What This Solves
Calculates the Froude number for open channel flow and classifies the flow regime as subcritical (Fr < 1), critical (Fr = 1), or supercritical (Fr > 1).
Best Used When
- You need to classify whether channel flow is subcritical or supercritical
- You are checking for the potential of a hydraulic jump at a slope transition or structure
- You want a quick check of flow regime from known depth and velocity
Do NOT Use When
- You need the actual critical depth value rather than just the Froude number — Use Critical Depth Calculator
- You need the normal (uniform) flow depth for a given discharge — Use Normal Depth Calculator
Key Assumptions
- Froude number is calculated as Fr = V / sqrt(g * D_h) where D_h is hydraulic depth
- Hydrostatic pressure distribution exists at the cross-section
- Velocity is uniform across the cross-section (or average velocity is representative)
- The channel has a well-defined cross-section geometry
Input Quality Notes
For non-rectangular channels, the hydraulic depth (A/T) is used rather than the flow depth. Ensure you are using the correct depth definition for your channel shape.
Froude Number Overview
The Froude number is a dimensionless parameter representing the ratio of inertial forces to gravitational forces in open channel flow:
Fr = V / sqrt(g * Dh)
where:
- V = flow velocity
- g = gravitational acceleration
- Dh = hydraulic depth = A / T
Flow Regime Classification
| Regime | Fr Range | Characteristics |
|---|---|---|
| Subcritical | Fr < 1 | Deep, slow flow; downstream control |
| Critical | Fr = 1 | Unstable transition; minimum energy |
| Supercritical | Fr > 1 | Shallow, fast flow; upstream control |
Source: Chow (1959), Open-Channel Hydraulics.
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Last verified: February 2026