How Do I Size a Culvert?

[LBrown59]

A summer or 2 ago.
I put in 110' of 20' x 18'' double wall to fill in the road ditch along front of my property.
I think it was about 134... dollars a joint.
I also have a 130 foot section of 6'' plastic flex pipe in a ditch on one side of my land and another 80 foot long piece of it in the ditch on the other end of the property.
The key to keeping the flex pipe from clogging up is to keep the leaves out of it.

 

[horse7]

I put in a pair of 22' (give or take) 30" steel culverts next to each other to get my driveway over an intermittent stream. People ask why the overkill?

Turns out that the previous owner had a 30 wash out (sold the land to me, had never replaced the 30).

All the rules of thumb and book calculations from weather charts won't replace knowing the area.

4" of rain over 48hr on frozen ground fills the culverts 100%.

This spring, 6" of rain on saturated ground overflowed the culverts, and demonstrated the driveway design worked-- part of the driveway is slightly lower than the area with the culverts and backfilled with cobbles. The idea is that a broad flat layer of water will flow over the driveway, and
a) if shallow enough can be driven through
b) won't wash out the driveway like a narrower faster flowing spot would. The lack of fines helps resist erosion also. The down side is the ride isn't exactly the smoothest.
The principle idea though is to not lose the culverts, which are harder to replace than just backfilling a gully washout in the driveway.

Being steel, unfortunately, the culverts are rusting on the bottoms and will probably need to be replaced in not too many more years. Probably with plastic.

 

[Greyfields]

Regardless of what the salesmen say, all steel or alluminized CMP pipes do rust and just get nailed by root intrusion. And they pretty much just rot. Unless you are needing it for an extreme case where plastic isn't practical (think fish passable culverts) avoid CMP and other metals at all costs!

 

[Egon]

Instead of weight I should have said metal gauge.

 

[dqdave1]

If it calls for 12"; install 16" to be safe.