Nelson weighs up cost of mitigating future flood damage

As Nelson recovers from the devastating August floods, the full extent of slip damage across the region is becoming clearer.

Thirty-three houses were damaged by landslide debris from council-owned land.

One of those was Barry and Nicola McKay's home of nearly 20 years. Memories of the day they were forced to leave amid heavy rain remained fresh in their minds.

"We just heard a big crack, I assumed it was a tree coming down off the bank onto the roof, but in actual fact what it was was the wall breaking because of the force of the slip so we just ran out, in our PJs," Nicola said.

As they ran out, Barry watched the ceilings fall and a slip -taller than he was - come crashing into the lounge.

Nine months on, they have only been able to return twice. Many of their belongings are still inside.

On Monday, the home the couple planned to retire in was pulled apart by a digger. They chose not to watch the demolition.

They have bought another home, and are yet to decide what to do with their land once their former house is gone.

"Obviously the land can be remediated," Barry said. "You can't ever live in a place with no risk, because there's no such place. End of the day, a rock could fall out of the sky. So it's managed risk, you know - people build on hills everywhere."

The McKays' former home is one of 33 properties in Nelson damaged last August by 18 slips that originated on council land.

Nelson Mayor Nick Smith said in the nine months since the floods, the council had engaged three geotechnical companies to come up with plans for remediation.

Two of the damaged properties have been purchased by the council, as the land is unable to be remediated.

The council's legal obligation was to reinstate the land where it could, at an expense of about $5m. But the cost of protecting against future landslides in the same areas was $17.2m.

Smith has written to Emergency Management Minister Kieran McAnulty asking him to consider splitting the cost of bettering the remediation works, an additional $12m, between the council and the government.

"The risk is that in a future weather event we're only going to have people in the community put back through the sort of trauma that we had nine months ago.

"The geotechnical assessments are that we can do extensive engineering and remediation work to get the level of risk which is never zero, but an unacceptable level of low risk."

Avoiding repeating past mistakes

Nelson City Council infrastructure manager Alec Louverdis said it was important remediation increases resilience in the face of more frequent severe weather events.

"If we were going to replace like for like, we would just revegetate some of these slips.

"We know that if we get a similar event that we got in August we'd end up with the same result, and that is unacceptable because we'd be facing the same issues the residents would have the same anxieties and we need to look ahead."

More extensive remediation involves retaining walls, catch fences, soil nails and deflection structures.

"There's a whole range of things that are unique to each slip and what we've done is work with consultants to figure out what the best solution is for those that will give us a degree of resilience and peace of mind."

"Every time it rains now, the days are long gone where we welcome that rain on the roof."

McAnulty said the recovery from increasingly severe weather events would require a partnership between local and central government - and while the current priority was the recovery from Cyclone Gabrielle, he was committed to working on longer-term resilience across the country.