How to properly size a culvert for Drainage ditch..?

[Drifthopper]

One of my projects for this summer is to put a culvert pipe in this ditch / low area and make it where i can drive my tractor across it.
It's not really a fast flowing ditch, but the whole low area is real soft.... even in the summer.
I want to put the culvert in, basically right where the 4 wheeler is.
But.... not sure what diameter I can get away with so it won't wash out.
Any input on sizing is appreciated...!

 

[GreensvilleJay]

If you want to do it just ONCE, toss in a 24" . The important details are getting rocks upstream as a 'V' to direct all the water into the culvert .Use at least a 12' section (20' is std ). The wider the access, the more stable it'll be over the next 3-5 decades....

 

[fried1765]

If you want to do it just ONCE, toss in a 24" . The important details are getting rocks upstream as a 'V' to direct all the water into the culvert .Use at least a 12' section (20' is std ). The wider the access, the more stable it'll be over the next 3-5 decades....

I would suggest a 20ft 20" or 24" steel culvert, as you can likely get by with only a foot of cover if using steel.
20" dia. should be sufficient, as you describe the water to be not fast flowing.
You will need a LOT of fill though, to build up the access at either side of the culvert pipe.

 

[Drifthopper]

Why metal over black plastic...?
And... why such a long length...... i was hopeing to get by with a 1/2 section : 10'

 

[mcfarmall]

Why metal over black plastic...?
And... why such a long length...... i was hopeing to get by with a 1/2 section : 10'

Stronger. Culverts are like gate openings and pole buildings...they are always too small

 

[Daren Todd]

Why metal over black plastic...?
And... why such a long length...... i was hopeing to get by with a 1/2 section : 10'

I prefer the plastic ones myself. They don't rust out on the bottom from standing water.

 

[Mark_BX25D]

Why metal over black plastic...?
And... why such a long length...... i was hopeing to get by with a 1/2 section : 10'

Whatever the length you use, you'll get a few feet less of usable driving area. The edges won't be safe to drive on. (Unless you build concrete walls at each end, I guess.)

These old adages apply here with enthusiasm:

• Go big or go home.
• Do it right, do it once.
• Pay now or pay later.

As for strength, I'm not sure the plastic takes a back seat to metal. The new stuff is pretty strong, but then, I haven't done a comparison of the numbers.

If you go plastic, get the dual wall HDPE plastic . The smooth inside won't trap water and debris like corrugations will, and won't provide a breeding ground for mosquitoes.

 

[fried1765]

I prefer the plastic ones myself. They don't rust out on the bottom from standing water.

I prefer the plastic culverts too (I wrote the steel culvert response) but.....
as I pointed out, the steel culverts require less cover, because they are stronger.

Poly culverts are much more easily crushed!
Whatever the culvert type, there will be a need for a lot of material to be trucked in to make that very wide (low) wet crossing usable.
Why haul in it any more material than absolutely necessary?

10 ft. pipe length, with the necessary cover may provide for a 6' crossing width (more, if the OP uses rock for wing walls).
If 6' crossing width is enough, then go for it.

 

[North Idaho Wolfman]

It really depends on what you really want to accomplish.
I would rather do (2) 12" sections 10 feet long than one 24", Less digging, less filling.
A two foot tall piece of pipe take a lot of work to setup and over fill.
I do plastic pipe, plenty strong enough, I have semi trucks over mine all the time.
Easy to work with and doesn't rust out!

 

[Daren Todd]

It really depends on what you really want to accomplish.
I would rather do (2) 12" sections 10 feet long than one 24", Less digging, less filling.
A two foot tall piece of pipe take a lot of work to setup and over fill.
I do plastic pipe, plenty strong enough, I have semi trucks over mine all the time.
Easy to work with and doesn't rust out!

The only issue with the 2-12" Culvert's is less flow than one 24" line.

2- 12" Culvert's equal around the equivalent of one 18" Culvert in regards to flow.

I wonder if it would be better to form up each side and install a cattle guard instead. Granted, that's a more expensive and labor intensive project.
Guess he'll have to get his wife on the phone with a stop watch down stream. Drop a marshmallow in up stream and calculate the flow rate.

I'm sure there's a YouTube video on how to do it

 

[fried1765]

It really depends on what you really want to accomplish.
I would rather do (2) 12" sections 10 feet long than one 24", Less digging, less filling.
A two foot tall piece of pipe take a lot of work to setup and over fill.
I do plastic pipe, plenty strong enough, I have semi trucks over mine all the time.
Easy to work with and doesn't rust out!

Poly pipe is excellent, but the OP will need to use a lot more cover fill if using poly pipe.
Running a semi truck over poly pipe with 12" of cover is not the same as running that semi truck over steel pipe with 12" of cover.
More cover material means more dirt hauling for the entire approach, and departure.
2-12" diameter pipes will work, but small branches/trash will plug smaller pipe readily...even if there are two pipes.

 

[North Idaho Wolfman]

I agree with both of you, flow will be less.
But just from his pic's it looks like it would take a lot of fill to get a 2 foot culvert in and a foot of cover.
Also a 2 foot pipe is $1000 give or take, and a 12" is $300 give or take, so I'll deal with cleaning out the opening a few times to make up for that.
This is all based on being a secondary temporary use drive.

If it was a main driveway access then:
16' min width 20' is better!
Then take the average depth multiplied by the average width to obtain the cross-sectional area and divide by four to determine the rough diameter of pipe needed to pass the average storm. The diameter of pipe(s) used must add up to the total diameter needed without using a pipe taller than the average depth

 

[Russell King]

You also need to consider how wide the stream is where you want to cross. If you collect a wide slow moving stream into a narrow channel it will have to speed up and then when it exits the narrow channel it will start to dig a narrow channel below the exit point. Then you will have a gully that will expand downward the flow path. I would say you will need to use several smaller diameter culverts but I am no civil engineer either.

 

[D2Cat]

Find someone scrapping some flat bed trucks. Can often get a bed fairly reasonable. Build up for each end and lay it in place. A 14' or 16' bed would make a nice bridge.

 

[jyoutz]

After years of experience with stream crossings on logging roads, I have a few thoughts. One culvert isn’t going to be sufficient for the width you want to span. You have 3 options:

1. Multiple culverts spanning the width of the swale with lots of rock fill. I mean a lot of fill.
2. Hardened wet water crossing (these never wash out if done correctly). Construct a base of larger rock across the wet area, extending onto dry soil. Then top that with three inch rock to make a roadbed that will support the machine as you drive through the water.
3. Some kind of bridge section (flatbed trailers work well).

 

[GeoHorn]

Old railroad flat-cars are even better (than flat trailers) for making a bridge. They have heavier trusses.

That water appears to have little or no movement, so approaches to either culverts OR bridgework will be required. If you can locate any building or hauling outfit who needs to rid themselves of construction-rubble… and needs a place to dump it…..

 

[Daren Todd]

I agree with both of you, flow will be less.
But just from his pic's it looks like it would take a lot of fill to get a 2 foot culvert in and a foot of cover.
Also a 2 foot pipe is $1000 give or take, and a 12" is $300 give or take, so I'll deal with cleaning out the opening a few times to make up for that.
This is all based on being a secondary temporary use drive.

If it was a main driveway access then:
16' min width 20' is better!
Then take the average depth multiplied by the average width to obtain the cross-sectional area and divide by four to determine the rough diameter of pipe needed to pass the average storm. The diameter of pipe(s) used must add up to the total diameter needed without using a pipe taller than the average depth

The bonus with poly is he can by a 20ft stick and cut it with a chainsaw.

 

[dirtydeed]

My vote would be double walled plastic pipe. Its plenty strong and the smooth inside is very beneficial in slow/low flow conditions. And no, you will not need much cover to drive your tractor over it.

 

[fried1765]

The bonus with poly is he can by a 20ft stick and cut it with a chainsaw.

There should not be a need to cut any type of 20' pipe for that proposed application.

 

[PoTreeBoy]

One of my projects for this summer is to put a culvert pipe in this ditch / low area and make it where i can drive my tractor across it.
It's not really a fast flowing ditch, but the whole low area is real soft.... even in the summer.
I want to put the culvert in, basically right where the 4 wheeler is.
But.... not sure what diameter I can get away with so it won't wash out.
Any input on sizing is appreciated...!

Before I decided to build a bridge, I considered a culvert. I found a site using Google that sizes culverts based on the drainage area it services, design rainfall, etc. Try searching 'culvert sizing' or 'how to size culvert'.
Use 12" bare minimum to reduce plugging. And set the bottom of the culvert 3-4" below the ditch invert. It'll fill up a little to match the bottom of the ditch.