Short Answer:

Weirs and notches are both used to measure the flow of water in open channels by allowing water to flow over a barrier. The main difference is that weirs are used in larger channels with permanent structures, while notches are used in small-scale, laboratory, or temporary setups.

A weir is a broad structure built across a stream or canal, while a notch is a precisely cut opening on a thin plate. Both work on the same principle of measuring water head above the crest, but they differ in size, purpose, and application scale.

Detailed Explanation:

Difference between weirs and notches in flow measurement

In hydraulic engineering, both weirs and notches are devices used to measure discharge in open channels. They work by forcing the flow to pass over a specific shape and measuring the height (head) of water above the crest. While their basic principle is the same, they are different in terms of design, usage, and scale of application.

Understanding their differences helps engineers choose the right device depending on the flow condition, available space, and required accuracy.

Key Differences Between Weirs and Notches

1. Structure and Size
   • Weir: A large barrier or wall built across open channels, rivers, or streams. It is usually made of concrete or stone and can handle large water flows.
   • Notch: A small, thin plate with a precisely shaped opening, often made of metal or wood, used in laboratories or small channels.
2. Usage and Application
   • Weirs are permanent structures used in field conditions, such as irrigation canals, rivers, and treatment plants.
   • Notches are used in experimental setups, small-scale testing, and educational laboratories.
3. Material and Setup
   • Weirs are generally constructed as part of the channel itself.
   • Notches are temporary and placed vertically in a container or tank wall to measure flow.
4. Flow Measurement Principle
   • Both use the same principle: measuring head of water above the crest to determine flow.
   • Equations used for both may be similar, but notches require more precision in shape and dimensions.
5. Examples of Shapes
   • Common weir types: Rectangular weir, Broad-crested weir, Ogee weir
   • Common notch types: V-notch, Rectangular notch, Trapezoidal notch

Summary of Practical Use

• Weirs:
  • Used in canals, rivers, and reservoirs
  • Built for long-term use
  • Suitable for large discharge rates
• Notches:
  • Used in labs or small water channels
  • Lightweight and movable
  • Suitable for low discharge rates

Despite their differences, the flow over both weirs and notches is calculated using formulas derived from Bernoulli’s theorem, and both need free-flowing conditions (not submerged).

Conclusion:

The difference between weirs and notches lies in their size, construction, and area of application. While weirs are large structures used in the field for controlling and measuring high water flows, notches are smaller and mostly used in laboratories for low-flow measurements. Both serve the same function but are applied based on the scale and precision required in hydraulic systems.