New South Wales Floods 2021

On the evening of Monday 22 March, 10 million Australians were under a severe flood warning, with floods forecast to affect most states excluding Western Australia (The Guardian, 2021d). At least 18,000 people were evacuated from New South Wales (BBC, 2021a). As of Wednesday morning (24 March, GMT +10), 17,000 insurance claims had been lodged in New South Wales and South East Queensland (Insurance Council Australia, 2021).

A period of prolonged rainfall in New South Wales (NSW) was firstly caused by a tropical low pressure system off the Kimberly coast in Western Australia, and secondly by a coastal trough – a low-pressure system lying close to the coast and causing localised heavy rainfall (Bureau of Meteorology, 2019). This was followed by a high-pressure system stationed between Tasmania and New Zealand which pushed strong winds towards the coast of NSW for over a week (The Guardian, 2021b).

The extreme weather affected not only the east coast of NSW, but also parts of Queensland including Brisbane and the Gold Coast which received high rainfall over the weekend (20–21 March). During this period Brisbane received more than 100mm of rainfall (The Guardian, 2021c). Mount Seaview received the highest recorded rainfall totals during this event (over a 7-day period from 17 to 23 March), at 907mm. The maximum daily rainfall recorded at this site was 272mm, that is equivalent to an estimated return period of 50-years. The estimated return period for Mount Seaview over 168 hours is 200-years (Table 1)(Design Rainfall Data System, 2016).

In addition to having prolonged period of rainfall, it is also the intensity that is likely to result in such devastating floods. At Mount Seaview, the total amount that fell during this period of 907mm is four times the mean rainfall for March (243.5mm) since 1949 (Bureau of Meteorology).

Major cities along the east coast such as Sydney, Wollongong, Grafton and Newcastle were placed under warning as heavy rainfall continued on Monday and Tuesday. The heavy rainfall only started to taper off around Tuesday. Even though the weather systems that caused an excessive amount of rain to fall on NSW have passed, the rivers can still continue to swell and pose a flood threat to residents. As of Wednesday, flood warnings were still in place for the Clarence, Hawkesbury, Nepean, Colo, Macintyre and Gwydir rivers (The Guardian, 2021f). Based on insurance claims that were filed by Tuesday 23 March, the worst affected areas included Kempsey, Port Macquarie, Laurieton, Taree, Penrith, Richmond and Windsor. The last three are located in the Hawkesbury-Nepean basin located to the north-west of Sydney (The Guardian, 2021e).

Figure 1: Spatial distribution of total rainfall received across New South Wales, Australia between 17 and 23 March 2021 (9am GMT +10). Return period based on 7-day rainfall total at the selected sites illustrated in the figure. Mount Seaview gauge site received one of the highest amounts of rainfall over the 7-day period, at 907mm. [Source: NASA PPM, 2021; processed by JBA Risk Management.]

Impacts by town/city

Kempsey

By Wednesday, Kempsey town was declared safe as the flood water levels on the Macleay River receded (The Guardian, 2021f). Having earlier peaked on Saturday at 6.5m. The estimated return period based on the river flood height was 1 in 13-years (Table 2) based on river gauge data gathered by JBA. Rainfall totalled 740mm over the 7-day period and the return period is equivalent to a 1-in-30-year event. Based on the maximum 1-day rainfall of 174.6mm, the return period is 1-in-5-year (Table 1)(Bureau of Meteorology, Australia).

Port Macquarie, Laurieton and Taree

Figure 2: Flooded areas as identified in aerial video footages near Gordon Street in Port Macquarie; JBA 30m Australia 1-in-50-year river flood map illustrates the extent of the event. [Source: Images of flooded regions: https://www.youtube.com/watch?v=NO7_S54okbU; Map: JBA Risk Management.]

The Hastings and Manning River run through Port Macquarie and Taree, which were inundated over the weekend. In Kendall, a town south of Port Macquarie, 700mm of rain was recorded in four days (The Sydney Morning Herald, 2021a). Many news outlets have described the floods in Port Macquarie, Taree and nearby towns as a 1-in-100-year event (The Guardian, 2021a). From drone video footage, and images captured by the public, it can be seen that there were significant floods in north Port Macquarie, at Settlement Point (0.5 – 1.5m), at Shoreline Drive, and at The Anchorage. Many houses along the river channels and bays were built on elevated ground, and were not inundated.

The estimated return period of the flood event in Port Macquarie and Laurieton based on 168-hour rainfall period is approximately 1 in 30-years (Table 1). Port Macquarie received approximately 497mm of rainfall between 17 and 23 March, with the maximum daily rainfall being 174.6mm, with a return period estimate of 5-years (Bureau of Meteorology). The estimated return period based on 168-hours rainfall, is in agreement with observations made at specific sites and JBA’s 30m Australia Flood Maps.

In Laurieton, houses at Camden Haven were inundated, with flood depths estimated to be between 0.5m and 1.2m based on videos and images captured (Wauchope Gazette, 2021). At Camden Haven river flows were estimated to be approximately 1 in 50-years. River flood height peaked at Camden Haven River on Saturday morning (20 March), at two locations. At Laurieton, water level peaked at 2.47m and has exceeded the major flood threshold of 1.7m. At North Haven, water levels peaked at 1.69m, just slightly below the major threshold of 1.7m (Bureau of Meteorology)(Table 2).

Figure 3: (Top) JBA 30m 1-in-50-year Australia river flood map on the west of Laurieton. (Bottom) The orange shaded regions represent flooded areas captured by Sentinel-1 satellite data; the flooded regions were separated to distinguish between standing water bodies and areas inundated during the March 2021 floods. [Source: Copernicus Emergency Management Service (© 2021 European Union), [EMSR504] Laurieton: Delineation Map, Monitoring 2; JBA Risk Management Flood Maps © 2021.]

Windsor, Richmond and Penrith (north-west of Sydney)

River heights peaked on Wednesday morning (24 March) at the Hawkesbury River, near Richmond and Windsor, in the Hawkesbury-Nepean catchment north-west of Sydney. The Hawkesbury River at North Richmond peaked at 14.05m on Sunday afternoon (21 March); that is almost 4m above the major river height of 10.5m. The estimated flow at this location is estimated to be 6,717cumec, and has a return period of 46 years based on the river discharge (Table 2). The previous maximum river flow recorded at this site was 5,818cumec recorded on 1 May 1988 (Bureau of Meteorology).

Floods levels observed at Windsor (2021) might be similar to 1990 records (The Sydney Morning Herald, 2021b). Severe flooding occurred near the newly-built bridge in Windsor as the Hawkesbury River peaked at 12.9m on Wednesday morning (24 March), and has exceeded the major flood classification of 12.2m. It was reported that water reached depths of about 0.5m in some houses (The Sydney Morning Herald, 2021c).

In addition to high rainfall levels, the Warragamba Dam may have contributed to severe floods in the Hawkesbury-Nepean River basin, affecting towns like Penrith, and surrounding areas such as Windsor and Richmond. Due to the heavy rainfall that fell over the region, the Warragamba Dam is the only Sydney dam that spilled as it was near full capacity, at 99.2% full by the afternoon of Saturday 19 March. Following the spill, the Warragamba Dam has been a topic of contention as the NSW government has proposed to raise the dam wall by up to 17m to make it safer. The Hawkesbury-Nepean valley is likely to see an increase in population given its location west of Greater Sydney (The Sydney Morning Herald, 2021d).

Rainfall and river gauge information (Return period analysis)

There is a range of return period estimates across various flooded locations based on daily rainfall maximum and the total accumulated rainfall over a 7-day period. However, there is a great degree of consistency between the estimated return period based on rainfall, river gauges and flood maps.

Table 1: Total rainfall accumulated at selected sites in New South Wales, Australia between 17 and 23 March 2021, and the maximum daily recorded during this period. A return period has been estimated at the following sites, based on the Design Rainfall Data System (2016) provided by the Australia Bureau of Meteorology. (Data processed by JBA Risk Management.)

Table 2: Peak water level and flood classification at selected sites in New South Wales, Australia between 19 and 24 March 2021. A return period has been estimated at the following river gauge sites, based on the intensity-frequency-curve provided by the Australia Bureau of Meteorology. (Data processed by Jeremy Benn Pacific.)

Ongoing challenges

Australia has been seeing a pattern of more extreme events with increased numbers of hot days and heatwaves, and more extreme rainfall over short duration. As stated by Andy Pitman, from the ARC Centre of Excellence for Climate Extremes at the University of NSW, there is a strong link between global warming and the increase in rainfall (NYTimes, 2021). In addition, flood is the most expensive natural catastrophe for insurers (The Guardian, 2021e). Between November 2019 and February 2020, the insurance industry lost more than AUD$ 5 billion due to natural catastrophes and extreme weather events (The Guardian, 2021e).

Based on the average insured losses from similar historical events, total claims for NSW and SE Queensland would amount to AUS$ 254.2 million (Insurance Council Australia, 2021). Insurance penetration rate in Australia for non-life insurance is 3.5% of GDP in 2019 (Swiss Re Institute, 2019). The Insurance Council of Australia has estimated that 80% of households across the country are likely to be underinsured (The Guardian, 2021e). Based on trend indexes published by the Insurance Council of Australia, it is notable that premiums for both home – buildings, and home – contents, have increased over the years. However, there was greater fluctuation year-on-year for insured claims, and a much more significant increase in the average claim size for homes and the buildings line of business (Figure 3) (Insurance Council Australia, 2020).

Outlook

River heights peaked at Grafton and Ulmara on the Clarence River on Wednesday evening (24 March). With new satellite data available in these recently flooded regions, JBA Risk Management will investigate and provide an update on the extent of impacts and evaluate the return periods in Grafton, Ulmara and Moree.

JBA's Event Response team has produced a flood footprint for the event, estimated flood extents and depths for fluvial flooding. To access the footprint, email the team at EventResponse@jbarisk.com.

If you like to find out more information about this event, our new 5m flood mapping for Australia, and our existing climate change data for Australia, please get in touch. To ensure you are kept up to date with all future event response and company updates, subscribe to our mailing list using the form below.

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