Providing safe public access to the Caversham Tunnel could mean more sewage might flood Surrey St properties, Dunedin city councillors have been told.
Access to the tunnel, through which the Dunedin Tunnels Trail Trust hopes to build part of a cycle trail connecting Dunedin and Mosgiel, is restricted because of the utilities it houses.
One of the main issues is that sewage overflows and dangerous sewer gas could escape from two manholes inside the tunnel.
Sealing the manholes would meet the council's obligation under a memorandum of understanding with the trust to help provide access to the tunnel, but would create issues elsewhere, councillors heard.
Water department staff said the sewer overflowed in the tunnel two or three times a year.
Blocking the manholes would force that overflow to escape elsewhere, in this case into Surrey St, where it could overflow into the stormwater, which could then overflow into the street, and people's properties.
Water and waste manager Dr Laura McElhone said staff had looked closely at how that might be avoided, but concluded the only way to fix the problem was to replace the ageing sewerage system.
''The most cost-effective and efficient way to fix this is to address the root cause of the problem, and that is the ageing pipes.''
After many years of delays, the council had embarked on a programme of pipeline renewals, but it would be at least five years before and difference was noticed and 10 to 15 years before it was fully fixed, she said.
Wastewater already overflowed into Surrey St once every two years or so. Sealing the tunnel manholes could cause that to happen more frequently, though how often was unclear, she said.
''[Sealing the manholes] is not something that would be done lightly, but without sealing the manholes we won't be able to use the tunnel.''
Councillors offered all sorts of technical advice and options, but only one, from Cr Staynes, will be considered further: that staff look into whether part of the tunnel, possibly either end, could be opened to the public at least, and any overflow in other parts of the tunnel be stored and taken away.
At the end of the day, it was the levels in the manholes that were the issue, Dr McElhone said.
''We have looked at this carefully and to really address the overflow situation a sustained programme (of pipe renewals) over the next 10-15 years is the only option.''
In the end Cr Richard Thomson called a halt to an already lengthy conversation, suggesting it was a matter that was better dealt with outside the budget process, given it involved about $40,000 worth of work, which could be spent from existing budgets.
Councillors agreed staff should report back to the infrastructure services committee with more detailed options in due course.
