Citizens have taken to the streets in frustration at there being no report yet on flood levels experienced in the devastating February 2022 flood. We do, however, have river gauge levels at Federation Bridge. A crucial question for fluvial hydraulics in Mullumbimby was whether the Federation Bridge over the Brunswick River in the middle of town restricted flood flows and thus caused the 2022 flood to spill via routes that leave the main river channel upstream of the bridge.
Turns out, it did.
The government-operated river gauge on Federation Bridge peaked at 6.43am on 28 February 2022 at a water level of 4.98m AHD. All the following levels are to Australian Height Datum (AHD). They were cross-checked via two datum sources, but are not certified by a registered surveyor.
One problem with the government gauge is that it measures water levels on the downstream side of the bridge. Water levels upstream of the bridge can get way higher – especially when the bridge opening is ‘filled’, which happens if the river surface touches the underside of the bridge.
Turns out, it did.
The level of the underside is about 4.8m AHD. Big floods do reach up to the underside of Federation Bridge. Gauge heights above 4.8m don’t tell us what is happening upstream (west) of the bridge. This happened in 2022 and appears to have happened in 2017 too. Flood levels marked on the upstream side of the bridge indicate that levels there peaked at 5.45m in 2017 and 5.8m in 2022. This 2022 level is 800mm above the level recorded at the gauge. It explains why, in 2022, unprecedented amounts of floodwater spilled from the river channel west of Federation Bridge and flowed through precincts from Mullumbimby High School to Dalley Street. At the high school end, Mullumbimby Creek may have contributed as well.
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