---
title: "Classification of hydraulic jump by Froude number"
subject: "Hydraulics"
chapter: 9
key: "semester_4/hydraulics:hydraulic-jump-classification-froude"
source: https://www.bctengineer.com/q/semester_4/hydraulics/hydraulic-jump-classification-froude
site: BCT Engineering
---

# Classification of hydraulic jump by Froude number

Hydraulics · Chapter 9: Non-uniform rapidly varied flow (RVF)

- Asked in: 2080 Chaitra, 3 marks
- Video: [Open Channel 1b - Froude number](https://www.youtube.com/watch?v=XNaRJOZyKac) — CE340Hydraulics
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/hydraulics/hydraulic-jump-classification-froude
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-9

## Question

Classify the hydraulic jump based on Froude number.

## Short answer

Based on upstream Froude number Fr₁: undular (1.0–1.7), weak (1.7–2.5), oscillating (2.5–4.5), steady (4.5–9.0), and strong (>9). Steady jump dissipates 45–70% energy and is preferred for stilling basins.

## Answer

Hydraulic jumps are classified by the upstream Froude number $Fr_1$ (USBR classification):

| $Fr_1$ | Type of jump | Features |
|---|---|---|
| 1.0 | No jump | critical flow |
| 1.0 to 1.7 | **Undular jump** | surface shows undulations (standing waves); little energy loss (under 5%) |
| 1.7 to 2.5 | **Weak jump** | series of small rollers; smooth downstream surface; loss 5 to 15% |
| 2.5 to 4.5 | **Oscillating jump** | jet oscillates between bed and surface, producing irregular waves that travel far downstream; loss 15 to 45% |
| 4.5 to 9.0 | **Steady jump** | well-balanced, stable and well-defined; best for energy dissipation; loss 45 to 70% |
| above 9.0 | **Strong jump** | rough, choppy surface, high energy dissipation (over 70%), but the downstream waves can be damaging |

The percentage of energy lost, $\Delta E/E_1$, rises with $Fr_1$. Stilling basins are designed to produce a steady jump ($Fr_1 = 4.5$ to $9$); oscillating jumps are avoided.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which Fr1 range gives a steady jump, the best for energy dissipation?
   A. 1.7 to 2.5
   B. 2.5 to 4.5
   C. 4.5 to 9.0
   D. Above 9.0
   Answer: C. A steady jump (Fr1 4.5 to 9) is well balanced and stable, and stilling basins are designed for it. (Key point: Steady jump range)

2. What characterises an undular jump (Fr1 1.0 to 1.7)?
   A. Violent roller with 70% energy loss
   B. Surface undulations (standing waves) with very little energy loss
   C. Jet oscillating between bed and surface
   D. Rough choppy surface
   Answer: B. Near critical flow the jump is only waves with under 5% loss. (Key point: Undular jump)

3. Why are oscillating jumps (Fr1 2.5 to 4.5) avoided in stilling basins?
   A. They dissipate no energy
   B. They occur only at Fr1 = 1
   C. The jet oscillates and produces irregular waves that travel far downstream
   D. They produce a very strong roller that damages the bed
   Answer: C. The unstable jet creates waves that travel far downstream. (Key point: Oscillating jump)

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