Maintenance advice heard

The Clutha Mata-Au river running through Roxburgh. PHOTO: ODT FILES
The Clutha Mata-Au river running through Roxburgh. PHOTO: ODT FILES
Geography and geology mixed with massive rainfall were responsible for past flooding in Roxburgh, a meeting in the town heard on Wednesday night.

Otago Regional Council natural hazard analyst Julion Wright said he was part of a team working to mitigate events like those that created widespread damage and cut access to the town in 2017.

Sediment from the Old Man Range behind the town came down creeks and streams forming alluvial fans, which were a dynamic land form, he said.

A slurry of water, rock and soil, gets washed down when there is intense rainfall, like the event back in 2017.

As a result, State Highway 8 is at risk as it runs between the hills and the Clutha Mata-Au river, Mr Wright said.

Since the 2017 event, NZ Transport Agency Waka Kotahi had replaced and increased the size of culverts along SH8 and increased the size of the concrete chute that channelled water from Reservoir Creek to the river.

The flooding had eroded land and increased the likelihood of landslips. Banks had been undercut and there was a lot of sediment ready to go if there was another massive rainfall. It was possible climate change could bring more rainfall events, he said.

The regional council was monitoring and developing creek maintenance plans to allow channels to operate as efficiently as possible, Mr Wright said.

People at the meeting had plenty of questions and advice for the council.

Several said the culverts that had been replaced were still not big enough and the bridges were not suitable for the area.

"They are playing at it," one man said.

However, that work was done by Waka Kotahi and was not the council’s responsibility.

Fish ladders, which the council required in culverts and weirs to allow fish to move up and down stream between their various habitats, were cited as a problem.

A man in the audience said the fish ladders were creating problems in culverts.

"There’s no water in them for eight or 10 months of the year but they are creating a problem with debris."

The alluvial fans used to be moved as the gravel was used for various purposes but "rules and regulations" meant that was no longer possible and it had built up, an audience member said.

"You need to pull the plug out of the sink," she said.

Mr Wright said local knowledge was invaluable alongside the scientific data his team were gathering.