What is a hydraulic jump and where does it occur?

A hydraulic jump is a rapid transition from supercritical (high velocity, shallow) to subcritical (low velocity, deep) flow, accompanied by significant energy loss and turbulence. It occurs when downstream conditions force subcritical flow but upstream conditions create supercritical flow. Common locations: at base of spillways, below sluice gates, in stilling basins. The jump dissipates excess kinetic energy preventing downstream erosion. Jump height ratio depends on upstream Froude number.