CHAPTER 7 - WEIRS

3. Weir Nomenclature and Classification

The overflow section shape cut with a sharp upstream corner into a thin plate is the weir notch, sometimes called the overflow section. If the notch plate is mounted on the supporting bulkhead such that the water does not contact or cling to the downstream weir plate or supporting bulkhead, but springs clear, the weir is a sharp-crested or thin-plate weir.

A weir in the form of a relatively long raised channel control crest section is a broad-crested weir. The flow control section can have different shapes, such as triangular or circular. True broad-crested weir flow occurs when upstream head above the crest is between the limits of about 1/20 and 1/2 the crest length in the direction of flow. For example, a thick wall or a flat stoplog can act like a sharp-crested weir when the approach head is large enough that the flow springs from the upstream corner. If upstream head is small enough relative to the top profile length, the stoplog can act like a broad-crested weir. Wide, flat, triangular weirs exist that have wall sills with beveled corners. These short-crested weirs are in frequent use for hydrologic watershed research. Section 14(f) discusses these weirs.

Weirs are commonly named by the shape of their blade overflow opening shape (figure 7-1) for sharp-crested weirs or the flow control section shape for broad-crested weirs. Thus, weirs are partially classified as rectangular, trapezoidal, triangular, etc. In the case of sharp weirs, the triangular weir is also called a V-notch weir, and one kind of trapezoidal weir is the Cipoletti weir. In the case of rectangular or Cipoletti weirs, the bottom edge of the notch in the thin plate is the crest, and the side edges (which are vertical or flare up and outward) are the sides or ends (figure 7-1). The point of the triangle is the crest of a V-notch weir. The lowest elevation of the overflow opening of the sharp-crested weirs or the control channel of broad-crested weirs is the head measurement zero reference elevation.

When the distances from the sides of the weir notch to the sides of the weir pool are greater than two measurement heads, the water will flow relatively slowly along the bulkhead face toward the overflow opening. As the water from the sides of the channel nears the notch, it accelerates and has to turn to pass through the opening. This turning cannot occur instantaneously, so a curved flow path or side contraction results in which the water springs free to form a jet narrower than the overflow opening width.

Flow coming along the bottom of the weir pool and up a sufficiently high bulkhead and weir plate springs upward and forward in the curved, underside jet surface or crest contraction. The falling sheet of water springing from the weir plate is the nappe.

After passing the head measuring station or about a distance of two head measurements upstream from the overflow opening, the water surface drops more and more as flow approaches the crest. This continuing drop of water surface or drawdown results from the acceleration of the water as it approaches the weir. The drop in water surface between the measuring station and the notch is equal to the change of velocity head, or V²/2g, between these stations as explained in section 7 in chapter 2.

The term vertical contraction includes both crest contraction and drawdown at the weir plate. When approach conditions allow full contractions at the ends and at the bottom, the weir is a contracted weir. For full contraction, the ends of the weir should not be closer to the sides and bottom of the approach channel than a specified distance. Full side contractions on a thin-plate Cipoletti weir are shown on figure 7-2. If the specified distances are not met, then the weir is partially contracted.

When sides of the flow channel act as the ends of a rectangular weir, no side contraction exists, and the nappe does not contract from the width of the channel. This type of weir is a suppressed weir and is shown on figures 7-1 and 7-3. To reproduce the full vertical rating, contraction of the suppressed weir that existed during its calibration requires full air ventilation under the nappe and the proper crest elevation.

Velocity of approach is equal to the discharge divided by the flow section area at the head measuring station. Velocity of approach is important because it can change weir calibrations by effectively reducing the crest length and/or measuring head. In addition, a variable discharge coefficient results as increasing velocity changes the curvature of flow springing from the weir plates.

Free flow occurs when a thin-plate weir allows free access of air under the falling jet sheet or nappe. With free flow, head measurements at one upstream location determine discharge with knowledge of weir size and shape.

Downstream water rising above the weir crest elevation produces a submerged weir condition. When the downstream water surface is near or above the crest elevation of a sharp-crested weir, accuracy of measurement should not be expected. "Submerged flow correction methods" or "submerged calibrations" only produce estimates of discharge. The use of a submerged weir as a water measurement device is not good practice and should only be done as a temporary, emergency procedure. Because of the large loss of accuracy, designing thin-plate weirs for submergence should be deliberately avoided. However, submergence may happen unexpectedly or may be temporarily necessary. In such cases, flow can be estimated using special techniques discussed in Skogerboe et al. (1967), but not on a long-term basis.

A weir discharge measurement consists of measuring depth or head relative to the crest at the proper upstream location in the weir pool, and then using a table or equation for the specific kind and size of weir to determine discharge. Commonly, a staff gage, described in chapter 6, having a graduated scale with the zero placed at the same elevation as the weir crest, measures head. Putting staff gages in stilling wells dampens wave disturbances when reading head. Using vernier hook point gages in stilling wells produces much greater accuracy than staff gages. These staff gages must be zero referenced to the weir crest elevation. Section 7 in chapter 8 provides more information regarding measuring head and related errors.