In an open channel, what is the specific term for the abrupt transition from supercritical to subcritical flow, characterized by a sudden rise in water depth?

The specific term for the abrupt transition from supercritical to subcritical flow, characterized by a sudden rise in water depth, is a hydraulic jump. A hydraulic jump is a phenomenon in an open channel where rapidly flowing, shallow water, known as supercritical flow, suddenly converts into slower, deeper water, known as subcritical flow. This transition is accompanied by intense turbulence and significant energy dissipation.

Supercritical flow occurs when the flow velocity is high and the water depth is relatively shallow. In this regime, the Froude number, a dimensionless parameter that represents the ratio of inertial forces to gravitational forces, is greater than 1 (Fr > 1). When the Froude number is greater than 1, disturbances cannot propagate upstream against the flow, meaning that the flow conditions are primarily controlled by upstream factors.

Subcritical flow occurs when the flow velocity is low and the water depth is relatively deep. In this regime, the Froude number is less than 1 (Fr < 1). When the Froude number is less than 1, disturbances can propagate upstream, indicating that the flow conditions are influenced by downstream controls, such as weirs, gates, or changes in channel geometry.

The Froude number is calculated as the flow velocity divided by the square root of the product of gravitational acceleration and the hydraulic depth. Its value dictates the flow regime.

During a hydraulic jump, the sudden change from supercritical to subcritical flow results in a significant increase in water depth and a corresponding decrease in flow velocity. This process involves a large amount of kinetic energy being converted into potential energy and thermal energy, manifesting as considerable turbulence, air entrainment, and a substantial loss of mechanical energy (or head loss). Hydraulic jumps typically form when fast-moving, supercritical flow encounters an obstruction, a change in channel slope, or a region where downstream conditions force a slower, deeper flow, such as at the base of a spillway or downstream of a sluice gate. This phenomenon is often utilized in hydraulic engineering to dissipate excess kinetic energy to prevent erosion and scour, and for mixing purposes in water treatment processes.