Flow measurement in drains must cope with high velocities.   Many of the techniques measure drain velocity by putting a scanning probe in the flow.  The probe not only disturbs a supercritical flow pattern, it soon becomes fouled.   Also, velocity profiles are turbulent and their measure­ment in drains is highly uncertain.

These problems are avoided by hydrometric struct­ures.   The U-Weir limits the drain velocity and controls the flow distribution in the drain to ensure reliable and accurate results.