CHAPTER 9 - SUBMERGED ORIFICES

8. Discharge Adjustment for Contraction Suppression in Submerged Orifices

Because effective discharge coefficients are not well defined where suppression exists, use of a standard fully contracted orifice is desirable wherever conditions permit. However, a bottom suppressed orifice allows the sediment to pass the structure. To avoid accumulations of sediment on the upstream side of the orifice, the bottom contraction may need to be suppressed by placing the lower side of the orifice at canal grade. In occasional instances, use of orifices with both bottom and side contractions suppressed may be necessary.

For rectangular submerged orifices having partially suppressed perimeter contraction and negligible velocity of approach assured by providing an approach area at least 8 times larger than the orifice opening, the approximate discharge may be computed by:

Q1 = 0.61 (1+0.15r)a * sqrt(2gh)

where:

Qs = discharge of the suppressed orifice in ft3/s, velocity of approach neglected
r = ratio of the suppressed portion of the perimeter of the orifice to the total perimeter

The variables h, A, and g have the same significance as in equation 9-1b. The term in the parentheses can be thought of as a factor that adjusts the fully contracted effective coefficient of discharge used in equation 9-1b in terms of amount of perimeter with suppressed contraction. This method is expected to produce a coefficient correct to within about +/-3.0 percent.

As a temporary estimating procedure, this method can be used to estimate discharge when sediment deposits are present in the approach to an orifice that was meant to be fully contracted. If the deposits were similar in effect to a smooth invert at the bottom of the orifice, the correction would be good. However, the accuracy of the estimated correction is usually in doubt because of the shape, depth, and location of the deposits. Best practice would require removing the sediment immediately rather than making the discharge adjustment.

Equation 9-2 can be used for sluice gates when they are in effect bottom and/or side suppressed rectangular orifices with variable opening area. Table A9-3 gives discharge versus head for orifices that are both bottom and side suppressed for orifice areas of 2 ft2 to 125 ft2. Other more exact and complex approaches can be used for determining discharge with sluice gates. These approaches are discussed by Bos (1989), who states that equation 9-2 agrees closely to relationships developed by Henry (1950) and Franke (1968).