Short Answer:

Open channel flow is the movement of liquid with a free surface exposed to the atmosphere, such as in rivers, canals, or drains. It is driven mainly by gravity. In contrast, pipe flow occurs in closed conduits where the fluid fills the pipe entirely, and pressure differences drive the flow.

The key difference is that open channel flow has a free surface and flows under atmospheric pressure, while pipe flow is completely enclosed and generally under pressure. These differences affect how each type of flow is analyzed and designed in civil engineering systems.

Detailed Explanation:

Open channel flow 

Open channel flow refers to the flow of a liquid, usually water, in a channel that has a free surface exposed to air. This type of flow occurs naturally in rivers and streams or artificially in canals, ditches, and drainage systems. The driving force for open channel flow is gravity, which causes the liquid to flow from a higher elevation to a lower one.

In contrast, pipe flow occurs when the fluid is completely enclosed within a pipe, and the flow is usually driven by pressure differences created by pumps or height differences in a closed system. There is no free surface in pipe flow, and the fluid is in contact with the pipe walls all around.

Characteristics of Open Channel Flow

• Free Surface: The top surface of the fluid is open to the atmosphere.
• Flow Due to Gravity: Water flows due to the slope or incline of the channel.
• Variable Cross-section: The shape and size of the flow area can change with water depth.
• Non-uniform Pressure: Atmospheric pressure acts on the free surface, while pressure at the bottom depends on depth.

Characteristics of Pipe Flow

• Fully Enclosed: Fluid is enclosed within the pipe with no free surface.
• Flow Due to Pressure: Driven by pressure differences or pump force.
• Constant Cross-section: The flow area remains constant as long as the pipe diameter doesn’t change.
• Internal Pressure: Pressure varies along the pipe length and affects the flow.

Key Differences

1. Free Surface:
   • Open channel: Has a free surface.
   • Pipe: No free surface.
2. Driving Force:
   • Open channel: Driven by gravity.
   • Pipe: Driven by pressure.
3. Flow Depth:
   • Open channel: Varies with time and flow.
   • Pipe: Constant when full.
4. Pressure Distribution:
   • Open channel: Hydrostatic pressure distribution.
   • Pipe: Varies based on system pressure.
5. Analysis Methods:
   • Open channel: Manning’s or Chezy’s equations.
   • Pipe flow: Darcy-Weisbach or Hazen-Williams equations.

Applications in Civil Engineering

• Open channel flow: Used in rivers, canals, irrigation systems, stormwater drains, and sewers.
• Pipe flow: Used in drinking water distribution, wastewater collection, and pressurized pipelines.

Understanding the differences helps civil engineers design the appropriate system for different conditions—whether it’s controlling floodwaters, supplying clean water, or collecting sewage.

Conclusion:

Open channel flow occurs in systems with a free surface exposed to atmospheric pressure and is primarily driven by gravity, while pipe flow occurs in enclosed conduits and is driven by pressure. Their differences in behavior, analysis, and application are important for designing water transport and drainage systems in civil engineering.