Ex-Tropical Cyclone Ellie triggers record-breaking floods in Western Australia: December 2022
Ex-Tropical Cyclone Ellie caused havoc in Western Australia (WA) as it brought heavy rain and damaging wind to the region, cutting off towns, forcing road closures and transforming areas of the landscape into an inland sea (24). The cyclone saw homes and businesses being swamped, people evacuated and animals fleeing the affected areas (15). On 3 January 2023, Stephen Dawson, Emergency Services Minister, declared an emergency flood situation in Kimberley, WA (23). Over 200 people have been evacuated so far from the desert town of Fitzroy Crossing (18).
Event timeline
Ellie first formed as a tropical disturbance on December 20 at 18:00 UTC over the Timor Sea. Over the course of the next two days Ellie intensified into a Category 1 Tropical Cyclone, finally making landfall at 11pm local time on December 22 close to the Daly River (southwest of Darwin), bringing strong winds and heavy rain across parts of WA and the Northern Territory (NT). A few hours later, Ellie began to weaken slightly and was downgraded to a Tropical Low (Depression). During the final days of 2022, the ex-tropical cyclone travelled west across the Kimberley coastline in WA and became mostly stationary throughout the first week of January, dumping continuous quantities of rain over the area. The weather system crossed east into the NT through 7-8 January 2023, before finally clearing south-east.
Meteorological overview
Heavy rainfall is not uncommon across northern parts of WA and NT at this time of year. The Australian monsoon, which occurs between the months of November to April, is characterised by seasonal changes in wind direction that are associated with greater-than-average rainfall in the north of the country.
The prevailing winds in this area usually originate from the east or south-east. During the summer months, the ground begins to heat up, producing an area of low pressure that causes the wind direction to switch to a northwesterly. These northwesterly winds pull in moisture-laden air from the Indian Ocean which acts as fuel for deep clouds and storms. Because the Australian monsoon is an annual occurrence, communities and ecosystems alike have adapted their lives according to it and are in fact dependent on the arrival of the rain (28).
What makes this event so unusual is the interaction between Ex-Tropical Cyclone Ellie and the monsoon. Usually once a cyclone makes landfall, the system rapidly weakens as its supply of moisture from below is removed. In this case however, Ellie’s storm system became embedded within the monsoon trough (an elongated region of low pressure), which acted to offset the weakening effects of the cyclone making landfall. The interaction between the two weather systems resulted in an unusually long period of unsettled weather, catching local residents off-guard.
Impacts
The Kimberley region of WA experienced torrential rainfall, recording seven-day totals of 200-800mm. A 700km section of the Great Northern Highway - one of the major transport networks across the north of WA – was closed by authorities, isolating communities and prompting concerns over food and medical supplies. The Australian Government declared an emergency situation, resulting in evacuations across the region (16), (17), (21).
The town of Fitzroy Crossing was severely affected as the Fitzroy River reached its highest ever recorded levels. By 05 January, the Fitzroy Barrage station was recording a height of 17.2 metres, indicating major flooding. Over a thousand homes were inundated with floodwater, prompting evacuations to the town of Broome 400km west (8), (12).
Remote communities across NT were also affected, becoming isolated as transport networks were made impassable by floodwaters (1). Sections of the Victoria Highway near Timber Creek in NT were also made impassable by flood waters, and a woman had to be rescued after she was washed off the Bradshaw Bridge (13), built in 2008 with the aim of withstanding up to a 1-in-20-year flood (26). The Aboriginal community of Peppimenarti experienced severe wind speeds and heavy rain that destroyed the temporary housing of dozens of people displaced in 2020 due to ongoing civil unrest (2). However, despite some of the negative aspects of the flooding, for some cattle producers the rain was a welcome break after a long dry season (3).
Historical context
According to a report published by the Water and Rivers Commission in 2000 (31), the average annual flood damage cost for Australia is AUD $350 million, of which $17 million (5%) comes from WA. Insurance industry analysis by McBride (20) discovered that tropical cyclone activity (which includes both flood and wind perils) results in direct economic losses between AUD $40-100 million every year in WA. Table 1 lists the major impacts from three notable historical flood events across WA and NT caused by tropical cyclones and monsoonal rains.
Back in April 2021, Tropical Cyclone Seroja made landfall across mid-western parts of Western Australia. The Category 3 storm affected Kalbarri and Northampton the most with about 70% of both towns’ structures damaged according to Insurance Council of Australia (ICA) (19). By August 2021, ICA had reported up to AUD $281 million worth of claims, 43% of which were personal while the remaining 57% were commercial. The recovery following the event was slow and difficult due to the prevalent COVID border restrictions. By November 2022, ICA concluded the event with a total of AUD $400 million of claims from Tropical Cyclone Seroja.
Extreme value analysis
JBA conducted Extreme Value Analysis on both rainfall and river streamflow data in order to estimate the return period of the event. We fitted a Generalised Pareto Distribution (GPD) curve to a 44-year daily rainfall climate data timeseries from NOAA (25) to produce a rainfall exceedance probability curve at Fitzroy Crossing. The curve is shown by the solid orange line in Figure 4, with the 70th percentiles given by the dashed lines. The observed daily rainfall at Fitzroy Crossing on 01 – 02 January (shown by the blue line) suggests a 1-in-250-year rainfall event.
The exceedance probability of streamflows along the Fitzroy River were calculated using daily maximum watercourse discharge data from the Department of Water and Environmental Regulation, accessed via BOM (11). The data were fitted with a GPD curve to produce exceedance probability curves at Fitzroy Crossing and Noonkanbah and are shown in Figures 5 and 6. The analysis suggests a streamflow return period of approximately 50 years.
Fully comprehensive flood hazard maps
JBA have comprehensive river and surface water hazard maps covering all of Australia at 30m spatial resolution and six return periods: 20, 50, 100, 200, 500 and 1,500 years. Figure 7 shows a sample of our 1-in-50-year river flood map along the Fitzroy River, capturing the locations of Fitzroy Crossing and Noonkanbah. We additionally have high resolution (5m) maps covering major built-up areas including Adelaide, Brisbane, Melbourne, Perth and Sydney. If you are interested in licensing our hazard maps for Australia, please get in touch via the email below.
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