Weirs are used in open channels and in conduits where the fluid has a free surface, such as canals, streams, rivers, tunnels, nonpressurized sewers, and partially filled pipes.
1. Meter Designs: Weir designs are classed as sharp crested and not sharp crested. Sharp crested weirs are used for flow measurement. Not sharp crested weirs are used for flow control and are not discussed in this manual.
1.1 Sharp-Crested Weirs: Sharp-crested weirs (Figure 7-1) are classified according to the form of the notch or opening as follows:
  • Rectangular notch, original form.
  • V-notch, used to provide higher head readings at low flow rates.
  • Trapezoidal, hyperbolic, and parabolic, special notch types, intended to have a constant discharge coefficient or to have a head directly roportional to the flow rate.
1.2 Broad-Crested Weirs: Broad-crested weirs have a considerable thickness of crest as measured along, and parallel to, the channel. The crest should be thick enough to prevent the nappe from springing free at the upstream edge (Figure 7-2). This requires a thickness equal to at least twice the maximum head if the upstream edge of the crest is square. Thickness of the crest may be reduced one-fourth if the upstream edge is rounded.
1.3 Round-Crested Weirs: The curvature of the crest surface of a round-crested weir may be radial or some other geometric curve such as a parabola (Figure 7-3). The upstream face of these weirs may be vertical or sloping. Both the degree of the crest rounding and the amount of slope to the faces affect the rate of discharge. Weirs of this type are seldom built with the primary purpose of measurement.
2. Operating Principles: A weir is a dam-like device that obstructs liquid flow and creates a controlled nappe and head of liquid which can be measured. For flow measurement, the weir is considered a primary device to be used in conjunction with a level measuring device.
LIMITATIONS: The basic limitations of a weir type open channel flowmeter are as follows:
  • Conditions must be present to provide for development of a head of liquid.
  • Weirs are not recommended for flows that contain large amounts of suspended solids or debris which could accumulate to Impede flow.
Sharp-Crested Weir
FIGURE 7-1. Sharp-Crested Weir
Broad-Crested Weir
FIGURE 7-2. Broad-Crested Weir
Round-Crested Weir
FIGURE 7-3. Round-Crested Weir
INSTALLATION: To obtain reliable flow information, weir installation must include sufficient upstream and downstream preparation. The following paragraphs are general suggestions only. A design engineer is required to provide specific details.
1. Upstream: A weir should be preceded by a straight uniform section of channel to ensure uniform velocity distribution in the flow. The length of this channel should be 4 to 6 times the width of the approach channel upstream of the weir.
2. Downstream: On the downstream side of a weir plate, the edges of the channel end wall, to which the plate is mounted, should be cut away so that the nappe will fall freely without adhering to the end wall. Also, the width of the downstream channel at the weir should be sufficiently greater than the crest length, so that the sides of the nappe fall free.
MAINTENANCE: All types of head and area meters are used for open flow measurement, and their operation depends on the absence of any kind of interference at the discharge opening. Weirs require the following maintenance procedures.
1. Daily Maintenance: Check the weir edge daily to make sure it is clean and free of algae growth and other interfering material.
2. Monthly Maintenance: Make sure the breather pipe is open. In cold weather make sure frost is not blocking the opening.
3. Annual Maintenance: Drain the weir and make sure that water breaks evenly over the crest. Check the weir edge for irregularities and correct if found.
ACCURACY AND RELIABILITY: The accuracy of flow measurements made with sharp-crested weirs varies depending upon the sharpness of the weir edge, the exactness with which the weir dimensions were determined, and the method and accuracy of head determination. Weir accuracy is determined to vary between 0.5 and 4.0 percent. The reliability of weirs is not a quantitative characteristic since the crest must be free of debris to function correctly.
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