Flow calc help: non-pressure, tortuous path + pipe size reduction

[Evan5]

Open channel flow:
I have a 12" corrugated steel pipe with known flow, Manning's n value 0.010, slope = 0.02.
I need to connect a 10" inner diameter HDPE (n = 0.012) that has three 90 degree bends... on a 0.1 slope to the end of the 12" pipe.
I'm looking for suggestions on how to tell if the smaller/smoother pipe will accomodate the flow or back up the flow. Can I just use a Manning's comparison...or do I have to take into account the pipe length/bends?
Thanks,
Evan

 

[cvg]

corrugated steel n=0.010?

you need to confirm if the steeper downstream pipe hydraulic control is a)at the inlet b)not at the inlet but further down

that will require calculation of bend losses, outlet losses and friction losses plus transition losses at the connection point

 

[Evan5]

corrugated steel n=0.010?...oops typo, 0.022

Thanks

 

[gbam]

Which flow regime is your pipe in, sub or super? If sub then you need to account for headlosses if super you need to check if the bend will drown out the super regime upstream (hope I stated that correctly)

 

[Evan5]

Ugh...I need to freshen up. Feel like I'm starting from square one.

 

[cvg]

on first blush I would think you are OK, but I don't know what your tailwater depth is on the downstream pipe, how you plan to make the transition, length of the pipe, nor your discharge. These all affect the flow

 

[Evan5]

Ok...I'll give a run-down of the flow.

1) flows over a weir and dumps down...into a confined area where it...
2) flows into 12" dia corrugated metal pipe, n = 0.022, design Qin = 134.4 cfm, slope = 2%, length = 47'
3) Transition to 10" dia HDPE, slope = 10%, n = 0.012, slope = 10%...goes a couple feet & then left hand 90, goes a few feet & then right hand 90, goes a foot or two & then right hand 90, goes several feet & discharges into a drain box from an elevation (never submerged exit).

 

[cvg]

2.24 cfs is pushing it. your 10 inch may flow full. depending on the transition type that may backup your 12 inch a bit.