Floods strike Sydney in July 2022 – the second major event to hit the region in recent months
Following March’s devastating floods in Queensland and New South Wales, covered in our Event Report, the area was hit by another wave of flooding in July, causing at least one fatality and affecting around 50,000 people who were asked to evacuate their homes (NBC, 2022). This was, in fact, the fourth major flood to hit greater Sydney in two years, with heavy rain falling on ground already saturated after an unusually wet summer (Guardian, 2022a).
Heavy rain began to fall on 2 July, continuing through the following two days, with 552mm of rain in 48 hours reported at Brogers Creek in the Illawarra region and 318mm in 24 hours at Wattamolla (Floodlist, 2022).
A man died when he fell from his kayak on the Parramatta river in western Sydney, which also saw neighbourhoods cut off when roads were flooded, and evacuation orders being served on residents. Local politicians said they were facing dangers from flash flooding, riverine flooding and coastal erosion concurrently (BBC, 2022).
By 5 July, 50,000 residents had either been told to evacuate or warned they might receive evacuation orders (Reuters, 2022). As landslides caused further problems in the Blue Mountains, meteorologists announced that not only was Sydney seeing its wettest year-to-date, but it is already the 11th wettest year since records began in 1859 – with another five months to go (9news, 2022).
As floodwaters began to recede and roads and bridges were reopened, questions began to be asked about the causes of these recent events, with a major factor thought to be the surge in development and the building of houses on swampland making the ground in the affected areas of Sydney less able to absorb runoff (Guardian, 2022b).
Impacts
The suburbs of Sydney, New South Wales, were again some of the main affected areas, with 100 troops and two helicopters deployed by the Australian government to help rescue efforts (Al Jazeera, 2022). By 5 July, 102 evacuation orders were in place, affecting around 50,000 people across New South Wales (9news, 2022). As of 9 July, more than 2,000 properties had been damaged by the floods across areas of eastern New South Wales (ABC News, 2022a). The flooding triggered disaster recovery payments from the federal government for 37 local government areas declared as disaster zones (ABC News, 2022b).
According to the Australian Bureau of Meteorology (BoM), many rainfall gauges between Nowra on the south coast and Newcastle and the Hunter Valley further north saw daily rainfall records for July (BoM 2022a). The Hawkesbury and Nepean River catchment received more than 400mm across four days at several locations including Beaumont, Robertson and Fitzroy Falls (BoM, 2022a). Rainfall totals were highest to the south of Sydney, including at Brogers Creek, Illawarra, where 933mm of rain was recorded in the four days from 1-5 July (Weatherzone, 2022).
In Windsor and Richmond, north-west Sydney, the Hawkesbury River level peaked at around 14m by 5 July, which is above the major flood stage (Floodlist, 2022). Similarly, the Nepean River level rose above 15m on 3 July at Menangle in south-west Sydney, also above major flood stage (Floodlist, 2022). River flooding in the Hawkesbury and Nepean catchments inundated properties in Camden, Richmond and Windsor. Upstream on the Nepean River, heavy rainfall caused the Warragamba Dam to spill over on 3 July (Al Jazeera, 2022).
Evacuation orders were also issued in the Hunter River valley for 400 residents near Singleton (ABC News, 2022c). Further downstream, extensive flooding from the Hunter River, which peaked on 8 July, inundated properties around Maitland, isolating semi-rural communities (ABC News 2022d; Guardian, 2022c).
Many of the affected areas were inundated with flood water earlier this year (JBA, 2022), which resulted in an estimated $4.8 billion AUD ($3.3 billion USD) in insured damage according to the Insurance Council of Australia (CNA, 2022). For some residents along the Hawkesbury and Nepean Rivers this is now the third or fourth time their homes have been flooded in the past 16 months (New York Times, 2022).
Event origins
There are several factors that have contributed to the remarkable flood extent and associated impacts across New South Wales. Here we explore three of these.
Meteorology
The combined presence of a seasonal East Coast Low (see JBA, 2022 for a more detailed description) and a deepening coastal trough (in simple terms - an elongated area of relatively low pressure) encouraged large masses of moisture-laden air to rise through the atmosphere, eventually cooling and allowing the moisture within to plummet to the surface as rain.
Climatology
As the author Robert Heinlein famously said, “Climate is what you expect; weather is what you get”. While the weather may change unpredictably from minute to minute, the climate is more predictable and is measured by long-term averages in atmospheric conditions. Climate indices refer to broad-scale alternating patterns of weather across the Earth that flip between two extremes or ‘phases’ (World Climate Service, 2021).
One such climate index is the El Niño Southern Oscillation (ENSO), which oscillates between a positive El Niño phase and a negative La Niña phase based on sea surface temperature anomalies in the Southern Pacific Ocean. La Niña is associated with increased rainfall along the eastern coast of Australia and has been linked to the higher-than-usual rainfall over the past few months. Although BoM have confirmed that the most recent La Niña event ended in June (BoM, 2022a), the long-term increase in rainfall over the past few months left dams at a critical level, with the ground saturated and unable to retain additional water (Guardian, 2022a; CNA, 2022).
Another climate index that influences weather conditions across Australia is the Southern Annular Mode (SAM), which has three phases: neutral, positive and negative (BoM, 2022b). The positive phase of SAM is associated with weaker-than-average westerly winds, more frequent East Coast Lows and increased rainfall along the east coast during the southern hemisphere winter (BoM, 2022b; NCAR, 2018).
A third climate index, The Indian Ocean Dipole (IOD), is measured by the difference in sea-surface temperatures (SSTs) between western and eastern parts of the Indian Ocean (BoM, 2022c). The IOD is currently in its negative phase, meaning that warmer waters are concentrated in eastern parts of the ocean basin which results in increased rainfall across Australia during the winter and spring months (BoM, 2022a).
In addition to this combination of climate indices whose phases have aligned to bring increased rainfall across New South Wales, SSTs around the Australian coastline are generally higher than normal, also associated with an increased chance of above average winter-spring rainfall (BoM, 2022d).
Exposure
Impacts of the recent flooding in New South Wales have been exacerbated by the fact that many regions are still recovering from the previous floods in March this year and the ongoing COVID-19 pandemic (CNA, 2022).
Emergency services across NSW are stretched, which resulted in reported delays to evacuations in Shanes Park, West Sydney (Sydney Morning Herald, 2022).
Additionally, rapid population growth has resulted in expanding urban development along the Hawkesbury-Nepean floodplain (CNA, 2022). The valley is particularly prone to flooding, owing in part to the many tributaries that feed it as well as the presence of natural ‘choke points’ created by narrow sandstone gorges (NSW Government, 2019). Increased exposure along this floodplain has inflated the risk to human life and damage to property in the event of flood, and many lower-income residents are unable to afford flood insurance (CNA, 2022). Indeed, research by Booth and Tranter (2018) concludes that a similar pattern is observed across Australia, with under-insurance “(re)produced along socioeconomic and geographical lines, with those of lower socio-economic status or living in cities more likely to be under-insured.”
Rainfall analysis
As mentioner earlier, Brogers Creek, Illawarra, received a total of 933mm of rain over a four day period from 1 to 5 July (Weatherzone, 2022). Based on Extreme Value Theory (EVT) analysis of historical records of daily mean rainfall data at Brogers Creek, this equates to around a 1-in-80-year rainfall event. This is consistent with statistical analysis of BoM’s annual exceedance probability data, which estimates a 1 to 2 percent chance of this event occurring in any given year – this translates to a rainfall event with a return period between 50 and 100 years (Weatherzone, 2022).
Future outlook
A common misconception is that a “1-in-X-year flood” means that a flood of a given magnitude will occur only once every X years; a more meaningful way of interpreting this phrase is to say that there is a 1-in-X chance of such an event occurring each year, independently of each other. For example, Margaret Cook - an environmental historian from the University of the Sunshine Coast - noted that Brisbane experienced two 1-in-100-year floods within a fortnight during the late 1800s (ABC News, 2021).
Dr Thomas Mortlock, a Senior Catastrophe Analyst at Aon and Adjunct Fellow at Macquarie University, notes that La Niña events increase the chance of catastrophic floods across New South Wales (Guardian, 2022d). Although the recent La Niña event ended back in June, BoM have announced that there is a 50% chance of it reforming later this year, bringing further risk of flooding (CNA, 2022). In addition to this, a warming atmosphere under climate change is able to hold greater amounts of moisture, thereby further increasing the risk of flooding in NSW and along Australia’s east coast during La Niña events (Guardian, 2022d).
Around 130,000 people in NSW still reside on floodplains, and thousands of homes lie within an area at risk of a 1-in-100-year flood (Nikkei Asia, 2021). Moreover, the population of Greater Western Sydney is estimated to rise from 2.5 million today to 3 million by 2036 (Nikkei Asia, 2021). Further urbanisation in flood-prone areas will only increase the amount of risk faced by inhabitants.
Given the forecasted increases in precipitation during La Niña events combined with projected exposure growth (in the form of both buildings and population), the likelihood of parts of NSW experiencing a 1-in-100-year flood more than once annually is set to increase.
The report is accompanied by a flood footprint for the event, detailing extents and depths of the flooding in areas affected. The Event Response team also produced a flood footprint for the previous March 2022 flooding - request both footprints here.
JBA helps re/insurers across Australia manage their risk. We have nationwide return period flood maps at 30m resolution for Australia and an Australia Flood Model, which enables quantification of risk across the country.
If you're interested in finding our more about how our products or services can improve your management of flood risk, please get in touch for more information.
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