Calculate Gravity Flow Through Pipe

Manning's Equation:

\[ V = \frac{1}{n} \times R^{2/3} \times S^{1/2} \]

Flow Velocity (V):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Manning's Equation?

Manning's equation is an empirical formula that calculates the flow velocity in open channels and pipes flowing under gravity. It's widely used in civil engineering for designing drainage systems, irrigation channels, and sewer systems.

2. How Does the Calculator Work?

The calculator uses Manning's equation:

\[ V = \frac{1}{n} \times R^{2/3} \times S^{1/2} \]

Where:

\( V \) — Flow velocity (ft/s)
\( n \) — Manning's roughness coefficient (unitless)
\( R \) — Hydraulic radius (ft)
\( S \) — Slope of the energy line (ft/ft)

Explanation: The equation relates flow velocity to channel characteristics, where roughness coefficient represents the friction factor, hydraulic radius is the cross-sectional area divided by wetted perimeter, and slope represents the energy gradient.

3. Importance of Flow Velocity Calculation

Details: Accurate flow velocity calculation is essential for designing efficient drainage systems, preventing sediment deposition, ensuring adequate scouring velocity, and optimizing pipe/channel dimensions for various flow conditions.

4. Using the Calculator

Tips: Enter Manning's roughness coefficient (typical values: 0.012-0.015 for smooth pipes, 0.022-0.030 for rough channels), hydraulic radius in feet, and slope in feet per foot. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What are typical values for Manning's n?
A: For smooth concrete pipes: 0.012-0.013; for corrugated metal pipes: 0.022-0.026; for natural streams: 0.030-0.050.

Q2: How is hydraulic radius calculated?
A: Hydraulic radius = Cross-sectional area of flow ÷ Wetted perimeter. For full circular pipes: R = D/4, where D is the diameter.

Q3: What are typical slope ranges for gravity flow systems?
A: Sewer pipes typically have slopes between 0.5-2.0%; drainage channels may have slopes from 0.1-5.0% depending on design requirements.

Q4: When is Manning's equation not applicable?
A: Manning's equation is not suitable for pressurized flow, very steep slopes where flow becomes supercritical, or when sediment transport significantly affects flow characteristics.

Q5: How does pipe material affect the roughness coefficient?
A: Smoother materials (PVC, HDPE) have lower n values (0.009-0.012), while rougher materials (concrete, corrugated metal) have higher values (0.012-0.024).