It doesn’t always take superstorms to get supersurges

By Thomas Mortlock (

Severe Tropical Cyclone Debbie made landfall at Airlie Beach on the Whitsunday Coast earlier on in the year, with an estimated property insurance market loss estimate over AUD $1.6 billion (PERILS, 2017). Debbie had all the ingredients for a large storm surge potential – a low and dropping pressure before landfall (down to 943 mB), high and sustained onshore wind speeds (landfalling as a Cat 4 system), a track perpendicular to the coast, and a very slow forward moving speed (7 km/hr at landfall).

Debbie also coincided with a relatively high state of tide (landfall occurring 2 hours after high water) and large waves (> 9 m), to produce a storm tide inundation, according to Risk Frontiers’ own survey estimates, of around 5 m above mean sea level. This was roughly equivalent to the height of most coastal foredunes, meaning direct coastal inundation damage to property was limited.

While the storm tide inundation could have been much higher if Debbie had made landfall two hours earlier, the storm surge itself (a combination of elevated coastal water levels due to high wind speeds and low atmospheric pressure, minus tides and waves) should have been bigger. The open question posed since March has been – why was it not?

A similar question has been asked of storms and surges in the Wadden Sea, a fringe basin in the North Sea between the Netherlands and Denmark. The article below, written by Giordano Lipari in the Netherlands, makes the point that superstorms don’t always lead to supersurges, especially in coastal areas fronted by islands. In the case of the Wadden Sea, and the article below, these are “barrier [sand] islands”, but in the case of Debbie and the Whitsunday region, the same effect may also be caused by the numerous rock and coral islands and reefs that fringe the mainland coast.

Below is an edited version of Lipari’s article. The original version can be found at

Figure 1 The Wadden Sea

1. Big can fail, little can hit 

The Wadden Sea (Figure 1) is a fringe basin of the North Sea delimited by a strip of barrier islands. When it comes to storms and surges, the Wadden Sea stages an intriguing three-way interaction between physiographic features (in plain language: the water container), atmospheric systems (weather), and flow patterns (water). In the Dutch part of basin, in particular, this interplay defeats the intuition that the most severe surges are caused by the most severe storms when ranked by wind speed alone.

2. Back by the beach

When raging winds raise the water against the coast, it is generally taken as ground truth that the higher the peak wind speed, the higher the peak water level. Some tide gauges in the Dutch Wadden Sea, however, showed that record-breaking surges were not caused by the most severe winds in the same control period.

The unlimited presence of water is self-evident on a shore squarely facing the ocean’s expanse. In contrast, the water volume contained in the Wadden Sea depends on the course of the waters flowing in and out across its several tidal inlets. However hard the wind pushes in the water across one tidal inlet, some water may still escape from another, leading to no noteworthy accumulation of water inside the basin. In the extreme, there is no surge if no extra water stays in and for long enough. Hence, there could be much barking in the wind, little biting in the water.

Figure 2 Hydrodynamic modelling in the Wadden Sea – the fringing barrier islands and the complex flow directions behind may be an indication of the type of situation that occurred to the lee of the Whitsunday Islands during Debbie

In a basin delimited by barrier islands [or rock islands and reefs, in the case of the Whitsunday coastline] the surges are significantly modulated by the physical geography. Only those storms causing a substantial piling-up of water behind the islands can cause severe surges, once they have managed to bring in the excess water to raise in the first place.

The arrows in the picture above, based on computer simulations, indicate qualitatively where water is going in and out at a storm’s given moment: clearly, it’s not the same everywhere, nor will it stay unchanged while the storm unfolds itself.

In sum, the Wadden Sea evidence is that high wind speeds alone are neither necessary nor sufficient to cause, or expect, record-breaking surges. Since the container drives both water storage and motion, the Wadden Sea itself determines effectively which storms result in a surge with a certain level of flood hazard with possibly counter-intuitive outcomes. The scope and degree of generality to which the cautionary tale is applicable to all/other situations is matter of orderly scientific discourse. Certainly, the severity of storm surges is pretty much a situation-specific matter, and it cannot be reduced to a single number defining the storm alone, such as the Beaufort or the Saffir-Simpson scale, except in the simplest configurations.

3. Thinking onwards and upwards

There are many ways in which the Wadden Sea insights might be helpful beyond the specifics. Societal concerns for the coastal areas are justified due to the growing concentration of the global population, to the weather anomalies and outliers expected to increase after climate change, and to the land subsidence aggravating flood-proneness.

The investigations on the Wadden Sea made it at least clear that the excess of simplification in the superstorm-supersurge anticipation could mishandle the exposure and vulnerability of certain coastal areas.

As often said — and hence attributed to Albert Einstein for good measure —, every problem statement should be as simple as possible, but not simpler. In the case of storm surge prediction – this may not be that simple.

Risk Frontiers staff and associates have significant experience in coastal process and hydrodynamic modelling, in particular, understanding the dynamics and impacts of extreme waves and water levels in Australia. In association with a consortium of eight other research institutes and government agencies, we are coordinating the analysis of an unprecedented number of coastal impact observations post-Debbie, to be published soon.


Have We Increased our Vulnerability to Big Floods?

By Chas Keys.

In New Orleans: 11 Years after Katrina (Briefing Note 317: May 2016), John McAneney and Foster Langbein cite an observation from sociologist Shirley Laska, Professor Emerita at the University of New Orleans. Laska argues that decisions and actions taken over three centuries had reduced the vulnerability of the city of New Orleans to small and moderate floods but increased its vulnerability to very large ones. The reduction of sediment loads in the Mississippi River had led to the destruction of the marshlands and barrier islands that once protected the city from storm surges. The import of this was that ‘routine’ floods were kept out of built-up areas by the levees but the reduction of the coastal landmass increased the likelihood of the embankments being overwhelmed when very big floods struck. The disaster that was Hurricane Katrina, which led to the breaching of the levees and well over a thousand deaths, supports Laska’s point.

Laska piqued a thought that I have long harboured about flood management in NSW. We have, over the past 60 years, invested heavily in mitigating the effects of the flood threat. Much has been achieved and dozens of communities are now better off than they once were in terms of exposure to flooding. Structural protection by way of levees, flood bypasses, the rock-armouring of stream banks and the construction of mitigation dams and retarding basins has been much improved as have warning services, rescue and other response capabilities, land use management, better flood modelling, insurance coverage and, to a degree, community education about flooding and the steps people can take to manage it in their own interests. Without doubt, communities are more able to live with the flood threat created by virtue of their locations and developmental histories.

But what, precisely, is the nature of the improvement? Laska’s thinking provides a clue. What NSW has done since the late 1950s has contributed greatly to the mitigation of the effects of modest sized floods, which for many urban communities, have ceased to exist: they have been contained to nearby rural areas. But bigger, less frequent, floods are the most consequential in terms of loss of life and damage to private and public assets. No levees can be guaranteed to keep out these, partly because there is never complete certainty about levee integrity and partly because very few levees are built to exclude floods larger than the 1-in-100 year event. At the same time, we have not accompanied our engineering efforts with measures to ensure that community members understand the level of protection provided and what incomplete protection inevitably means.

In effect we have allowed, indeed encouraged, people to believe that the levees have overcome the flood problem and made it benign. This is not true.

Take the case of Maitland. On the Hunter River, its Central Business District, and more than 3000 residents in central and South Maitland, Horseshoe Bend, Lorn and part of East Maitland, are protected by levees. With the exception of Lorn, which has experienced inundation only once in more than a century, these areas have long flood histories with many killed and much property damage over the decades. The period 1949-55 saw parts of these areas flooded several times, catastrophically so in 1955, and Maitland became part of the reason that drew the state government and then the commonwealth into the field of flood mitigation. Hitherto, flood mitigation (along with warning and response) had been the responsibility of private and local council efforts and local funding. Often, it was managed poorly.

Maitland’s modern flood mitigation scheme was completed in about 1970 and has done a fine job of protecting the community from floods that have flowed past, rather than into the built-up areas. Some areas would have experienced inundation several times had the primitive levees of previous times not been superseded by well-engineered ones after the floods of the 1950s.

None of these recent floods, however, has come close in peak height or volume to those of the February, 1955 flood. That event, if repeated today, would overtop some of the levees and inundate much of urban Maitland. The so-called ‘ring levee’ which partly surrounds the town on its southern edge is designed to be overtopped in a 1-in-50 year flood, half a metre lower than the 1955 flood, which is thought likely to be equalled or exceeded only in a 1-in-200 year flood (AEP = 0.5%).

The problem is that there is a strong feeling in the community that the levees have rendered Maitland flood-free. (Andrew Gissing and his team at Risk Frontiers have seen the same sentiments expressed in Lismore.) The fact that the ring levee is designed to admit floodwaters in floods much smaller than the 1955 event is unknown to many, probably most. One indication of this was in 2007, when a flood for a time thought likely by the Bureau of Meteorology to be the highest since 1955, produced a rather desultory property protection and evacuation response from many members of the community.

During the 1950s, with flood after flood assailing them, Maitlanders became expert at neighbourhood-level self-help endeavours like lifting furniture in situ, trucking it to the nearby hill suburbs of East Maitland and Telarah and evacuating people to safety. Some had to evacuate eight times between 1949 and 1955. Of necessity a strong flood culture existed in those days as it had in earlier times. When floods were approaching, groups of men would move from house to house, helping residents lift furniture and other home contents or carrying them out to drays and trucks for transporting to the nearby hill suburbs. Families followed, staying with friends and relatives while waiting for the floodwaters to recede.

Since the 1950s only two floods have produced forecasts that would have justified raising or removing belongings and leaving for high ground. These were the floods of 1971 (peaking nearly a metre lower than in 1955 and coming close to overtopping the ring levee) and 2007 (when the peak, thought initially to have been likely to slightly exceed that of 1971, turned out to have been substantially over-predicted). On the evidence of 2007, a big flood now would see a substantial under-response on behalf of community members, with material damage and perhaps deaths higher than would have been the case had the the behavioural modes of the 1950s been in place.

And there is further reason for pessimism about Maitland: the local council, in its concern about the commercial viability of the Central Business District, has sought to reverse some of the land use restrictions that have been in place since the 1950s. The ‘old city’ has lost population steadily over the decades through out-migration and the expansion of commercial land uses into residential areas, and the CBD’s market has shrunk considerably. To bolster the viability of the CBD the council now seeks to restore the residential population of nearby areas to the level of 1954, when well over 5000 people lived in them compared with fewer than 1800 today.

Faced with severe state-instituted restrictions on building in these areas, the council proposed in 2015 that the restrictions on residential floor height construction in levee-protected areas be abandoned provided that new dwellings were built with at least 50% of their habitable flood space above the flood standard (the modelled 1-in-100 year flood level plus half a metre freeboard). The reasoning was that residents, on hearing a flood forecast and being advised to raise items of value, would have the opportunity to move items from the lower floors of their dwellings to the higher floors.

An appeal to the state Minister for Planning for a relaxation of the building restrictions was, however, rejected. The Minister’s decision implied a recognition that the proposed change might easily have led to increased flood damage. Now the Office of Environment and Heritage has argued that new residential development − even development which adheres to the existing planning restrictions − should not be undertaken until the road infrastructure that will support evacuation from the ‘old city’ is upgraded. The council’s ambitions for population growth in the old city are being thwarted. Had the council’s preferred solution to the woes of the CBD been implemented then the community’s vulnerability to flooding would have been increased simply by virtue of many more people becoming residents of flood prone areas.

Given the reality of the flood situation, there is a strong case for community flood education to include messages about the inevitability, in very large floods, of inundation of areas behind the levees. One initiative, undertaken in the early 1980s by the Department of Public Works, involved the fixing to power poles of markers indicating the heights reached in the flood of 1955. A few of these in the built-up areas were more than four metres above ground level; many were more than two metres above. The council was never enthusiastic about the markers, and when Public Works vacated the field of flood education, they slowly disappeared ─ the victims of power pole replacement, people concerned about the value of their properties, and the rusting of the nails that held them in place. Today there are fewer than ten of the several dozen original markers left. An inexpensive, easy-to-maintain means of reminding or informing residents and others of the potential for severe flooding has gradually disappeared.

Since 2001, the State Emergency Service has assumed the role of providing community flood education. Yet these worthy efforts there is much to suggest that the community at large does not comprehend the flood risk implied by the potential for levee failure or overtopping. What we have in today’s Maitland is levees designed to let water into built-up areas in big floods, a community that is inexperienced in flood management, many residents who are oblivious to the threat that big floods pose and a council that seeks to increase the population in areas that will be severely affected by big floods and which shows little interest in flood education. This is a potentially lethal combination.

The levees have done an excellent job in protecting the community but carry the downside of an altered perception of the flood risk. The policy message is that in building levees we should also build an understanding of their limitations and stress that they can only mitigate, not eliminate, the flood threat. Co-ordinated, properly resourced and appropriately evaluated programmes seeking to do this do not exist in Australia.

The education needs to extend to elected councillors to help them understand that their decisions can contribute greatly to the oft-demonstrated ‘levee paradox’ in which the provision of structural protection too easily leads to intensified development in the protected areas. Councils are accustomed to dealing with the tension between developmental and environmental considerations, but less so in managing conflict between community safety and developmental objectives.

In New Orleans it was largely the progressive erosion of natural coastal defences that increased the city’s vulnerability to big floods. Many died in Hurricane Katrina as a consequence. In Maitland we risk the same effect being wrought. By pursuing a land use management policy that will put more people in harm’s way when big floods occur, and at the same time by not making a fully-fledged effort to ensure that people comprehend the nature of the threat posed by such floods, the level of the community’s flood vulnerability has been increased. A big flood, even one not as big as the flood of 1955, will demonstrate this some day. Maitland’s story also has the potential to be reproduced in many other leveed areas in Australia.

Happy Graduates – Stuart and Tetsuya

From left: Christina Magill, Stuart Mead, Tetsuya Okada, Kat Haynes

Congratulations to Stuart Mead and Tetsuya Okada pictured here with their supervisors Christina Magill and Kat Haynes.

Tetsuya Okada’s PhD investigated recent disaster recovery and risk reduction processes in Australia and Japan. Read more.

Stuart Mead’s PhD developed and integrated computational models of lahar hazard in order to quantify the risk and potential losses caused by lahars. Read more.

Where, Why And How Are Australians Dying In Floods?

This article by Freya Jones, published in Asia Pacific Fire Magazine, October 3, refers to research undertaken the CRC research team led by Katharine Haynes, Risk Frontiers.

Fatalities from floods are a major cause of natural hazard deaths around the globe. Here in Australia, floods are ranked second only to heatwaves in terms of the total number of natural hazard fatalities since 1900. Recent cases over the last two years, such as June 2016 in New South Wales and Tasmania, along with the aftermath of Severe Tropical Cyclone Debbie in northern NSW in April this year, highlight the significant dangers of floodwaters and as the research suggests, many of the flood deaths are avoidable.

To gain a greater understanding of human behaviour and why people choose to enter floodwaters, the CRC research project Analysis of human fatalities and building losses from natural disasters has measured the impacts of floods. The research looks at the toll on human life, injuries and building damage while analysing trends over time. Read more.

A Machine Learning Model Of Tropical Cyclone Wind Risk

This article by Thomas Loridan, Risk Frontiers, was published in Asia Pacific Fire Magazine, October 3, 2017.

Extreme winds from tropical cyclones (TCs) regularly threaten communities worldwide. In recent decades significant efforts have been put towards improving our understanding of the mechanisms involved. In particular detailed analysis of satellite imagery and observations from aircraft reconnaissance missions have allowed formulation of a well-accepted framework whereby asymmetries in the TC wind field structure are attributed to the forward motion of the system: stronger winds occur to the right (left) of a moving TC in the northern (southern) hemisphere with the magnitude of the left/right asymmetry increasing as the storm moves faster. Read more.

Disaster Risk Management: Australian Challenges

This article by Andrew Gissing, Risk Frontiers, was published in Asia Pacific Fire Magazine, October 3, 2017.

Australia is exposed to a variety of natural and technological disaster risks, which vary in their significance across the nation. Communities are faced with the increasing costs of disaster losses due to higher wealth and the increasing development of hazardous areas, whilst Government budgets are under pressure. Climatic, demographic, economic, political and technological changes are acting to shape future disaster risks.

Internationally, the Sendai Framework for Disaster Risk Reduction exists with the goal to: “prevent new and reduce existing disaster risk through the implementation of integrated and inclusive economic, structural, legal, social, health, cultural, educational, environmental, technological, political and institutional measures that prevent and reduce hazard exposure and vulnerability to disaster, increase preparedness for response and recovery, and thus strengthen resilience”.  Read more.

Could Sydney be the next Houston?

Andrew Gissing and Dr Chas Keys.

Devastating floods have occurred last month in Texas, USA, inundating large parts of America’s fourth largest city, Houston, as a consequence of Hurricane Harvey. Thousands of people have required rescue, and as of the 14th of September 82 people had died. It had been some ten years since a large hurricane had crossed the US Gulf Coast. Much of the blame for the disaster is being placed on the significant increases in urban development in flood liable areas.

According to research by the Bushfire and Natural Hazards Research Centre and Risk Frontiers, flooding in Australia has been the second largest contributor to natural hazard deaths since 1900, behind fatalities attributed to heatwaves. The Productivity Commission in 2014 reported that Australian floods have also contributed over five billion dollars in damages between 1970 and 2013.

Is Australia’s largest city, Sydney, prone to similar catastrophic flooding scenarios in the future? Sydney has a different geography and climate to Houston but has numerous populated river and creek catchments that have experienced flooding historically, but not for some time. A possible Sydney flood scenario would see heavy rainfall from perhaps a severe east coast low pressure system first drenching the city’s impervious streets and small creek and river catchments, resulting in significant stormwater and flash flooding. This flooding would occur with little specific warning, but rapidly subside. The greater metropolitan area has seen significant flash flooding before as a result of severe rainfall in 1984, 1986 and 1988. Areas that could be impacted include the northern beaches, eastern suburbs (Randwick and Rose Bay), the Inner West (Marrickville, Strathfield, Canterbury and Annandale), Parramatta, Ryde, Woronora and Fairfield areas. The impacts would significantly disrupt the city’s transport systems and undoubtedly lead to countless flood rescues.

Following initial flash flooding, rivers could rise to severe levels, threatening communities along the Georges, Hawkesbury and Nepean Rivers, necessitating large scale evacuations across the southwest and western parts of the city. These areas were hit by severe flooding in 1867, and experienced major flooding in the 1980s and 1990. If similar flood levels to 1867 were to occur today, over 11,000 homes could be flooded in western and south western Sydney, their occupants requiring evacuation. Essential infrastructure would be damaged resulting in disruption to transport, energy, water supply and businesses for days to weeks following. It would take those affected years to recover. More extreme flood events are also possible.

Other communities outside of Sydney could be impacted too. The 1867 event also affected Wollongong, Nowra, Moruya, Tamworth, Bathurst, Mudgee, Dubbo, Forbes and Wagga Wagga (Yeo et al., 2017). Such a scenario would place significant demand on the state’s emergency services.

Unlike the current experience of Houston, local governments in New South Wales do regulate development of flood prone areas with a policy of ensuring new development is limited to areas outside zones which would likely be flooded, on average, once every hundred years. This standard, however, does not provide immunity against larger flood events, which may include some high risk areas and many properties.

There are still large legacy issues in areas that have already been developed. The NSW Government has recently released a flood management strategy to tackle flood risk along the Hawkesbury and Nepean rivers. In other areas, local government complete floodplain risk management plans detailing intended methods to address flood risk.

Flood risk management is a game of balance, requiring the careful management of what can often be competing objectives of building prosperous communities where there are economic opportunities versus maintaining public safety and the resilience of the community. To avoid the amplification of flood risk and suffering experienced in Houston, land use planning controls will remain key.

It has been some time between large floods in Sydney, with communities becoming largely apathetic towards the risk. It is, however, inevitable that they will return. The risk is serious, requiring both prudent flood risk management by governments and action by individuals to ensure household and business preparedness.

For more information please contact Andrew at


YEO, S., BEWSHER, D., ROBINSON, J. & CINQUE, P. 2017. The June 1867 floods in NSW: causes, characteristics, impacts and lessons. Floodplain Management Australia National Conference. Newcastle, NSW.