Shepparton’s river level, including its flood peak in October 2022, may have been as much as 0.08 metres higher than officially recorded after the Goulburn Broken Catchment Management Authority found a discrepancy between the peak of the 2022 and 1974 floods.
The disparity would make the 2022 event Shepparton’s biggest flood on record.
An email leaked from the GBCMA about the flood said the flood level in October peaked at 12.14 metres, not 12.06 metres as reported at the time.
The email said from 1968 to “at least” 1983, the gauge measuring the Goulburn River was located upstream of Dainton’s Bridge.
The gauge is now eight metres downstream of the bridge, which results in water being caught up by the causeway between Shepparton and Mooroopna which, despite several floodways and bridges, acts similarly to a dam on the Goulburn River.
The email said there was a head loss of 0.08m from one side of the bridge to the other.
It is unclear if emergency plans for Shepparton and Mooroopna and flood planning are based on the 1974 flood level or have been adjusted for current measurements.
Similarly, it is unclear what the measurement disparity means for the operation at Loch Garry, which is triggered when the flood measurement reaches 10.36m in Shepparton.
The adjustment would also make 2022 flood Shepparton’s biggest flood, with the adjusted level of 12.14m higher than the 1974 flood, which peaked at 12.09m.
“The 2022 flood level of 12.14m at the 1974 gauge site has been estimated based on the head loss of 0.08m through the bridge structure as determined from the Shepparton Mooroopna 1% AEP Flood Mapping Project,” the email said.
Greater Shepparton City Council’s Flood Emergency Plan, in a footnote on page 21, notes the gauge was moved downstream of the bridge in 1986.
The GBCMA was contacted for comment, but did not respond by deadline to questions about how the error was noticed and what it meant for historical flood data.