This post is borrowed from our Innovyze blog and modified for #SWMM5
You may have noticed, that in some instances, the Pipe full capacity value on a pipe in #SWMM5 is less than the maximum simulation flow in a surcharged pipe. How can there be more flow in a pipe than its full capacity?
The Qfull in the link input summary table is calculated from the Manning’s equation. Manning’s equation is simpler than the full solution St Venant equations used by the SWMM5 simulation engine used to generate the model results. Therefore, there can sometimes be differences between Qfull (based on the slope) and the actual flow that can discharge as calculated using the 1D St Venant Equations. The Qfull value 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.
Manning’s equation assumes 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 meters can carry much more flow than one of 100 meters, given the same gradient, roughness etc.due to the longer length and the increased friction loss.