Rains stir familiar lament on lack of preparation

A return of flooding rains that have inundated towns and homes along the east coast of Australia demonstrates how poor planning can exacerbate the dangers and squander the opportunities that natural events can bring. Bad design, inadequate infrastructure and unchecked urban expansion increase the cost of disasters. A lack of foresight to prepare for events that occur on a regular if not totally predictable basis means that precious resources go to waste. It is a reasonable lament that when big rains come, billions of litres of water are left to flow out to sea, often causing great damage along the way. Too little effort is made to harvest floodwater for use at a later date. The quality of public infrastructure built in flood-prone regions often is not sufficient for the task. As Floodplain Management Australia president Ian Dinham writes on Tuesday, the current flooding in the Hawkesbury-Nepean area in western Sydney is not unexpected as various studies and reports have highlighted the extreme flood risks since the 1990s.

Nonetheless, the people of the Hawkesbury-Nepean Valley will be left with the heartache and repairs when the television cameras and politicians move on as the threat from the latest flooding episode subsides. Thousands of households, businesses and livelihoods are at unnecessary risk because development has been allowed in areas known to flood. In the Hawkesbury-Nepean Valley there are 5000 houses built below the one-in-100-year flood level and a further 7000 below the one-in-500-year flood level. Current flooding is estimated to be a one-in-50-year event in Sydney and a one-in-100-year event on the NSW mid-north coast. Flooding has been severe enough to spark the evacuation of more than 18,000 people and for 38 areas of the state to be declared disaster zones.

Before the floodwater had peaked, debate started about NSW government plans to allow about 134,000 more people to settle in Hawkesbury-Nepean Valley flood plain by 2050. This development is contingent on a plan to raise the Warragamba Dam wall to transform the dam from water storage only to flood mitigation as well. A report by Infrastructure NSW says the raised wall would mitigate floods by creating “airspace” in a dedicated flood mitigation zone around 14m above the current full water supply level. The flood mitigation zone behind the wall would be used to capture and temporarily hold back floodwater coming from the large Warragamba catchment. After the flood peak, floodwater would be released in a controlled way, reducing flood levels downstream and alleviating potentially devastating impacts on communities. The mitigation zone would be used only during floods. The current water supply level would not change. Engineering estimates are that increasing the dam wall would mitigate nearly 100 per cent of the risk to homes from the most serious floods estimated to happen once in 200 years.

Questions are being asked about why more water was not released from Warragamba Dam in anticipation of the most recent rains. The answer in NSW is that Warragamba Dam was not designed or approved to operate as a flood mitigation dam. Pre-releasing water ahead of a predicted flood inflow would have limited effectiveness, with minor benefits for small floods and none for larger floods. Lowering the dam wall by 5m would have limited benefits for larger floods and lowering the wall by 12m would provide moderate flood mitigation but have a significant impact on water security for Greater Sydney. Opponents of the plan to raise the dam wall are concerned about the impact it would have on some areas of the World Heritage-listed Blue Mountains National Park. They say raising the wall would not prevent flooding from smaller rivers and creeks below the dam. One option is to remove houses already located in the flood zone and return the draining system to its original state. Another is to trust that the calculated flood mitigation benefits of raising the dam wall make it a preferable option for existing houses as well as offering the potential to ease development congestion pressures in Australia’s biggest city.

Whatever that decision may be, there is a demonstrated need to improve the quality and durability of urban and rural infrastructure to cope with the impacts of heavy weather. One recommendation of the Royal Commission into National Natural Disaster Arrangements was to establish a standing entity that would enhance national natural disaster resilience and recovery, focused on long-term disaster risk reduction. Scott Morrison announced in November last year that the new agency initially would incorporate the functions of the National Bushfire Recovery Agency, established in response to the 2019-20 Black Summer bushfires, and would integrate the functions of the National Drought and North Queensland Flood Response and Recovery Agency, and the disaster recovery and risk reduction functions within the Department of Home Affairs. Disasters aside, efforts must be focused on building infrastructure that can withstand harsh conditions and harvest the abundance of water during periods of heavy rain and flood so it can be used when conditions inevitably turn to drought.