Pearls and Irritations

The floods on the eastward-flowing rivers of New South Wales have abated, but when they were at their height there were some alarming differences between state government ministers on the important matter of development on the floodplains of the Hawkesbury-Nepean River system.

Worryingly, the mainstream media failed to pick up the inconsistencies and their relationship to the contentious matter of the potential raising of Warragamba Dam as a mitigation measure to protect downstream communities from floods.

David Elliott, the Minister for Emergency Services, casually suggested on Triple M radio that raising the dam would “probably” allow more land downstream to be released. He may have been suggesting that the intended mitigation benefit could mean that land below the currently-assessed 1% AEP (Annual Exceedence Probability) flood level (17.3 metres at the Windsor gauge) might be made available for residential development. Elliott’s was a careless, dangerous remark given that raising the dam will provide only modest mitigation in genuinely severe floods.

Stuart Ayres, the Minister for Western Sydney, wants to raise the dam. He also supports the government’s plan of a near-doubling of the population in the Hawkesbury valley by 2050. The extra 120,000 people will live in dwellings whose floor levels will presumably be above the 1% flood level but many (perhaps all) will be within reach of bigger floods which can reach much higher levels.

Ayres’ and Elliott’s cabinet colleague, Minister for Planning Rob Stokes, takes a strong stand and a completely different approach. He announced a “pause” on development while the “evacuability” of what is already there is assessed. Stokes recognises a serious problem which others in the government seem to have ignored: the need for better evacuation infrastructure to allow all residents to escape before a very big flood engulfs them. Ayres, late in the flood, was quoted in the media as saying that investment in additional evacuation routes might be necessary in addition to raising the wall. A costly dam raising and costly road investment as well, then, to support massive residential development which will inevitably experience flooding at some stage. This might not seem totally sane or economically ideal to everybody.

Let’s look at the word “floodplain” and the risk of floodplain land being flooded on occasions in the future. Just how much danger is courted? In the case of the Hawkesbury area around Windsor, the answer to that question is both surprising and frightening.

A river’s floodplain can be defined as the area which would be flooded by the highest flood thought possible on that river. Windsor lies on seemingly high ground between the Hawkesbury River and the tributary South Creek. But it is built on a “crinkly” floodplain which is far from flat. Windsor’s soil is alluvial, laid down over the aeons in floods bigger than the biggest seen there in European times.

With no flooding, the water level at the Windsor gauge on the Hawkesbury River is at about 1 metre. The flood in late March peaked at just under 13 metres, or 12 metres above non-flood flow. Back in 1867, the peak in the worst flood seen over the past two centuries was more than 19 metres (the equivalent of a six-storey building in depth), and vast areas were flooded by “backwater” flowing far up the tributary South and Eastern creeks near which much housing is being built.

Such a flood, occurring now, would inundate areas like Marsden Park and Riverstone where huge numbers of houses have recently been built and are still being constructed. In a bigger flood than 1867’s, Windsor could be completely inundated. Potentially tens of thousands of floors of houses in nearby suburbs would be flooded above (in some cases well above) floor level. With the growth that the government envisages, many more dwellings will be at risk.

The so-called “bathtub effect” caused by the choke in the valley of the Hawkesbury below Sackville is relevant here. Because of the choke, floodwaters upstream can reach very high levels. In 1867 Windsor became an island which would have disappeared with a rise not much beyond that flood’s peak. The residents, and evacuees from lower-lying areas nearby, would have been swept away.

Under current regulations, new buildings in and around Windsor must have their floors at a minimum level equivalent to 17.3 metres (plus 50cm freeboard) at the gauge. Some, even those built to today’s rules, will thus be inundated if floodwaters reach a level somewhat lower than was reached in 1867. Others, built at lower levels before the current regulations were adopted, will be very deeply inundated indeed. Roofs will disappear beneath the floodwaters.

In a flood approaching the highest level thought possible at Windsor (the so-called Probable Maximum Flood, estimated to peak at above 26 metres at the gauge), every building in the town would be inundated. Such a flood would be extremely rare and current residents are most unlikely to experience it, but it should be recognised as possible, even inevitable ꟷ one day. Everywhere, flood records are broken periodically, and Windsor’s alluvial soils are undeniable evidence that higher floods than the highest measured there since European settlement have occurred.

There is absolutely nothing to suggest that such floods will not occur in the future. Indeed, scientists argue that very big floods, say of the scale of the 1867 Hawkesbury flood and larger, will become rather more common in times to come because a warmer atmosphere will hold more moisture to be triggered as rainfall to produce flooding.

It was because of the very high potential reach of floods in and around Windsor that the state Labor government early this century built the high bridge from Windsor to Mulgrave. The intention was to enable everybody to escape before a very big flood struck. Now, though, planned increases in the residential population seem likely to overwhelm the evacuation capacity of the bridge and require the additional provision of escape routes. One could be the long-proposed “Castlereagh Connection” across the valley between Penrith and Windsor.

It is, of course, impossible to fully “depopulate” or “sterilise” the entire floodplain. For one thing, Windsor would have to be abandoned. But it is clear that we should be very wary of allowing further growth in the area, even on the higher parts of the floodplain (those above 17.3 metres). Further land releases would be most unwise. In fact we should consider raising the planning level, perhaps to the height reached in 1867 (19.3 metres) or higher.

The ministers’ prescriptions are inconsistent. Stokes comprehends the danger, Ayres wants no new land released but advocates a dam-raising measure that will reduce flood levels by only modest amounts in very big floods (and proposes a greatly enlarged floodplain population downstream). Elliott flirts with releasing even more land that is certain to be flooded at some stage.

There is recklessness here as well as inconsistency. The government is floundering on an issue that is critical to human life and property in very big floods that will not occur frequently but which are inevitable at some stage. The lesson of 1867, that huge floods well beyond living memory do occur, has not been learned, and disaster is being courted on Sydney’s fringe. Stokes aside, does this show that development and developers’ interests trump public safety in this government’s thinking?

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