A Tunnel under Cairns Inlet

Submitted by Webmaster on Wed, 27/07/2005 - 04:43

A tunnel under Cairns (Trinity) Inlet to allow the City ofCairns to expand onto the southern side of the inlet has recently beensuggested by Warren Entsch.  The purpose of this article is simplyto provide some basic information on what such a tunnel might look likeand how it might be constructed.  The purpose of this site to putthe facts on the table for public discussion and that includesdevelopment issues as well as conservation issues. Ihave beenin a tunnel of similar type and dimension to what would be needed toservice a 20 000 person community on the other side of the inlet. The Niigata Port tunnel is located in northern Japan and passes underthe Shinano River, which is Japan's longest river.  The soils aresandy and the river has an average annual discharge of about 900 mcubic metres of freshwater per second! Trinity Inlet has similar width, soil types and maximum current velocities. The picture below isof one of the ventilation towers that have been constructed at eitherend of the tunnel.  A brief summary of how the tunnel is constructed can be found here. There is even an association for tunnelling that maintains a catalogue of immersed tunnels.Despite this tunnel forming a link across a major river that dividesa city of nearly a quarter of one million people, there is publicperception that the tunnel was a waste of tax payers money. That isvery surprising. I liked the tunnel, however it did occur to me howstrange it was that I was the only one using it. In Japan I would rideeverywhere by bicycle and it was a sensation to ride down the almost 1in 10 grade of the bike lane at high speed past the vertical lighting -it was sort of like speeding through the time tunnel. Even car trafficwas light. It is likely that any tunnel under Trinity Inlet would belightly used for years, even decades before growth eventually caught upand made the link essential. Before a tunnel could be build across the inlet, it would be nice to have answers to the following:

  • Does it make sense to settle the other side of the inlet even with a tunnel - previously this option has been rejected - why?;
  • It there enough room at the Sheridan Street end to constructthe approaches to the tunnel - for the Niigata Port Tunnel, these areabout 400 m long, as the bottom of the tunnel will have to be at least20 m below water level before it leaves the shore to allow ships todock; 
  • Could Sheridan Street be reengineered to avoid becoming amonumental traffic bottleneck during construction of the project andafterwards, when 2 major traffic streams converge on the same street;
  • What does such a tunnel cost in round figures;
  • Is such a tunnel compatible with large ships (I don't know iflarge ships still pass over the Niigata Port Tunnel as the City hasalternative ports);

My main concerns are economic, social and environmental.  Wouldsuch a tunnel actually be in the tax payers interest?  Japan has apolitical-industrial complex and the industrial giants sponsorpoliticians who promote large and expensive infrastructureprojects.  Sometimes I think that Cairns also runs on thisprinciple.  If a tunnel does not stack up economically in the taxpayers favour, I don't think it should happen.  It costs hugedollars to fund such projects and these dollars could oftenbe invested into education or railways or something of greatersocial benefit.  On the social front, I would hope that if there acity expansion on the eastern side of the inlet, then theenvironment will still be available to the people.  Theenvironmental assets of the East Trinity area should remainin public ownership. If we fail to do this, only a few hundred peoplecould lock up most of the coastline and most of the watercourses andeffectively exclude the rest of the population from natural openspaces.  Development of East Trinity would also causeeither impacts on agriculture or natural area or most likely toboth.  It would be good to see a proper planning framework forsuch a thing.  However, given the ease in which the Far NorthQueensland Regional Plan has been swept aside by the Mayor of Cairns(Kevin Byrne), I have little faith in the value of such plans. Would we lose the green mountain vista that touristslike so much?   

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