Heaviest rains in 140 years bring floods to Hong Kong and cities across southern China
Two people died and more than 100 were injured when the remnants of Typhoon Haikui brought flooding to Hong Kong in early September 2023. The storm continued its path into southern China, where the city of Shenzhen experienced nearly 500mm of rainfall in a single day (Guardian, 2023). Transport services across the region were affected, with Hong Kong’s cross-harbour tunnel inundated, subway stations closed, and landslides blocking highways (BBC, 2023).
Event Overview
Authorities in Hong Kong issued a ‘black rainstorm warning’ for the first time in two years on 7 September 2023 as heavy rain associated with Typhoon Haikui swept across the Pearl River delta (Sky News, 2023). Areas of Hong Kong received over twice their average September rainfall in 24 hours, with 600mm being recorded at the Hong Kong Observatory and 800mm over the Eastern and Southern districts, compared to the mean monthly September average of 321.4mm (Floodlist, 2023).
Two people are known to have been killed by the floods, with 132 more receiving hospital treatment (South China Morning Post, 2023). Schools and clinics were closed across Hong Kong, whilst shopping malls and train stations were also inundated; the city’s stock exchange was suspended on Friday 8 September (Guardian, 2023).
The next wave of flooding was reported in the neighbouring Guangdong province, where heavy rain brought devastation to cities including Shenzhen and Meizhou. A record 24-hour rainfall total of 469mm was recorded in Shenzhen, whilst 11,000 people were evacuated in Meizhou (Floodlist, 2023). Transport links and businesses were particularly badly affected in Shenzhen, a city of over 17 million inhabitants known for being a technology hub (Guardian, 2023).
Although the heavy showers had begun to ease by the end of 8 September, the black warning lasted for over 12 hours; as a comparison, the previous record length of a black rainstorm warning, set back in 1999, was less than six hours in duration (South China Morning Post, 2023).
As the region began to count the cost of the event, further heavy rain falling at a rate of at least 50mm an hour on 14 September led to a red warning and school closures across the city (Reuters, 2023).
Figure 1: Satellite-observed rainfall animation showing Typhoon Saola and Typhoon Haikui’s paths between 01 September 2023 and 17 September 2023. Rainfall data source: NASA GPM (2023). Typhoon Saola and Haikui track data source: IBTrACS (2023). Animation produced by JBA Risk Management (2023).
Return Period Estimation
JBA has conducted Extreme Value Analysis on historic rainfall data from Hong Kong Observatory (HKO) to estimate the return period of the rainfall at that location. We fit an exponential distribution to close to 140 years of daily rainfall climate data timeseries from HKO (2023a) to produce a rainfall exceedance probability curve. The curves are shown by the solid orange line in Figure 2, with the 70th percentiles given by the dashed lines. The observed 24-hour rainfall total of 425mm at the observatory on 8 September 2023 (shown by the blue line in Figure 2, HKO 2023a) suggests close to 1-in-100-year rainfall event at this location.
Figure 2: Estimated extreme rainfall return period analysis for Hong Kong. Observed daily rainfall data used in the analysis was provided by HKO (2023a), the observed rainfall of 425mm on 8 September 2023 is shown by the blue horizontal line (HKO, 2023a).
Historical Events
Despite not having many rivers, Hong Kong is highly susceptible to typhoons and severe rainstorms. Strong typhoons often result in severe storm surge along its coast, resulting in inundation of low-lying areas from sea level rise and seawater backflow. As for the inland regions, torrential rainstorms can result in large surface water accumulations and landslides. This is evident in recent past events in Hong Kong that caused severe flooding and resultant impacts.
The table below shows recent significant historical events affecting Hong Kong.
The Hong Kong government has implemented various measures to mitigate the impact of flooding, such as improved drainage systems and early warning systems (Fire Services Department, 2021). This most recent event has been likened to Typhoon Mangkhut, with AM Best estimating that the losses could rival the 2018 event at HK$3.1 billion ($400 million USD) (Insurance Journal, 2023).
JBA Flood Hazard Maps
JBA has 30m resolution river and surface water flood hazard maps for every country in the world. This report is accompanied by a flood footprint for the event - detailing extents and depths of the flooding in areas affected. Download it via our Client Portal or request a copy by emailing eventresponse@jbarisk.com.
References
BBC, 2023. Hong Kong and southern China battle widespread flooding from record rains. [Online]. Available at: https://www.bbc.co.uk/news/world-asia-china-66748239 [Accessed 15 September 2023]
Choy, C. W., Lau, D. S., & He, Y. (2022). Super typhoons Hato and Mangkhut, part I: analysis of maximum intensity and wind structure. Weather, 77(9), 314-320.
Fire Services Department, 2021. Flooding [Online]. Available at: https://www.hkfsd.gov.hk/eng/cep_edu/cep/disaster_preparedness/natural_disasters_hk/flooding.html. [Accessed 21 September 2023]
Floodlist, 2023. Hong Kong – Floods Cause Chaos After Record 158.1mm of Rain in 1 Hour. [Online]. Available at: https://floodlist.com/asia/hong-kong-floods-september-2023 [Accessed 18 September 2023]
Guardian, 2023. Two dead after Hong Kong’s heaviest rain in at least 140 years. [online] Available at: https://www.theguardian.com/world/2023/sep/08/hong-kong-weather-record-rain-flooding-after-typhoon-haikui [Accessed 15 September 2023]
Hong Kong Observatory (HKO), 2001. The Weather of September 2001 [Online]. Available at: https://www.hko.gov.hk/en/wxinfo/pastwx/mws200109.htm [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2008a. Rainstorm on 7 June 2008 [Online]. Available at: https://www.hko.gov.hk/en/wxinfo/news/2008/pre0607e.htm [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2008b. Typhoon Neoguri (0801) 15 - 20 April 2008 [Online]. Available at: https://www.hko.gov.hk/en/informtc/neoguri/report.htm [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2009. This Year’s Weather - 2008 [Online]. Available at: https://www.hko.gov.hk/en/wxinfo/pastwx/ywx2008.htm [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2023a. Daily Total Rainfall All Year - Hong Kong Observatory [Online]. Available at: https://data.gov.hk/en-data/dataset/hk-hko-rss-daily-total-rainfall/resource/7266ded8-4670-4235-b9b4-7fdc66cf59ea [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2023b. [Online]. Available at: [Accessed 21/09/2023]
Hong Kong Observatory (HKO), 2023c. Casualties and Damage Caused by Tropical Cyclones in Hong Kong since 1960 [Online]. Available at: https://www.hko.gov.hk/en/informtc/historical_tc/cdtc.htm [Accessed 21/09/2023]
Insurance Journal, 2023. Hong Kong’s Floods to Hit Insurers’ Earnings, Could Pressure Reinsurance Renewals [Online]. Available at: https://www.insurancejournal.com/news/international/2023/09/18/740830.htm [Accessed 19 September 2023]
International Best Track Archive for Climate Stewardship (IBTrACS), 2023. [Online]. Available at: https://www.ncei.noaa.gov/products/international-best-track-archive [Accessed 15 September 2023]
NASA Global Precipitation Measurement (GPM), 2023. Precipitation Data Directory. [Online]. Available at: https://gpm.nasa.gov/data/directory [Accessed 18 September 2023]
National Oceanic and Atmospheric Administration (NOAA), 2008. June 2008 Global Hazards and Significant Events [Online]. Available at: https://www.ncei.noaa.gov/access/monitoring/monthly-report/hazards/200806 [Accessed 21/09/2023]
Reuters, 2023. Hong Kong shuts schools as it issues heavy rain warning. [Online]. Available at: https://www.reuters.com/world/asia-pacific/hong-kong-shuts-schools-it-issues-heavy-rain-warning-2023-09-14/ [Accessed 18 September 2023]
Sky News, 2023. Two dead after Hong Kong flooded by heaviest rainfall in almost 140 years, prompting black rainstorm warning. [Online]. Available at: https://news.sky.com/story/hong-kong-flooded-by-heaviest-rainfall-in-almost-140-years-as-black-rainstorm-warning-issued-12957117 [Accessed 15 September 2023]
South China Morning Post, 2023. Hong Kong floods: 132 people sent to hospitals, Observatory cancels all warnings after city battered by heaviest rainfall on record. [Online]. Available at: https://www.scmp.com/news/hong-kong/health-environment/article/3233782/hong-kong-observatory-issues-red-rainstorm-alert-warns-significant-road-flooding [Accessed: 18 September 2023]
Xinhua News Agency, 2008. Heavy rain brings chaos in HK [Online]. Available at: http://www.china.org.cn/china/2008-06/07/content_15684165.htm [Accessed 21/09/2023]
