11. Constant-Head Orifice (CHO) Turnout

(a) General Description

A water measuring device frequently used in irrigation is a combination regulating gate and measuring gate structure. This device uses an adjustable rectangular gate opening as a submerged orifice for discharge measurement and a less expensive circular gate downstream. This system is called the CHO turnout. For convenience, it is operated by setting and maintaining a constant head differential across the orifice. Discharges are set and varied by changing the gate opening. These structures may be used in place of meter gates or turnout gate-and­weir combinations to regulate and measure flows from canals and open laterals into smaller ditches. The turnouts are usually placed at right angles to the main canal or open lateral. Typical CHO turnouts are shown on figures 9-2 and 9-3.

Figure 9-2 -- Schematic diagram of a CHO turnout.

Figure 9-3 -- A single-barrel CHO turnout.

The CHO turnout consists of a short entrance channel leading to a headwall containing one or more gate-controlled openings, a head measurement stilling basin section, and a downstream headwall with one or more gate-controlled barrels that release the flow into the delivery channel (figure 9-4). The rate of flow is measured by using the principle that a submerged orifice of a given size operating under a specific differential head will always pass the same known quantity of water. The upstream gate or gates serve as orifices. The orifice area can be increased or decreased by adjusting the upstream gate or gates. Usually, the head differential is maintained at a constant value, usually 0.20 ft (deltah on figure 9-4) measured by staff gages or stilling wells located upstream and downstream from the orifice gate headwall.

Figure 9-4 -- Schematic view of a single-barrel CHO turnout with a horizontal inlet channel.

To set a given flow, the opening of the orifice for the desired discharge is obtained from discharge tables (tables A9-4 and A9-5 for the older 20- and 10-ft3/s sizes). With the upstream gates set at this opening, the downstream gates are adjusted until the differential head across the orifice as measured by the staff gages or stilling wells is at the required constant head (usually 0.20 ft). The discharge will then be at the desired value.

Two sizes of orifice gates, 24 by 18 in and 30 by 24 in, have been used extensively in the past. Both sizes are provided in single-barrel and double-barrel designs. The capacity of the single-barrel 24- by 18-in turnout is 5.0 ft3/s. The capacity of the single-barrel 30­ by 24-in turnout is 10 ft3/s. Double-barrel installations have twice the capacity of the single-barrel ones. Newer designs (Aisenbrey et al., 1978) provide standard CHO turnouts for discharges of 2-,4-, 6-, 9-, 12-, 15-, 18-, 24-, and 30-ft3/s with corresponding opening widths of 1.5, 1.5, 2.0, 2.5, 2.5, 3.0, 3.5, 4.0, 4.0 ft. The gate sizes for these turnouts vary from 18 to 48 in.

Table A9-6 gives discharge versus gate opening for these turnout sizes with a differential head of 0.2 ft.

(b) Discharge Calibrations

Calibration tests for the original design sizes were conducted in the Bureau of Reclamation (Reclamation) laboratories in Denver, Colorado, on one-half-scale models of 24- by 18-in CHO turnouts (Blackwell, 1946). The effective coefficient of discharge varied from 0.68 to 0.72 as gate opening increased from 0.2 to 1.5 ft. These tests covered ratios of approach head to gate opening of from 6 to 2, respectively. To produce tables A9-4 and A9-5, the effective coefficient of 0.70 at the ratio of 4 was used in tables for both single-barrel and double-barrel structures with a standard set head differential of 0.2 ft. Thus, the table values are good to +/-3.0 percent.

Discharge tables for the newer CHO sizes (Aisenbrey et al., 1978) are also based upon a coefficient of 0.70. Discharge for standard head differential of 0.2 ft is provided on standard drawings and in table A9-6. For the 2-ft3/s CHO turnout, with minimum canal water surface elevation and maximum recommended orifice gate opening, the submergence ratio (approach depth to opening) is about 4. As the turnout size increases, the minimum approach submergence ratio decreases to become about 2 for 15-ft3/s and larger sizes.

Differential heads other than 0.2 ft can be used, but equation 9-1b and an effective coefficient of 0.70 must be used to compute dis-charges or to generate new tables.

To provide CHO calibrations and structural designs for sizes not actually calibrated using a discharge coefficient of 0.70 and to attain "3 percent equation accuracy, Aisenbrey et al. (1978) give the following design criteria for smaller and larger CHO turnouts with capacities up to 30 ft3/s:

An important detail of the Reclamation orifice gate design is a 1-1/2- by 1-1/2-in angle iron brace projecting upstream on the face. The projecting leg of angle iron is located 1-3/4 in from the gate lip. Some of the smaller gates were built without this brace and were field calibrated with weirs. They were found to have an effective coefficient of 0.65. When this bracing is missing, equation 9-1b and this lower coefficient must be used to calculate discharges or tables.

Colorado State University (CSU) tests (Kruse, 1965) determined that the effective discharge coefficient is about 0.65 for the normal operation where the depth upstream from the turnout is 2.5 or more times the maximum gate opening. This coefficient is the same value that Reclamation determined for no angle iron bracing at the bottom of the upstream gate face.

CSU also investigated the effects of changes in upstream and downstream water levels, sediment deposits, plugging of the orifice gate with weeds and debris, and approach flow conditions.

For discharges larger than about 30 ft3/s, special structures involving multiple gates and barrels are designed for the particular site and flow requirements.

(c) Effects of Upstream Water Depth

When the depth of water upstream from the orifice gate is four or more times the height of the opening of the orifice, the coefficient of discharge, C, remains essentially constant at 0.65 (Kruse, 1965). When the depth of water upstream is less than four times the orifice opening, the coefficient increases. The rate of increase is moderate at submergence ratios between 4 and 2.5, but rapid at submergence ratios below 2.5.

Attempting to predict the coefficients for different installations having low submergence ratios is impractical and inaccurate, and doing so is not recommended. Structures should be installed so the minimum water depth in front of the orifice gate will be at least 2.5 times, but preferably 4 or more times, the maximum expected gate opening. In some cases, to place the structure low enough, the inlet channel may need to be sloped downward as shown on figure 9-5. An alternative design in which the inlet floor is abruptly stepped downward is also used.

Figure 9-5 -- Schematic view of a CHO turnout with a sloping inlet channel and with piezometers and stilling wells.

(d) Effects of Upstream and Downstream Water Depth

Because of its name, the CHO is often mistakenly thought to maintain the constant head differential after setting when the water surface changes in the supply canal. However, the CHO cannot maintain this constant differential because the orifice gate coefficient varies with upstream submergence and differs from the downstream control gate coefficient.

A change in tailwater depth or downstream submergence on the control gate after a discharge has been set also can cause a significant change in the flow rate. The set differential changes because the two gates have different coefficient characteristics relative to their shape and response to amount of submergence. If considerable drop exists in the channel downstream from the turnout, tailwater will have no effect on flow measurement. However, if the CHO turnout is placed at about the same grade as the ditch it is supplying, the discharge may be affected as the water level changes.

Therefore, whenever tailwater or downstream delivery depth can affect the rate of flow, the ditchrider must make the necessary and frequent adjustments until flow conditions in the ditch become stable.

(e) Effects of Sediment and Weeds

In common irrigation use, sediment is usually swept through the orifice and the downstream gates during normal operation (Blackwell, 1946). The small sediment accumulations that occur in the stilling basin between the gates have little or no effect upon performance. Thus, sediment is usually not a problem in CHO turnouts.

Choking caused by weeds that become lodged within the measuring orifice can be serious. Moreover, choking can be difficult to detect when silty water is flowing because the orifice cannot be seen. The principal cause of choking is the presence of waterlogged weeds that catch in the gate opening. These weeds may trap other particles and eventually plug the turnout. The measuring accuracy of CHO turnouts is greatly reduced by the presence of even a few weeds. Care must be taken to ensure that the orifice and the area upstream from the orifice are kept completely clear of weeds and other debris. Trashscreens or trashracks are sometimes placed at the inlet to the CHO turnout.

(f) Effects of Approach Flow Condition

The turnouts are usually placed at 90 degrees to the canal centerline (figure 9-3). As a result, when the flow in the canal moves past the turnout, an eddy and related flow disturbances occur at the turnout entrance. This eddy and the other flow disturbances affect the flow into the turnout. The intensity of the disturbances depends largely upon the velocity of the passing supply canal flow. For small gate openings, the discharge coefficient, C, for the turnout increases from a value of 0.64 for a canal flow velocity of 1 ft/s to a value of 0.69 for a canal flow velocity of about 3 ft/s (Blackwell, 1946). On the other hand, with large gate openings, increasing the canal flow velocity near the turnout decreases the coefficient from high values of about 0.74 for canal flow velocities of 1.0 ft/s to low values of about 0.63 for canal flow velocities of 3.0 ft/s. This appreciable, but inconsistent, effect upon the measuring accuracy of CHO turnouts must be recognized. This error is greatest at the larger orifice openings. Whenever possible, installations should be designed so that relatively low flow velocities prevail at the turnout, especially if larger openings are to be used. Fortunately, the normal flow velocity distribution in canals provides relatively low velocities near the banks.

(g) Head Measurements

In the standard CHO turnout, the head differential across the orifice, or upstream gate, is determined by reading staff gages just upstream and downstream from the headwall on which the upstream gate is mounted (figure 9-4). Rough water surfaces at these gages can easily result in large head reading errors. These errors are particularly bad during large flows when the water surface in the stilling basin downstream from the orifice opening may be quite unsteady or tilted. Head reading errors can cause significant errors in flow measuring accuracy, and every reasonable effort should be made to avoid them. Chapters 6 and 8 show other ways of stilling the water surface to make head measurement more accurate.

Stilling devices to reduce water surface fluctuations at the staff gages can reduce head measurement errors. External stilling wells connected to piezometers upstream and downstream from the orifice gate greatly increase the potential accuracy of head readings and of the discharge measurements (figure 9-5). Additional information regarding stilling wells can be found in chapters 6 and 8. For existing structures, small wooden or metal shelf-type stilling devices installed within the flow area across the inlet and across the stilling basin near the staff gages will help reduce reading errors caused by vortices and waves (figure 9-6).

Figure 9-6 -- Baffles to reduce water surface fluctuations at staff gages in CHO turnouts.