Toma Lateral con doble compuerta.doc

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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-and-weir

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.


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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


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decreased by adjusting the upstream gate or gates. Usually, the head

differential is maintained at

a constant value, usually 0.20 ft (h on figure 9-4) measured by staff gages or

stilling wells

located upstream and downstream from the orifice gate headwall.

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-ft

3

/s sizes). With the upstream


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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 singlebarrel

24- by

18-in turnout

is 5.0 ft

3

/s. The capacity of the single-barrel 30 by 24-in turnout is 10

ft

3

/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-ft

3

/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


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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-ft

3

/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-ft

3

/s and larger sizes.

Differential heads other than 0.2 ft can be used, but equation 9-1b and an

effective coefficient of


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0.70 must be used to compute discharges 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

ft

3

/s:

For maximum capacities of 10 ft

3

/s and less, the length of gate basin should be at least

2.25 times the maximum gate opening or 1.75 times the gate support wall

opening,

whichever is greater. However, no basin length should be less than 3.5 ft.

For capacities between 10 ft

3

/s and 30 ft

3

/s, the gate basin length should be at least 2.75

times the maximum gate opening. The bottom of the gate basin should be

level.

The gate opening should be less than or equal to 0.8 times the wall gate

support wall

opening.

The distance from the gate lip to the top of the gate support wall opening

should be at

least equal to the wall thickness.


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The approach flow submergence above the top of the opening should be 1.78

times the

velocity head plus 0.25 ft.

The set head differential should be at least 0.2 ft.

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 ft

3

/s, special structures involving multiple gates and barrels

are designed for the particular site and flow requirements.

(c) Effects of Upstream Water Depth


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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.


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(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.


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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.

(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


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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).

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