CHAPTER 7 - WEIRS

10. Standard Suppressed Rectangular Weir

A standard suppressed rectangular weir has a horizontal crest that crosses the full channel width. The elevation of the crest is high enough to assure full bottom crest contraction of the nappe. The vertical sidewalls of the approach channel continue downstream past the weir plate, preventing side contraction or lateral expansion of the overflow jet.

Special care must be taken with suppressed weirs to secure proper aeration beneath the overflowing sheet at the crest. Aeration is usually accomplished by placing vents on both sides of the weir box under the nappe (figure 7-8).

 Figure 7-8 -- Section through suppressed weir with air vent in wall.

Other conditions for accuracy of measurement for this type of weir are identical to those of the contracted rectangular weir, except those relating to side contraction and the crest height. The crest height should be equal to at least 3h1max. A suppressed weir in a flume drop is illustrated on figure 7-3.

(a) Equation for Standard Suppressed Rectangular Weirs With Full Bottom Contraction

The Francis equation for the standard suppressed rectangular weir (figure 7-1) is:

(7-5)

The variables in this equation have the same significance as in the equations for contracted rectangular weirs discussed in section 9. Francis obtained the coefficient of discharge from the same general set of experiments as those stated for the contracted rectangular weir. No extensive tests have been made to determine the applicability of these equations to weirs less than 4 ft in length. Similar to the contracted rectangular weir, heads less than 0.2 ft do not give accurate flow readings because the nappe of water going over the crest may not spring free of the crest. Also, at smaller head depths, heads that are large, relative to precision of head measurement, cannot be measured. The equation should not be used to compute discharges for heads less than 0.2 ft or greater than one-third the crest length.

(b) Discharge of Standard Suppressed Rectangular Weirs

Table A7-3 contains discharges in cubic feet per second for full bottom contracted suppressed rectangular weirs. These discharges were computed from the Francis equation for lengths and heads commonly used in measuring small quantities of irrigation water.

(c) Limits of Standard Suppressed Rectangular Weirs

Equation 7-5 must not be used beyond the maximum discharges shown in table A7-3 or for measuring heads greater than one-third the crest lengths. All the requirements in section 5 apply. All the approach flow conditions in chapter 2 apply. The crest height, p, should be at least equal to 3h1max (three maximum heads). Head is measured at an upstream distance of at least 4h1max from the weir. The sidewalls must extend at least a distance of 0.3h1max down-stream from the crest, and the overflow jet must be adequately ventilated to the atmosphere.

However, the Kindsvater-Carter method discussed in section 6 is ideally suited for use with suppressed rectangular weirs. This method provides the capability of using partially bottom contracted suppressed weirs and automatically corrects for velocity of approach. This method is recommended for general use and is discussed in section 6. This method provides the opportunity to conserve delivery head by using crest heights less than 3h1max, within limits.