NCC plans to check key sewer after leak
The Nelson City Council is looking at spending $100,000 to use high-tech equipment to investigate the cause of a raw sewage leak into the harbour.
It is also considering bringing a long-term planned system upgrade forward.
The leak this month, caused by fragile old concrete pipes, resulted in the area between Tahunanui Beach and the port being closed to the public.
The council said the spill happened after the failure of the Atawhai rising main - a drain or sewer through which sewage and/or surface water runoff is pumped to join with the main sewerage system - at Marybank.
The council's group manager of infrastructure, Alec Louverdis, has asked the council to put an extra $100,000 into investigating the leaks and the condition of the Atawhai rising main's pipes as part of the coming year's annual plan.
Work to duplicate the Atawhai rising main is planned to start in 2024-25 and finish in 2033-34, with an estimated $12.5 million price tag, but high-tech equipment will be used to see whether the pipes require immediate repairs or need to be upgraded earlier.
Louverdis said possible options included using a "pig" - a large, bullet-shaped, abrasive object the same size as the internal diameter of a pipe, which can be passed through the pipe to clean out accumulated debris or material.
Others included CCTV or "smart ball" technology, which uses a small sphere containing sensors to check the inside of the pipe.
"The ball is released into the pipe to identify any large leaks or gas pockets. As most of the failures to date have resulted from hydrogen sulphide gas forming weak sulphuric acid on the top of the inside of the pipe, we are primarily interested in these gas pockets," Louverdis said.
Once the council had the information, it could either use a robot with more sensors to check any changes to the pipe, or repair or replace weak sections of pipe, he said.
Nelson Mayor Rachel Reese said it was appropriate to do the investigation work, because the planned long-term upgrade for 2024-25 was a major capital project and the council needed to know what work was required.
"To help us with our forward planning of infrastructure, we need to have a good understanding of the condition of that pipe network.
"Ideally, you don't want to be in a situation where you have sewage leaks into an estuary," she said.
"We need to do something about it, so the question now is, how quickly do we need to undertake that work?
"Can we manage the pipe network with some repairs over the next couple of years, or do we need to look at a major upgrade earlier than we thought?"
The council expects the investigation work to be carried out in the next 12 months, once a contractor is appointed.