Short Answer:

The function of a notch in flow measurement is to measure the discharge of liquid flowing in open channels by allowing water to pass through a specially shaped opening. The flow over the notch depends on the height (head) of the water above the bottom of the notch, which is directly related to the discharge rate.

Notches are commonly used in laboratory experiments and small-scale irrigation channels. By observing and measuring the head, the flow rate can be calculated using standard formulas based on the notch shape, such as rectangular, triangular (V-notch), or trapezoidal.

Detailed Explanation:

Function of a notch in flow measurement

A notch is a precisely cut opening, usually placed on the side of a tank or across a small open channel, that allows liquid to flow over it in a controlled manner. The main function of a notch is to measure the flow rate (discharge) of water or other fluids in open channel systems, particularly in small-scale applications such as irrigation channels, lab setups, or water distribution studies.

The flow rate over the notch depends on the height of the water (head) above the bottom edge of the notch, which is easy to observe and measure. The simplicity and accuracy of this method make it one of the most widely used tools in fluid mechanics, especially in civil engineering for practical flow analysis.

Principle Behind Flow Measurement Using Notch

The basic principle is that when water flows over a notch, it accelerates and forms a falling sheet or nappe. The velocity of flow increases as the head increases. This relationship between the head (H) and discharge (Q) is used to calculate the flow rate. Each type of notch has its own discharge formula depending on its shape.

General Formula:

Q=Cd⋅L⋅HnQ = C_d \cdot L \cdot H^{n}Q=Cd​⋅L⋅Hn

Where:

• QQQ = discharge (flow rate)
• CdC_dCd​ = coefficient of discharge
• LLL = length of the notch
• HHH = head of water above the notch bottom
• nnn = exponent depending on notch type (e.g., 1.5 for rectangular)

Types of Notches and Their Use

1. Rectangular Notch
   • Straight horizontal crest
   • Common in labs and small irrigation systems
   • Suitable for moderate flow measurement
2. Triangular or V-notch
   • Angled notch (usually 90° or 60°)
   • More sensitive to small flows
   • Ideal for precise low discharge measurement
3. Trapezoidal (Cipolletti) Notch
   • Modified rectangular notch with sloping sides
   • Designed to reduce errors from side contraction
   • Used when consistent accuracy is required

Applications in Civil Engineering

• Irrigation water management
• Hydraulic experiments in labs
• Flow monitoring in open channels
• Water supply system design and testing
• Educational demonstrations and research work

Advantages of Using Notches

• Simple and cost-effective
• Easy to install and observe
• High accuracy with proper calibration
• Requires minimal maintenance
• Suitable for a wide range of flow conditions

Limitations

• Accurate results only in free flow (not submerged) conditions
• Sensitive to improper installation or damage
• Not suitable for very high flows or wide channels
• Requires calibration for different fluid properties

Conclusion:

The primary function of a notch in flow measurement is to accurately determine the discharge of water in open channel conditions by relating the water height over the notch to the flow rate. With different shapes available for different flow ranges, notches serve as essential tools in civil engineering for both practical and educational purposes.