Short Answer:

A hydraulic jump occurs when fast-moving water (supercritical flow) suddenly slows down and becomes deeper (subcritical flow), causing a sudden rise in water level. This change results in turbulence, energy loss, and mixing of the water.

Hydraulic jumps usually happen in open channels like spillways, canals, and stilling basins when the flow transitions from high velocity to a slower state. It is an important concept in civil engineering as it helps in energy dissipation and controlling erosion in hydraulic structures.

Detailed Explanation:

How hydraulic jump occur

A hydraulic jump is a sudden and visible rise in water surface that occurs when supercritical flow (fast, shallow flow) transitions into subcritical flow (slow, deep flow). It is a common and important phenomenon in open channel flow, used by engineers to dissipate excess energy and control erosion downstream of hydraulic structures such as spillways, sluice gates, and culverts.

The hydraulic jump forms because the supercritical flow carries a lot of kinetic energy due to its high velocity. When this fast-moving water encounters a downstream condition that forces it to slow down—such as a flatter slope or deeper channel—the flow abruptly changes, creating a turbulent region where energy is lost and water depth increases sharply.

Conditions for Hydraulic Jump

A hydraulic jump typically occurs under the following conditions:

1. Supercritical Upstream Flow:
   The flow upstream must be supercritical, meaning the Froude number (Fr) is greater than 1. This type of flow is shallow and fast.
2. Downstream Obstruction or Resistance:
   A change in slope, an increase in depth, or a structure downstream slows the flow. This resistance forces the water to change to subcritical flow.
3. Energy Loss Zone:
   The sudden change creates turbulence and mixing, which dissipates the flow’s excess kinetic energy.
4. Subcritical Flow After Jump:
   After the jump, the flow becomes slower and deeper, with a Froude number less than 1.

Steps in Hydraulic Jump Formation

1. Water flows at high speed in a shallow depth (supercritical).
2. It hits an area where it can no longer maintain that speed (e.g., a deeper channel or obstacle).
3. The flow slows suddenly, and its depth increases.
4. A rolling turbulent region forms, known as the hydraulic jump.
5. Water flows downstream at lower speed and higher depth (subcritical).

Importance in Civil Engineering

• Energy Dissipation:
  Hydraulic jumps reduce the energy of high-speed water and prevent damage to structures.
• Erosion Control:
  They protect riverbeds, canals, and downstream areas from scouring and erosion.
• Mixing of Water:
  Helps aerate water and mix suspended solids.
• Flow Control:
  Engineers use jumps in stilling basins to manage water flow effectively.

Visual Indicators

• Sudden rise in water level.
• Rolling, frothy turbulence.
• Often seen downstream of gates, spillways, or sloped channels.

Conclusion:

A hydraulic jump occurs when fast, shallow (supercritical) flow suddenly transitions into slow, deep (subcritical) flow due to downstream resistance. It forms a turbulent region that helps reduce flow energy, prevent erosion, and stabilize open channel systems. Hydraulic jumps are not just natural occurrences but are intentionally created in many civil engineering projects for safety and efficiency.