The pipeline in 2022. Photo: LDR / Supplied
The pipe supplying the only source of drinking water to Featherston remains at high risk of failure and an urgent solution is needed, experts say.
South Wairarapa councillors will meet next week to reconsider the replacement of a section of the pipeline put forward by Wellington Water in 2022.
The cost estimate from that time was $2.75 million.
The option, which involved replacing and trenching the section of pipeline that crosses the Tauwharenīkau River, was not endorsed by the previous council at the time because further information was requested.
A temporary fix, costing about $300,000, was pursued while this information was gathered.
The section of the pipeline across the river was labelled at high risk of failure after it was seen to be exposed and leaking in 2021.
At the time, about 23 metres of pipeline was exposed in the Tauwharenīkau River.
Of this, 15m was concrete-encased and 8m was exposed steel pipe.
After temporary solutions were pursued in 2022, Wellington Water informed South Wairarapa District Council (SWDC) in September last year that a section of the exposed concrete pipe casing had now dislodged from the pipeline.
Crews from Wellington Water repair the pipe in 2022. Photo: LDR / Supplied
Subsequently, Wellington Water commissioned an independent assessment from a consulting engineer who said there was a high scour and erosion risk to the pipeline.
He also said remedial works or the renewal and trenched installation of the pipeline should be "progressed with urgency".
Analysis of replacement methods supports a trenched installation under the river at 4m deep.
Three long-term options have been put forward for consideration at SWDC's Strategy Working Committee, which meets on Wednesday morning.
All associated cost estimates in the report are from 2022.
Option 1 is to "do minimum" and involves keeping the existing pipe as is and doing annual maintenance.
This option would cost $130,000 in capital expenditure and $3.08m in estimated maintenance, based on a 100-year design life.
SWDC would carry a high risk of failure until the pipe is replaced and the risk remains that a large river flow event could expose and damage the pipe further.
The council would still need to replace the pipe in 20-30 years.
Option 2 is to reinforce the existing pipe, which provides some resilience to high river flow events and scour protection, but the condition of the internal pipe is still unknown.
This option would cost $5.39m in capital expenditure and $1.62m in estimated maintenance.
The council would carry a medium risk of failure until the pipe is replaced.
The third option, which is recommended by council staff and Wellington Water, is the replacement of the pipe using a trenched installation under the river.
This would cost $2.75m in capital expenditure.
The estimated maintenance costs are $0.
With this option, the short-term effects of working in the river would need to be managed and the removal of existing pipe may need consent.
Further consultation with mana whenua is also required, the report says.
If Option 3 is endorsed, a detailed design would be completed by June and construction would start in September.
The drinking water supply for Featherston is currently supplied from the Waiohine water treatment plant near Woodside, Greytown.
The pipeline was originally installed under the riverbed in 1976 but in recent years, the bed of the river has degraded, exposing the pipeline.
LDR is local body journalism co-funded by RNZ and NZ On Air.