# Hydraulic jump A hydraulic jump is the abrupt step where fast shallow [[Water]] turns slow and deep. It happens wherever a supercritical flow has to meet a subcritical one — below a spillway, at the foot of a weir, in the ring around a tap stream hitting a sink. The transition is violent and irreversible: [[Energy]] is dissipated in the churn, which is precisely why [[Hydraulic_engineering|hydraulic engineers]] build structures to force one on purpose. ## The one-way step The jump can only run from fast-and-thin to slow-and-deep, never the reverse, because the reverse would require [[Energy]] to appear from nowhere — the [[Second_law_of_thermodynamics]] enforcing direction in a [[Fluid_dynamics|fluid]] problem you can see in a kitchen. The incoming [[Froude_number]] sets the character: just above one it is a smooth undular ripple, higher and it becomes a steep frothy roller entraining air. The depth ratio across the jump follows from conservation of momentum alone, which makes it one of the few [[Turbulence|turbulent]] phenomena with a clean analytic answer. ## Built on purpose A stilling basin below a dam exists to trap the jump in a lined pocket where its [[Energy]] can be spent harmlessly rather than eroding the channel downstream — otherwise the flow scours its own foundation, a [[Reliability_engineering|failure mode]] with a long history. The same structure aerates the [[Water]], which matters for [[Ecology|downstream]] oxygen. Tidal bores on rivers are travelling jumps, and the same mathematics describes shocks in compressible gas and traffic jams in [[Traffic_engineering_(transportation)|traffic flow]]. **Microsim brief (next pass):** Side-on channel; sliders for inlet speed and downstream gate height. The jump forms, slides upstream and downstream, and switches between undular and roller as Fr changes. **On the spine:** [[Froude_number]] · [[Water]] · [[Turbulence]] · [[Energy]] · [[Fluid_dynamics]] · [[WT!Thury_Hydrodynamics_Compendium]]. <!-- COMPENDIUMLINK:BEGIN g19 — generated from _registry/plans/THURY_COMPENDIUM_SECTIONS.md; do not hand-edit inside --> **Part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]]** — main article for section 28, *Open channels and the hydraulic jump*. Related sections: [[Hydropower]] · [[Compressible_flow]] · [[Dam]]. <!-- COMPENDIUMLINK:END --> <!-- THURYSIM:BEGIN g21 — Thury Compendium microsim (framework build, specs/sims/Hydraulic_jump.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Hydraulic jump* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/thury/Hydraulic_jump.html" data-title="Hydraulic jump"></div> *Built from `MICROSIM_GUIDE/specs/sims/Hydraulic_jump.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.* <!-- THURYSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hydraulic_jump) : [Wikitube](https://en.wikitube.io/wiki/Hydraulic_jump) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*