Manning Equation Calculator
Open channel flow software
Consulting available March 2024. Call or text Ken Edwards, PhD, PE (Owner of LMNO Engineering) for help with your project 740-707-2614. Email: LMNO@LMNOeng.com
Units in Manning calculator: ft=foot, m=meter, s=second.
Manning Equation:
k is a unit conversion factor: k=1.49 for English units (feet and seconds). k=1.0 for SI units (meters and
seconds).
A=Flow area of the pipe, culvert, or channel.
P=Wetted perimeter which is the portion of the circumference that is in contact with water.
Q=Discharge (flow rate).
S=Downward (longitudinal) slope of the culvert.
V=Average velocity in the pipe, culvert, or channel.
Manning n varies with the roughness of the pipe, culvert, or channel. The higher the n, the rougher the material. Table of Manning n values.
The Manning Equation is the most commonly used equation to analyze open channel flows. It is a semi-empirical equation for simulating water flows in channels and culverts where the water is open to the atmosphere, i.e. not flowing under pressure, and was first presented in 1889 by Robert Manning. The channel can be any shape - circular, rectangular, triangular, etc. The units in the Manning equation appear to be inconsistent; however, the value k has hidden units in it to make the equation consistent. The Manning Equation was developed for uniform steady state flow (see Discussion and References for Open Channel Flow). S is the slope of the energy grade line and S=hf/L where hf is energy (head) loss and L is the length of the channel or reach. For uniform steady flows, the energy grade line = the slope of the water surface = the slope of the bottom of the channel.
The product A/P is also known as the hydraulic radius, Rh.
© 1998-2024 LMNO Engineering, Research, and
Software, Ltd. All rights reserved.
Please contact us for consulting or questions about the Manning equation.
LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd. Athens, Ohio 45701 USA Phone: (740) 707-2614
LMNO@LMNOeng.com https://www.LMNOeng.com
LMNO Engineering home page (more calculations)
Related calculations:
Design of Rectangular Channels
Design of Trapezoidal Channels
Circular Culverts using Manning Equation
Culvert Design using Inlet and Outlet Control
Q=VA simple flowrate calculator
Other information: