6. Dimensions for Fully Contracted Submerged Rectangular Orifices

The most suitable proportions for standard submerged rectangular orifices are those in which the height, D, is considerably less than the width, L. Usually, these proportions are preferred because of shallow flow depths in some canals and open laterals. Figure 9-1 and table 9-1 provide recommended dimensions for wood or concrete submerged-orifice structures.

The size of the orifice selected for a particular situation is governed by the quantity of water to be measured and the head available. The precision of the head measuring system relative to the minimum required differential needs to be considered when selecting size. For example, if staff gages are used which can only read head to within +/-0.005 ft, the uncertainty of the differential is +/-0.01 ft. This uncertainty in the head causes a discharge error of +/-2.5 percent at a differential head of 0.2 ft. Chapter 3 gives more detailed examples of error calculations at low heads for selection of head measuring devices. Chapter 8 contains head error information for different measurement systems