ACook Strait bridge? Really?
Bridging the gap between the islands. Can it be done?
The Cook Strait is a violent body of water. It’s an exception. Unlike other straits around the world, it has opposite tidal flows at either end. When it’s hightide on the Tasman side, it’s roughly low-tide on the Pacific side and vice versa.
Before the end of the last Ice Age, you might have been able to walk between the two islands – if there had been anyone around to do it. But for the last 20,000 years this troubled strait has divided New Zealand in a way most countries have never known.
What if the country could become physically connected again? Is a Cook Strait bridge or tunnel pure fantasy?
There would be far more to gain that just the novelty of being able to take a 27-hour 2000km drive from Cape Reinga to Bluff. Immense increase in traffic between the two islands, the untold billions saved in shipping and flying costs, the Marlborough and Wellington areas thriving and booming from increased commerce, the tourist dollars, the sheer convenience of replacing a three-hour ferry ride (and it’s associated on and off-loading times) with a short drive.
It’s an idea that’s so outrageous even some of our more seasoned politicians have never even heard it being raised before.
The idea was pitched to Transport Minister Phil Twyford.
‘‘This is the first time I’ve heard the idea. I know there is a successful tunnel between the UK and France, but I would have thought our fault line would rule out a tunnel. It is also a very rough stretch of water, and I’m no engineer but I suspect that would rule out a bridge,’’ he said.
Judith Collins, who has National’s transport portfolio, was impressed by the idea but pointed out an obvious flaw.
‘‘Wow, this is a huge ambitious and audacious idea. Where would be the fun of a Cook Strait pie in the middle of a howling gale though?,’’ she said.
These days something like 1.1m people and 350,000 vehicles cross the strait every year with the two ferry companies, the Interislander and Bluebridge. Many more would be guaranteed to use it who would normally fly or not take the trip at all.
With a bridge or tunnel, suddenly the prohibitively expensive and time-consuming trip from somewhere like Palmerston North to somewhere like Nelson becomes a drive that could be done in under five hours.
Ask someone in the know and they’ll quickly explain that it’s a pipe dream.
‘‘I think given the geometry and the morphologies of the strait, a conventional bridge is not possible. The water is extremely deep and the cost will be prohibitive. Tunnelling will also be very expensive.’’ That from Professor in Structural Engineering and Materials Alessandro Palermo at the University of Canterbury – one of New Zealand’s top specialists in bridges. It’s tempting to look at the strait’s narrowest point of just 22 kilometres for a potential crossing from Cape Terawhiti to Arapaoa Island in the Marlborough Sounds. That, however, means building a state-highway tier road through the extremely hilly country behind Karori all the way to the coast, another such road across undeveloped Arapaoa Island in the Sounds, a bridge across the Tory Channel and another road to get back to State Highway 1.
A 27km bridge or tunnel from Cape Terawhiti directly to the mainland and bypassing Arapaoa Island, landing on the peninsula east of Picton, would still involve significant roadworks.
If, on the other hand, you wanted to build a link between the two closest developed points (Wellington city and either Picton or Blenheim), the distance is 64km to Picton and 65km to the shoreline just off Blenheim.
Bridges are much cheaper than tunnels. The catch? A bridge would have to withstand a highly turbulent Cook Strait, probable earthquakes and be high enough for shipping to get through (or at least able to open up).
A 65km-odd bridge would be New Zealand’s biggest bridge by far.
It’s so long, in fact, that if it existed it would be the sixth longest bridge in the world.
The longest is in China is 165km long.
Tunnels are much, much more expensive.
The 2.4km Waterview Tunnel in Auckland cost $1.4b – which equates to about $583m per kilometre. For a 27km tunnel, that’s $15.7b. For a 65km tunnel, that’s $37.9b.
Even if the government were to spend 50 per cent of the country’s capital allowances for transport over the next four years, a tunnel starting at the Wellington CBD might not even get to the water.
Palermo has another, more modern idea. A ‘submerged floating tunnel’ – a tunnel which floats on or near the surface of the sea and is anchored to the ground.
One has never been built but the idea is being developed and explored in places overseas like Japan and the United States.
Palermo said the main challenges are sea currents, earthquakes and tsunamis but:
‘‘I think the concept could be feasible. Construction will not be easy but not far different than building an off-shore petrol platform. The bridge could be manufactured with innovative ultra-high performance concrete and segments of the tunnel prefabricated in a specialised precast yard. The great challenge will be the anchors but it will not be more challenging than an offshore platform.’’
If New Zealand were to really bridge a 20,000-year-old gap over one of the more problematic stretches of water the world has to offer, it may be time, rather than money, which might be the best bet.
‘‘Wow, this is a huge ambitious and audacious idea. Where would be the fun of a Cook Strait pie in the middle of a howling gale though?’’ Judith Collins