Once the system data has been entered/opened/edited the user can first run network checks on the system to ensure all necessary data is in place, before commencing with the hydraulic computation of the flow (and velocity). The latter is achieved by selecting from the main menu Analysis > Time Simulation > SWMM > Analyse (or clicking the 
SWMM Time Simulation toolbar button).
A progress bar will appear, while the system is analysing the network. In case there are possible faults in the network, the Model Check box will appear with a list of faults/warnings.