14. Meter Gates

In the past, meter gates have been used for controlling and measuring irrigation flows (figure 9-9) (Schuster, 1970; Ball, 1961). These gates are basically modified, submerged, variable area orifices (slide gates) at the upstream end of a length of smooth or corrugated pipe. The gate leaf has either a round or square bottom over the entrance. Wells provide a means of measuring the head upstream and downstream from the gate, designated h1 and h2, respectively. The upstream head, h1, is measured in the well connected to the headwall located a certain distance to the side of the gate opening. The downstream head, h2, is measured in the other well, which is connected to the pipe a short distance downstream from the gate. The difference in head in the two wells is the effective operating head across the gate. The discharge is then determined by the proper equation or a table provided by the manufacturers.

Figure 9-9 -- Typical meter gate installation.

Meter gates are usually purchased from commercial suppliers who furnish discharge tables for their product. Generally, the tables are reasonably accurate. In some instances, errors of 18 percent or more have been found. A number of characteristics of meter gates influence their performance, particularly when they are operated at openings larger than 50 percent or the upstream submergence above the crown of the pipe is less than the inside pipe diameter.

The discharge table being used should be checked to ensure that it applies to the brand and type of gate being used. Tables for round-bottom gates must not be used for square-bottom gates and vice versa. Gate settings must be made and read accurately, which requires that the gate position indicators be in good condition and show the true openings. The stilling wells should be periodically flushed to make sure they are operating properly and are free of obstructions and silt. Staff gages or scales should also be checked to be sure they have been secured in the proper zero position.

Weeds, trash, and sediment must be removed from the approach to the gates because they can cause flow disturbances that result in erroneous head differential readings. This requirement is particularly important along the wing walls because these walls have more effect upon the flow than does the alignment of the bottom. The approach effects are particularly important at gate openings greater than 50 percent.

Low head wall submergence over the meter gate entrance can also result in discharge errors, particularly at gate openings greater than 75 percent. Considerable error results when the head is less than one pipe diameter above the top of the pipe. Sufficient submergence must also occur at the downstream end of the conduit to ensure that the conduit flows full and that a readable water surface is present in the downstream stilling well, which will usually require at least 1 ft of water depth above the pipe crown. This amount of submergence will normally prevent scour damage downstream in earthen ditches.

Large errors in discharge determination can be introduced if the differential head (difference in water surface elevation between the two stilling wells) is small. For example, in reading the two water surface elevations in the stilling wells, an error of 0.01 ft could be made in each reading, giving a possible value of 0.10 ft for a true differential head of 0.08 ft. For a true discharge of 1.10 ft3/s through an 18-in meter gate open 5 in, the difference in the indicated discharge would be about 0.12 ft3/s, an error of about 11 percent. If the gate opening was reduced to 2 in, and the upstream pool could be allowed to rise to pass the same discharge, the differential head would be 0.40 ft, and the same head-reading error of 0.02 ft would indicate a change of only 0.03 ft3/s. The error in discharge determination would be reduced from about 11 percent to less than 3 percent.

The head in the downstream measuring well can vary widely depending upon the longitudinal and lateral location of the pressure tap in the pipe. Placing the pressure tap of the downstream measuring well 12 in from the gate is a special case requiring special calibration for each size gate unless the maximum gate opening is limited. A better location for the downstream piezometer would be at a distance D/3, measured from the downstream face of the gate. The pressure gradeline here is lower and flatter. Minor variations in piezometer locations would not result in major measuring errors, and the measured head differential would be greater. However, if the piezometer is moved to this point, the meter must be recali-brated because the manufacturer's published tables will not apply.

Laboratory tests have been conducted on square-bottom, flat-leaf meter gates to determine the coefficient of discharge, Cd, for a pressure tap located at a distance D/3 downstream from the gate (Ball, 1961). This curve, shown on figure 9-10, is valid for all sizes of square-bottom, flat-leaf meter gates under the following standard conditions:

Figure 9-10 -- Coefficient of discharge curve for meter gates with downstream pressure tap at D/3..

It should be noted that the coefficient Cd is used with A, which is the area of the pipe and not the gate opening. Discharges may be computed from this equation with an accuracy of +/-2.5 percent. The degree of downstream submergence does not affect the accuracy of the meter if water rises sufficiently in the downstream well to obtain an accurate reading and the pipe runs full at the outlet.