Gravity Pipe Flow Rate 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):

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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 relates the flow rate to the channel's cross-sectional area, hydraulic radius, slope, and roughness coefficient.

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 (square feet)
\( n \) — Manning's roughness coefficient (dimensionless)
\( R \) — Hydraulic radius (feet)
\( S \) — Slope of the energy line (feet per foot)

Explanation: The equation accounts for the geometry of the pipe/channel and the frictional resistance to flow.

3. Importance of Flow Rate Calculation

Details: Accurate flow rate calculation is essential for designing drainage systems, irrigation channels, sewer systems, and other gravity-flow applications to ensure proper capacity and prevent flooding.

4. Using the Calculator

Tips: Enter cross-sectional area in ft², Manning's coefficient (typical values: 0.013 for smooth concrete, 0.015 for cast iron), hydraulic radius in feet, and slope in ft/ft. All values must be positive.

5. Frequently Asked Questions (FAQ)

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

Q2: What are typical Manning's n values?
A: Common values: smooth concrete (0.012-0.014), cast iron (0.013-0.017), corrugated metal (0.022-0.026), natural streams (0.03-0.05).

Q3: When is Manning's equation applicable?
A: For turbulent flow in open channels and partially full pipes. Not suitable for pressurized pipe flow or laminar flow conditions.

Q4: What units should be used?
A: This calculator uses US customary units. For metric units, replace 1.486 with 1.000 in the equation.

Q5: How accurate is Manning's equation?
A: It provides good estimates for most engineering applications but accuracy depends on proper selection of the roughness coefficient.