build them up to form a much stronger system than humans ever could.

Instead of coasts, beaches and real estate being washed away, new land and larger beaches may be formed as a result of these processes. This requires design thinking, insights into the resilience of the coastal system, and understanding of the natural forces at play.

Second, urban design should reconsider the way we build our cities. Most urban areas do not have the capacity to “welcome” lots of water. And it is about lots of water, not the average shower or two.

Until cyclones are gone, these enormous amounts of water need to be stored for a short period in dense urban areas. This goes beyond water-sensitive urban design.

Despite the benefits of water-sensitive design in many urban developments, when the going gets tough, this is just not enough. Water-sensitive urban design can barely cope with average rainfall peaks. So, in times of severe weather events, cities need to have additional spaces to store all this water.

The general rule here is to store every raindrop as long as possible where it falls.

How and where should we redesign our cities?

So, what can be done to cyclone-proof our cities? We can:

  • Create larger green spaces, which are connected in a natural grid, increase the capacity of these green systems by adding eco-zones and wetlands, and redesign river and creek edges. Remove the concrete basins from every creek in the city.

  • Use large public spaces, such as parking spaces near shopping centres, ovals and football pitches, for temporarily capturing and storing excess rainwater. Small adjustments at the edges of these places are generally enough to capture the water.

  • Turn parking garages into temporary storage basins.

  • Redesign street profiles and introduce green and water-zones in streets. Out of every three-lane street, one lane can be transformed into a green lane, which can absorb rainwater.

  • Redesign all impervious, sealed spaces and turn these into areas where the water can infiltrate the soil. Use permeable materials.

  • Think in an integrated way about street infrastructure, green and ecological systems, and the water system.

These design interventions are not new and have been done abroad in cities such as Rotterdam, Hamburg or Stockholm. If we could add to these the redesign of roofs and gardens of industrial and residential estates and turn these into green roofs and rain gardens, the city would start to operate as a huge sponge.

When it rains, the city absorbs the huge amounts of water and releases it slowly to the creek and river system after the rain has gone. This way, green spaces and water spaces not only play an important role during and just after a cyclone, but they then add quality to people’s immediate living environment.

And maybe the best of all this: the bill Debbie and other natural disasters would present to government, industries and insurers could be much lower.

Rob Roggema, Professor of Sustainable Urban Environments, University of Technology Sydney

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

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What happened in New Zealand’s magnitude 7.5 earthquake


Brendan Duffy, University of Melbourne and Mark Quigley, University of Melbourne

At least two people have died in the magnitide 7.5 earthquake that struck New Zealand’s South Island early on Monday, local time.

Preliminary modelling suggests that the earthquake was caused by a rupture of a northeast-striking fault that projects to the surface offshore.

But this may be a complex event, involving several faults on the South Island.

The northern part of the South Island straddles the boundary between the Pacific and Australian tectonic plates.

The jostling between these plates pushes up rocks that create mountains including the Southern Alps and the beautiful Seaward Kaikoura Range, one of New Zealand’s most rapidly uplifting mountain ranges.

The plate motion forces the oceanic crust of the Pacific plate beneath the Australian plate on thrust faults, and also causes the plates to slide laterally with respect to one another on strike-slip faults.

The region affected by the recent earthquake has been one of the most seismically active in New Zealand over the past few years, including earthquakes that occurred as part of the Cook Strait earthquake sequence in 2013. It is likely that these sequences are related given their close spatial and temporal association.

What slipped during the earthquake?

The preliminary analysis strongly suggests that most of the energy release during this earthquake was sourced from the rupture of a roughly 200km-long fault system. This fault system is aligned northeast and dips to the northwest, beneath the northern part of the South Island. It coincides roughly with the subduction thrust in this area.

The potential for large earthquakes on the subduction fault in the lower North Island and upper South Island of New Zealand was recently highlighted by GNS Science, New Zealand’s geological survey. It published evidence for two similar events in the Blenheim area roughly 520-470 years ago, and 880-800 years ago.

Given its setting, this latest earthquake may be structurally complex, involving a mixture of plate boundary thrusting, lateral slip on strike-slip faults, and thrusting within the Pacific plate close to the epicentre, some 15km northeast of Culverden.

The largest aftershocks suggest a mixture of thrusting and strike-slip movements.

The damage caused by the earthquake

Because the fault system was large, and the earthquake apparently started at the southwest end of the fault and propagated to the northeast, the seismic energy was released over a period of up to two minutes.

Large earthquakes produce more long period wave energy than smaller events. The 2011 Christchurch earthquake contained a lot of high-frequency energy and very strong ground accelerations, exposing more than 300,000 people to very strong to intense ground shaking.

In contrast, this recent earthquake was manifested in Christchurch as lower-frequency rolling, and due to the sparse population density in the earthquake region, roughly 3,000 people in the upper South Island experienced strong ground shaking equivalent to the Christchurch earthquake.

Reports are emerging of at least one major fracture in the ground surface that could be related to strike-slip faulting in the Clarence region.

More traces may yet be found given the complexity of the earthquake. Tide gauge analysis will help to understand if a similar trace offshore caused the tsunami.

The earthquake has also triggered liquefaction in coastal areas and in susceptible sediments, and landsliding of up to a million cubic metres along steep susceptible cliffs in the northern South Island.

There are reports of extensive road damage including in the area between Hanmer Springs and Culverden, much of State Highway 1 and even Wellington, on the North Island.

Most of this damage is probably caused by strong ground shaking, which causes weak ground to move en masse and has resulted in numerous slips and road closures in the central and northern South Island.

Earthquakes, aftershocks and the pull of the moon

Given the earthquake happened on the eve of a supermoon full moon, and the closest the Earth and moon will be since 1948, it wasn’t long before some tried to make a connection.

But the tidal triggering of earthquakes has been investigated since the 19th century and remains a challenging and controversial field.

Small amplitude and large wavelength tidal deformations of the Earth due to motions of the sun and moon influence stresses in Earth’s lithosphere.

It is possible that, for active faults that are imminently close to brittle failure, small tidal force perturbations could be enough to advance rupture relative to the earthquake cycle, or to allow a propagating rupture to travel further than it might otherwise have done.

But the specific time, magnitude and location of this or any other large earthquake has not been successfully predicted in the short-term using tidal stresses or any other possible precursory phenomenon.

Deliberately vague predictions that provide no specific information about the precise location and magnitude of a future earthquake are not predictions at all. Rather, these are hedged bets that get media air time due to the romantic misinterpretation that they were valid predictions.

Most earthquake scientists, including those that research tidal triggering of earthquakes, highlight the importance of preparedness over attempts at prediction when it comes to public safety.

To this end, GNS Science uses a system of operational earthquake forecasts to communicate earthquake risk to concerned New Zealand residents during an aftershock sequence such as we are now entering.

These forecasts are based on earthquake physics and statistical seismology. The current operational forecast indicates an 80% probability of:

A normal aftershock sequence that is spread over the next few months. Felt aftershocks (e.g. M>5) would occur from the M7.5 epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few weeks and months.

This aftershock sequence will probably (98%) include several large aftershocks (some greater than magnitude 6 have already occurred), and for each magnitude 6 aftershock we expect 10 more magnitude 5 aftershocks over the coming days and weeks.

The Conversation

Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne and Mark Quigley, Associate professor, University of Melbourne

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