How should a weir be oriented relative to the flow axis to obtain accurate measurements?

• A. Parallel to Axis
• B. Perpendicular to Axis of Flow
• C. At a 45 Degree Angle
• D. Twice the Channel Slope

Answer: (B)

For accurate weir flow measurements, the flow must strike the weir face head-on, perpendicular to the flow axis. This orientation creates a well-defined hydraulic head across the crest, so the discharge is controlled by that head and is relatively independent of the velocity distribution upstream. When the weir is perpendicular, the approach is symmetric and stable, yielding repeatable readings that match the standard weir discharge relationships. If the weir were parallel to the flow, water would tend to ride along the crest, causing an unreliable head and inaccurate measurements. A 45-degree angle introduces asymmetric flow and extra energy losses, also degrading accuracy. Channel slope isn’t a factor in achieving a correct measurement orientation.