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The most common types of sharp-crested weirs are rectangular, trapezoidal (Cipolletti), and 90

The weir selected should be that most adapted to the circumstances and conditions at the sites of measurement. Usually, the rate of flow expected can be roughly estimated in advance and used to select both the type of weir to be used and the dimensions of the weir. The following facts should be considered when a specific type of weir is selected for a given application. The head should be no less than 0.2 feet and no greater than 2.0 feet for the expected rate of flow. For the rectangular and Cipolletti weirs, the head should not exceed one-third of the weir length. Weir length should be selected so that the head for design discharge will be near the maximum, subject to the limitations in 1 and 2. Measurements made by means of a weir are accurate only when the weir is properly set, and when the head is read at a point some distance upstream from the crest, so that the reading will not be affected by the downward curve of the water. That distance should be at least 4H. The proper method of measuring H is shown in Figure 1.

The rectangular-notch weir is illustrated in Figure 3. This is the oldest type of weir now in use. Its simple construction makes it the most popular.

The discharge equation for the rectangular-notch weir is :

Equation 1 gives discharge values for rectangular-weir notch lengths of up to 4 feet and depths of flow or head of up to 1.5 feet.

The Cipolletti weir, illustrated in Figure 4, is trapezoidal in shape. The slope of the sides, inclined outwardly from the crest, should be one horizontal to four vertical.

The formula generally accepted for computing the discharge through Cipolletti weirs is :

Equation 2 where parameters are as defined in equation 1. The selected length of notch (L) should be at least 3H and preferably 4H or longer. Discharge values for Cipolletti weir notch lengths of up to 4 feet and depths of flow up to 1.5 feet are given in Table 1.

The 90

The basic formula for discharge through the 90

Equation 3, where H = vertical distance (ft) between the elevation of the vertex (lowest part of the notch) and the water surface at least 4H upstream from the weir, and other parameters are as previously defined. Table 2 gives discharge values for 90