Hydraulic Jump in Open Channel Flow: Definition, Types, and Energy Loss Explained

Hydraulic jump is an important phenomenon in fluid mechanics and open channel flow. It occurs when fast-moving water suddenly changes into slow-moving water, resulting in a noticeable rise in the water surface. This phenomenon is commonly observed in rivers, canals, spillways, and downstream of dams.

In civil engineering, understanding hydraulic jumps is essential because they play an important role in controlling water flow and reducing excess energy. Engineers use this phenomenon in hydraulic structures to protect channel beds and riverbanks from erosion.

In this article, we will explain how to define hydraulic jump, discuss the different types of hydraulic jump, understand energy loss in hydraulic jump, and explore its applications in hydraulic engineering.

What Is a Hydraulic Jump? Define Hydraulic Jump

A hydraulic jump is a sudden transition of water flow from a supercritical state to a subcritical state in an open channel. During this transition, the water depth increases rapidly while the flow velocity decreases.

In simple words, when fast-moving and shallow water changes into slow-moving and deeper water, a hydraulic jump is formed.

A hydraulic jump generally occurs when the upstream flow has a Froude number greater than 1 and downstream conditions require subcritical flow.

The main characteristics of a hydraulic jump include:

• Sudden increase in water depth.

• Rapid decrease in flow velocity.

• Strong turbulence and mixing.

• Formation of surface rollers and air bubbles.

• Significant loss of mechanical energy.

Hydraulic jumps are particularly important in the design of spillways, dams, weirs, and stilling basins.

How Does a Hydraulic Jump Occur?

To understand how a hydraulic jump occurs, it is important to know the three basic flow conditions in open channels.

1. Subcritical Flow

Subcritical flow occurs when the Froude number is less than 1. In this condition, water flows at a relatively low velocity with greater depth. Gravity effects dominate the flow behaviour.

2. Critical Flow

Critical flow occurs when the Froude number is equal to 1. It represents the transition between subcritical and supercritical flow.

3. Supercritical Flow

Supercritical flow occurs when the Froude number is greater than 1. Water flows at a high velocity with relatively small depth. In this condition, inertial forces dominate the flow behaviour.

When supercritical flow meets a downstream condition that requires subcritical flow, a sudden increase in water depth takes place. This transition creates a hydraulic jump.

Types of Hydraulic Jump

The types of hydraulic jump are generally classified according to the upstream Froude number. The United States Bureau of Reclamation (USBR) classification commonly identifies five types of hydraulic jumps in horizontal rectangular channels.

1. Undular Hydraulic Jump

An undular hydraulic jump occurs when the upstream Froude number is approximately between 1.0 and 1.7.

In this type, the water surface rises gradually and forms a series of small waves or undulations. The flow transition is relatively smooth, and turbulence is very limited.

Energy loss in an undular hydraulic jump is generally small compared with other types.

2. Weak Hydraulic Jump

A weak hydraulic jump occurs when the upstream Froude number is approximately between 1.7 and 2.5.

Small rollers begin to develop on the water surface, but the turbulence is still relatively low. The water depth increases across the jump, while the velocity decreases.

The amount of energy dissipated in this type is relatively small.

3. Oscillating Hydraulic Jump

An oscillating hydraulic jump occurs when the upstream Froude number is approximately between 2.5 and 4.5.

In this condition, the water flow becomes unstable and produces irregular oscillations. Large waves may travel downstream and cause erosion along channel beds and banks.

This type of hydraulic jump is generally undesirable in hydraulic structures because of its unstable behaviour.

4. Steady Hydraulic Jump

A steady hydraulic jump occurs when the upstream Froude number is approximately between 4.5 and 9.0.

In this type, a relatively stable roller develops, and the water surface becomes highly turbulent within the jump region.

A significant amount of energy is dissipated through turbulence and mixing.

Steady hydraulic jumps are particularly useful in stilling basins because they provide effective energy dissipation.

5. Strong Hydraulic Jump

A strong hydraulic jump occurs when the upstream Froude number is greater than 9.0.

This type is characterized by intense turbulence, considerable air entrainment, and a highly disturbed water surface.

A large amount of mechanical energy is converted into turbulent energy and eventually dissipated.

Strong hydraulic jumps can provide substantial energy dissipation, but their design requires careful consideration of structural stability, flow conditions, and possible cavitation.

Energy Loss in Hydraulic Jump

Energy loss in hydraulic jump is one of the most important concepts in open channel hydraulics.

When water passes through a hydraulic jump, its velocity decreases and its depth increases. During this process, a considerable amount of mechanical energy is dissipated due to turbulence, mixing, air entrainment, and internal friction.

This energy loss is important because excessive flow energy can damage the downstream channel bed and riverbanks.

Formula for Energy Loss in Hydraulic Jump

For a hydraulic jump in a horizontal rectangular channel, the energy loss can be calculated using the following equation:

Energy Loss:

[
\Delta E = E_1-E_2
]

Where:

• (\Delta E) = Energy loss in hydraulic jump

• (E_1) = Specific energy before the jump

• (E_2) = Specific energy after the jump

The specific energy equation is:

[
E=y+\frac{V^2}{2g}
]

Therefore:

\left(y_2+\frac{V_2^2}{2g}\right)
]

For a horizontal rectangular channel, the energy loss can also be expressed as:

[
\Delta E=\frac{(y_2-y_1)^3}{4y_1y_2}
]

Where:

• (y_1) = Initial water depth before the jump

• (y_2) = Final water depth after the jump

• (g) = Acceleration due to gravity

• (V_1) = Velocity before the jump

• (V_2) = Velocity after the jump

This equation is applicable to a classical hydraulic jump in a horizontal rectangular channel under the usual assumptions of the simplified theory.

Why Does Energy Loss Occur in a Hydraulic Jump?

Energy loss occurs mainly because of the following reasons:

1. Turbulence: The sudden change in flow conditions produces strong turbulence and irregular water movement.

2. Mixing: Different layers of water mix rapidly, resulting in the dissipation of mechanical energy.

3. Air Entrainment: Air becomes trapped within the turbulent water, creating bubbles and increasing mixing.

4. Internal Friction: Friction between moving water particles contributes to the conversion of mechanical energy into heat.

Importance of Energy Dissipation

Energy dissipation is one of the main reasons hydraulic jumps are used in hydraulic engineering.

Below dams and spillways, water may flow at extremely high velocities. If this excess energy is not controlled, it can cause serious erosion and scour downstream.

A properly designed hydraulic jump helps reduce the velocity of water and dissipate excess energy before the flow enters a natural river or canal.

Applications of Hydraulic Jump in Civil Engineering

Hydraulic jumps have several practical applications in water resources and civil engineering.

1. Stilling Basins

Stilling basins are specially designed structures constructed downstream of spillways and dams. They help create and control hydraulic jumps to dissipate excess energy.

2. Downstream Protection of Dams

Hydraulic jumps reduce the energy of high-velocity water released from dams, helping protect downstream beds and banks from erosion.

3. Irrigation Canals

Hydraulic jumps are used in irrigation systems to control flow conditions and reduce excessive water velocity.

4. Flow Measurement and Control

Hydraulic jumps can help engineers understand changes in flow conditions and manage water movement in open channels.

5. Wastewater Treatment

The turbulence and mixing associated with hydraulic jumps can be useful in certain water and wastewater treatment processes, particularly for aeration and mixing.

Hydraulic Jump and the Venus Atmosphere: Is There a Connection?

The keyword "Venus atmosphere hydraulic jump" may appear in searches related to planetary science and fluid dynamics, but it is important to distinguish atmospheric flow phenomena from hydraulic jumps in civil engineering.

A classical hydraulic jump occurs in open-channel water flow when supercritical flow changes into subcritical flow. It is a standard topic in fluid mechanics and hydraulic engineering.

Venus has a dense atmosphere with complex atmospheric circulation, strong winds, and various wave phenomena. However, a conventional open-channel hydraulic jump is not a standard description of Venus's atmospheric circulation.

In geophysical fluid dynamics, similar jump-like transitions can occur in certain stratified atmospheric flows, but these are different from the classical hydraulic jump studied in civil engineering.

Therefore, the Venus atmosphere should not be confused with the hydraulic jump phenomenon used in open-channel flow analysis.

Conclusion

A hydraulic jump is an important phenomenon in open channel flow in which supercritical flow suddenly changes into subcritical flow. It is characterized by an increase in water depth, a decrease in velocity, and significant energy dissipation.

The five commonly studied types of hydraulic jump are undular, weak, oscillating, steady, and strong jumps. Each type has different flow characteristics depending on the upstream Froude number.

Energy loss in hydraulic jump is particularly important because it helps reduce excess mechanical energy and protects hydraulic structures from erosion and damage.

For civil engineering students and professionals, understanding the definition, types, energy loss, and practical applications of hydraulic jumps is essential for studying fluid mechanics and designing safe hydraulic structures.