There will always be a risk of flood in Brisbane. There will always be a risk of flood on the Gold Coast, a city built on a flood plain and at the convergence of a number of tropical river systems.
But it is western Sydney, say emergency management and flood experts, that might be the most vulnerable area to floodwater in the country, at least in its potential to claim life.
''Absolutely it is the biggest flood risk in the state,'' says Steve Opper, the director of community safety at the State Emergency Service of what is known as the Hawkesbury Nepean flood plain.
''It is probably close to the worst flood risk in Australia.''
It might seem strange to hone in on a hypothetical flood in the suburbs of Sydney at the end of a week of heartbreak in northern NSW and Queensland.
But a combination of political factors and a general heightened sensitivity to risk has served to revive interest in what might happen if masses of rain concentrated outside Sydney - and the controversial measures some propose to deal with it.
Archaeological records speak of bigger floods before European settlement, but the largest recorded event in the Hawkesbury Nepean - which includes Windsor and Richmond, and parts of Emu Plains, Penrith, Blacktown and the north-west suburbs - struck in June 1867.
Settlements at Windsor, Riverstone, Pitt Town, and parts of Blacktown were submerged or shrunk to tiny islands in an inland sea.
''Everybody is astonished at the tremendous accumulation of water, and it will seem incredible to all who have not actually seen it,'' the Herald reported at the time.
''Places which since the settlement of the colony have never been known to be flooded are now lost to view.''
The 1867 flood may have been the worst in recorded history, but there have been many lesser since. Between WWII and 1990, floods struck the area at least once a decade.
The absence of a serious flood in the past 20 years, therefore, is more historical anomaly than norm, and a survey of the geography of the area helps explain why.
The Hawkesbury Nepean flood plain sits as the receptacle of a vast water catchment, stretching from south of Goulburn to the Southern Highlands and the Blue Mountains.
This catchment sends rainwater downstream or north through a number of rivers, including the Hawkesbury. But about half-way between Windsor and Wisemans Ferry, at Sackville Gorge to the north-east of Sydney, the path of the Hawkesbury narrows dramatically. In a flood, the water flow stalls, where it ''ponds'' or spills back downstream into the plains of Sydney's west.
An often-used metaphor to describe the dynamic is a bath: water is released into the flood plain or bath tub, but its one drainage point - at Sackville - can't do the job of getting it out.
This geographical quirk endows the area with its unique risk property: waters in the valley can rise very high, cutting off roads, submerging or making islands of whole neighbourhoods.
Water at the Windsor Bridge, for instance, sits generally at about 1.5 metres. The 1867 flood would have reached about 19.2 metres on the bridge's gauge. The probable maximum flood would be about 26 metres. ''The risk to life is so much greater because of the potential depth of flooding,'' says Opper.
After the August 1990 flood, in which water at Windsor rose to 13.4 metres, the potential for a 1867-esque catastrophe sat firmly on the political agenda. (In the mid-1990s, studies showed that a one-in-a-100-year flood at Windsor could be produced by 340mm of rainfall over three days on the catchment; a one-in-a-200-year event about the size of the 1867 flood could be produced by an average of 380mm rain in three days on the catchment.)
Concerns about the flood risk were also tied to a justified fear that Warragamba Dam - Sydney's main water supply - could fail in the event of major pressure. By 1993, the then chief executive of Sydney Water, Paul Broad, wanted to tackle the structural integrity of the dam and its potential use for flood mitigation.
The Fahey government supported the idea and an Environmental Impact Statement was prepared recommending thickening the wall, extending a spillway to release pressure from the dam and raising its height by 23 metres. Lifting the wall would help mitigate a flood, but not prevent one occurring.
It also would have consigned vast areas of upstream wilderness area - land that contains a rare Camden White Gum - to destructive periods of inundation in the event of serious rainfall.
The result was a powerful environmental movement against the proposal. As opposition leader, Bob Carr went for a bushwalk along the lower Kowmung River to look at the area potentially threatened by the flood. And on becoming premier, he kept the plan to strengthen the dam but shelved the idea of lifting the wall.
But that plan is now back on the agenda, partly because Broad is now chief executive of Infrastructure NSW and has told the O'Farrell government to consider lifting the wall.
And it is also back because the de facto plan - to improve roads in the flood plain to allow evacuation in case of an emergency - has been over-run by time and development.
One study says 22,000 people may not be able to get out in a flood because roads would be clogged.
''If we just apply all of our arrangements we currently wouldn't be able to get people out,'' says Opper.
The O'Farrell government says it is conducting a review into its options for the Hawkesbury Nepean.
But is there a chance a flood could hit Sydney, endangering the lives of thousands and destroying hundreds of millions of dollars in property? Sure there is.
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