The ‘levee paradox’: why has development in floodplains exploded?

Apr 7, 2021

In March we were reminded that flooding is a problem in Australia. It is a problem partly because of the way we accommodate our growing population on floodplains.

For decades we have been among the fastest-growing countries in the Organisation for Economic Co-operation and Development that club of developed nations with which we like to compare ourselves when checking on our economic and social ‘development’. On both sides of Australian politics strong population growth has been sought, and we have pursued pro-immigration policies to achieve it.

The results of high immigration have been several. Our major metropolitan areas, Sydney and Melbourne, have grown apace, doubling their populations over the past 40 years, and Brisbane, Perth, Newcastle, Canberra, the Gold Coast and other cities have also grown strongly. The states have had to accommodate the growth, and they to a degree have passed the responsibility for managing the creation of the necessary housing stock to the councils of local government which have accepted the task with enthusiasm. Embracing population growth has meant more business for their commercial sectors and more ratepayers’ contributions for their own coffers.

The emphasis on growth has also meant more stress on land resources, especially on the fringes of the metropolitan areas which, despite state government efforts to encourage higher densities (‘consolidation’) in their established inner-city and suburban areas, have continued to be major foci of population growth. In some of these areas the growth is or will become problematic because they are not well suited as residential environments. The problem for some is flooding.

Let’s take two areas of many in Australia in which this problem applies. They are the valley of the Hawkesbury-Nepean river system, in north-western Sydney, and Maitland which lies on the edge of the expanding Hunter City centred on Newcastle.

Both have large tracts of land which are liable to flooding, respectively from the Hawkesbury-Nepean and Hunter rivers. Both are also targets and magnets for rapid residential growth, in the Hawkesbury on very large greenfield tracts formerly given over largely to agricultural land uses and in Maitland on land which has been built on for decades but which over recent decades has declined in population and is now seen as ripe for re-development. Both areas have experienced disastrous flooding in times past and both have strong local council backing to cater for substantial influxes of new residents.

Flood prone ‘inner’ Maitland, a site of some of the earliest European settlement outside the Sydney Basin, had a population of approximately 5400 in 1954 notwithstanding a long history of periodic inundation, frequent evacuation to nearby higher ground, financial loss, demoralisation and painfully distressing clean-up operations. In 1955 the area was hit by a particularly severe flood, and development thereafter was severely restricted for some decades by state regulations and council policies.

By 2006 the population of the area was down to 1800, scores of houses had been bodily removed to higher ground and some of the dwellings that remained were nearing the ends of their useful lives. In the meantime, the levees which protected inner Maitland, built by residents and the council over several decades, had been rebuilt to higher standards by the state government.

In 2008 the Maitland City Council commissioned a report, the Central Maitland Structure Plan, which recommended that the area be redeveloped for a population of the scale of the years before 1955. The council, worried by signs of decline and decay in its heritage-rich, architecturally attractive Central Business District, assailed by competition from modern planned shopping centres in the suburbs, accepted the recommendation as a means of stimulating the CBD. In 2010 it embarked on a policy designed to stimulate new housing development and bring people back to the old core.

But little happened. Growth did not take off. So, after a few years, the council petitioned the state government to relax the regulations which are intended to improve the safety of housing on floodplains. Where the regulations prohibited the building of residential floors below the height reached by the 1955 flood, the council sought a modification whereby up to half of the defined ‘living space’ in new dwellings could be below this level. The council also sought to mandate ‘hitching’ facilities for flood boats at the first-floor level, ostensibly to make it easier for residents to be evacuated during floods.

The New South Wales Minister for Planning rejected both proposals. A decade after the council adopted the goal of enhancing growth in ‘old’ Maitland, little has been achieved.

There is a silver lining to this cloud. It is that few additional people have been placed in the way of potential flooding. Central Maitland’s levees have been built to various heights, relative to flood levels, but some of them are designed to be overtopped in a 2% Annual Exceedance Probability (on average once every 50 years) flood. Such a flood would see much of inner Maitland inundated.

The council’s preferred approach to development, as encapsulated in its attempts to persuade the minister to weaken the building regulations, could only guarantee that the community’s vulnerability to floods was increased. Worse yet, redeveloping the floodplain for more houses and accompanying infrastructure guarantees that the so-called ‘levee paradox’ will strike at some stage.

The paradox is well known in areas with histories of inundation. It works like this: the people petition government (for example, their council) for a levee, when it is built there is confidence that the flood problem has been solved or contained and there is increased impetus for investment in the areas now ‘protected’. Then the levee is overtopped in a flood which exceeds its design standard, and the economic losses are greater than would have occurred had the levee not been built and in addition, often intensified development been encouraged in the first place.

Given that virtually all levees are eventually overtopped the economics and aesthetics of levee construction almost always militate against them being built high enough to keep out occasional large floods the levee paradox is bound to be demonstrated at some stage. A more populated inner Maitland, if it eventuates, will one day pay in heavy financial losses and possibly in lives.

A parallel looms in the Hawkesbury. There, the state government wants to raise Warragamba Dam by 14 or 17 metres to give protection to downstream areas from flooding. Arguably the government has exaggerated the degree to which floods (including floods much larger than have just been seen on the Hawkesbury) can be protected by raising the dam. But worse still, the Minister for Emergency Services (David Elliott) indicated as the recent floods were receding that raising the dam will make it possible for more land to be released for development on the floodplains downstream. More growth, more developers’ profits, more revenue and more vulnerability for residents.

The spectre here is precisely as is embodied in the levee paradox. Big floods, brought about by intense rainfall largely over the sub-catchments of rivers not controlled by the dam, will occur in the ‘protected’ areas. To the degree that housing and other development is enhanced by the promise of mitigation, there will be more assets exposed to damage and loss and more lives risked too.

Dangerous paths are being followed in Maitland and in the Hawkesbury. Ill-considered flood mitigation and land use development strategies will ensure that flood losses increase in the future. It should not be thus. We have to do better at accommodating our growing population.

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