Pressure Pipe question

[TheEnginerd]

Good morning all!
I have a situation where a drainage area flows towards and existing inlet with a 10" pipe discharge.
I setup hydrocad to show that the 10" pipe is the primary discharge, the parking around around the inlet is a small "pond" with a weir created which represents the low point where and water that doesn't get into the 10" pipe would start to spill overland. The overland flow is the Secondary discharge.
I get approximately 1.49 CFS during the 25 year storm event discharging through the 10" pipe.
I wanted to check if the orifice equation was giving me accurate numbers because I know that when the 10" pipe has say 2 feet of head above it, the flow through the 10" would be much greater than if it had no head above it... So using the Hazen-Williams formula in another program (flowmaster), i find that with the same amount of head, the flow through the pipe is 3.5 cfs.
Is it possible that HydroCAD and the other methodology yields such different results?
It behooves me to show that the existing conditions are as accurate as possible and that the existing number is as high as they really are. So i'm considering setting up a primary custom discharge table with discharges at different head elevations.
Does this sound like the correct thing to do?
Thanks very much.
Andy

 

[psmart]

HydroCAD performs a full culvert analysis, including entrance loss, frictional loss, and possible tailwater control. In contrast, your Hazen-Williams calculations considers only frictional loss within the barrel, and is based on pipe-full conditions, which may not be valid and is not assumed by HydroCAD. So there are a number of reasons for different results.

As a guess, the entrance loss is the primary reason for the difference. With a common Ke=0.5 the entrance will usually be the control, rather than the frictional loss. Try reducing the Ke to near zero and I suspect the numbers will come into closer agreement. However, the calculations are very different to begin with, so differences are to be expected. For further details you can read about the culvert flow calculations in the HydroCAD Owner's Manual.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TheEnginerd]

thank you as always psmart! that's great information.