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June 23, 2024

What’s missing from the Mullum Pod Flood Report?

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The Resilience NSW (ResNSW) Flood Report on the impact of the fill at the emergency housing site at Mullumbimby was finally released to the public on 7 November.

The report details the impacts that the fill, built up to current 1-in-100-year flood level under selected Scenario A, will have on flood levels for existing housing, in particular on Prince, Poinciana and Station Streets. 

‘According to the report, there are 11 properties that will see an increase in flooding in a 1-in-100-year event, and 85 properties that will actually see a reduction in flooding in this type of event,’ said Byron Shire Mayor, Michael Lyon.

‘They might not want the fill to be removed.’

Two properties identified in the ResNSW Flood Report, with six units that were severely impacted by flooding in 2022, will see a 3cm increase of above-floor flooding as a direct result of the fill-in a 1-in-100-year flood (as labelled in 2020 by the North Byron Floodplain Management Study and Plan). 

The temporary pod site will provide 40 units, for up to 160 people who were affected by the devastating February floods. However, there are key areas where the ResNSW Flood Report by BMT fails to provide adequate information on how their conclusions are drawn regarding the impact on existing houses and residents in these areas.

Local Councillor and hydrologist Duncan Dey pointed out that, ‘At 40 pages this is a very thin technical report and it has not provided the modelling and details needed to allow the public to see how they reached, or to confirm, the conclusions they have put forward. There is also no clue as to who did the actual modelling, or authored the report’. 

3–6cm not a small increase

In their November Construction Update, ResNSW say that this is a ‘small increase in flood levels’. However, Cr Dey says that ‘in the profession, rises of 3cm or 6cm are not considered small’,

‘The government should accelerate the many flood mitigation options at its disposal, as described in the adopted North Byron Floodplain Risk Management Plan. That plan is a joint venture of Council and the NSW government. Work on those measures might well achieve a 3cm drop in flood levels at this and many other sites throughout the north of the Shire. Government should pursue that rapidly, before the next flood.’ 

Fill creates a levee

‘The flood report deals with current climate conditions only. It doesn’t deal with future flooding, which will be worse in 2050 or 2100. It doesn’t have to, because the fill is only there until the middle of this decade… or is it?  

‘It looks at the 385m long fill site that runs parallel to the railway, plus an 80m northward extension as shown in Figure 4.1 of the report.

‘This fill acts as a levee bank. It totals 465m parallel to the railway and acts as a barrier to flow when the Brunswick Valley floods,’ explained Cr Dey. 

‘The water flows west to east down the Brunswick Valley, that is, it flows from the Mullumbimby Showground across town towards the Industrial Estate.’ 

Flood velocity overlooked

The impact of the velocity, the speed that the water moves during the flood event, has not been presented in the report. 

‘The reality is that these velocities have to have been modelled to obtain the water levels,’ said Cr Dey. 

‘The ResNSW Flood Report contains no information about flood velocities and hence doesn’t consider their impacts. If you block a 465m width of a floodplain like this, you get still water behind the levee (on the east side) but you get a raging torrent around the two edges of the levee. By not examining velocities, government doesn’t have any picture of how they will impact Poinciana and Argyle Streets, which are the streams that the high velocity water will run down. The result could be that people who were able to get out of harm’s way under the pre-fill scenario may no longer be able to. One family escaped on 28 February by floating their kids to a neighbour’s elevated house using a kid’s three-ring pool as a life raft. Flood velocity must always be considered as well as flood depth. 

‘Why were the velocities not reported and made publicly available to the community? They sit there in the computer model – it won’t run without them.  We don’t know what the consultant was asked to do or report on as this has not been made public. The community is in the dark about the parameters being considered on their behalf by ResNSW.’ 

Long-term site? 

The Mayor, Cr Lyon told residents when the report was released that there had ‘been talk of houses and other purposes here [on the fill site] for 20 years… Those conversations [regarding future removal of fill] are not for right now’, he said. However, the risk to existing houses if the fill remains long-term are significantly increased. 

‘Under the state’s own Floodplain Development Manual, constructing works on a floodplain is only allowed after investigation through a proper Floodplain Study and Plan. We completed one in 2020. It doesn’t support a levee bank anywhere in the floodplain of the Brunswick River,’ explains Cr Dey. 

‘As shown on Table 4 of the ResNSW Flood Report, the 100-year flood level is lowered under Scenario A for 85 properties while being raised for 11 properties. The story for rarer floods, like the 2022 flood, is far more unacceptable however, and must not be ignored, especially if the fill stays after 2025. 

‘The report estimates 280mm of water above the floor level for one of the negatively impacted houses during a 1-in-100 year flood. However, they just experienced 800mm above floor level in the February 2022 flood. The report did consider the 2022 flood. Figures like Drawing 2.2 in the report indicate that a 100-year flood is 0.5m deep in Poinciana Street. However, flood marks indicate the 2022 level at half a metre higher. 

‘It is likely that when the North Byron Floodplain Plan is reviewed for the 2022 flood, that review will raise the 100-year level for this area to the level experienced in February. That is an increase of half a metre above what was studied for the ResNSW Flood Report. This report has studied the wrong flood. 

‘In planning law the 100-year flood is used to for setting floor heights for new constructions. When considering impacts of mitigation works, like levee banks, on existing residences, all floods should be considered, especially the floods of most concern to the people affected. In this case, that is the flood they just had. 

‘The ResNSW Flood Report doesn’t consider the 2022 flood and how a repeat of that flood would behave with the fill in place. 

Climate change

‘The ResNSW Flood Report ignores climate change, because it is for a two-year project, not the one the mayor is speaking about in relation to longer-term housing on the site. Climate change will make what is now the 100-year event occur more frequently. And similarly, the future 100-year flood is likely closer to the current 500-year flood. 

‘For the current 500-year flood, the report shows that the fill of Scenario A (which is effectively a levee bank) lowers the flood level at 57 properties while raising it for 56 properties. 

‘For the “Probable Maximum Flood”, the fill lowers flood levels at only two properties, while raising it at 52 properties. Most of those affected properties are west of the railway line, around Station Street. 

‘ResNSW modelling shows a significant increase in flooding in Station Street for the 1-in-500-year flood scenario. If the fill remains long-term, these figures would be the ones that count. They show that this levee bank would be deemed unacceptable under normal scrutiny.’ 

The right consultant?

It is understood by The Echo that work done last decade on the North Byron Floodplain Management Study by BMT, previously known as WBM-BMT, had to be redone by a second consultant before it could be used. Byron Shire Council resolved (19-036) in February 2019 ‘that Council recognise the weakness of service provided by the consulting company which prepared the Flood Study [will] and consider that in future engagements’. So why did ResNSW choose this same consultant?  

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