Costly dilemma: $20m price tag to replace South Wairarapa bridge

A century-old bridge linking Featherston to the South Wairarapa coast will cost about $20 million to replace or $375,000 for a "do-minimum" band-aid option.

A recent engineering report showed the Lower Valley bridge on Kahutara Rd was unable to carry high productivity motor vehicles and required strengthening.

It had been under a 10kph speed limit since December last year and had a gross weight limit of 44,000kg.

It was previously used as an alternative access route into Martinborough when the State Highway 53 bridge was closed due to flooding.

Due to its vulnerability it now also closed when the SH53 bridge did, following advice from engineers.

In a report to South Wairarapa District Council, which was set to meet on Thursday, group manager of infrastructure and community operations James O'Connor said initial investigations suggested a range in costs to remedy the Lower Valley bridge.

This varied from about $375,000 "at what is essentially the do minimum option", through to about $19.8m for a full bridge replacement.

The bridge was currently being monitored by a specialist surveying consultant and engineering firm Stantec to assess any impacts resulting from ongoing weather events.

O'Connor said the bridge was one of 141 structures in the district that were aged and required "a robust inspection and maintenance programme to ensure they are fit for purpose".

He said this was a common situation across the country and that the condition of the aged infrastructure had at times contributed to disrupted access on the roading network during bad weather events.

The council's focus was largely on renewals rather than addressing all maintenance needs "as renewals will reduce the need for maintenance in the longer term", he said.

"However, this is still balanced with an increase in maintenance being required as in some cases this will extend the asset life before a renewal is needed."

Over the next three years, bridge projects would be prioritised to improve roading network resilience on alternate routes, such as access into Martinborough when State Highway 53 was closed, or routes that provided sole access for a community such as Cape Palliser or Lake Ferry, O'Connor said.

"Estimated maintenance costs per bridge structure range from thousands of dollars to hundreds of thousands, noting that full replacement can be into the millions of dollars," he said.

In the report to South Wairarapa councillors, Ruamāhanga Roads - which managed Carterton and South Wairarapa's roading network - highlighted that the current budget allocated by South Wairarapa District Council was "insufficient to address all maintenance needs identified through previous years' inspections".

"This shortfall is further compounded by the increasing costs associated with retaining wall structures which are placing additional pressure on the existing structural budget."

Ruamāhanga Roads would continue to prioritise critical works and deliver maintenance activities within the available annual budget, the report said.

As part of the upcoming Long-Term Plan and Asset Management Plan, Ruamāhanga Roads was working towards developing new strategies to mitigate the existing maintenance backlog.

This included addressing the growing demand and cost pressures associated with retaining wall structures, as well as improving the condition and integrity of ageing assets to build a more resilient structure network across the region into the future.