Gravity Drain Flow Rate Calculator

Manning's Equation:

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

Manning's Roughness (n):

Cross-sectional Area (A):

m²

Hydraulic Radius (R):

Slope (S):

m/m

Flow Rate (Q):

1. What is a Gravity Drain Flow Rate Calculator?

Definition: This calculator estimates the flow rate in open channels or drains using Manning's equation, which considers channel geometry, roughness, and slope.

Purpose: It helps engineers, hydrologists, and planners design and analyze gravity-driven drainage systems.

2. How Does the Calculator Work?

The calculator uses Manning's equation:

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

Where:

\( Q \) — Flow rate (m³/s)
\( n \) — Manning's roughness coefficient (dimensionless)
\( A \) — Cross-sectional area of flow (m²)
\( R \) — Hydraulic radius (m) = A/P, where P is wetted perimeter
\( S \) — Slope of the energy grade line (m/m)

Explanation: The equation balances gravitational forces driving flow against frictional resistance from channel boundaries.

3. Importance of Flow Rate Calculation

Details: Accurate flow rate estimation ensures proper drainage system design, prevents flooding, and maintains water quality in channels.

4. Using the Calculator

Tips: Enter Manning's n (default 0.013 for concrete), cross-sectional area, hydraulic radius, and slope. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What are typical Manning's n values?
A: 0.012-0.015 for concrete, 0.022-0.030 for natural streams, 0.03-0.04 for vegetated channels.

Q2: How do I calculate hydraulic radius?
A: Divide cross-sectional area by wetted perimeter (R = A/P).

Q3: What slope should I use?
A: For uniform flow, use channel bottom slope. For non-uniform, use energy grade line slope.

Q4: Can this be used for pressurized pipes?
A: No, this is for open channel flow only. Use Darcy-Weisbach for pressurized pipes.

Q5: What units does this calculator use?
A: Metric units (meters, m³/s). For English units, convert inputs appropriately.