Manning Equation Explained 2

[me2001]

Hello everyone, I have performed calculations using the manning equation to obtain pipe slopes for a 1.5 and a 2 inch pipe. My calculated values show that for the larger pipe my slope is smaller than the smaller pipe even thought my velocity for the larger pipe is larger. I am having some difficulty trying to get this through my "common sense" explanation. I would assume that my values would show that with a smaller velocity you would have a smaller slope. I am assuming the hydraulic radius has something to do with the value, but I am failing to recognize how this is so. Could someone please clarify this phenomenon. I thank you for your assistance.

 

[katmar]

Your pipe is obviously not full if the velocity in the 2" pipe is higher, presumably for the same flowrate as in the 1.5" pipe. You are correct that the hydraulic radius is the important factor. The ratio of cross sectional area to wetted perimeter is higher for the 2" pipe than the 1.5" pipe because the perimeter increases with the diameter, but the area increases with the square of the diameter.

This means that the 2" pipe will have less resistance, because the resistance is due to the wetted perimeter. With less resistance you will need less slope and will obtain higher velocities.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[me2001]

Thank you Katmar for the explanation, it was very helpful.

 

[LHA]

katmar's explanation is correct, with respect to R's effect on v.

However, to clarify, if you solve for slope at a given flow, you end up with the full-flow velocity. Both pipes are full, but the 2" carries that water at a flatter slope.

Engineering is the practice of the art of science - Steve