The height of convenience

HOW high is the new Bridgewater Bridge? The answer is 16.2m. But why?

The new bridge emerged from initial engineering calculations and designs back in 2011. Here are the basic questions. How much traffic is going over the bridge and, at the same time, under the bridge?

The answers dictate dimensions, particularly width and height. There’s a cost consideration, too. Higher and wider the bridge, higher the price.

More height/width means more concrete, more weight, more wind load. Critically, the main navigation span of the Bowen Bridge, 15km south, is 16.2 metres above the water. So from a planning perspective, there’s not much point in new Bridgewater Bridge being any taller. So, 16.2 metres it is.

To put this in perspective, the mast of the Gweilo, winner of the 2022 Sydney to Hobart, is 22.5m. So, the new bridge is too short for the Sydney-Hobart boys and girls, but large enough for most water traffic. (By the way, the 100mm – four inch – shift in the tide at Bridgewater wouldn’t make much difference.)

Another perspective. The existing lift-span at Bridgewater can be raised some 30m above the water. But when you consider it was built in 1946, (and it’s been stuck in the not-up-not-down position in recent years) that’s probably not a good idea.

Currently 22,000 vehicles use that bridge daily. You do not want all those people in front of you when things go pear-shaped with 75-year-old technology. Over the past two years, the lift span has been put to work about 90 times.

But for nine months from March 1 this year, it’s going to get a rest while construction work ramps up on the new bridge on its downstream side.

Whether it comes back to work, and when, we’ll have to wait and see.