Auckland flooding: Landslips, sediment close West Auckland dam

Monday, 6 September 2021

Auckland mayor Phil Goff says flooding in Kumeū on top of Covid is a 'big burden' for many. (Video first published August 31, 2021)

The wild storm that ravaged homes, flooded Auckland’s western suburbs and left 400 homes without power has also filled one of the city’s drinking water dams with sediment, resulting in its temporary closure.

Intense, localised downpours affected suburbs including Huapai, Waimauku, Kumeū, Ranui and Piha, with Kumeū noting its second wettest day since records began.

The severe rainfall caused landslips in the west which pulled debris and dirt into the catchment, muddying the dam’s waters and blocking its access ways.

Watercare’s higher western catchments recorded between 220mm and 270mm of rainfall over twelve hours.

Last Monday and Tuesday, rain gauges at Watercare’s higher western catchments recorded between 220mm and 270mm of rainfall over the course of twelve hours.

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The severe rainfall resulted in landslips and flooding in Auckland’s western suburbs.

The landslips and flooding has blocked the dam’s accessways, and muddied its waters.

The catchments, which feed into Upper Huia, Upper Nihotupu and Waitakere dams, normally receive between 150mm and 170mm of rain in the entire month of August.

An aerial survey carried out by Watercare, which provides water and wastewater services to Auckland, found some of the larger slips had taken out large trees and sent murky water into the supply lakes.

“Upper Huia Dam appears to have fared the worst in terms of water quality, and we suspect the raw watermain may have been moved by a slip,” Watercare chief operations officer Mark Bourne said.

“Given we can’t access this dam at the moment to inspect it, we have taken it out of service.”

Bourne said the Upper Huia Dam was overflowing into the Lower Huia Dam and dirtying the water, making the turbidity readings – which measure the clearness of the water – “off the charts”.

The water at the bottom of the Lower Huia storage lake was the dirtiest, he said, “with turbidity readings up to 500 times greater than normal”.

The water was being treated at the Huia Water Treatment Plant before it would be ready the public to drink.

Bourne said Watercare’s diverse range of water sources and increased treatment capacity at the Waikato River plants meant the Huia plant could be run with low production levels while “still meeting Auckland’s water needs.”

Clean up operations are ongoing throughout the city’s western suburbs, where more than 5500 houses are currently being assessed for damage.