Need to calculate divergent flow

[harboe]

I have a project where due to limited vertical clearance I have been asked by the City plan checker to neck a 24-inch RCP down to twin 18-inch lines in the downstream direction. I have not been able to find any standard details for structures designed to split flow from one into two lines. So, I have been forced to do it on my own and design my own structure which consists of an approximately 6'Wx4'Hx8'L generic utility vault with two 18" outlets on one end and a single 24" on the other end. I am going to have a false floor grouted on the bottom with semi circular channels to ease the transition through the box.

I would appreciate any advice on the following:
1. Is a standard detail for this available?
2. I have WSPG and HEC at my disposal but from what I can see they do not do divergent flow.

This project is in southern California.

Thanks.

 

[RWF7437]

This is a fairly common situation in sanitary sewer design. It commonly occurs in the design of inverted siphons although there the two outlets are usually the same size as the single inlet pipe.

"Channeling" the false bottom should improve hydraulic efficiency. I would try modeling this in HEC-RAS. That should give you at least some confidence that your design can work. You could then "tweak" the design until you felt comfortable with the result.

I don't know of any reason HEC-RAS couldn't handle "divergent" flow. Rivers diverge all the time and they are less predictable than your situation.

good luck

 

[RWF7437]

Two 18" pipes are NOT hydraulically equivalent to a single 24" pipe at the same slope.

Conveyance of a single 24" pipe ( n=.013) = 56.53 cfs
Conveyance of Two- 18" pipes ( n=.013) = 33.07 cfs

If the single 24" carries 27 cfs and a minimum velocity of 2.5 ft/sec it would need to be laid at a slope of 0.0012.

To carry the same flow with two 18" pipes each carrying 13.5 cfs would require those pipes to be laid at a slope of 0.1667. This results in a velocity of about 30 ft/sec.

Check my calculations, but it appears to me the Reviewer is suggesting an unreasonable design even if you can make it work. Velocities from 2.5 to, perhaps, 10 ft/sec are more typical of storm drain design and even those over 5 ft/sec may require special protection to prevent erosion, pipe separation and thrust problems .

good luck

 

[harboe]

I am aware that two 18's are not the same as a single 24 but I was trying to get the flow "moving" in the larger 24 first and then once it has some momentum tranistion it into the twin 18's. I also realized my WSPG model was not working properly and have corrected the problem.

Also I think I am going to push an alternatives onto the City where we use elliptical pipe or RCB so that we can make the vertical clearances while still using a single conduit.

Thanks