Manning Open Channel Flow Calculator
Free Manning open channel flow Calculator for civil engineering projects. Enter dimensions to get material lists and cost estimates.
Reviewed by Abdullah, Technical Content Specialist
Formula
V = (1.486/n) x R^(2/3) x S^(1/2) | Q = V x A
The Manning equation calculates flow velocity (V) in feet per second using the roughness coefficient (n), hydraulic radius (R) in feet, and channel slope (S) in ft/ft. The constant 1.486 converts to US customary units. Discharge (Q) in cubic feet per second equals velocity times the cross-sectional flow area. The hydraulic radius is the flow area divided by the wetted perimeter.
Worked Examples
Example 1: Rectangular Drainage Channel
Problem:Calculate the flow rate in a 6 ft wide rectangular concrete channel with 3 ft flow depth, slope of 0.5%, and Manning n = 0.013.
Solution:Area = 6 x 3 = 18 sq ft\nWetted Perimeter = 6 + 2(3) = 12 ft\nHydraulic Radius = 18/12 = 1.5 ft\nV = (1.486/0.013) x 1.5^(2/3) x 0.005^(1/2) = 10.59 ft/s\nQ = 10.59 x 18 = 190.62 cfs
Result:Velocity = 10.59 ft/s, Discharge = 190.62 cfs
Example 2: Trapezoidal Earth Channel
Problem:Calculate flow in a trapezoidal channel with 8 ft bottom width, 2 ft depth, 2:1 side slopes, slope 0.3%, Manning n = 0.030.
Solution:Area = (8 + 2x2) x 2 = 24 sq ft\nWetted Perimeter = 8 + 2(2)sqrt(1+4) = 16.94 ft\nHydraulic Radius = 24/16.94 = 1.417 ft\nV = (1.486/0.030) x 1.417^(2/3) x 0.003^(1/2) = 3.44 ft/s\nQ = 3.44 x 24 = 82.56 cfs
Result:Velocity = 3.44 ft/s, Discharge = 82.56 cfs
Frequently Asked Questions
What is the Manning equation and when is it used?
The Manning equation is an empirical formula used to calculate the average velocity of water flowing in an open channel under uniform, steady-state conditions. The equation relates flow velocity to the channel roughness coefficient (n), the hydraulic radius, and the channel slope. It is one of the most widely used formulas in civil and environmental engineering for designing drainage channels, storm sewers, irrigation canals, and natural stream assessments.
How do I select the correct Manning roughness coefficient (n)?
The Manning n value depends on the channel material and surface condition. Smooth concrete channels typically use n = 0.012 to 0.015, while corrugated metal pipes use 0.022 to 0.025. Natural earth channels range from 0.020 for clean straight channels to 0.035 for winding channels with vegetation. Heavily vegetated floodplains can have n values of 0.050 to 0.150. Published tables from FHWA and USGS provide n values for hundreds of channel conditions.
What is the difference between subcritical and supercritical flow?
The flow regime is determined by the Froude number, which compares the flow velocity to the wave propagation speed. When the Froude number is less than 1, the flow is subcritical (tranquil) and disturbances can travel upstream. When it exceeds 1, the flow is supercritical (rapid) and disturbances cannot propagate upstream. Critical flow occurs at a Froude number of exactly 1. Most designed drainage channels operate under subcritical conditions for stability and safety.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy