Talking Point: Bridgewater crossing still our weak link two centuries on

LEWIS GARNHAM: There are cheaper and safer ways to span the Derwent

VITAL LINK: Cruising under the Bridgewater Bridge in 2005. Picture: Tony Palmer

The tide of troubles has ebbed and flowed at the Bridgewater crossing since settlement in 1803.

Lieutenant Governor George Arthur commissioned a crossing in 1829. About 200 convicts started work, initially to build an arched viaduct. This build changed to 2 million tons of packed earth fill for the present rebuilt causeway. In 1849 the first swing bridge was opened to replace the punt. Progress in the late 1870s included a rail link. Our present lift steel bridge was completed in 1946.

More than a century later, another arched bridge is proposed! The massive costs and foundations for this concept are also problematic, except when electors see visions of our own Arc de Triomphe. It is easy to say “we need a new bridge” but this will not span the real issues.

The important question before building any bridge is what will happen to the causeway and heritage-listed steel bridge.

An independent engineering assessment last year found the lifespan of the existing bridge could be extended another 50 years with repairs and maintenance. It is still the weak link, being unreliable and not designed for predicted road loads. There are closure concerns from a one-in-200 year seismic event! I am certain the new high piers would also be affected.

Infrastructure Australia has assessed the proposed new bridge. On July 8 the business case did not demonstrate that the benefits would outweigh its costs. There are unresolved engineering issues, environmental externalities, crash costs and maintenance. An elevated bridge is dangerous and costly to build, and also to maintain.

If the present roadway/causeway is closed, the present two traffic lanes will be relocated to the new bridge. For $576 million there will only be two extra new lanes, but it would lose an important multifunctional link of rail and easy low level pedestrian/cycle traffic. Clearly we need to maintain this causeway link.

Why not strengthen and weld shut the present steel bridge and build a reliable opening bridge in the present roadway approach for road, rail and pedestrian/cycle traffic.

It will be difficult for pedestrians and cyclists to access and use the new high exposed bridge pathways.

The proposed new arches fail to provide for a much needed rail link for easy cheap bulk passenger transport. We have no alternatives to using roads. When used they clog up and the kneejerk reaction is to build more … also to clog up. A rail link alternative is required.

The 16.2m maximum air draft (above-water clearance) equals the restricted centre span of the Bowen Bridge. It is less than provided on the western span of at least 17.3m that yachts actually use. Even this is too low.

For extra traffic capacity, for now and future, build a hollow-piled concrete causeway parallel to the present one. It will also have with a rapid acting opening bridge. Our predicted smaller road and river traffic volumes could cope with occasional bridge openings.

Lake Pontchartrain in the US has a 24-mile causeway and a bascule bridge. It was built in 1955, taking only one year, using prefabricated techniques. High-span bridge proposals were seen as uneconomic. Perhaps there are lessons here.

The cross-linked four lanes of the present causeway carry about 12 million vehicles a year, and bascule bridges open about 60 times a year. With that traffic volume why all the fuss here for an occasional opening? Our 1.6km new bridge has two structurally isolated traffic lanes in either direction. A mid-bridge multi-vehicle crash will block the isolated two lanes like being in a tunnel. The one way traffic will be trapped and emergency vehicles would find it very difficult to access. The vehicles banked up could be in danger. There will be multiple injuries and possible deaths from the following collisions. All two-way traffic will have to be diverted to the other open 1.6km dual carriageway open bridge; how dangerous will this be?

A causeway upgrade and parallel duplication with two opening bridges will be safer and meet all needs.

The New Zealand 4.8km SH16 Causeway was also built on soft marine mud and faced worse problems with flooding and gross subsidence. It was raised 1.5m, widened to accommodate five lanes in each direction, with an added pedestrian/cycleway. The cost for a 4.8km section was $220 million. Translating this into our 0.8km causeway is only $34 million of the proposed $576 million.

With population expansion along the Midland Highway and increasing local use, the two dual carriageways are a retrograde step, especially if the causeway is closed. The Tasman Bridge soon reached capacity as will this plan.

Because it is essential the causeway remains open we should concentrate on the benefits and implement plans to make it work properly for all.

Hobart’s Lewis Garnham is a retired general practitioner, former navy cadet and a keen sailor on small yachts and square-rigged ships.