Gravity Pipe Flow Calculator

Manning's Equation:

\[ Q = A \times \frac{1.486}{n} \times R^{2/3} \times S^{1/2} \]

Cross-sectional Area (A):

ft²

Manning's Coefficient (n):

dimensionless

Hydraulic Radius (R):

Slope (S):

ft/ft

Flow Rate (Q):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Manning's Equation?

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

2. How Does the Calculator Work?

The calculator uses Manning's equation:

\[ Q = A \times \frac{1.486}{n} \times R^{2/3} \times S^{1/2} \]

Where:

\( Q \) — Flow rate (cubic feet per second)
\( A \) — Cross-sectional area of flow (square feet)
\( n \) — Manning's roughness coefficient (dimensionless)
\( R \) — Hydraulic radius (feet) = A/P (Area divided by wetted perimeter)
\( S \) — Energy slope or channel slope (feet per foot)

Explanation: The equation relates flow rate to channel geometry, roughness, and slope. The constant 1.486 converts the metric version to US customary units.

3. Importance of Flow Rate Calculation

Details: Accurate flow rate calculation is essential for designing efficient drainage systems, preventing flooding, sizing pipes correctly, and ensuring proper water conveyance in irrigation and municipal systems.

4. Using the Calculator

Tips: Enter all values in US customary units. Common Manning's n values: concrete pipe (0.012-0.015), corrugated metal (0.022-0.026), natural stream (0.03-0.05). Ensure hydraulic radius is correctly calculated.

5. Frequently Asked Questions (FAQ)

Q1: What is hydraulic radius?
A: Hydraulic radius (R) is the cross-sectional area of flow divided by the wetted perimeter. For full circular pipes, R = D/4 where D is diameter.

Q2: How do I choose Manning's n value?
A: n depends on pipe material and condition. Consult engineering tables for specific materials. Smoother surfaces have lower n values.

Q3: Can this be used for partially full pipes?
A: Yes, but you must calculate the actual flow area and wetted perimeter for the specific depth of flow.

Q4: What are typical slope values?
A: Sewer pipes typically have slopes between 0.5% to 2%. Storm drains may have steeper slopes depending on design requirements.

Q5: How accurate is Manning's equation?
A: It's an empirical equation that works well for turbulent flow in rough pipes. Accuracy depends on correct selection of n value and proper hydraulic radius calculation.