Working out Flow through a stormwater pipe with only top water level known

[kazzy]

Hi, I am having a bit of trouble working out how to figure out the maximum flow through a pipe network when the water is at a specified level? I know all the dimensions etc of the network and I am having trouble figuring out how to find the maximum flow that will travel downs the pipes at the head. The level is above a full pipe so I can't use Mannings. Any help would be much appreciated. Thanks

 

[froude]

If it were just a culvert I would suggest using one of the FHWA nomographs. Since it's an entire network, my thought would be to set up a model of your network (in StormCAD or similar) which includes all your pipes, junctions, and appropriate losses. Then (I'm assuming you only have flow entering at an upstream inlet, headwall, etc.) you can vary your Q value until you match your known HGL elevation.

I'm sure there are other ways to get your answer, but depending on the size of your network, this may be a good option. Good luck.

 

[cvg]

if the pipes are steep enough and tailwater is not controlling, then the flow is controlled at the inlet - in which case, no need to model the entire thing.

 

[jgailla]

You should be able to use Manning's to solve the flow in a simple network with both ends submerged. Manning's slope is the slope of the hydraulic grade line.
The elevation of the water at either end can be used to calculate the HGL slope if both ends are submerged.
You can account for entrance and exit losses with a K-factor and iterate to find a solution given the water level at each end and full flow pipes.
The flow is easier to calculate when both ends are submerged because you don't have to deal with headwater and tailwater effects.
cvg and froude give good advice depending on your situation.

 

[gbam]

without knowing the specific layout of the system and where additional flows come into the system it is difficult to provide feedback. Both Froude cvg and jgailla give good information. I suggest a model of some sort to vary discharges.

 

[LincolnPE]

Put the network into EPA SWMM and let it run. You can specify a starting water elevation at the top of the network like you are asking.