Short Answer:

A siphon is a pipe or tube that allows liquid to flow from a higher elevation to a lower elevation using gravity, even if it has to rise above the original liquid level in between. It works based on the difference in height and atmospheric pressure.

The working of a siphon starts once the tube is filled with liquid. Due to gravity and pressure difference, the liquid starts flowing continuously from the higher level to the lower level without using any pump, as long as the outlet is lower than the inlet.

Detailed Explanation:

Siphon and how it works

A siphon is a simple hydraulic device used to transfer liquid from one container to another, especially when the second container is at a lower level than the first. It is widely used in civil engineering applications such as drainage, irrigation, and water distribution systems.

The siphon works without a pump and relies on two main principles: gravity and atmospheric pressure. Once the liquid completely fills the pipe, gravity pulls the liquid down on the discharge side, creating a low-pressure zone at the top of the siphon. This pressure difference causes the liquid to rise up the inlet side and continue flowing over the high point.

How a Siphon Works

1. Initial Filling
   The siphon pipe must first be filled with the liquid to eliminate air gaps. This can be done by priming the siphon—sucking the liquid manually or using a vacuum.
2. Start of Flow
   Once the pipe is full and both ends are submerged, gravity starts pulling the liquid down on the longer outlet side. The falling liquid creates a suction effect at the top bend of the siphon.
3. Pressure Difference
   Atmospheric pressure acts on the liquid surface in the source container, pushing the liquid up the shorter inlet leg of the siphon to replace the falling liquid.
4. Continuous Flow
   As long as the outlet is lower than the inlet and the siphon remains air-free, liquid continues to flow without any pump. The flow stops if air enters the pipe or if the outlet is raised above the source level.

Conditions for Proper Siphon Operation

• The siphon tube must be completely filled with liquid at the start.
• The outlet end must be lower than the liquid level of the source.
• There must be no air gaps or leaks in the tube.
• The siphon height (from liquid level to top bend) must be within the atmospheric pressure limit (around 10.3 meters for water).

Applications in Civil Engineering

• Irrigation Canals: To transfer water across embankments without disturbing the soil.
• Stormwater Drainage: Used to bypass roads, railways, or structures.
• Reservoir Overflow Systems: For safe discharge of excess water.
• Sewage Systems: To create flow without the need for pumping.
• Water Tanks: To empty or transfer liquid between tanks at different levels.

Siphons are especially useful in remote areas or in systems where power is not available for pumps.

Conclusion:

A siphon is a simple and effective tool that uses gravity and pressure difference to move liquid from a higher to a lower level. Once the tube is filled, the flow continues automatically as long as the outlet stays lower than the source. It is a cost-effective and energy-free method widely used in civil engineering and daily applications.