Wellington Scoop

3000 tonnes of sediment: the consequences of Transmission Gully

Three thousand tons of sediment flowing into Porirua Harbour … one of the consequences of building the new Transmission Gully motorway which has been given draft approval by the government’s board of inquiry.

The main reason for the draft approval is that the coastal road is seen as

inadequate, congested … and could close in an earthquake or tsunami.

The new route will offer … an alternative four-lane route which will avoid congestion, reduce travel times and achieve consistency in travel times.

However the price to be paid – as well as the $930million – includes

the destruction, modification, or diversion of 10km of streams … and … sediment from construction effecting in particular the nationally significant Pauatahanui Inlet

The inquiry was given different estimates of how much sediment could potentially flush into the inlet. The figure of 3000 tonnes seems to have been a contentious one.

But it’s undeniable that while the Porirua City Council has begun a campaign to get rid of pollution in its harbour, the motorway-building work of the NZ Transport Agency will put back these efforts by many years.

The DomPost today says of the decision to build the new road: little better news could be heard for the capital…The sooner construction starts the better.

And of the 3000 tonnes of sediment, it peers hopefully into a distant future:
Increased sediment flow will damage the Pauatahanui Inlet – the only large area of salt marshes and seagrass in the Wellington region – but the damage will be offset by the longterm benefit of reduced siltation once trees planted in association with the project are established.

What about the streams and fish?

Dedicated to cleaning up Porirua harbour

We want cleaner water

New trust pushing for faster harbour improvements

IMAGES: how sediment damaged the Makara coastline


  1. Nick, 8. May 2012, 10:31

    I don’t think the problem is with the current coastal route. The problem is that the traffic backs up from the Waikanae lights at rush hour. I’m not sure how Transmission Gully will make that any better?

    No one seems to know how much Pauatahanui Inlet will be affected – that is a concern for what the city could lose. The token environmental tree planting gestures are a slap in the face.

  2. traveller, 8. May 2012, 15:49

    Three thousand tonnes of mud – is a better description of what will be the devastating result of Transmission Gully.

  3. Sridhar, 8. May 2012, 20:35

    So what! who cares! isn’t the ability to keep driving uninterrupted at 100kmph more important? who cares about damage to the environment. Who cares that petrol prices will rise by the time transmission gully is ready for use. It’s speed man! Just think about that.

  4. James, 8. May 2012, 22:20

    You may want to read about Marinetti and futurism >:->

  5. Elaine Hampton, 9. May 2012, 12:24

    What NZTA refuses to acknowlege is that car use has peaked.
    These roads are not being built for cars but for large trucks to keep our goods on the road, presumably powered by diesel made from Lignite mined in the South Island.
    Doing nothing more with roading is really a good idea
    Meanwhile the railway line tracks are pulled up (see SH 2) – now that is a really stupid idea.
    3000 tons of mud is a side issue to the road builders. Environment is a side issue. People are a side issue.
    Goods trains could move freight and smaller vehicles could be used to move them to the final destination.
    Then trains could be used for public and goods transport when the oil is too highly priced and the train could again stop at Taumaranui on the main trunk line.

  6. Sridhar, 10. May 2012, 7:05

    @ Elaine! Like your comment. Wish scoop had a “like” button.

  7. Chris Horne, 13. May 2012, 21:55

    If Transmission Gully Motorway were to be built, and in the highly unlikely event that petrol and diesel prices are about the same as they are now, then the extra traffic such a road would generate would guarantee gridlock in the capital far more severe than we’ve ever experienced.

    Politicians and transport planners should be future-proofing the nation and the region, to the best of their ability, against the inevitable spiralling increases in the prices of petrol and diesel, now that peak oil passed c. 2008, and in preparation for the inevitable mandating of drastic cuts in the transport sector’s emissions of greenhouse gases.

  8. Janice, 14. May 2012, 11:06

    Interesting that neither Scoop, nor anybody commenting here, seems to have bothered to read the Board of Inquiry’s decision. The Board determined that the additional 3,000 tonnes of sediment (or mud if you prefer) during construction would be offset by all the vegetation planting as part of the project, while will reduce sediment/mud by 450 tonnes per year, every year forever after the construction finishes. They concluded that it only takes 7 years before the sediment/mud from construction is fully offset and after that ongoing effect is a positive one, not a negative one.

    Since this article is about the environmental effects on the Porirua Harbour (rather than the traffic ones that most people here are talking about), then people like me who care about the harbour should be informed that the real impact of transmission gully on sediment/mud is a good news story, not another bad news story, which Scoop always loves telling people about.

  9. filosofos, 17. May 2012, 1:34

    Really, Janice, how gullible can you get? Both the Board of Inquiry and the Environmental Protection Authority are appointed by the same government that is set on pushing through this white elephant at all costs.

    As for the EPA – do you know who the chair is? Kerry Prendergast, ex-mayor of Wellington with the worst possible environmental record imaginable, and a rail hater to boot.

    How can these people predict whether the new vegetation will offset the mud and by how much? Have they done any studies or simulations? Is it even possible to do so or is this just another paper exercise coming up with the result they want? And in the end will all the planned vegetation actually be planted or will they decide that costs have overrun, so let’s scale it down or forget it altogether?

    Make no mistake, this road is all about keeping the National Party in power. Perhaps you don’t know their biggest financial contributor is the Road Transport Forum which represents the truckies. While rail has to pay for its own infrastructure, the truckies have a free road to travel on. They don’t pay anything like the true cost of road building and maintenance. In effect, you and I are subsidising their businesses.

    Interesting, isn’t it, when you delve a little deeper?