Step 1: Understanding the Concept:
Weirs are structures used in open channels to measure the rate of flow (discharge). Different shapes of weirs are suitable for measuring different ranges of discharge. The question asks which type is commonly used for "medium discharges."
Step 2: Detailed Explanation:
Let's analyze the characteristics of each type of weir:
(D) V-notch weir (or Triangular weir): This weir has a V-shaped opening. It is most accurate for measuring low discharges. The narrow opening at the bottom allows for precise measurement of the head (water level) even when the flow rate is very small. At high flows, the head increases rapidly, making it less suitable.
(C) Rectangular weir: This weir has a simple rectangular opening. It is suitable for measuring high discharges. However, a standard rectangular weir with vertical sides experiences "end contractions," where the nappe (the sheet of overflowing water) narrows as it passes through the opening. This complicates the discharge formula.
(B) Contracted weir: This is a general term for a weir (like a rectangular one) where the sides of the opening are located away from the channel walls, causing the end contractions mentioned above.
(A) Cipolletti weir: This is a special type of trapezoidal weir. It is designed to simplify the discharge calculation by compensating for the end contractions that occur in a rectangular weir. The sides of the Cipolletti weir are sloped outwards at a specific angle (1 horizontal to 4 vertical). This slope provides an additional flow that exactly cancels out the reduction in flow caused by end contractions. The result is that the simple discharge formula for a rectangular weir without end contractions can be used. Because it is essentially a modified rectangular weir, it is suitable for measuring medium to high discharges and is very commonly used in irrigation channels for this purpose.
Comparing the types, the V-notch is for low flows, the rectangular is for high flows, and the Cipolletti is a very practical and common choice for the medium-to-high flow range typically found in irrigation channels, making it the best answer for "medium discharges."
Step 3: Final Answer:
The Cipolletti weir is a modified rectangular weir that is commonly used to measure medium to high discharges, particularly in irrigation systems. Therefore, option (A) is the correct answer.