$200m for an extra 8 metres: Debate rages over design for new Bridgewater Bridge
Updated
Questions are being asked about the value for money of the proposed half-a-billion-dollar upgrade of the Bridgewater Bridge after revelations the bulk of movements under the current bridge are a handful of pleasure craft.
Key points:
- The Tasmanian and Federal governments have committed $576 million to replace the Bridgewater Bridge
- The Government’s chosen 16.2-metre design allows vessels on the river to pass underneath
- An Infrastructure Australia assessment recommended a second bridge design that’s lower and costs $200 million less
Replacing southern Tasmania’s Bridgewater Bridge has been billed as the largest federally-funded infrastructure project in the state’s history, with $576 million in state and federal funds committed to the project.
Those funds are expected to cover the cost of the Tasmanian Government’s preferred option: a brand new bridge with four lanes, 1.6 kilometres long, with a speed limit of 110 kilometres per hour.
However, Infrastructure Australia’s (IA) assessment of the project found that a second, 8m-high option costed at $200m less should be further considered because of its “lower cost and potentially stronger economic merit.”
That option is to widen the existing causeway and build a replacement bridge, also with two lanes in each direction, and a speed limit of 100 kilometres per hour.
One of the main differences between the two options is the height above the water allowing vessels to travel underneath: the less-expensive bridge would be 8m, while the brand-new bridge would be 16.2m — the same height as the Bowen Bridge further south.
IA documents show the lower height would “restrict a small number of vessels each year from travelling upstream of the bridge.”
The police boat Van Diemen would be one of those vessels.
Treasurer Peter Gutwein confirmed the preferred replacement bridge option had been chosen with boating in mind.
“It was obviously designed with a view to ensuring there were able to be movement of boats further up the river,” he said.
The Department of State Growth said lifts of the current Bridgewater Bridge are “normally always for pleasure craft,” with Infrastructure Australia documents revealing the current Bridgewater Bridge was required to lift for just 34 return journeys in one 12-month period, which allowed 151 vessels to travel through.
Building a lower bridge ‘exceptionally short-sighted’
Glenorchy Mayor Kristie Johnston said although she supported a new Bridgewater crossing, a discussion was needed to determine what it should look like.
“We need to have a really good conversation with our communities about whether building a billion-dollar bridge represents good bang for buck,” she said.
Ms Johnston said there was a risk building a new bridge and decommissioning the existing bridge could remove rail access and end the possibility of light rail extending through there in future.
“The Brighton community desperately need better public transport facilities, and to cut the rail access across the river is incredibly short-sighted,” she said.
Tasmanian Liberal senator Eric Abetz said he did not believe Infrastructure Australia had fully assessed the benefits of building a higher bridge.
“It would be a tragic mistake if a decision in 2019 to limit the height of the bridge were to block the river off for a full century,” he said.
“To make a decision today to lock off the Derwent River all the way up to New Norfolk and beyond from, let’s say, vessels like the Cartela and from tourism would be exceptionally short-sighted.”
According to the IA assessment, the 8m option has been costed with a P50 cost estimate of $271 million, compared to the preferred 16.2m option’s P50 estimate of $471m.
The P50 estimate refers to the 50 per cent probability of the cost estimate not being exceeded, while a P90 estimate refers to a 90 per cent probability of the cost estimate not being exceeded.
The preferred option’s P90 estimate is $633m, and total state and federal funding committed to the project is $576m.
All options cost more than they’re worth
The IA assessment identified other issues with both options.
The Government’s preferred and funded $576m bridge plan requires further geotechnical work to ensure the bridge can be supported by the river bed, with a risk of significant cost increase if it cannot.
There is also a risk the cost will be significantly higher due to a change of bridge design standards since the cost estimates were developed.
IA determined that option would cost about $103m more than the value of benefits it would provide.
But on the other hand, the IA assessment said widening the causeway as part of the less-expensive option two may not be feasible due to heritage values.
The cost of that project would also outweigh its benefits by $47.9m.
Ultimately, the assessment said: “Given its lower cost and potentially stronger economic merit, we recommend further exploring option 1 (the cheaper option) as part of a revised submission.”
A spokesman for State Growth said all options that would deliver a crossing that meets the agreed funding would be considered.
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