Variables
Description
What is this formula?
The Manning open channel flow equation estimates the flow rate in an open channel using the flow area, hydraulic radius, slope, and channel roughness.
When to use it
Use this formula for irrigation canals, drainage ditches, furrows, open agricultural channels, and gravity-fed water conveyance systems.
Example
An irrigation canal has:
A = 1.20 m²
R = 0.45 m
S = 0.0015
n = 0.025
Formula:
Q = (1/n)·A·R^(2/3)·S^(1/2)
Substitution:
Q = (1/0.025) × 1.20 × 0.45^(2/3) × 0.0015^(1/2)
Q = 1.10 m³/s
Result:
The estimated open channel flow rate is approximately 1.10 m³/s.
Applications
- Irrigation canal design
- Agricultural drainage channels
- Surface irrigation systems
- Open channel hydraulic analysis
- Gravity water conveyance
Note
Manning’s equation is an empirical hydraulic formula. It gives reliable estimates for steady, uniform open channel flow when the roughness coefficient and channel geometry are reasonably representative of real field conditions.