Which type of weir is most effective in low flow conditions?

A V notch weir is specifically designed to measure flow rates in low flow conditions effectively. The shape of the weir creates a sharp point at the top, which results in a narrow throat. This design allows the water to flow over the weir with minimal turbulence, leading to a more accurate measurement of flow even at lower volumes. The relationship between the head of water above the weir and the flow rate is more sensitive in V notch weirs, making them ideal for applications where flow is low and precise measurements are necessary.

In contrast, rectangular weirs are better suited for moderate to high flow conditions due to their larger surface area, which can lead to more variability in measurements at low flows. Compound and broad crested weirs are generally used for varying flow conditions and larger volumes, respectively, making them less effective in accurately measuring lower flow rates compared to the V notch design. Thus, for scenarios where water flow is minimal, the V notch weir offers the most reliable and accurate measurements.