bridge drainage outlet velocity calculation in close vertical drainage system

Question

I was trying to calculate vertical close pipe drainage system velocity in stormCAD. I deigned vertical pipe as manhole. I see that the velocity on the ground outlet does not change with the change of the height of the pipe (manhole). What is the right way to design vertical pipe network in StormCAD.

Craig Calvin · Answer

Hello Fatema, 
 When the Calculation Option "Average Velocity Method" is set to "Actual Uniform Flow Velocity" then the conduit property "Velocity" is calculated at normal depth. The conduit property "Velocity (In)" and "Velocity (out)" are the actual velocities. Therefore if the average velocity appears to be larger than the velocity in and out of the conduit, then conduits are not flowing near normal depth. 
 See: 
 What are the different velocity methods used in SewerCAD and StormCAD? 
 The average velocity appears to be larger than the velocity in and out of the conduit. 
 After looking into this further, you aren't going to be able to model a vertical pipe and get an accurate velocity out of it. As discussed in this article , conduits are assumed to be horizontal. 
 Using a manhole as the vertical pipe won't work either. It will look vertical in profiles, but it's not going to compute the velocity as if water is falling down vertically. Manholes are assumed to have flow pass through them horizontally. The velocity out of the manhole will be the velocity of water flow horizontally through the bottom of the structure. 
 I recommend using one of the approaches described in the following article and assume the inlet opening is the restricting factor and the vertical pipe will have as much or greater capacity to drain the water than the inlet opening captures. 
 See: How to model a bridge scupper