Effective use of water for crop irrigation requires that flow rates and volumes be measured and expressed quantitatively. Measurement of flow rates in open channels is difficult because of nonuniform channel dimensions and variations in velocities across the channel. Weirs allow water to be routed through a structure of known dimensions, permitting flow rates to be measured as a function of depth of flow through the structure. Thus, one of the simplest and most accurate methods of measuring water flow in open channels is by the use of weirs.

In its simplest form, a weir consists of a bulkhead of timber, metal, or concrete with an opening of fixed dimensions cut in its top edge. This opening is called the weir notch; its bottom edge is the weir crest; and the depth of flow over the crest (measured at a specified distance upstream from the bulkhead) is called the head (H). The overflowing sheet of water is known as the nappe.


Types of Weirs

Two types of weirs exist: sharp-crested weirs and broad-crested weirs. Only sharp-crested weirs are described here because they are normally the only type used in the measurement of irrigation water. The sharp edge in the crest causes the water to spring clear of the crest, and thus accurate measurements can be made. Broad-crested weirs are commonly incorporated in hydraulic structures of various types and, although sometimes used to measure water flow, this is usually a secondary function. The components of a sharp-crested weir.

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.

Rectangular-Notch Weir
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.