Otago Regional Council trying to ensure flood protection scheme viable in long-term

An Otago scheme aimed at keeping floodwaters at bay is under review with climate change looming as a potential challenge.

The Lower Taieri Flood Protection Scheme is a buffer for roughly 13,000 hectares, including the townships of Mosgiel, Outram, Momona and Dunedin Airport.

Some of the scheme dates back to 1870, but ageing infrastructure is only part of the problem.

When the Taieri River flooded last January, it hit Outram Berry Farm during peak season.

The farm sat nearly level with the Taieri River, and was only a property away.

Its owner, Alex Cunningham, said most of their berries were submerged and some of the young plants went completely underwater.

"It also submerged farm implements and vehicles at the time, or it got up into their engine bays, and then the residue of the flood, which of course stopped us picking for a couple of weeks while we waited for the bushes to dry out, then left a silt residue behind on the berries which was tastable."

Otago Regional Council has earmarked about $45 million over the next three decades to protect the Taieri Plain from flooding.

The council's engineering manager, Michelle Mifflin, said the Lower Taieri Flood Protection Scheme was currently fit-for-purpose but it was facing more challenges.

Ensuring viability in 30 years' time

Towns were spreading out with development encroaching further into the floodplain, more floods were predicted and the river dynamics were changing, she said.

"It's nothing to be concerned or alarmed about. It's future thinking. It's not only for the integrity of the scheme now, but we are looking to the future of the scheme in a 10, 20, 30-year plan."

The lowest parts of the plains were about one-and-a-half metres below the mean sea level.

Flood banks were only part of the picture as raising or widening them could make them more vulnerable, she said.

"But it might mean that we look at spillways, we might look at retreat in some areas where the scheme reaches the sea so there's a lot of options, and the review will help guide those decisions."

Vulnerable home and property owners still needed a flood plan, Mifflin said.

"You have to have flood mitigation. The scheme provides a level of protection and service, but there has to be that awareness ... and that responsibility for any owner in a rural or urban environment that they need to be prepared."

Dunedin Airport chief executive Richard Roberts was glad the review was underway as flooding events became more frequent.

"The inundation that we've had has been around the peripheries of the airport ... Carparks, some of the tenant's rental car garages have been flooded and not been able to be occupied for a number of days."

He was keen to see a spotlight on the scheme's management to ensure it was also in tip-top shape for the future.

"They've got these pump stations that drain the whole valley. Are they turned on early enough in a weather event? Are they robust enough if we lose power supply? Is there back-up?

"All those operational things, we need to address those."

Call for iwi involvement

Tangata whenua and farmer Ian Bryant's dairy and beef property is next to the airport.

He could get surface flooding from heavy rainfall but Taieri River has not spilled over the flood banks onto his property since the June 1980 flood that closed the airport for weeks, he said.

His tipuna was based down in a kaik near Henley on the riverbank.

"So the flood never affected us down at Henley because the Taieri Plain acted as a big basin to contain the flood water."

He hoped iwi would have an active seat at the table during the review.

"As soon as the water was channelled by flood banks and drainage systems were put in, our Henley kaik came into that unprotected area and so it immediately became not viable.

"It just disappeared within a short number of years once the flood protection scheme went in."

Mosgiel-Taieri Community Board chair Joy Davis said flooding was a hazard they were learning to live with but some good work had been done in recent years to improve the protections.

She was pleased with the community consultation which has included some initial meetings.

"The way they're listening to the locals and finding out from people who live on the floodplain where the pinch points are, why things haven't worked in the past, what might work in the future, so I think there's a lot more two-way conversation.

"And that gives some sense of sense of security to people as well to know that their local knowledge is being listened to."

The council is gathering together recommendations to analyse and then taking them to the community and the council chambers later this year.