Northern NSW and SEQ Flooding

This week has seen extreme flooding in Northern NSW and South-East Queensland, which has been record-breaking in many areas. We have been working alongside JBA teams globally to produce our initial flood report and flood footprint.


Find the initial report here. To access the footprint or for more information, email the team at: EventResponse@jbarisk.com, or read on for a summary of findings.


Figure 1: Newstead Flooding in Brisbane (Source: JBP)


Flood zone

Our analysis shows that the worst affected areas span from Gympie, QLD to the Clarence River, NSW, which covers 450km (north/south). If you consider the Local Government Areas within this zone, the area covers around 30,000 km2 and extends into the inland catchments. This really puts into perspective the region that the SES, Councils and other first responders have had to manage as the event evolved.


Our event map offers a more detailed look at the scale of the flooding. We expect the final numbers to show an area of around 4,000 sq.km was inundated – meaning around 10% of all land in the region was exposed to some degree of flooding. This flooding has been a combination of pluvial and fluvial inputs – and whilst coastal communities experienced fluvial-tide interaction, storm surges were not the primary mechanism for inundation.


Event Rainfall

At this early stage, we’ve looked at the daily rainfall totals at each gauge prior to the 28th February to consider 24-hour rainfall statistics. The results are staggering – with so many areas recording over 100-year rainfall intensities for a 24h period. This is a statistic-changing event.


Figure 2: Estimated return periods from the maximum daily rainfall measured between 22-28 February 2022 at rainfall stations in Queensland and New South Wales. Data from Bureau of Meteorology Commonwealth of Australia (2022). Data analysis conducted by JBP based on typical IFDs for each region.


Whilst interesting in their own right, the 24-hour gauge statistics are not a great indicator for all catchments, as it includes stations with missing data due to the many gauge issues experienced during the event. Also, because many catchments respond to different concentration times, statistics on shorter bursts (e.g. 3-hour totals) or even longer sustained rainfall (e.g. 48 hours) will be more meaningful for individual areas. To get an understanding of the highest rainfall areas, we have developed a heatmap created using the NASA Global Precipitation Measurements (GPM) below, which uses integrated multi-satellite data combining precipitation from infrared and microwave sensors.


Figure 3: Rainfall accumulation along the east coast of Australia over a seven-day period between 23 February and 1 March 2022. Data source NASA GPM, 2022 (image produced by JBA Risk Management, 2022).


Water levels

Our initial analysis has used water level records, flood study data and gauging information to analyse return periods across the affected regions. Again, we consider this to be a statistic-changing event in many areas. Our initial analysis indicates return periods for major river systems to be approximately a 40-year event in the Clarence River system, well beyond 100-year in the Richmond River, 50 to 100-year in the Brunswick, 50 to 100-year in the Tweed, generally 20 to 50-year through the Gold Coast/Logan systems, >20-year for the upper Brisbane, and around 40-year for Gympie. Expect more analysis to follow.


Figure 4: Plot of water levels at Tumbulgum, NSW, from BoM. For further information see here.



Flood mapping

Our event footprint has been developed using a review of rainfall and return period analysis, with validation through the emerging satellite image analysis. An example of this validation information is shown below of Grafton, where indicative extents have been produced using the Sentinel Application Platform (SNAP), which is an Earth Observation tool that processes and analyses satellite imagery.


If you are an existing JBA/JBP client, please get in touch with your project manager to access the footprint data. For others, to access the footprint email the team at: EventResponse@jbarisk.com


Figure 5: Satellite imagery analysis of flooded areas around Grafton, produced using Sentinel-1 data available from the Copernicus Open Access Hub.



Going forward


Its been less than one week since the flooding, and many rivers are just passing their peak now. Expect a lot more analysis in the upcoming weeks, months, and years.


Remember, this has been an initial analysis only, and you can read our report here. To access the footprint or for more information, email the team at: EventResponse@jbarisk.com


For any site-specific flood investigations, LGA-wide mapping, or just a general chat about flooding, please get in touch.


Dan Rodger

daniel.rodger@jbpacific.com.au







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