Manning's Velocity Calculation

http://www.engineeringtoolbox.com/mannings-formula-gravity-flow-d_800.html

What part are you having trouble with?

The use of these graphs should help to answer your question. These graphs can be found in any civil engineering handbook. Match the pipe diameter with the pipe slope and draw a line. This will give your flow and velocity. Manning's is always done for full flow, then, using the next graph you can determine the flow characteristics for pipe running at less than full conditions. The images aren't very legible but as I said these graphs can be found in any handbook. I would never recommend using an internet calculator to do this sort of thing unless you have a lot of experience with it and can verify the results.

Hope this helps!