16 April 2008

A Cook Strait tunnel?

You may or may not find this hard to believe, but I was pondering the idea of a tunnel under Cook Strait the other day. When I did the usual follow-up, plugging the words 'Cook Strait tunnel' into a well-known search engine, I found absolutely nothing. Well, there was this quick discussion on Public Address System, which is a good headline but nothing more. This spurred me into action. So, for no good reason other than to fill a void and secure my place in blogging posterity as (possibly, sort of) the first poster on the topic, here's a brief discussion of the prospects for a Cook Strait tunnel.

It's often mentioned that New Zealand has a similar land area to that of the United Kingdom, the country from which many Pakeha colonists originated. New Zealand has a land area of 268,000 sq km and the UK has 241,500 sq km. While the UK does have the outlying territory of Northern Ireland over the Irish Sea, most of the land area of the UK is easily accessible by road or rail. However, New Zealand is an archipelago split by Cook Strait and Foveaux Strait, which hampers communications and commerce and generally makes getting about a real nuisance. In my years living in Wellington I certainly bemoaned the lack of driving opportunities from the capital – it’s north or nothing – and the car ferries to Picton in the South Island are expensive.

Another island archipelago nation is Japan (land area 374,750 sq km), which is divided amongst four main islands. Japan has the advantages of having relatively narrow channels between its three southern islands, Honshu, Kyushu and Shikoku, and the considerable wealth required to connect those islands. Even the broad Tsugaru Strait between Honshu and the northern island of Hokkaido has been conquered by the 53km Seikan Tunnel, 23km of which is underwater.

New Zealand’s Cook Strait is the major break point in the New Zealand economy and the transportation network. On days with foul weather and high winds – and there are many of those days – ferries are held in port and aircraft are grounded, thereby shutting down the only links between the North and South Islands. The cost to business and travellers alike is high.

So I started wondering: has anyone seriously considered a Cook Strait Tunnel? Obviously it would be ridiculously expensive, perhaps beyond the ability of a small country to afford. But in practical terms, is it a workable proposition? The factors to be considered are distance, depth, geology and cost.


On this front, it’s good news! In theory, that is. Tunnels far longer than the width of Cook Strait have been in operation for years. The shortest distance across the Strait is about 22km from Cape Terawhiti on the North Island to Perano Head on Arapawa Island in the Marlborough Sounds at the top of the South Island.

When we look for examples in other countries, there are several tunnels longer than 22km. The Seikan Tunnel, mentioned above, tops the list. Next is the most well-known to New Zealanders: the Channel Tunnel linking England and France, which has a total length of 50km, of which 39km is underwater. This list shows seven tunnels – six rail tunnels and one road tunnel – longer than the 22km between Terawhiti and Perano.


Cook Strait has an average depth of 128m. Surveyors charting a course for the Saikan Tunnel found that the western neck of Tsugaru Strait had a maximum depth of 140m, so they sited the tunnel there. The English Channel is much shallower at its narrowest crossing though – only 45 to 60m deep. One consolation is that it’s certainly possible – the admittedly much shorter Eiksund road tunnel in Norway is the deepest undersea tunnel, reaching a depth of 287m below mean sea level at its deepest point. So a Cook Strait tunnel might have to be built at the cutting edge of engineering technology.


Here I’m on shaky ground, if you’ll excuse the pun. I can draw a straight line from A to B, but I can’t tell you whether the intervening material that has to be drilled through is good or bad. Given the environment of the strait itself, I can't imagine the rocks underneath it will be particularly easy to tunnel through. Not a particularly substantive analysis is it? Perhaps one of these bods might know something.


There's another factor that makes a Cook Strait tunnel quite challenging. While the narrowest crossing point is 22km wide, the topography of the terrain on either side of the strait is particularly rugged. If a rail tunnel to the South Island was considered, extending the Wellington railhead from the Thorndon station westwards to the coast would take the rail lines up steep slopes to residential Karori (which could certainly do with some livening up, but perhaps not from a busy freight line) and on through hilly terrain that would require numerous tunnels to pierce the ridgelines, which are aligned southwest to northeast.

On the other side of the strait it's just as tricky. Arapawa Island has the same steep hills to negotiate, and it would also require a tunnel or bridge across the Tory Channel to reach the South Island railhead at Picton. Blimey. Oh well, here's a nice picture anyway.


Okay, here's where the idea falls down. I suspect this will only ever be affordable in the far distant future when silver-suited engineers will lazily instruct a cyborg drill monster to go away and dig the tunnel, leaving them in peace to carry on the crossword in the digital Dominion Post.


I think as a result of this investigation into the prospects for a tunnel under Cook Strait the only thing we can be certain about is that I wouldn't have made a good engineer.

But don't get me started on the idea of a Cook Strait bridge... now that's an idea with potential!

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