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Reports of sediment, mud running into Auckland stream from major motorway project


By Aaron Dahmen

Local residents say measures put in place to stop sediment runoff into the Papakura Stream are "not fit for purpose." Source: Supplied

There are serious concerns a New Zealand Transport Authority (NZTA) development on Auckland’s Southern Motorway is causing sediment to run into waterways, affecting wildlife and water quality.

1 NEWS understands Auckland Council environmental protection staff will be asked to check on reports of sediment and mud entering the Papakura Stream.

David Hopkins, chairperson of Wattle Downs Residents and Ratepayers Association, said the stream needs further protection from runoff, rubbish and erosion.

The project is NZTA’s estimated $268 million dollar Southern Corridor Improvements development, which started in October 2015, and covers the Southern Motorway stretch from Manukau to Papakura.

It includes adding additional lanes in both directions, upgrading the Takanini Interchange and building a 4.5km pedestrian/cycle path.

Mr Hopkins said, before the project began, the land above the Papakura Stream was covered in trees.

“Most of them had to be knocked down, exposing a lot of soil.”

While he understands the decision to remove some of the trees for construction purposes, he wants more care taken to stop rubbish from the road falling down the slope.

Before NZTA's Southern Corridor Improvements project started in October 2015, the land above the Papakura Stream was covered in trees. Source: Supplied

According to Mr Hopkins, NZTA added protection after further vulnerabilities to silt erosion were revealed at the site in April this year.

He said contractors covered the lower parts of the site with coir matting and erected a small erosion trap fence.

“This was always going to be insufficient in the event of heavier rainfall… it’s just not fit for purpose.”

NZTA Project Delivery Senior Manager Andrew Thackwray told 1 NEWS the fence undergoes weekly inspections by the contractor’s environmental team.

He said erosion and sediment control has been monitored throughout the project.

NZTA was first made aware of the latest sediment runoff last Sunday after heavy rain. It intends to increase inspections, particularly during wild weather events. 

“We have taken immediate steps to repair the environmental controls and ensure the area is clean and controls are in good condition.”

Mr Thackwray admits the silt fencing was damaged, causing "a small amount of erosion on the lower bank." 

Replanting of the area with native species was also planned to start this week.

But Mr Hopkins said, while NZTA has agreed on the need to protect the slope from runoff, the response should be more proactive.

"There need to be adequate physical barriers so that rubbish and discarded material from vehicles cannot get into the stream."

He is asking for higher expectations of NZTA’s road designers and contractors to totally protect sensitive environments.

Mr Hopkins said eels, fish and wildlife depend on the waterway. 

Sediment and mud from Auckland's Southern Motorway can run down the steep slope. Source: Supplied

Papakura Local Board chair Brent Catchpole accepts growth cannot come at the environments’ expense, but said the contractors, Australasian construction company CPB, hold an excellent consent management record and no problems have been reported so far.

“They use sustainable approaches and are aware of the sensitivities involved in working so close to precious waterways."

Mr Hopkins said Auckland Council staff have been active in requiring NZTA to achieve better standards.

A strategic plan that includes driving the reversal of environmental decline caused by freshwater sediment runoff was endorsed by Auckland Council’s Environment and Community Committee in December 2018.

Mr Catchpole believes protecting the environment is a cornerstone of his board’s work.

“It's simple. Cause the pollution, provide the solution.”

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