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