Enter the roughness coefficient, flow area, hydraulic radius, and channel slope into the calculator to determine the manning flow.

## Manning’s Flow Formula

The following formula is used to calculate a manning’s flow rate.

Q = (1.49/ n) * A * R^(2/3) * Sqrt (s)

• Where Q is the manning flow rate (ft^3/s)
• n is the roughness coefficient
• A is the flow area (ft^2)
• R is the hydraulic radius (ft)
• s is the channgel slope (ft/ft)

## What is Manning’s Flow?

Mannings flow is a metric of the volumetric flow rate of water that is moving through an open channel. The volumetric flow rate is calculated using Manning’s Equation which is stated above.

The equation itself was developed as an alternative to the Chezy Equation for flow through channels, to try and achieve more accurate calculations.

## How to calculate Manning’s Flow?

The following example outlines how to calculate open channel flow using Manning’s Equation.

First, determine the roughness coefficient. This is typically gathered through tables. For this example, the coefficient is .040.

Next, determine the total flow rate. In this example, the area is calculated to be 20 ft^2.

Next, measure the hydraulic radius. This is found to be 3 ft.

Next, determine the slope of the channel. This channel has a slope of 5ft/ft.

Finally, calculate the flow rate using the formula:

Q = (1.49/ n) * A * R^(2/3) * Sqrt (s)

Q = (1.49/ .040) * 20 * 3^(2/3) * Sqrt (5)

Q = 3456 ft^3/s