Should bridges be built with floats and propellers?

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Paul Anderson

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Paul Anderson  asked the Naked Scientists:
Hi Chris and team,

I have just been listening to the BBC news about the Pakistan floods and they mentioned the bridges being washed away.

The thought occurred to me of building a bridge with pontoon supports which float when flooding occurs and have propellers at the base which act similar to reverse thrust in aircraft to keep them stable and prevent them from being washed downstream.

I admit it is only a thought. Obviously the question must be asked whether or not the power of the propellers would be sufficient to keep the bridge supports stable.

Anyway, I'm throwing up that idea to be shot down by anyone and everyone.

I must get off to work.

What do you think?
« Last Edit: 02/08/2010 09:30:06 by _system »


Offline doppler1

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Should bridges be built with floats and propellers?
« Reply #1 on: 03/08/2010 14:23:08 »
I would say that bridges should be more accomodating of the changing weather conditions.....not sure if props are the way to go because usually with flood water comes lots of debris and cr*p that can damage the system. anyway, lets see what other contributions are made to this one.


Offline SeanB

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Should bridges be built with floats and propellers?
« Reply #2 on: 03/08/2010 21:21:34 »
The major problem with bridges in floods is that the approaches wash away, and isolate the bridge. Also the water is restricted going under the bridge, and this tends to scour the river sediments away from the bridge supports. To make a bridge resistant to flooding ( note resistant, not flood proof) adds considerably to the cost of building, as you need to pile down to bedrock, and need more spans that elevate the bridge above the flood plain so that any water can flow under the bridge without eroding the approaches. You also have to build above the highest recorded flood level, and you will find that most of the time the bridge span is very high above the ground.


Offline chris

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Should bridges be built with floats and propellers?
« Reply #3 on: 04/08/2010 00:12:36 »
All the tree trunks, cars, rocks and houses being washed down the river in the flood would instantly foul, if not totally obliterate, the propellers; thanks for "floating" this interesting suggestion Paul, but I honestly don't think, from a practical perspective, that it will "hold water"...

I never forget a face, but in your case I'll make an exception - Groucho Marx


Offline ccheric

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Should bridges be built with floats and propellers?
« Reply #4 on: 08/08/2010 15:25:02 »
I live in Seattle, both the I-90 and I-520 consist of a floating bridge. We have seasonal wind storms (not hurricane) that reaches >60mph, and the bridges are holding pretty well.
Among other problems, bridges are heavy, to use propellers for station keeping would require a tremendous amount of power. I believe it would always be cheaper to just build a stronger bridge to start with.

I've been to Phoenix, Arizona. The city is essentially in a desert, but the city infrastructure includes a system of huge canals, for the sole purpose of flash flood drainage. And you'll see bridges over these canals that may get flash flooded once per 100 years.