Could Sydney be the next Houston?

By Andrew Gissing. This article first appeared in Issue One 2018 of Fire Australia.

It had been some ten years since a large hurricane had crossed the US Gulf Coast, but Hurricane Harvey hit Houston, America’s fourth largest city, hard in August last year. Devastating floods inundated large parts of the city. Thousands of people required rescue, and the death toll was more than 90 people. Much of the blame for the disaster is being placed on the significant increases in urban development in flood liable areas. 

According to research by the Bushfire and Natural Hazards CRC and Risk Frontiers, flooding in Australia has been the second largest contributor to natural hazard deaths since 1900, behind fatalities attributed to heatwaves. The Productivity Commission in 2014 reported that Australian floods have also contributed over five billion dollars in damages between 1970 and 2013.

Is Australia’s largest city, Sydney, prone to similar catastrophic flooding in the future? Sydney has a different geography and climate to Houston but has numerous populated river and creek catchments that have experienced flooding historically, but not for some time. A possible Sydney flood scenario would see heavy rainfall from perhaps a severe East Coast Low pressure system first drenching the city’s impervious streets and small creek and river catchments, resulting in significant stormwater and flash flooding. This flooding would occur with little specific warning, but rapidly subside. The greater metropolitan area has seen significant flash flooding before, as a result of severe rainfall in 1984, 1986 and 1988. Areas that could be impacted include the northern beaches, eastern suburbs (Randwick and Rose Bay), the Inner West (Marrickville, Strathfield, Canterbury and Annandale), Parramatta, Ryde, Woronora and Fairfield. The impacts would significantly disrupt the city’s transport systems and undoubtedly lead to countless flood rescues.

Following initial flash flooding, rivers could rise to severe levels, threatening communities along the Georges, Hawkesbury and Nepean Rivers, necessitating large scale evacuations across the south west and western parts of the city. These areas were hit by severe flooding in 1867, and experienced major flooding in the 1980s and 1990. If similar flood levels to 1867 were to occur today, over 11,000 homes could be flooded in western and south western Sydney, their occupants requiring evacuation. Essential infrastructure would be damaged resulting in disruption to transport, energy, water supply and businesses for days to weeks following. It would take those affected years to recover. More extreme flood events are also possible.

Other communities outside of Sydney could be impacted too. The 1867 flood also affected Wollongong, Nowra, Moruya, Tamworth, Bathurst, Mudgee, Dubbo, Forbes and Wagga Wagga. Such a scenario would place significant demand on the state’s emergency services.

Unlike last years’ experience in Houston, local governments in New South Wales do regulate development of flood prone areas with a policy of ensuring new development is limited to areas outside zones which would likely be flooded, on average, once every hundred years. This standard, however, does not provide immunity against larger floods, which may include some high risk areas and many properties.  

There are still large legacy issues in areas that have already been developed. The NSW government has recently released a flood management strategy to tackle flood risk along the Hawkesbury and Nepean rivers. In other areas, local government complete floodplain risk management plans detailing intended methods to address flood risk.

Flood risk management is a game of balance, requiring the careful management of what can often be competing objectives of building prosperous communities where there are economic opportunities versus maintaining public safety and the resilience of the community. To avoid the amplification of flood risk and suffering experienced in Houston, land use planning controls will remain key.

It has been some time between large floods in Sydney, with communities becoming largely apathetic towards the risk. It is, however, inevitable that they will return. The risk is serious, requiring both prudent flood risk management by governments and action by individuals to ensure household and business preparedness.