Slip could create dam

The huge Rawene car park slip could create a dam endangering the iconic Chelsea heritage park below it, council engineers say.

At the Kaipatiki Local Board meeting on Wednesday, Auckland Council’s geotech lead Ross Roberts said they were monitoring the situation ‘‘carefully’’ as there was a risk of the landslide causing a dam.

‘‘The real problem we are considering is that there will be a load of silt coming down,’’ he said.

Member Lindsay Waugh asked: ‘‘On a worst-case scenario could it take out the bank above the lower lake/cooling pond?’’

Roberts said he had visited the Chelsea Estate Heritage Park below the slip and the worst-case scenario was ‘‘highly unlikely’’ as any flooding from a dam establishing and breaching would result in over-land flow rather than underground erosion.

‘‘It would spread out and be a bit of a mess on the surface,’’ Roberts said.

‘‘I might hold you to that one,’’ Waugh said.

Six weeks before the landslide swallowed the back end of the Auckland City car park, Auckland Council began an ‘‘essential’’ $9-million project to strengthen unstable land that leads to the Chelsea Heritage Estate.

Auckland Council said, in an email dated August 15, the Chelsea Heritage Estate stormwater project was to renew the ageing and damaged stormwater pipes and to reduce the risk of collapse and blockage.

In a prescient email, project spokeswoman Liz Kirschberg said: ‘‘Surface run-off from the Huka Rd and Rawene Rd stormwater drain flows over the former landfill and may cause slope instability during large rain events.’’

Currently, the pipelines running under the former landfill of the Chelsea Sugar Refinery are in poor condition and cannot be repaired, she said.

Local board chairwoman Danielle Grant said in August, the stormwater improvements to Chelsea Estate Heritage Park should see the land stabilised, the water quality improved and an overall improvement to the main walking tracks in the construction area.

Construction began at the end of August 2017 and was planned to end December 2018.

The stormwater project is ongoing but engineers are now ‘‘monitoring the situation’’ at the slip-site above.

A potential dam caused by run-off from Birkenhead’s slip could threaten an on-going stormwater project, which was designed to strengthen the land.