January's 6.2 magnitude earthquake has created the danger of 4 metre-diameter rocks falling on the access road to Palmerston North's water treatment plant and storage dams.
City council water and waste services manager Rob Green said there was a "very high threat" that loose rocks could tumble onto the upstream section of Turitea Rd.
Staff and contractors on the road could be in danger, and there was a risk access to or from the plant could be cut.
Cracks in a steep rockface above the road, on the section used only by those with business at the plant or reserve, were noticed shortly after the quake, which was centred just 27 kilometres from the dams.
The dams were checked after the earthquake and no damage was identified.
Geotechnical reports have confirmed the risk of a rockfall above the road in the event of further quakes or heavy rainfall.
Mr Green said the road was a crucial link to the treatment plant.
The city's principal water main lay below the road, but was unlikely to be damaged by falling rocks.
A protective barrier has already been put on the road to reduce the risk of falling rocks landing on vehicles, and a warning sign is in place.
But Mr Green said the precautions could only be temporary, and the barriers further narrowed a road already only 4m to 5m wide and used by some large truck-and-trailer delivery vehicles.
He recommended removal of the large rocks most at risk of falling.
In a process known as "scaling", the rocks would be loosened using hand tools, airbags and explosives.
The cost of scaling and installing protective nets across the entire rockface, similar to what has been done in the Manawatu Gorge, was estimated at $80,000.
There was $30,000 that could be diverted from a project to build retaining walls along the access road, but using that money, and the additional $50,000, needed full council approval.
Mr Green will put the case for the spending to the finance and performance committee on Monday.
- Manawatu Standard
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