single culvert equivalents for multiple barrel culverts?

[proletariat]

I am trying to come up with a coefficient that will express the efficiency lost when going from a single culvert to a dual culvert system. I am doing this, because my stormwater program doesn't allow parallel culverts, and I'm trying to model a dual culvert within a system.

For example, If I am running two 18 inch pipes across the road, there is a cumulative area of 3.5sf. One pipe with a diameter of 2.12 ft would have the same area, but we all know that the 2.12' pipe will flow more efficiently because of reduced influence of the pipe walls.

Is there a standard percentage reduction when going from a one-pipe crossing to two pipes. I don't want to use HY8 either, because this is all part of a larger system.

 

[RWF7437]

You need to calculate the conveyance for each culvert system, not just the area.

K = A*1.486/n*R^2/3, where

n = Manning's roughness
A = area in sq. ft.
R = hydraulic radius = area/wetted perimeter- ft.
K= Conveyance in cfs

Good Luck

 

[proletariat]

THanks. I'll give it a whirl.

 

[MPENN]

When considering your equivalent pipe sizes, don't get lost in hard numbers; keep in mind that there are standard pipe diameters.

 

[francesca]

You also have inlet losses which will probably be much larger for two smaller pipes than one large pipe.

 

[BRIS]

You can solve the mathematics - but why not just run a sample model with a culvert at 50% and 100% flow and determine the coefficient ?

 

[ccor]

If you have equivalent pipes; same size, roughness, inverts, etc. each pipe must carry half the flow. Model a single culvert with half the flow. If the pipes are not identical an iterative approach must be used. Model each culvert separately and converge on a solution which the flow from the two culverts equals your total flow when the model indicates the same headwater elevation for both culverts. Here headwater elevation is the true elevation (in the case of different sizes or inverts) and not depth above invert.

 

[cvg]

don't forget to calculate and consider your tailwater condition (using the full flow in the channel...)