Toma Lateral con doble compuerta.doc
Transcript of 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).