How Do I Size a Culvert?

[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.

 

[Greyfields]

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.

 

[RonRock]

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.

 

[2manyrocks]

NCDFR - Culvert Sizing Instructions

 

[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.

 

[RonRock]

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

 

[Greyfields]

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.

 

[RonRock]

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.

 

[LBrown59]

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.

 

[2manyrocks]

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?

 

[Greyfields]

Racks over the inlets on culverts tend to cause more clogs than they prevent; but for safety reasons you still see them. Essentially your best bet to avoiding clogs is not laying the pipe too flat (so that sediment settles out and builds up) and sizing it large enough to pass stray objects. I've seen more basketballs in pipes that you could ever imagine. But, in the end, there are outfits out there that can run a little robot up the pipe and clear any obstructions.

 

[LBrown59]

Racks over
the inlets on culverts tend to cause more clogs
than they prevent

But clogs on the out side of the pipe are easier to clean out than those half wan down through say an 80 foot long culvert.

 

[MarkV]

Better be sitting down when they give you the price on 80’ of 10 or 12” culvert.

MarkV

 

[RonRock]

Better be sitting down when they give you the price on 80 of 10 or 12 culvert.

MarkV

Give me your best guess. I haven't checked yet.

 

[LBrown59]

Give me your best guess. I haven't checked yet.

Probably around 125 to 150 bucks for a 20 foot joint give or take a few bucks.

 

[Greyfields]

That sounds like the right ballpark. Consider an installed price for a 12" culvert would be in the ballpark of $30 per LF.

 

[drssg]

A picture of the area may be useful. Are you set on the idea of a long culvert? I've only used culverts to go under the drive. For other areas, I use a swale and/or french drain.

This goes against the whole "do it yourself" philosophy, but if you get a guy in with an excavator with laser integrated leveling, he can put it on a perfect grade in no time.

 

[bigballer]

i recently paid $5.75/ft for 12" ADS double-wall (smooth inside).. comes in 13ft and 20ft lengths at my local supplier.

 

[Egon]

Would not the cost of culvert depend on the type and weight?

 

[Greyfields]

By type, yes. Weight? No. That's why you rarely see concrete pipes used anymore. When given the choice of joining 4' long sticks of concrete vs. 20' long sections of HDPE; plastic pipes win out even if the prices is more since you get far better productivity. Unless you have specific issues with scour, I would avoid concrete pipes at all costs. ADS and Hancor are your best bets (which are corrugated PE pipes, smooth bored).