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Raising the wall won’t eliminate the flood risk: Sydney’s Warragamba Dam
“The valley could fill like a bathtub in the right conditions, with water levels rising by 50cm an hour for several hours and topping a metre an hour...”
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by up to five meters, so there is more space to store floodwater, and changing the operation of the gates. But these actions would have a minor impact that is helpful only in small floods. Dredging and diversion projects are rejected because they would cost as much as raising the dam while providing half the benefit and having large environmental effects. The review finds limited potential for local mitigation using levees because of the extreme depth of flooding in the valley, particularly in the Richmond-Windsor area. Complexity is created by the region’s geology. The Hawkesbury-Nepean Valley is different from other Australian coastal floodplains because large upstream catchments and narrow downstream sandstone gorges can cause water to back up behind natural choke points. The valley could fill like a bathtub in the right conditions, with water levels rising by 50cm an hour for several hours and topping a metre an hour for short periods. A house on the lower areas of the floodplain could be submerged in six hours or less. The river reached 19.7 metres above sea level at Windsor during the worst floods on record in 1867, while sediment analysis shows at least one flood before European settlement was a one-in-1000 year event. The potential for fast-rising water, the size of the population and low-lying escape routes mean that to get everyone out in time, evacuation would have to start based on uncertain rainfall and river forecasts, and before residents can clearly see the danger. Hawkesbury-Nepean roads have areas insuranceNEWS
June/July 2014
that are cut off before higher inhabited locations are inundated. It leads to isolated “islands” that could flood in extreme events, endangering the lives of people who delay their escape. “Most people are reluctant to do anything until they see the water coming through the front gate or the fire over the hedge,” Professor McAneney says. If the 1867 flood happened today, about 45,000 people would need to be evacuated, damage costs could reach $4 billion and economic impacts would be far-reaching. In a worst-possible flood around 73,000 people would have to be removed from the danger and more than 20,000 homes would be at risk of failure, the report says. The challenges are rising as Sydney’s hinterland attracts more residents. The area’s population has grown 67% since 1997 and more than 70,000 people are now living in flood-prone areas. The draft Metropolitan Strategy for Sydney says the west sub-region, mostly within the Hawkesbury-Nepean Valley, will have a population of 89,000 by 2031. Despite the risks, the community does not see flooding as particularly life-threatening. Fewer than 10% of people have a flood plan and 20% are unlikely to evacuate if directed to do so, surveys show. Richmond, Windsor, South Windsor, Bligh Park, Pitt Town and McGraths Hill are among towns that could become flood islands during large events, the report says. The paper uses the example of Windsor, where people would need to flee a major flood before the Jim Anderson Bridge is cut. The State Emergency Service says an evacuation could take 25 hours, a