Bridge design for lahar 1

Dear GHJ,

Thanks for the question. I had to look up what Lahar flow means. I found the following from the USGS Web Site:

"What is a Lahar?
Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun."

I would guess that the probable forces due to this type of extreme event would be hard to estimate without extensive investigation of the upstream environment. If the lahar is in fact a "large" one, I doubt any bridge would be able to withstand the associated forces (certainly not economically feasible). Sorry I wasn't any help, but at least I learned a little today. Good Luck.

Dear Rowe

Thankyou for your response. We have good data on expected "hydraulic" characteristics of the lahar, which include knowledge of a catastrophic flow from the same source in 1953. To date we are treating the lahar using conventional "hydraulic" tools, adjusting for anticipated flow density. My question was posted as it seems sensible to seek advice and precedent from the engineering community at large.

I suggest you also post your question in the "foundation engineering" forum. There may be someone who has some experience relating lahar flow to typical hydraulic parameters.

P.S. There's also a "Storm/Flood engineering" Forum and "Erosion Control" Forum.

GHJ,
you should look up "debris flow." This is a liquifaction of the soil and sounds like what your talking about.

As always you can resist it or avoid it. Resist it looks improbable if the flow is big, but maybe astounding piles might be enough. A classical solution, the entirely infilled arch looks too small for big flows.

Then to avoid it you need set the entire bridge over the big flow, and surely make it stand very significant dynamical forces. This looks like some sophisticated suspended bridge prepared to even take very significant changes of geometry and wholly a dampened thing, otherwise compatibility with the event and use before and after looks umpromising. Of course arches and beams can also be used and with some slippage devices or other. But all these things look dear and targeted to a daunting environment.