Introduction:
The Cipoletti weir is a type of trapezoidal weir used in open channel flow measurement. It is named after the Italian engineer Carlo Cipoletti, who developed the design. This weir is widely used in irrigation systems and water resource management to measure flow rates accurately.

Definition of Cipoletti weir:
A Cipoletti weir is a rectangular or trapezoidal notch structure with sloping sides. It consists of a solid rectangular or trapezoidal weir plate placed across a channel. The sides of the notch have a specific slope, which determines the flow characteristics.

Slope of the sides:
The slope of the sides of a Cipoletti weir is 1 horizontal to 4 vertical (1H:4V). This means that for every 1 unit of horizontal distance, there is a vertical rise of 4 units. The slope is also referred to as the side slope or batter slope.

Explanation of the slope:
The 1H:4V slope of the sides of a Cipoletti weir is chosen based on hydraulic principles and empirical studies. This slope allows for better hydraulic performance and accurate flow measurement.

Advantages of 1H:4V slope:
1. Efficient flow control: The 1H:4V slope ensures a smooth and efficient flow of water over the weir. It minimizes turbulence and reduces energy losses, resulting in accurate flow measurement.

2. Reduced clogging: The sloping sides help prevent debris and sediment from accumulating on the weir. This reduces the risk of clogging and ensures continuous and reliable flow measurement.

3. Improved accuracy: The 1H:4V slope provides a stable and well-defined nappe (water flow) profile, allowing for accurate measurement of the head-discharge relationship. This is crucial for determining the flow rate accurately.

4. Wide range of applications: The Cipoletti weir with a 1H:4V slope can be used for a wide range of flow rates. It can handle both low and high flows effectively, making it suitable for various irrigation and water management systems.

Conclusion:
In summary, the correct answer to the question is option 'C' - a Cipoletti weir has a slope of 1 horizontal to 4 vertical (1H:4V) on its sides. This slope is chosen to ensure efficient flow control, reduced clogging, improved accuracy, and versatility in flow measurement applications.