Hydraflow Velocity vs. Mannings Eq. Velocity 1

[cas4ku]

I have a question about velocities as calculated by Hydraflow. We have a project where the velocities in our storm drain pipes have been questioned as being to low. While working out a possible solution we used the pipe calculator in autocad (mannings equation) to do some quick calcs.

We are discovering that the velocities reported by Hydraflow are way less than the velocity calculated directly by manning's equation.

Can you provide a possible reason for this?

For example:

15" RCP (manning's n = .013)
0.8% slope
Q(10) = 0.88 cfs

Hydraflow provided a velocity of 1.0 fps in the output table.
Manning's calculator in autocad (LDD) provided a velocity of 3.4 fps.

Any help would be appreciated.

 

[cvg]

depends on depth of flow in the pipe. if there is no backwater (surcharge) than flow depth is about 4 inches and velocity would be 3.4 fps. However if you assume full flow (or nearly full) conditions, velocity would be closer to 1 fps.

However, I'm not sure it makes much difference. 3.4 fps is a reasonable velocity. You can't do much about the 1 fps if it is surcharged. What is the main objection?