Gravity Flow Velocity Calculator

Manning's Equation:

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

Flow Velocity (v): m/s

1. What is a Gravity Flow Velocity Calculator?

Definition: This calculator estimates the velocity of water flowing in an open channel using Manning's equation.

Purpose: It helps engineers, hydrologists, and planners determine water flow rates in rivers, streams, and man-made channels.

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 (meters per second)
\( n \) — Manning's roughness coefficient (dimensionless)
\( R \) — Hydraulic radius (meters)
\( S \) — Slope of the energy line (meters/meter)

Explanation: The equation relates flow velocity to channel roughness, cross-sectional efficiency, and slope.

3. Importance of Flow Velocity Calculation

Details: Accurate velocity calculations are essential for flood prediction, drainage design, irrigation systems, and environmental impact assessments.

4. Using the Calculator

Tips:

Typical n values: 0.013 (smooth concrete) to 0.15 (natural streams with heavy vegetation)
Hydraulic radius = Cross-sectional area / Wetted perimeter
Slope is the vertical drop divided by horizontal distance

5. Frequently Asked Questions (FAQ)

Q1: What are typical Manning's n values?
A: Common values range from 0.011 (glass) to 0.040 (natural streams). Concrete channels are typically 0.013-0.015.

Q2: How do I calculate hydraulic radius?
A: For full circular pipes, R = D/4. For rectangular channels, R = (width × depth)/(2 × depth + width).

Q3: What units should I use?
A: This calculator uses metric units (meters and m/s). For imperial units, use n = 1.49 instead of 1 in the equation.

Q4: When is Manning's equation not appropriate?
A: For pressurized flow (pipes), very steep slopes, or non-uniform flow conditions.

Q5: How accurate is this calculation?
A: Accuracy depends on correct n value selection and proper channel geometry measurement.