How far upstream should measurements on weirs be taken from the maximum expected head?

Measurements on weirs should be taken at a distance that allows for the most accurate assessment of flow conditions and to ensure that the influence of turbulence and other factors related to the flow are minimized. The correct distance to take measurements is typically four times the maximum expected head upstream of the weir.

This distance is established to allow the flow to stabilize and develop properly before it reaches the weir. At four times the maximum head, the effects of any disturbances in the water (such as turbulence caused by an approaching structure) are minimized, leading to more accurate flow measurements. As water flows towards the weir, it must do so in a uniform manner for the weir's readings to be reliable, and being at this specified distance ensures that the measurements reflect true flow conditions without interference from upstream irregularities.

In contrast, shorter distances such as two or three times the maximum expected head may not provide sufficient distance for the water profile to stabilize, while a five times distance may be excessive, potentially leading to inefficiencies without further benefits in measurement accuracy. Thus, four times the maximum expected head is the standard practice to maintain precision in flow measurement.