Hydraulic jump

A hydraulic jump forms where fast, shallow (supercritical) flow suddenly changes to slow, deep (subcritical) flow. The water surface rises abruptly with violent turbulence and rolling. It happens below spillways, sluice gates and steep chutes, and engineers deliberately create it in stilling basins to dissipate the huge energy of water released from a dam before it reaches the erodible river bed.

The key parameter is the Froude number, Fr = V / √(g y), comparing flow velocity with the speed of a small surface wave. Fr above 1 means supercritical flow, below 1 subcritical. Applying momentum conservation across the jump, since energy is not conserved, gives the relation between the depths before and after, called conjugate or sequent depths. The energy lost in the jump follows from those depths. The jump's character depends on Fr₁: weak and undular near 1, oscillating and producing waves between about 2.5 and 4.5 (undesirable), steady and efficient between about 4.5 and 9, and strong but rough above that.

Stilling basins use chute blocks, baffle blocks and end sills to stabilise and shorten the jump, with designs chosen by Froude number. The tailwater depth must match the required sequent depth across the range of discharges; if it is too low the jump sweeps out of the basin, and if too high it drowns. Jumps also aid mixing of chemicals at treatment plants.