HHM - Hydraulic Jump

Hydraulic Jump

A hydraulic jump is a sudden rise in water level due to decreasing velocity.

In flowing water, the water before the jump is moving much faster than the water after the jump. When the velocity of the water decreases to critical flow speed a jump occurs. Above this velocity, the water is moving too quickly to allow gravity waves to move upstream. At the point where the water reaches critical flow speed, the gravity waves become unable to move upstream, and build up at the jump, forming the sudden increase in water level, called a hydraulic jump.

Froude’s Number:

In order for a hydraulic jump to occur, the flow must be supercritical. The jump becomes more turbulent and more energy is dissipated as Froude’s number increases. A jump can only occur when the Froude’s number is greater than 1.0. Froude’s number (Fr) is a ratio relating inertia and gravity forces.

V=velocity

g=gravitational constant

y=depth of flow in open channel

Types of Hydraulic Jumps:

Name	Froude’s Number	Energy dissipation	Characteristics
Undular Jump	1.0-1.7	<5%	Standing waves
Weak Jump	1.7-2.5	5-15%	Smooth rise
Oscillating Jump	2.5-4.5	15-45%	Unstable; avoid
Steady Jump	4.5-9.0	45-70%	Best design range
Strong Jump	>9.0	70-85%	Choppy, intermittent

• In standing waves there is only a slight difference in depths y1 and y2. Near Fr₁ = 1.7 a series of small rollers develops.
• When the Froude’s number is between 1.7 and 2.5, the flow is in the pre-jump condition. The water surface is quite smooth, the velocity is reasonably uniform, and the energy loss is low.
• The transition region is when the Froude’s number is between 2.5 and 4.5. An oscillating action of the jump exists. Each oscillation of the jump produces a large wave of irregular period that can travel downstream for miles and damage earth banks. It is recommended to avoid this range of Froude numbers in the design of stilling basins.
• The best range for dissipating energy effectively is with a Froude number from 4.5-9.0. The jump is well balanced and the action is at its best. Energy losses range from 45% to 70%.
• Effective, but rough jumps occur at dissipating energy when Froude’s number is greater than 9.0. Energy losses range from 70% to 85%. Other types of stilling basins may be more economical.