Hydraulic jump

A hydraulic jump is an abrupt increase in the depth of a fast-moving liquid stream in an open channel, which is accompanied by a decrease in speed. The jump appears as a wavy or turbulent region between the high-speed upstream flow and the slower downstream flow. A common example is the circular jump formed when a tap runs into a kitchen sink.

Hydraulic jumps occur below dam spillways and in rivers. Hydraulic jumps may be stationary, as below a dam, or they may propagate as surges along a stream, as in a tidal bore. Civil engineers design spillways and stilling basins to create hydraulic jumps that dissipate the mechanical energy of water flowing over dams.

A hydraulic jump can form only when the upstream flow moves faster than shallow-water waves, so that small disturbances to the flow cannot travel upstream. For speeds only slightly above the wave speed, the transition is a rolling wave. As the flow speed increases, the transition becomes more abrupt, until at high enough speeds the front breaks and curls upstream. These regimes are characterized by the ratio of upstream speed to wave speed, which is called the Froude number.

Hydraulic jumps also occur in stratified flows, including in the atmosphere and the oceans. In rivers, they can create both recreational whitewater features and dangerous recirculating currents.