Hydraulic Jump — Reference
The rapid transition from supercritical to subcritical flow used to dissipate energy below spillways, chutes, and outlets. Subscript 1 = upstream (supercritical) section, 2 = downstream (subcritical) section.
Core Equations (Rectangular Channel)
The relation is reversible: y&sub1;/y&sub2; = ½(√(1 + 8·Fr&sub2;²) − 1). The two depths are conjugate (equal specific force), not equal specific energy — the difference is the dissipated ΔE.
Jump Classification by Upstream Froude Number
| Fr&sub1; | Type | Character & energy dissipation |
|---|---|---|
| 1.0–1.7 | Undular | Standing waves, minimal loss (< 5%) |
| 1.7–2.5 | Weak | Smooth surface, low loss (5–15%) |
| 2.5–4.5 | Oscillating | Avoid — jet oscillates, sends damaging waves downstream (15–45%) |
| 4.5–9.0 | Steady | Stable, well-balanced, best performance (45–70%) |
| > 9.0 | Strong | Rough, intense turbulence, very effective (up to ~85%) |
Sources: Chow, V.T. (1959), Open-Channel Hydraulics. USBR, Hydraulic Design of Stilling Basins and Energy Dissipators (Engineering Monograph 25). FHWA HEC-14.
Related cheat sheets and tools
Get the approach depth and velocity from Manning's with the channel geometry card, design the basin in the stilling basin tool, and protect the downstream channel with riprap sizing. For routing the spillway flow that feeds the dissipator, see HydroComplete.