The program online_cipoletti calculates the discharge through a Cipoletti weir. A standard Cipoletti weir is trapezoidal in shape (Fig. 1). The crest and sides of the weir plate are placed far enough from the bottom and sides of the approach channel to produce full contraction. The sides incline outwardly at a slope of 1 horizontal to 4 vertical. The computation procedure follows Section 12 of Chapter 7 of the USBR Water Measurement Manual.

Fig. 1  A Cipoletti weir.

The formula for the Cipoletti weir, in U.S. Customary units, is:

Q = 3.367 L H3/2

[1]

in which L = length of the weir crest, in ft, and H = head on the weir crest, in ft.

The accuracy of measurements obtained by Eq. 1 is considerably less than that obtainable with V-notch weirs. The accuracy of the discharge coefficient is ± 5 percent.

The height of the weir crest above the bottom of the approach channel should be at least twice the maximum head over the crest, and the distances from the sides of the notch to the sides of the channel should also be at least twice the maximum head. This weir should not be used for heads less than about 0.2 ft (6.1 cm) or for heads greater than one-third the crest length unless calibrations exist beyond this range. The head is measured at a distance of at least four heads upstream from the crest.