Flow Velocities

[Gearoid O Sullivan]

Hello,

I have modeled a sewer network using Infoworks ICM. I have based my pipe diameters and gradients from a static calculation spreadsheet which I produced based on achieving 0.7 m/s velocity. However the velocities in the infoworks model are up to 20% lower! Both are based on C-W equation with ks value of 1.5. I was expecting there to be a slight difference due to headloss in the model. Is there anything else that could be causing this difference?

Thanks in advance

[Kristian Ravnkilde]

Flow rate would do it. Pipe full velocity does not equal actual velocity at lower flows.

[Robert Dickinson]

Hello Gearoid,
ICM uses the full St Venant solution for the links with multiple computational points along the link. The additional terms such as the water surface slope, non linear term in the St Venant solution would allow the same flow at smaller depth of higher depth depending on the upstream and downstream depths, cross sectional areas. The static calculation is a conservative solution and assumes a flat water surface slope.

If you are comparing a full pipe then altering the slot width for the FULL solution or using the presssure solution will affect your velocities. The slot adds more area based on the 2 percent of pipe width default,
Regards,
Robert Dickinson