The long-awaited state government analysis of the 2022 flood in the shire’s north is now available on the SES website.
It is called The Post 2022 Event Flood Behaviour Analysis – Brunswick River.
It was finalised in February 2024, and despite Echo requests over subsequent months, it was not released publicly by the NSW Labor government.
It’s unclear why there was no media release on this important document, which details flood heights and modelling from the most devastating flood in living memory.
Council’s Floodplain Advisory Committee had been privy to the document, but were gagged from releasing it to the public.
Hydrologic modelling
The technical 115-page document was compiled by WMA Water for the NSW government, and draws on hydrologic modelling from various gauges across the shire’s north, including Durrumbul, Federation Bridge, Yelgun Creek, Billinudgel, Brunswick Heads and Orana Bridge.
Peak 2022 flood levels were surveyed at Mullum, South Golden Beach, Brunswick Heads and Ocean Shores/New Brighton.
Remarkably, the flood data supplied by the public was not included as levels, only as deviations from modelled levels, according to longtime Council watcher, Matthew Lambourne. He sits on Council’s Floodplain Advisory Committee, and told The Echo that data from the public had been included in past reports.
Gauge is not accurate
Lambourne believes that Main Arm’s Federation Bridge gauge is not accurate.
‘The gauge is on the on the downstream side of the bridge, and the flood level was higher, so an accurate flood height was not recorded’.
‘[Regarding the data collected], ‘The difference between recorded height and modelled height was recorded’, he says, ‘but not the actual flood height’.
‘All previous flood studies identified recorded and actual heights.
‘Obviously, the closer the levels between the two, the better. They are not all that close, particularly in Marshalls Creek, where it is off by an average of 200mm.
‘It’s been acknowledged that a new flood model is needed’.
New model needed
While the report says on page 15 that ‘The current hydrologic model that covers the Brunswick River does appropriately replicate recorded flows and levels’, the authors also recommend that, ‘A full review of the Intensity–Frequency–Duration (IFDs) within the Brunswick River and Marshalls Creek catchment should be undertaken to ensure appropriate representation of orographic [position of mountains] effects’.
Recommendations
Other recommendations include, ‘The design flow estimates at Durrumbul should be reviewed and updated as part of any future study; the hydrologic model should be extended to cover the entire Brunswick River system, including adjacent catchments which only interact in larger events; [and] the hydraulic model should be updated and extended to cover all areas that impact flood levels within the Brunswick River system, including adjacent catchments which only interact in larger events’.
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