HOW BIGA ?

[LBrown59]

This is definitely a loaded question with so many variables that we could discuss it for a week!!! First being, what is your intent???

I have a 15'' driveway culvert, but the ditch feeding it varies back and forth between 3 to 5 feet wide and deep.
Just seems to me that the ditch is way out of proportion to the culvert.
If it takes that much ditch to handle the run off then how is a 15'' culvert going to handle the ditch?
Why would it take a ditch bigger than a culvert to handle the same water flow as the culvert?

 

[LBrown59]

1*It almost seems as though the question is backwards.
2*You might want to ask "How big a culvert do I need for my ditch?". .

3*Ordernarly that would be rite.
4*but I already have the 15'' culvert so need to size the ditch to the culvert; not the other way around.
----->>

 

[schmism]

varies back and forth between 3 to 5 feet wide and deep.
Just seems to me that the ditch is way out of proportion to the culvert.

This is why you need to know a LOT more info.

your flow line (ie the bottom of the ditch) is the imporant thing. If you want to make the ditch flow downhilll, and you have to tie the ditch back into existing grade at 3to1 then your ditch might end up looking HUGE but infact "design" load may only be a foot of water. but the ditch looks huge because of exitisting ground you have grade back to.

make sence?

FYI some basic back-of-the-envolope calcs says you can flow just under 5 cfs (300 cfm) for that culvert (RCP). (again based on some rough minium assumptions) (if you got CMP corgated stuff, then its about half that)

 

[ctpres]

Re OVRSZD reply
Now thats what I call a great answer. Thank you. It was a big help to me and one of my do soon projects.

 

[LBrown59]

1*your flow line (i.e. the bottom of the ditch) is the important thing.
2*(if you got CMP corrugated stuff, then its about half that)

1*That's just it- the bottom is to far down due to 40 years of eroding the sides and bottom of the ditch.
2*It's 15'' Double Wall Smooth Bore

 

[Gatorboy]

How big of a ditch does it take to handle a 15 inch culvert ?

One that is big enough

 

[schmism]

2*It's 15'' Double Wall Smooth Bore

ah then it has the RCP value if not a bit better. In these parts Idot doesnt speicfically give a manning number for the DWP so we use the RCP number.

 

[ovrszd]

1*your flow line (i.e. the bottom of the ditch) is the important thing.
2*(if you got CMP corrugated stuff, then its about half that)

1*That's just it- the bottom is to far down due to 40 years of eroding the sides and bottom of the ditch.
2*It's 15'' Double Wall Smooth Bore

You can cut the banks and fill in your ditch to a respectable grade but make sure you work the dirt in and pack it well. You don't want the water soaking under the culvert and finding a crack and starting to run under your newly installe culvert. So pack the bulk of the fill. The last few inches of dirt fill should be left loose where the culvert is going so the culvert lies in this soft dirt bed. That will allow the corrugations to fill up with soil as you cover it and lessen the chance of a washout.

I have installed culverts in ditches so deep that I had to use a dozer and push the dirt out each end, lay the culvert and then push it all back in to fill the trench. But that didn't mean I had to install a 20ft diameter culvert. The depth of the ditch has nothing to do with the size requirement of the culvert.

If you contact your county road crew and give them your drainage information, such as how many acres, how steep, type of terrain, they will tell you whether your culvert is big enough.

I never install a particular sized culvert simply because that's what I have on hand. Unless of course you want to install it again when it washes over the roadbed because it was too little, or hump up the roadbed to cover it because it was too large.

If you buy a culvert from your county road barn, a 15" double wall, smooth bore will cost you around $6.00 per foot. If you are installing a culvert under a double laned roadbed and not very deep, 30 feet of culvert is plenty. So we are talking about $180 here. Don't use your 15" culvert if it isn't the size required simply because you don't want to spend a couple hundred dollars for one that is. You'll spend that money quickly retrieving dirt to reinstall if it's the wrong size.

 

[LBrown59]

One that is big enough

Ah Ha sort like i expected.
So a 100 foot wide ditch 100 feet deep meets that spec. However I'd like to narrow it down a little finer than that.

 

[CharlieTR]

A little confused here.

But here is my novice experience anyway.
When I built my place I went to the county's TxDOT for a permit.
(No charge for residential).
They gave me stakes and flags to mark the location of culvert.
Engineer came, mailed design plans with detail reqs.
Things like width limit, radius of driveway turn-in, etc.
The engineer was cool and advised he only cared about two things.
1. proper size of culvert
2. saftey ends and installed properly.

He advised that the ditch was 2 ft deep, and I must have 18" culvert, with 6" of cover on top. Design spec stated that water must not drain onto road surface.

No problems at all since. Have had 6 concrete mixers over it and 2 were loaded with 10 yards each. Not sure but I was told that is 78,000 pounds total. Never had a problem.

PS. The semi drivers usually tell me they are glad I have 30 foot long culvert.
Some even ask how long it is before they come out.