Thomas Mortlock and Paul Somerville
An East Coast Low (ECL) windstorm event impacted Southeast Queensland, coastal and inland NSW and the ACT over the period 5 to 10 February 2020, with the Insurance Council of Australia declaring the event a catastrophe (ICA, 2020). As of 10 February, over 10,000 insurance claims had been lodged, estimated to be worth $45 million. Most of the claims came from Queensland and coastal NSW for property damage caused principally by strong winds and heavy rain.
More positively, the deluge extinguished most of the remaining bushfires in NSW and made a significant contribution to water storage in the State. There is still some concern over water quality in dam storage due to hydrophobic soils and runoff due to bushfires. However, this is unlikely to be an immediate issue as reservoirs are thermally stratified at this time of year and authorities can draw down ‘older’ water from deeper depths.
In Sydney, heavy rainfall led to flooding of the two main river systems in the area, the Georges and Hawkesbury Rivers, and also Narrabeen Lagoon on Sydney’s Northern Beaches where the lagoon water levels exceeded those of the last major ECL event in June 2016 (Figure 1). Flooding occurred in the Hawkesbury-Nepean valley with a significant contribution from the Grose River. Evacuation orders were in place on 8 and 9 February in these areas. Strong winds also downed trees in the CBD and Potts Point during the afternoon of 9 February. Large wave conditions led to some significant beach erosion and coastal flooding on Sydney’s Northern Beaches, the Illawarra and Central Coast.
This briefing note evaluates the severity of flood and coastal conditions during this event for the Sydney area and provides some context regarding previous notable ECLs and the synoptic setting in Australia.
The spring/summer of 2019/2020 was dominated by the devastating bushfires and continuation of drought conditions in NSW. This was facilitated by the strongest positive Indian Ocean Dipole (IOD) in almost 60 years and an anomalously negative Southern Annual Mode (SAM) across Southeast Australia. Both the IOD and SAM combined to reinforce hot, dry conditions with prevailing westerly winds producing dangerous bushfire weather. Since the IOD and SAM both returned to neutral in January, conditions have become more favourable for localised convection. This has led to a spate of hailstorms along coastal NSW, Victoria and the ACT, and it has also coincided with the arrival of the monsoonal trough over Northern Australia.
The February 2020 ECL was an Inland Trough Low extending south down the eastern seaboard from the tropical monsoonal trough (Figure 2). In Figure 2, Tropical Cyclone Damien on the Northwest Shelf of Western Australia and a low-pressure system in the northern Coral Sea off Far North Queensland (later to form into Tropical Cyclone Uesi) were also both embedded in the monsoonal trough.
Coastal winds and waves around Sydney during the February 2020 ECL were predominately from the east to north-east. This wind direction not only caused damaging wave conditions, but also drew in moisture from an unusually warm Tasman Sea, which contributed to high rainfall over coastal NSW.
The East Australian Current is an eastern ocean boundary current that transports warm water from the Coral Sea south and extends poleward at this time of year. However, sea surface temperatures (SSTs) on 9 February were 1 – 2 °C warmer than normal around Sydney (up to 26 °C, Figure 3), providing a ready moisture source for this windstorm event.
Rain, wind and flooding
The most widespread impacts from this event arose from wind and wind-driven rain causing localised flash flooding. Figure 4 shows rain and wind observations at Observatory Hill (Sydney CBD) and Sydney Airport between 7 and 9 February.
During the evening of 9 February, a maximum gust of 87 km/hr was recorded at Sydney Airport, with hourly mean wind speeds reaching 60 km/hr. In the CBD, hourly wind speeds reached 65 km/hr (gusts are not measured at the Observatory Hill station). During this period, winds were from the ENE.
1-hour precipitation peaked at 38 mm at the airport and 31 mm in the city. According to Australian Rainfall Runoff Intensity Frequency Duration (IFD) data (Ball et al., 2019), this equates to a 5-year Average Recurrence Interval (ARI) at the airport and 2-year ARI at the city. While these are not uncommon rain rates, the ARI increases when accumulated over longer durations.
For example, the greatest 6-hour period saw falls of 93 mm at the airport and 84 mm in the city; this has an approximately 0-year ARI at the airport and 5-year at the CBD. When accumulated over a 24-hour period, 179 mm fell at the airport and 201 mm in the city with corresponding ARIs of 10-years at both locations.
In summary, this event was equivalent to a 5- to 10-year ARI with respect to rainfall at Sydney.
Storm surge, waves and coastal erosion
While winds and rain peaked in the evening of 9 February, coastal conditions were most hazardous in the morning of 9 February due to the coincidence of high waves with spring high tides (which reached approximately 2 m AHD). At HMAS Penguin in Sydney Harbour, a storm surge of over 1 m was recorded on the falling tide after the morning high water (note: this is from live data and is yet to be verified).
A storm surge is the product of low barometric pressure and onshore-directed winds raising water levels above the predicted tide height. Unlike with tropical cyclones, storm surge is often not the main component of coastal flooding with ECLs in Australia. This is because wind speeds are not as great during ECLs and the typical beach dune height along the NSW and Victorian coast is around 3 m AHD, which is usually sufficient to buffer against high water levels during these events.
During the February 2020 event, a maximum wave height of 14 m (that is over 45 feet!) was recorded on the morning of 9 February at the Sydney wave buoy, with significant wave heights (the highest one-third of all waves measured in one hour) peaking at around 7 m (Figure 4).
A significant wave height of 7 m at Sydney equates to a 5-year ARI, according to Shand et al. (2011), and is in fact a larger peak wave height than was recorded during the infamous June 2016 ECL event (6.4 m). However, the 2020 ECL led to relatively less erosion than in 2016. One of the reasons for this is wave direction.
The mean wave direction during the February 2020 event was from around 100 ° (ESE), although the spectral plot (left inset, Figure 5) suggests there was a wide spread of wave energy between 20 and 180 ° (NNE to S). This means wave energy was dissipated across all sections of the east-facing Sydney beaches, and not just focussed on the narrower and more vulnerable southern sections, as was the case in 2016 when waves predominated from the ENE. Coastal erosion was therefore more limited in the 2020 event, although still caused recession of up to 25 m at some locations (SMH, 2020).
Large waves and strong onshore-directed winds can also contribute to the flooding of coastal lagoons during ECL events. These lagoons – known as ICOLs (Intermittently Closed and Open Lagoons) – are often closed from the ocean by a bar of sand deposited by ocean waves. During storm events, waves and onshore winds can prevent floodwater from escaping the lagoon entrance and further contribute to elevated water levels in the lagoon. The areas adjacent to coastal lagoons are therefore particularly vulnerable to flooding during ECLs.
In order to avert the flooding caused by the lagoon entrance being closed in the 2016 storm, the lagoon entrance was bulldozed open on 8 February. Nevertheless, on 9 February the lagoon flooded prompting an evacuation of approximately 4,000 properties. Floodwaters also inundated parts of Pittwater Road, the main thoroughfare that connects Narrabeen with the City (Figure 6). Similar lagoon flooding was also observed at Fairy Lagoon in North Wollongong and the entrance to Lake Illawarra.
A summertime East Coast Low (Inland Trough Low) intensified over Sydney on 9 February causing heavy rain, flash and riverine flooding, localised downing of trees and some coastal erosion. Fortunately, there were no associated fatalities. Analysis suggests both rain volumes and coastal wave conditions were equivalent to a 5-to-10-year event magnitude. Unusually warm sea surface temperatures off the coast of Sydney provided a ready moisture source for this event. Given there is now no large-scale climate mode dominant at this time (the IOD, SAM and ENSO are all in neutral territory), we can expect variable weather driven by local-scale processes over the coming months.
Over the past decades, there has been no clear observed trend in the number of East Coast Lows. Climate model projections indicate that fewer East Coast Lows are likely to occur in the future, particularly during winter, with increased rainfall intensities in some cases (ESCC, 2020). However, there is more uncertainty associated with the projections of the future frequency of summertime ECLs such as this event.
Ball, J., Babister, M., et al. (2019). Australian Rainfall and Runoff: A Guide to Flood Estimation, Commonwealth of Australia.
ESCC (2020). East coast lows and climate change in Australia. Earth System and Climate Change Hub. November 2019, pp 4.
ICA (2020). Insurers delate Catastrophe for east coast storms and flooding. Insurance Council of Australia media release, 10 February 2020.
Shand, T., Mole, M.A., et al. (2011). Coastal Storm Data Analysis: Provision of Extreme Wave Data for Adaptation Planning; Water Research Laboratory (WRL) Technical Report 2011/242; University of New South Wales: Sydney, Australia.
SMH (2020). ‘Critical’ few hours for Sydney beach erosion as storm heads south. Sydney Morning Herald quoting measurements from UNSW Water Research Laboratory. 10 February 2020.