Surcharge Flow vs. Pipe Full Capacity in ICM

You may have noticed, that in some instances, the Pipe full capacity value on a pipe in InfoWorks ICM is less than the flow in a surcharged pipe. How can there be more flow in a pipe than its full capacity?

The Pipe full capacity (pfc) field is populated when the model network is validated. It is calculated from the Colebrook White or Manning equations. These equations are much more simplistic than the full solution St Venant equations used by the InfoWorks ICM simulation engine used to generate the model results. Therefore, there can sometimes be differences between the pipe full capacity field and the actual flow that can discharge. The value in the pfc field is only intended to be an approximation/reference for the user. It is not used by the engine to determine when a pipe goes into surcharge.

The Colebrook White or Manning equations assume that the pipe is infinitely long and therefore there is often more flow through a pipe than the quoted capacity, without it going into surcharge. To prove this is the case you can make the pipe a longer length or apply a constant max flow and you should see the pipe surcharging. The length of the pipe has a significant effect. You may find that a pipe of say 10 feet can carry much more flow than one of 300 feet, given the same gradient, roughness etc.

This pfc value has been in the software ever since we can remember, certainly since the early days of HydroWorks. It can be a useful reference, but sometimes creates confusion. We hope that this blog post provides some clarity on how the pfc field was intended to be used.

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