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# Difference between storm sewer pipe and a culvert pipe

## Difference between storm sewer pipe and a culvert pipe

(OP)
I am trying to understand the difference between storm sewer pipe that is designed by Mannings equation vs culvert design. Why can't we size a culvert using Mannings equation, considering that it is conveying flow from one end to other?
Can someone explain?

### RE: Difference between storm sewer pipe and a culvert pipe

You can if it acts as an open channel. The complication comes in when either the upstream or downstream end of the culvert is submerged.

### RE: Difference between storm sewer pipe and a culvert pipe

mannings can be used for calculating normal depth with uniform flow in open channels. culverts rarely flow with a uniform depth, so it really only applies to longer conduits or open channels.

you also need to factor in the tailwater and entrance losses. for a "short" culvert, these are highly critical and may totally override the normal depth. so most culvert capacities are a function of the inlet or outlet conditions.

### RE: Difference between storm sewer pipe and a culvert pipe

To expand on cvg's point, storm drains are typically hydraulically long; in that they can reach normal depth unless influenced by other controls like tail water. Storm drains also experience inlet & outlet losses which is alluded to by the term "other controls", above.

Culverts are typically short as cvg points out and experience more rapidly changing flow regimes.

Hopes this helps.

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