Irrigation in the Pacific Northwest

Cipolletti (Trapezoidal) Weir

This calculates the flow rate over a Cipolletti weir, a commonly used weir in many irrigation districts. The weir opening has a flat, level bottom and the sides that have a particular slope. The water before the weir should be held in a relatively calm and smooth pool. There should be air (not trapped) underneath the water leaving the weir. The Length is the bottom width of the weir. The height is measured from the bottom of the weir opening to the top of the water level ponded behind the weir (not the water level right as it leaves the weir). Learn more about the units used on this page.


Flow Rate:



* Note: 1 point = 1/100 ft.

The Equation

The equation to determine the flow rate for a Cipolletti (Trapezoidal) Weir is:


= Flow Rate in cfs.

= Width of the weir crest in feet.

= Height of the upstream water above the weir crest in feet.





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