North-eastern Australia hammered by heavy rain after Tropical Cyclone Jasper
Substantial rainfall in the wake of Tropical Cyclone Jasper has swamped northern Queensland, with several towns across Australia’s Great Barrier Reef coast being completely cut off (Reuters, 2023). Since the start of the weather event, the city of Cairns has received more than 2m worth of rain. The authorities were forced to call off evacuation in the remote town of Wujal Wujal due to adverse conditions (BBC, 2023). Australian Prime Minister Anthony Albanese pledged military support for relief operations and said that the government has provided financial aid for the people affected (CNN, 2023). The Insurance Council of Australia (ICA) declared the damage caused by the cyclone as an insurance catastrophe (Insurance Insider, 2023).
Event Overview
Cyclone Jasper made landfall on 13 December 2023 in the far north of Queensland. Thirty-six thousand homes suffered power cuts as north Queenslanders were met with strong winds and a life-threatening flooding situation (Guardian, 2023). The cyclone triggered more than 500mm of rainfall along the north tropical coast in 24 hours. Affected areas include Daintree, Mossman, Diwan, Wujal Wujal, Port Douglas, and Cairns. Water levels in the rivers spiked to levels that were unseen in over half a century (FloodList, 2023).
Cairns Airport was closed on 18 December after planes were submerged and so far, hundreds of people have been rescued from the floods (Sky News, 2023). Farmers were also affected as floodwaters blocked roadways, thwarting efforts to distribute farm produce (Nine.com.au, 2023).
Tropical Cyclone Jasper originated from a region of low pressure situated in the South Pacific Ocean. Initially, the system displayed a slow development while moving south-west through the South Pacific Ocean. The Australian Bureau of Meteorology (BoM) elevated the system to a Category 1 tropical cyclone on the Australian scale and assigned the name Jasper (BoM, 2023a). The storm rapidly intensified, reaching Category 4 status on 7 December 2023, with the Joint Typhoon Warning Center (JTWC) estimating sustained winds of 220 km/h (140 mph). Subsequently, Jasper encountered increased wind shear, leading to a gradual weakening. The low-level circulation centre became fully exposed, maintaining a poorly defined convective structure as it approached northern Queensland. After a period of re-intensification, Jasper made landfall as a Category 2 tropical cyclone on 13 December 2023 near Wujal Wujal on the east coast of the Cape York Peninsula. It weakened into an ex-tropical low, remaining detectable overland as it moved southeast.
Tropical Cyclone Jasper brought moderate to locally intense rainfall. The BoM weather station at Bairds on the Daintree River received 669mm of rain on 17 December and an accumulation of 2,252mm from 12-19 December (BoM, 2023a, Figure 3). The BoM announced that 21 stations had received over 1,000mm of rain since Jasper made landfall.
Extreme Value Analysis
JBA conducted an Extreme Value Analysis on rainfall data in order to estimate the return period of the event. We fitted a Generalised Pareto Distribution (GPD) curve to a 34-year daily rainfall climate data timeseries from BoM (2023a) to produce a rainfall exceedance probability curve at the Daintree River station at Bairds. The curve is shown by the solid orange line in Figure 3, with the 95th percentiles given by the dashed lines. The observed daily rainfall at Bairds on 17 December (shown by the blue line) suggests a 1-in-345-year rainfall event.
Jasper is the most rainfall-intensive tropical cyclone to affect Australia in recorded history with a 7-day total precipitation of 2,252mm recorded at the Bairds rainfall station. The other highest rainfall-producing storms are listed in the table below.
JBPacific Forecasting System
The Port Douglas Forecasting System uses Delft-FEWS, which is an open data handling platform, designed for building tailored hydrological forecasting systems. The system allows the integration of large datasets, provides specialised modules to process the data and runs fully automated.
Delft-FEWS manages the entire automated forecasting process which involves the following data:
- Recorded rainfall and water level gauge data
- BoM ADFD rainfall forecasts
- BoM Australian Water Resource Assessment – Landscape (AWRA-L) data
- Simulated hydrologic forecasts from URBS
- Established warning thresholds
The Forecasting System takes the 7 previous days of observed rainfall and the next 7 days of forecast ADFD rainfall and applies this to the URBS hydrologic model to get forecast flows at locations of interest. The flows are then applied to rating curves to get forecast flood levels. These flood levels are then used to indicate the severity of a forecasted flood by identifying whether a flood level will cross the minor, moderate or major threshold. The forecast level can also be linked to the JBA flood maps to indicate the extent of flood waters, impacted properties and road closures. Information from the Forecasting System can be issued through an API to the Guardian IMS platform to provide flood intelligence and information on disaster management.
The point selected to review the performance of the forecasting system was at the Foxton Bridge gauge near the centre of the town of Mossman. By Tuesday 12 December at 5am, the forecasting system indicated that the forecast water level would reach a major flood level on Thursday, 14 December. The forecast water level is shown compared to the observed water level in Figure 4. While the actual water level was higher than the forecast, the forecasting system was still able to predict the major flood level and give two days of warning to the Port Douglas Council.
The Foxton Bridge gauge reached a water level of 8.93m AHD on 18 December. From the Mossman River Flood Study undertaken by JBP in 2021, a water level of 8.93m AHD indicates a 1-in-100-year flood event. The 1-in-100-year JBP flood map for Mossman was validated against photographs and drone footage of Cyclone Jasper taken in Mossman.
Figure 5 shows the flood map over the town of Mossman with the location at which photos were taken and the direction they were taken. Both the flood map and images (Figures 8 to 10) show flooding to the north and south of Front Street and all through Mill Street.
JBPacific, a subsidiary of JBA Group, specialises in extreme weather engineering and focuses on natural disaster analysis, prediction, and protection. JBP offers a variety of services to tackle extreme events; the team has worked with governments, councils, international funding agencies and consultants across the Asia-Pacific such as Australia, South-east Asia, and Indonesia. For more information about the JBP forecasting system, please visit the JBP website here.
JBA 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. Hazard maps at 5m spatial resolution are also available for seven key metropolitan areas of Australia, including Port Douglas and Cairns.
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.
