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  1. #1
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    Aug 2007
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    Default How Do I Size a Culvert?

    Looks like I need a Culvert to drain the runoff from rain and snow between my Pole Barn and the house drive. I figured I will grade to a drain and pour a concrete slab with a funnel to a drain. Then a tube under the lane to Daylight. How could I even guess how large the tube needs to be? I'm guessing it'll be 60-80 feet long. I can figure Square footage if that would do me any good. But it looks to me that it will be just a guess whatever I do. Any ideas? Also how deep below grade will this need to be? I will have have heavy equipment crossing at times. Dump truck, Excavator, Tractors, you know all the cool Man Toys.

  2. #2
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    Mar 2007
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    Default Re: How Do I Size a Culvert?

    This is a question with a hundred or more variables. Where I live, we use a rule of thumb to reality check runoff calculations, which is 1 CFS per acre is usually on the high end. So calculate the contributary area which would lead to the pipe. Include the roof of the pole building and any area upstream of the barn where water could flow overland and eventually reach your pipe.

    Secondly, the diameter of the culvert is less important than the slope you install it on. So will it be flat as a pancake? 2%? 5%? 10%?

    Generally, 2' of cover will be safe for any kind of pipe, assuming you bed and backfill properly (compact every 12"). You will probably be finding PVC or HDPE (ADS or Hancor) are the most cost effective.

    If you want to know how culverts are sized professionally, then I'll tell you they're made as big as the agency can afford. Which is never nearly big enough; but a harsh reality in a Country where the infrastructure is in shambled.
    Greyfields
    Puget Island, Washington
    JD 4120

  3. #3
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    Default Re: How Do I Size a Culvert?

    Good reply to a very open question. I knew when I asked that I could get more questions than answers with this one.
    I was discussing this with a friend of mine and his suggestion was to "put in the biggest one you can afford" apparently sound engineering advice.

    I will look at the area that I need to be concerned with. And get some square footage to start with. The grade can be as steep as necessary it will Daylight on the side of an embankment so I could easily dig as deep as needed and just add enough pipe to get there.

  4. #4
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    Default Re: How Do I Size a Culvert?


  5. #5
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    Default Re: How Do I Size a Culvert?

    I should ask. After I get the Square Footage of the runoff area, what do I do with it? Not sure I understand how to use your Rule of Thumb.

  6. #6
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    Default Re: How Do I Size a Culvert?

    Thanks 2manyrocks. I was replyng now I'll check the link.

  7. #7
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    Mar 2007
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    148

    Default Re: How Do I Size a Culvert?

    Quote Originally Posted by RonRock
    I should ask. After I get the Square Footage of the runoff area, what do I do with it? Not sure I understand how to use your Rule of Thumb.
    Divide your SF by 43,560 and that gives you acres.

    The 1.0 cfs / acre X (# of acres) = XXX CFS.

    When you get that number, I can spit out the diameter you'll need for any given slope. Try not to go steeper than 5% if you can avoid it. I'm guessing not knowing a thing about your project that you'll end up using a 12" diameter pipe.
    Greyfields
    Puget Island, Washington
    JD 4120

  8. #8
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    Default Re: How Do I Size a Culvert?

    I looked at the chart and thought about what is available at the local Menard's. And I came up with 12" also. LOL I would like to figure it out though. I'll do some measuring tomorrow. Haven't priced it yet, may have to go with 1 1/2" PVC.

    Helpful info thanks. It'll be fun to figure it out.

  9. #9
    Super Star Member
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    Oct 2004
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    First organized permanent settlement in the northwest territory
    Tractor
    2003 Kubota BX1500/2004 Kubota Bx23/2005 Kubota BX1500

    Default Re: How Do I Size a Culvert?

    Quote Originally Posted by RonRock
    Looks like I need a Culvert to drain the runoff from rain and snow between my Pole Barn and the house drive. I figured I will grade to a drain and pour a concrete slab with a funnel to a drain. Then a tube under the lane to Daylight. How could I even guess how large the tube needs to be? I'm guessing it'll be 60-80 feet long. I can figure Square footage if that would do me any good. But it looks to me that it will be just a guess whatever I do. Any ideas? Also how deep below grade will this need to be? I will have have heavy equipment crossing at times. Dump truck, Excavator, Tractors, you know all the cool Man Toys.
    It don't have to be no bigger than the ditch.

    Come to think of it you can't put a culvert in a ditch that's smaller than it is.
    Tractors 2003 Kubota BX1500 / 2004 Kubota Bx23 / 2005 Kubota BX1500.
    Attachments 60'' Front Blade/48'' Rear Tiller/FEL/Back Hoe /
    60'' MMM/Clamp on Forks/48'' MMM
    South of Canton Ohio L .B

  10. #10
    Elite Member
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    Default Re: How Do I Size a Culvert?

    There's one thing that keeps bothering me and that's the anticipated 60-80 foot length of the culvert. If it gets clogged with leaves or whatever, that could be a problem. Maybe someone else has some suggestions about the culvert or its installation to avoid clogs?

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