1 in 100-year rainfall
causes flooding in Sydney

Torrential rain fell in Sydney on 28 November 2018, with reports of more than 100mm of rain falling across various areas of Sydney in less than two hours, inundating parks, roads and houses (The Sydney Morning Herald, 2018).

The amount of rainfall was comparable to the average rainfall that Sydney receives in the month of November- the historical average and median for November is 84.2mm and 66.9mm respectively (Reuters, 2018; Bureau of Meteorology Australian Government). Wind gusts of 96km/h were also reported at Wattamolla during the passage of the storm (The Sydney Morning Herald).

Figure 1: An extract of the JBA 1 in 100-year Australia 30m River Flood Map (in blue) and JBA 1 in 100-year Australia 30m Surface Water Flood Map (in purple). The area illustrated is Sydney, Australia. The 2018 Sydney flood is estimated to have a return period of 100 years by the Australian Government Bureau of Meteorology. (JBA Risk Management Limited™)

Two deaths have been attributed to the storm as it hit during morning rush hour. It resulted in chaos for many commuters, causing widespread disruption. The flood water and fallen trees resulted in power failure for several thousand households (The Sydney Morning Herald).

Figure 2: Accumulated rainfall from 27 November to 3 December 2018 from observation stations in and around Sydney, New South Wales, Australia. The majority of rainfall fell in less than two hours in the morning of 28 November 2018. Data source: Bureau of Meteorology Australian Government. (JBA Risk Management Limited™)

Figure 3: Daily precipitation recorded at Sydney Observatory Hill station for the month of November, 1858-2018. The 2018 Sydney storm saw a total of 105mm of rainfall within 24 hours. Data source: Bureau of Meteorology Australian Government. (JBA Risk Management Limited™)

Initial observations suggest this was the wettest day since November 1984, when rainfall totals of 235mm were recorded (the highest since 1858), and the first day since that event to achieve rainfall totals in excess of 100mm (ABC; Bureau of Meteorology Australian Government). The Observation Hill gauge in Sydney recorded 105mm of rainfall in less than two hours as a result of the 2018 storm. Based on historical data from the gauge, this would translate to a 24-hour rainfall return period of 20 years (calculated by applying the Generalised Pareto Distribution curve to the historical daily data for the month of November, as shown in Figure 4). 

Figure 4: Estimated 2018 storm return period for Sydney based on historical rainfall data between 1858 and 2018, obtained from Sydney Observatory Hill station. A Generalised Pareto Distribution (GDP) curve has been fitted to the historic data to give an estimation of the storm return period. Data source: Bureau of Meteorology Australian Government. (JBA Risk Management Limited™)

Table 1: A list of days that received more than 100mm of rainfall in the month of November between 1858 - 2018. Data source: Bureau of Meteorology Australian Government. (JBA Risk Management Limited™)

However, because the downpour occurred within a two-hour period, the event is estimated to have a 1 in 100-year return period by the authorities (Bureau of Meteorology Australian Government; The Sydney Morning Herald; Reuters, 2018). Insured losses for the storm are thought to be relatively low in comparison to other flood events of 2017 and 2018. Claims for the 2018 Sydney flood are estimated to be around AUD $13.5 million (USD $9.96 million), with the majority of these claims relating to damaged roofs and vehicles (InsuranceNEWS). 

Major flood losses in Brisbane, 2011

In terms of historical context, within the last decade, the most severe flooding in Australia has occurred in south-east Queensland in late 2010 to early 2011. Thousands of people living in the Toowoomba and Ipswich districts of Brisbane were evacuated, with total insurance pay-outs reaching over AUD $2.4 billion and total economic damage estimated at AUD $10 billion (Reuters, 2011). The floods occurred due to persistent heavy rainfall and a diminishing Tropical Cyclone Tasha in December 2010. At the same time, Australia was experiencing a strong La Nina event, which is typically associated with high rainfall and floods in eastern Australia. The floods claimed 44 lives and more than 200,000 people were affected (Munich Re). An estimated 18,000 properties were inundated in the affected regions. The mining, agriculture and tourism industries were amongst the hardest hit as losses in the Central Business District of Brisbane reached AUD $4.46 million.

However, from a hydrological perspective, the maximum flood level recorded during the 2011 floods was actually lower than the 1893 and 1841 floods, both of which peaked at 8.35m (Zurich Australian Insurance Limited).

Figure 5: A summary of past events that occurred in Queensland state, Australia, and the overall losses incurred by each event. The most significant event is the 2011 floods in Brisbane. Out of the AUD $6.94 billion overall losses, only AUD $2.45 billion was insured for. Data source: NatCatSERVICE, 2018; Sigma Explorer, 2018. (JBA Risk Management Limited™).

An overview of flood losses in 2018

Since the Brisbane floods of 2011, far more infrastructure has been developed to help mitigate and reduce flood risk in Australia. Insurance markets are estimated to have covered approximately 50% of the economic losses incurred by these floods. In 2018 alone, the total insured losses caused by floods and tropical cyclones has been estimated at AUD $261 million (USD $191 million) (Insurance Business Australia). Out of the total losses, the storm that hit Hobart and the surrounding suburban areas in Tasmania resulted in total claims of approximately AUD $100 million ($USD 73 million) (Insurance Council of Australia). In early 2018, Tropical Cyclone Marcus hit Darwin and resulted in insured losses totalling AUD $62 million (USD $46 million). Tropical Cyclone Marcus was named as the strongest cyclone in Darwin in the last 30 years (The Guardian). In comparison, the total insured losses for the Sydney storm is relatively lower, at an estimated AUD $13.5 million (USD $9.96 million).

JBA Risk Management has just released nationwide return period flood maps for Australia at 30m resolution. Please get in touch for further information.

References 

BBC, 2018, 'Sydney storms: Two killed amid flash-flooding chaos', viewed 3 December 2018

Clun, R., Keoghan, S. and Noyes, J., 2018, 'Two dead as heavy rain and storms cause commuter chaos in Sydney', The Sydney Morning Herald, viewed 3 December 2018

Davidson, H., 2018, 'Cyclone Marcus leaves tens of thousands in Darwin without power or drinkable water', Reuters, viewed 3 December 2018

Hancock, R. and Rea, M., 2018, 'Australian Storms and Floods: White Paper', Zurich Australian Insurance Limited, viewed 3 December 2018

Insurance Council of Australia, 2018, 'Tasmanian Severe Weather May 2018', viewed 3 December 2018

InsuranceNEWS, 2018, 'Claims low after giant Sydney rainstorm', viewed 3 December 2018

InsuranceNEWS, 2018, 'Sydney storm claims climbing, but Queensland firefighters keep losses low', viewed 3 December 2018

Keoghan, S., 2018, 'Sydney's 'one-in-100-year' storm causes $10 million in damages: insurers', The Sydney Morning Herald, viewed 3 December 2018

Martin, M., 2018, 'Australian catastrophe and weather losses hit $261m', Insurance Business Australia, viewed 3 December 2018

Mckinnell, J., 2018, 'Sydney weather sparks road chaos, as ‘mini cyclone’ blows part of building off, ABC News, viewed 3 December 2018

Munich Re, 2018, 'Flood facts: The biggest floods 2005–2015', viewed 3 December 2018

NatCatSERVICE, 2018, Munich Re, viewed 3 December 2018

Packham, C., 2018, 'Once-in-100-year' storm triggers Sydney chaos as heat fans Queensland fires', Reuters, viewed 3 December 2018

Sigma Explorer, 2018, Swiss Re, viewed 3 December 2018

Smith, M., 2011, 'Australia says economic cost of floods to top other disasters', Reuters, viewed 3 December 2018

Weir, B., Clun, R. and Noyes, J., 2018, 'Some of the worst I've seen': Sydney has wettest November day since 1984', The Sydney Morning Herald, viewed 3 December 2018