Online Standard Contracted Rectangular Weir Calculator 131108 The program online_standard_contracted_rectangular calculates the discharge through a standard contracted rectangular weir. To be fully contracted, the overflow plate sides and ends must be located at a distance of at least 2H from the approach flow boundaries. The head is measured at a distance upstream of at least 4H from the weir. The computation procedure follows Section 9 of Chapter 7 of the USBR Water Measurement Manual. Fig. 1 Definition sketch for a standard contracted rectangular weir. The formula for the standard contracted rectangular weir is the Francis equation. In U.S. Customary units, this equation is:
in which Q = discharge, in cfs, 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 Vnotch weirs. The accuracy of the discharge coefficient is Equation 1 has a constant discharge coefficient (3.33), which facilitates computations. However, the coefficient does not remain constant for a ratio of headtocrest H/L > 1/3, and the actual discharge exceeds that given by the equation. Francis' experiments were made on comparatively long weirs, most of them with crest length L = 10 ft and heads in the range 0.4 ft ≤ H ≤ 1.6 ft. Thus, the equation applies particularly to such conditions. USBR experiments on 6in, 1ft, and 2ft weirs has shown that the Francis equation also applies fairly well to shorter crest lengths L, provided H/L ≤ 1/3. Equation 1 is subject to the following restrictions:

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