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Q & A: air pressure on levee

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Most recent answer: 04/03/2011
Q:
Hi I am the public works director for our little town that is beautifully situated along the banks of a fairly good sized river and a smaller river that dumps into it in the middle of town.Well that makes for good fishing and great postcards for 10 months out of the year. The other 2 months,April and May, it is like living next to an active volcano. The threat from flooding is a yearly event. We have a levee system that protects our main business district. I happen to be the opperations manager for this levee and back in 2008 we had a record flood that pushed this 36 year old levee system to well above its design capacity. The levee was design to protect down town to a 29' river height plus a 3' freeboard cushion and we went to 30.5'. Now this is leading up to my question. The last 750' of the levee the Army Corp was not allowed to add the 3' freeboard and I had to Mcgyver my way out of that one to build a gravel burm 3' high by 750' long in the rain battling 1' of flood waters. It worked and since that spring we still haven't set up something to make it easier the next time the river decides to go crazy on us. My question to you is i want to design a system of interlocking jersey barriers made of cement that could hold back 3' of water preasure. This is how i did the math 1.5' depth of water wich is half of 3' depth that i want to be protected for times .433 lbs/sq. inch = .6495 lbs/sq. inch ( ? do i have to add atmospheric preasure "14.7" to this .6495 before multipling the sq. inch of each 10' jersey barrier to get how much force will be pushing against the barrier at 3 ' height. .6495 X 4320 sq."= 2805.84 lbs of force will be applied to each 10' section. I estimate that each section of barrier will weigh approx. 6000 lbs this should be enough to hold back 3' of river height. That is why your answer to my question is critical to my going ahead with the planning phase of the jersey barrier idea. Well sorry it took me so long leading up to my question but i felt you needed some back ground as to what i was doing. Thank you for your time and i am looking foward to your answer. Tony T.
- Tony Theriault (age 51)
Fort Kent, Maine
A:
Answering this question involves way more responsibility than we're used to taking. However, I'm confident that in calculating the pressure that the barrier will need to withstand, you do not need to include atmospheric pressure. Think of any little house of cards etc. If they had to support atmospheric pressure, they'd collapse. Fortunately that pressure comes in from all sides.

As for the other engineering issues involved, we are clueless.

Good luck!

Mike W.

p.s. Google maps says your town looks exactly as you described.

(published on 04/03/2011)

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