https://www.brighthubengineering.com/hydraulics-civil-engineering/65880-open-channel -flow-measurement-5-the-rectangular-weir/

The suppressed rectangular weir in the picture at the left is being used to meter flow of water in an open channel. The equation recommended by the Bureau of Reclamation in their Water Measurement Manual, for use with a suppressed rectangular weir is: Q = 3.33 B H3/2, where Q is the water flow rate in ft3/sec, B is the length of the weir (and the channel width) in ft, H is the head over the weir in ft. Use of this equation is subject to the condition that H/P < 0.33 and H/B < 0.33. Note from the diagrams above that P is the height of the weir crest above the bottom of the channel, and B is the channel width. For S.I. units the suppressed rectangular weir equation becomes Q = 1.84 B H3/2, where Q is the water flow rate in m3/sec, B is the length of the weir (and the channel width) in m, H is the head over the weir in m. The same condition for H/P and H/B apply.