4. Conditions for Accuracy of Fully Contracted Submerged Rectangular Orifices

The standard submerged rectangular orifice described on figure 9-1 and in table 9-1 has four sides, consisting of thin-edged plates, far enough removed from the sides, bottom, and top of the water prism in the approach channel that essentially full jet contraction occurs. The contraction which occurs is about equal to the maximum that could be obtained with the sides of the orifice at infinite distances from the water prism boundaries.

The following conditions are required to accurately replicate equation 9-1 coefficients and use table 9-1:

If all these requirements are satisfied, then the effective discharge coefficient is good to +/-2 percent. However, the accuracy of discharge measurement depends strongly on the secondary head measurement system selected and the magnitude of deltah. Chapter 8 contains error values for different head measurement systems.