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.