A Tonkin + Taylor Ltd report evaluating both the chances ageing floodbanks in the Taieri flood protection scheme would fail in a significant flooding event and the consequences of failure is to be discussed at an Otago Regional Council meeting next week.
The report rated two sections of the Waipori floodbank as "extreme" risk because of the very high likelihood that they would fail and the "catastrophic" consequences if they did.
Such a breach could lead to catastrophic damage to the terminal building at Dunedin Airport, inundate the runway and cause major long-term disruption to the region.
The sections rated extreme equated to 400m of the 109km stretch of floodbanks, but 15% were rated very high risk and 47% high risk.
Only 7% were rated low risk, and the rest were rated medium (31%).
The report was based on the estimated performances of the floodbanks — parts of which are more than a century old — in a flood event comparable to the one in June 1980, which caused widespread devastation across the Taieri Plain and closed Dunedin Airport for 52 days.
Council staff estimated there was a 26% chance of such a flood happening in the next 30 years and a 40% chance in the next 50 years, based on today’s climate patterns.
Council engineering manager Michelle Mifflin said once staff became aware of the extreme risk rating for the two sections, they were investigated immediately.
As a priority, trees growing near the floodbanks affecting their integrity would be removed in the next few months, which would require specialist equipment and geotechnical advice on how the banks should be repaired.
"For the remaining risks, a risk mitigation investigation is planned — triaged on the severity of their risk rating."
Mosgiel-Taieri Community Board chairman Andrew Simms said the community had every right to expect the regional council to use the rates it collected to protect life and property on the Taieri Plain.
"We will be making every effort to ensure that is a priority for the Otago Regional Council."
The board had had significant concerns for some time about inadequate maintenance on the Silver Stream, Mill Creek and Owhiro Stream.
His initial reaction to the report was flood protection loomed as the biggest issue for the community this year.
"We have various concerns and have raised these with the regional council throughout 2023.
"This report now is not doing anything to allay our fears."
Dunedin Airport operations and infrastructure general manager Nick Rodger said the report highlighted the vulnerabilities in the system of pumps, drains and floodbanks that protected the Taieri Plain.
"Dunedin Airport will continue to work closely with ORC to reduce the overall level of risk from flooding, with this issue also being analysed through our flood hydrology plan, which will also investigate site level mitigations in the event of a wider system failure."
Ms Mifflin said the council was planning to hold community information sessions about the report. Dates were to be confirmed and advertised.
While the flood protection scheme did work to reduce the flood risk, the risk of floodbanks being overtopped or breached under flood conditions could never be eliminated.
