City water supply remains vulnerable

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Opinion

News that Gisborne’s main water supply was identified as being at unacceptable risk of catastrophic failure months before it was broken in multiple places during Cyclone Gabrielle won’t come as a surprise to anyone familiar with the situation.

Gisborne District Council completed its first government-required water plan just three months before the cyclone struck.

The plan showed safeguards around the 40 kilometre supply pipe from Waingake to the city were not enough to reduce the high risk to an acceptable level. It rated a loss of supply as possible, with catastrophic consequences.

The council told RNZ that after the report it took two immediate steps. It did a stock take of critical spares and assessed the pipe for cathodic protection from corrosion.

But it did not address the problem of forestry slash and its water safety plan does not mention slash or what to do about it.

During Cyclone Gabrielle in February, slash debris smashed into bridges damaging or destroying nine of them. Sections of the pipe broke in 10 places.

A source hazard report said large areas of the Pamoa Forest had recently been harvested and added that areas of exotic plantation in proximity to the supply pipeline presented a very high risk during future harvest activity and post-harvest due to the steep topography and erosion of clear-felled land.

The vulnerability of the pipe was shown in Cyclone Bola in March 1988, long before forestry slash was an issue. It was also snapped by a landslide in 2014 which cut the city’s water supply for a week. The city is still under Level 3 water restrictions because of Gabrielle.

Again, none of this is a surprise and the problem for the council is that there are no quick fixes.

The council told RNZ it was considering getting its water from somewhere else or shifting the pipe.

Efforts to secure the pipeline had centred on the Waingake transformation programme which, in conjunction with iwi, will see 1200 hectares of pine plantation in the catchment eventually converted into native forest. The last of the pines in the council-owned block are due to be harvested by 2027.

A long-term plan for another reservoir elsewhere will be being reassessed now, with the main issues being the massive cost involved and similar land stability issues region-wide. But the essential situation is that we know Gisborne’s main water supply pipe is vulnerable, and there is not much immediately that can be done.