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?
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.
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