Gravity Pipe Flow - Manning's Equation



Gravity pipe flow of a partially filled pipe.

Hydraulic radius is calculated and all hydraulic properties of the pipe are calculated in a table.

Three charts are provided to highlight maximum and special conditions for hydsraulic radius (R), flow rate (Q) and velocity (V).

Manning's n, hydraulic radius, slope and velocity can also be calculated for existing conditions.




Gravity Flow in Pipes - The Manning Formula, William C. Dunn PE

Manning Equations,  Robert Pitt & Shirley Clark


Worked through  book examples & hand calculations.

Some variables in manning's equations have interesting properties

shown in the table & Charts on the Calcs sheet, providing a method

of self validation. Place cursor on dots on the charts to get values.

Calcs Sheet: 

Determine the hydraulic radius based on water depth in pipe.

Input is US or SI; use the units of your choice, the rest are

converted automatically.

Output is in US & SI units.

A Table based on percent of water depth points out some

interesting properties of hydraulic radius, flow rate and


Charts show visually the properties of hydraulic radius,

flow rate, and velocity plotted against percent of water

depth in the pipe.

Find Depth of Flow -- Q, n, S, r Known.  Enter n, S and r. Take a guess at d. 

Keep incrementing or decrementing d until Q = known value. You have found


n-Value Sheet:

Provides a Table of Manning's cofficient n.


Slopes Sheet:

Explains visually how slopes are determined.


n,R,S,V Sheet:

Helps to solve for n, R, S, V for existing conditions.

Provides calculations to determine pipe diameter with

known design requirements of  Q, Vallowable, Smin, and n.

Calculation Reference
Gravity Pipe Flow
Piping Calculations
Manning's Equation

Calculation Preview

Submitted On:
08 Sep 2017
File Size:
183.27 Kb
File Version:
File Author:
Tony DiRocco

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Comments: 4
Sar 8 months ago
Mr. Tony, can you please re-load this 'Gravity flow calc' excel file in dropbox again. Thanks in advance.
tony.only 3 years ago
Try downloading again. Everything worked when the calcs were accepted.
Here's a link to my Dropbox. I'll leave it there for a week.
WTJones 3 years ago
The worksheet downloaded fine but it seems none of the equations are in the calc cells. Am I missing something?
JohnDoyle[Admin] 4 years ago
A great debut calculation Tony I have awarded a free 6 month XLC Pro subscription by way of thanks.