Manning's Flow Calculator

Formula

To calculate the Manning's Flow Rate (\(Q\)):

\[ Q = \frac{1.49}{n} \cdot A \cdot R^{\frac{2}{3}} \cdot \sqrt{s} \]

Where:

\(Q\) is the Manning's Flow Rate (ft³/s)
\(n\) is the Roughness Coefficient
\(A\) is the Flow Area (ft²)
\(R\) is the Hydraulic Radius (ft)
\(s\) is the Channel Slope (ft/ft)

What is Manning's Flow?

Manning's flow is a metric of the volumetric flow rate of water 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.

Example Calculation

Let's assume the following values:

Roughness Coefficient (\(n\)) = 0.03
Flow Area (\(A\)) = 20 ft²
Hydraulic Radius (\(R\)) = 2 ft
Channel Slope (\(s\)) = 0.01 ft/ft

Using the formula:

\[ Q = \frac{1.49}{0.03} \cdot 20 \cdot 2^{\frac{2}{3}} \cdot \sqrt{0.01} \approx 157.68 \text{ ft}^3/\text{s} \]

The Manning's Flow Rate is approximately 157.68 ft³/s.