CHAPTER 7 - WEIRS

1. Background

The weir is one of the oldest structures used to measure the flow of water in open channels. Several rating equations were developed for standard rectangular contracted weirs by different investigators. Generally, the data of each investigator are within +1.5 to +2.5 percent with respect to their individual equations, but comparisons of the various equations differ as much as several percent (King and Brater, 1976; Ackers et al., 1978).

In the past, user organizations selected an equation, called it standard, and specified construction requirements and limitations of use. However, Kindsvater and Carter (1959) developed an improved method for computing rates of flow through rectangular, thin-plate weirs. Their method also applies to fully side suppressed, partially contracted, and fully contracted rectangular weirs. Kulin and Compton (1975) discuss the method and equation for rating fully contracted V-notch weirs with any angle between 25 degrees and 100 degrees. This method also rates partially contracted 90-degree, V­notch weirs. Sections 6 and 7 give references and discuss these improved rating methods in more detail.

The Kindsvater approach accounts for velocity of approach effects and the accompanying variation of discharge coefficient caused by changes of effective width and head. This method is preferred for calibrating or rating rectangular and triangular weirs. Also, this method will correct for excess approach velocity in standard weirs. Thus, this newer approach will accurately recalibrate some of the older weirs that are no longer operating as standard, as well as some that never were standard.

The previous editions of this manual presented considerably less accurate methods to correct for velocity of approach, all of which assumed that all the correction was accountable as a head adjustment alone. The Kindsvater relationships clearly show the defect of this assumption. Velocity of approach affects the effective crest length and the effective contraction coefficient, as well as the effective measuring head, all of which are accounted for in the Kindsvater approach. Thus, the older methods for correcting for velocity of approach are not contained in this edition of the manual.