HOW BIGA ?

[LBrown59]

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

 

[Dougryan]

Is this a trick question?

 

[schmism]

ummm no,

depends on your culvert type, and how much cover you need over you culvert based on how much weight you want to carry.

you also need to identify the ditch slope, ditch side slopes, and ditch material (as it affects your flow rates)

 

[AndyM]

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

More than 15 inches.

 

[LBrown59]

Is this a trick question?

The trick is to answer the question.

 

[LBrown59]

More than 15 inches.

So then would you say a ditch 4 to 5 feet wide and 3 to 6 feet deep would be a bIt of over kill?

 

[AndyM]

So then would you say a ditch 4 to 5 feet wide and 3 to 6 feet deep would be a bIt of over kill?

It gives you seat time, doesn't it?

 

[Thomas]

Morning LB. Where the water enters culvert even w/the bottom of the stream etc.. also add a slight down wards pitch....wide enough to pack fill w/BH or tamp.

 

[RobS]

It almost seems as though the question is backwards. You might want to ask "How big a culvert to I need for my ditch?". Around here, the county determines culvert needs for all driveways across roadside ditches. Even if yours isn't a roadside ditch, the county might be a good source of expertise.

 

[ovrszd]

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 would estimate that I've installed 400 culverts in my life, varying in size from 15" to 72". For the 30" and smaller I use double wall plastic. For bigger sizes I use galvanized steel. Double wall Plastic has larger corrugations and will not wash out as bad, plus plastic lasts thru my lifetime. In Missouri galavanized steel has a lifespan of about 12 years depending on location before it rusts out.

For dirt cover, it takes very little if it's installed in a hard packed roadbed. A plastic 15" culvert will survive with 6" of cover with daily traffic. My driveway culvert has about 4" of cover and has been there 25 years.

If you are controlling water without it running over the roadbed, you have to determine acres of drainage, degree of slope of drainage and type of soil cover of drainage area. If it's open fields with considerable slope you better have considerable freeboard above the culvert to catch and hold the runoff until the rain slows and it can catch up. If it's basically flat land with grass cover which will slow down the runoff, less freeboard is needed. Either way I build a catch basin at the input end to give the culvert some tolerance for flash floods.

I avoid putting in a culvert with slope. I just want enough slope that it's a few inches lower at the output end, just enough so water doesn't stand in the culvert. Over the years I discovered that culverts put in with a radical slope from input to output ends have a tendency to "leak" water down the outside of the culvert and eventually wash it out. Especially if using galvanized culvert with small corrugation. That's why I like plastic with larger corrugation.

If you want to significantly increase the flow capacity of a culvert, angle cut the ends, both ends, at 45 degree angle if possible. If the input end of a culvert is straight cut, it cannot fill and run to maximum capacity. If you need to check this, go out when you are getting heavy rain and look at the output end of a culvert with a flooded input end. The output end will be running about 70% full. If you angle cut the input you are increasing the intake capacity which will allow the culvert to run full.

Some DO NOTs. DO NOT use smooth plastic pipe as it will wash out easier if you are installing in a steep drainage area.

DO NOT use single walled, corrugated black plastic pipe like is used for terrace drainage. It has a very small crush limit and if buried shallow in a travelled roadway will eventually collapse.

DO NOT allow weeds, grass and/or brush to grow up around the input end, this reduces the capacity.

DO NOT burn the roadbank with a plastic culvert installed unless you are there to watch it. That black plastic pipe will burn like crazy. I've saw them completely burn out and just leave a hole in the dirt with a pile of melted plastic in the bottom. Really upsets the road crew if it's a county/township road.

I'm sure I've missed some great ideas and maybe someone can add them.

 

[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.