Short Answer:

A hydraulic jump is a sudden rise in the water surface level that occurs when fast-moving water (supercritical flow) changes to slow-moving water (subcritical flow) in an open channel. This sudden change creates turbulence, white water, and energy loss. It is commonly seen downstream of spillways, sluice gates, and slopes.

Hydraulic jumps are caused when the flow transitions from high velocity to low velocity due to an increase in depth or an obstacle in the channel. This transition results in energy dissipation and is used in civil engineering to control flow and prevent erosion.

Detailed Explanation:

Hydraulic jump

A hydraulic jump is a very important concept in open channel hydraulics. It is a sudden and visible change in flow condition, where water transitions from supercritical flow (fast and shallow) to subcritical flow (slow and deep). This process causes a sudden rise in the water surface, along with turbulence, air bubbles, and energy loss. The appearance of frothy, rolling water marks the location of the hydraulic jump.

What Happens During a Hydraulic Jump

When high-speed water flows in an open channel, it carries a lot of energy. But sometimes, due to changes in channel slope, width, or due to obstacles like gates or blocks, the flow slows down. This slowing of water increases the depth and changes the flow type.

The hydraulic jump acts as a natural energy dissipation process. The excess kinetic energy of supercritical flow is converted into turbulence and heat, resulting in a deeper and slower flow downstream. It is one of the most efficient and visible ways to dissipate energy in hydraulic structures.

Causes of Hydraulic Jump

Hydraulic jump mainly occurs due to the following reasons:

1. Flow Transition from Supercritical to Subcritical:
   When fast-moving shallow water needs to become slow-moving deep water (e.g., when the slope reduces), a jump is formed to adjust the flow.
2. Sudden Rise in Channel Bed or Downstream Obstruction:
   A weir, gate, or abrupt bed rise can reduce flow speed and increase depth, triggering a hydraulic jump.
3. End of a Steep Slope:
   After steep channels or spillways, water enters flatter regions. The sudden energy change causes a jump to form.
4. Tailwater Depth Conditions:
   When downstream water depth is higher than needed for supercritical flow, a jump forms to match the conditions.

Types of Hydraulic Jumps

• Undular Jump: Gentle jump with small waves, occurs at low Froude numbers.
• Weak Jump: Little energy loss and minor turbulence.
• Oscillating Jump: Unstable and moving, occurs at intermediate flows.
• Steady Jump: Strong and stable, with large energy loss, used in practical designs.
• Strong Jump: Very turbulent and powerful, occurs in high Froude number flows.

Applications in Civil Engineering

• Energy Dissipation: Prevents damage to channels by reducing flow energy.
• Downstream Flow Control: Helps maintain required flow depth for safe water management.
• Sediment Control: Reduces erosion and helps settle suspended particles.
• Design of Stilling Basins: Used after spillways or gates to safely absorb excess energy.

Importance of Predicting Hydraulic Jump

Engineers need to know when and where a hydraulic jump will occur to design protective structures. Without proper planning, the energy of fast-moving water can cause erosion, scouring, or structural damage downstream.

Conclusion:

A hydraulic jump is a sudden rise in water level that happens when supercritical flow changes into subcritical flow, mainly due to changes in slope, channel conditions, or flow control structures. It causes energy loss and turbulence but is helpful in reducing erosion and protecting channels. Understanding hydraulic jumps is essential in designing safe and efficient water-carrying systems.