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vinaosiys Sareea Aa tun Swsinas,
SIMS aTWad aS ine NF

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Ure

BV spe) THU18.

17



head, becomes 0.7 percent at 1.0-foot head and about 0.5 percent at
1.35-foot head, An extreme example of rounding of a weir crest is that
shown in the photograph on the preceding page.

Submergence of Weirs

For the measurement of irrigation water it is not the usual
practice to install weirs where submergence is anticipated. However,
changes in the regimen of the channel downstream may cause a weir to
operate under submerged conditions, Submerged flow, at its best, is
relatively unstable. Therefore, the results of the studies of submerged
weirs are not in good agreement and it may be concluded that measure-
ments made by a submerged weir should be considered as approximate
only. One solution is to remove the cause of submergence from the
downstream channel if this is practicable.

Aeration of the Downstream Nappe of a Weir

One of the general conditions for accurate and consistent meas-
urements by contracted weirs is that air circulates freely on all sides
of the flow issuing from the weir notch, Such conditions ordinarily are
not difficult to obtain, The weir bulkhead in irrigation structures is con-
structed of concrete in many instances. The use of metal weir blades
which do not project a sufficient distance from the concrete, or an im-
proper bevel of the concrete downstream from the blades can easily re-
strict the desired air circulation, The effect of this restriction of air
is to increase the flow rate for a given head, The increase in discharge
will depend on the degree of restriction of air and can be appreciable,

The problem is more pronounced when suppressed weirs are
used, For standard suppressed rectangular weirs used in irrigation
practice, the sidewalls are generally carried straight through the struc-
ture. Thus, auxiliary means must be provided to supply air to the under-
side of the nappe. Unless adequate air is provided to this area to replace
that carried away by the jet, a partial vacuum will be formed, The re-
sult is a lowering of the nappe and an increase in discharge over that ob-
tained with adequate aeration, A condition of instability may also exist
in which some erratic measurements will be obtained, One investigator
found that the discharge would be increased about 3-1/2 percent at 0. 5-
foot head and about 2 percent at 1.0-foot head when the pressure under
the nappe was reduced only 0. 8-inch of water below atmospheric. When
the pressure was further reduced to 1,2 inches of water, below atmos-
pheric, the increase in discharge was about 5 percent and 2-3/4 percent
for heads of 0.5 and 1, 0-foot, respectively. The size of vents adequate
to relieve this negative pressure will depend on conditions at the weir,
Two investigators have developed solutions for calculating the size of
vents. The important consideration is to design the vents of adequate
proportions to relieve the low pressure insofar as it is possible,

11



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Mr. Thomas concludes in his article that equipment designed
to give accurate measurements may not do so unless due care is exer-
cised in fabrication, installation, operation and maintenance; that ab-
solute accuracy may be unattainable, but the reduction of errors is
possible,

Two Classes of Errors

Errors in water measurement may be considered of two classes:
(1) avoidable errors which result from carelessness and can be elimin-
ated by thorough supervision and strict attention to details, and (2) un-
avoidable errors which are errors of degree; and although possibly not
of a type that can be eliminated entirely, they can, by use of extreme
care and a knowledge of their nature and magnitude, be alleviated and
satisfactory overall results can be obtained,

Faulty Fabrication or Construction

There are numerous possibilities for introduction of errors in
flow measurements resulting from faulty construction of measuring de-
vices, The error caused by incorrect di-

TABLE I mensions of some of the structures is read-
ily evaluated and may be used as an example
Discharge Error for to demonstrate the error,
each 0, 01-foot error
in length, Table I shows the discharge error

for rectangular or Cippoletti weirs for an
Discharge incorrect measurement of length of the weir
Weir Length Error crest of only 0.01 foot compared to the
(feet) Percent standard which was calibrated and from
which the flow formula was derived, Since

1.0 1.0 discharge is directly related to length in
2.0 0.5 the flow equation, (Equation "A" in notes),
3,0 0.33 an error in length of 0.05 foot would cause
4.0 0,25 the discharge to be in error by five times
the values shown in the table for any ob-
served head,
TABLE II The discharge error
caused by 0.01-foot error in meas-
Discharge Error for urement of the width of the throat
incorrect measurement of Parshall flumes of standard
of width of Parshall widths from 1 to 4 feet is shown in
flume, Table II, A constant head of 0.2

feet has been assumed, Also
[Discharge Error - Percent shown in this table is the error in-
Width for width error of troduced by faulty measurements

(ft.) [O-0T ft. JO. 02 ft. 0.03 Tt. of 0,02 and 0,03 feet, The error

is essentially constant for different
1,0 0. 86 2.0 3.1 values of measured head. Flow
2.0 0.45 -- -- equation ''B", used in development
3.0 0.30 -- -- of the table, is given in the notes.
4.0 0.23 -- --



PRECAST CONCRETE BLOCKS FOR MINOR STRUCTURES

One of the major problems on operating projects in many areas
is the inability to perform the concrete work for minor water control
structures during the frost free period of the year. The use of precast
concrete blocks to build minor structures, instead of placing concrete
on the job, affords many conveniences, as well as a saving in construc-
tion costs, especially when the jobs are located in a remote area. The
Uncompahgre Project in western Colorado has utilized precast concrete
blocks in the replacement of minor structures since 1954. Typical ofa
drop structure constructed with the blocks is that shown below.

The first structure using precast concrete blocks was con-
structed in 1954; since that time an additional 25 structures have been
completed. Jesse R. Thompson, now retired, former Manager of the
Project for the Uncompahgre Water Users' Association, Montrose, Col-
orado, designed a set of forms to precast 2! x 2' x 4" and 1' x 2' x 4"
concrete blocks to be used in the construction of drops, chutes, head-
walls, and other minor water control structures. A drop of 6 feet, ina
4-foot wide channel with the structure 10 feet long can be placed with the

18


CABLES SUPPLEMENT GUARDRAIL PROTECTION

Galvanized cables have been added to the guardrails in the
vicinity of the Shasta Power Plant, Central Valley Project, California,
as shown below, to provide additional protection to visitors, particu-
larly small children. The additional protection afforded by the cables
is quickly and easily obtained where guardrail posts of the type shown
are in existence.

24

GPO 852796


or ‘0100 7SCHLNOW * 28H

Ae IS

GOUT. FLIYINOD
WL ANGI Tod
M04 HOO 243LINOD
SMOKL IIS ONY Nv 7d
+ ON SNIMVEO

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20



This grease is free of tars and gummy substances that will flake off on
drying and is of a consistency that will penetrate to the center of the
cable.

We suggest that all stainless steel ropes now in service be ex-
amined minutely, at the convenience of operating personnel, for possible
breakage of individual wires as shown in the photographs above. If any
such breakage is discovered, it should be reported along with the condi-
tions of operation and service. An alarm is not being sounded, as the
one case of failure was due to very unusual operating conditions. How-
ever, due to the possibility of extensive damage to facilities in case of
rope breakage, a careful investigation is warranted; and operating per-
sonnel who may be using or considering the use of stainless steel rope
should be informed of its inherent work-hardening characteristics, and
its limitations under continuous flexing.

KOK KOK

15


Errors in measurement can easily occur if the gages used ina
Parshall flume are not placed in the manner and location developed in
the standards, The ratings for this flume include a calibration of the
gage positions, The gages are located in drawdown areas, Under these
conditions, movement of the gage upstream or downstream from the
standardized location will change the head reading and an error in dis-
charge will result. For similar reasons, if a stilling well is used, the
type and location of the entrance to the wells should be as specified.
Substantial errors in field measurements have been traced to changes
in location or design of the still well entrances,

Similar remarks apply to the location of the two gages used in
the constant-head orifice turnout, The discharge tables developed
from the calibration of this device are accurate only if the gages are
placed in the location given in the standard drawings.

Discharge Errors Due to Neglecting Velocity of Approach to Weir

In practical application, the cross sectional area of the approach
channel can usually be made sufficiently large in comparison with the
weir notch to render the effect of velocity of approach negligible. If,
however, the approach velocity is not maintained at or below 0.5 feet
per second, it must be taken into account and a correction applied, In
other words, if the normally used equation, charts, or tables are used,
without correction for obtaining discharge from measured head an error
will result.

In irrigation practice the velocity of approach to a weir is usu-
ally increased over that for which it was originally designed by: (1) a
general restriction of the cross sectional area of the weir pool by de-
posits of vegetal growth, or (2) sediment or other accumulations in the
bottom of the weir pool, Either will change the standards to which the
weir installation should conform,

A general reduction of the cross sectional area of the weir
pool will cause an increase in approach velocity which is directly re-
lated to the degree of restriction, The percentage error for a range
of approach velocities and heads of weirs, except the V-notch type, is
given in Table III on the next page. The error is such that the dis-
charge is actually greater than that obtained from the discharge
tables by the percentages given in Table II.

Authorities agree that the crest of the weir should be a distance
not less than two times the depth of water over the crest above the bot-
tom of the approach chamnel for accurate results. A greater height of
weir crest is to be preferred when practicable. A weir installed in an
irrigation channel in accordance with this standard may retain its accu-
racy for only a short period because of reduction of depth of the weir
pool by sediment deposits, as shown in the photograph on page 6. The
regularly used tables will no longer apply. The error may be reduced
or possibly eliminated by use of Rehbock's formula for computing dis-
charge from the head observations.



instead of using the average head over the weir. An inclination of about
6 degrees will cause an error in the order of 1 percent. An angle of
this magnitude should be detectable by eye, and corrective measures
taken,

Error in Discharge Resulting from Errors in Reading the Head

Perhaps the most common error in measuring irrigation water
is to misread the head. This may result from incorrect location of the
gage, or because the head gage is dirty, a stilling well is not used and
there is considerable fluctuation of water surface, or carelessness on
the part of the reader in not obtaining a good average reading at the time

the gage is observed,

Ey
The figure,
20 upper left, shows the
error in discharge re-
Siete sulting from a 0.01-
a} STN. Ss. lS. yp HEAD LESS THAN 0.20 FOOT foot incorrect head
5 4 SS reading on 12- to 48-
$y nN IN Suc) inch Cippoletti and 90°
2 ca GSES 4 eo V-notch weirs, This
& etary ey figure clearly illus-
@ zy x4 ke trates that even with a
7 Korg N small error of 0. 01-
rey | foot, an error of ap-
< rh proximately 7-1/2 per-
3 cent in discharge re-
6 o,0i-Fet incorrect |  SultS when the lower
Head Reading on .
ol Weirs. heads are being meas-
On 2 a 3 678910 2 * 4 8 67ee0 ured, For greater

DISCHARGE - GFS heads this error is

less. Also, it can be
hed noted that for the
longer weirs this slight
N error in head reading
NS results in quite large
INA FHEAD LESS THAN 0.20 FOOT errors in discharge
es measurements.

5 ens As in the case

of weirs, the head at

the throat of a Parshall

flume is quite easily

A misread in the field.

Si The error in discharge

| NN resulting from mis-

| rT reading the gage only

Hite aerial 0.01 foot is shown in

Parshall Flusaes. the figure at lower

vot tHHE +i left. Parshall flumes

DISCHARGE - CFS of throat widths of
from 6- to 36-inches

20

so a y00d

7
7|

DISCHARGE ERROR - PERCENT



Similarly, it may be shown that an error in measurement of
the width or the breadth of a rectangular submerged orifice will cause
considerable error in discharge. Since the discharge is directly re-
lated to the area of the orifice, the magnitude of the error is similar
to that for the weir, and an error in either length or breadth measure-
ment will be constant for various heads on the orifice or gate.

Error in Discharge Measurements Due to Transverse Slope of Weir Crest

When installing Cippoletti and rectangular weirs in the field, it
is necessary to set the crest exactly horizontal. If it is known that the
crest is not level, itis
\ common practice to con-

\ sider the effective head
to be the average head
6 on the weir. The error
N \ ey, caused by this practice

% ‘is shown in the figure
at upper left.

HEAD LESS THAN 0.20 FOOT--|_

Z

aT)
bo)

%,
7 NX 7 SY
a X NN Actually, a
Wo. more precise method
_% is to calculate the dis-
charge using the head
at the low end and the
“ 1oos? 2 Error in Discharge | e@d at the high end
ERROR «SoHE Seeofepreias | and average the dis-
$= SLOPE OF WEIR'= 5.01 Weir Crest. charge derived from
r bop yy |_| these two calculations.

i Lit]
7 23458 7 #810 23+ 67 8810 aan
DISCHARGE - CFS If this is done, the
error is reduced to

minor significance.

DISCHARGE ERROR - PERCENT

If it is not
* {SLOPE OF 0.01 ON WEIR CREST known that the weir is
» inclined and the gage
NY zero is referenced to
NS Sy 4 either the high or low
Ly

end, resulting error is
Ke considerably greater.

HEAD LESS THAN
0.20 FOOT ---}--

Le,

The figure,
lower left, shows the
magnitude of the error
in discharge resulting
from measurement of °
For nea Discharge head at either end of
Weir nsteadot 12-, 24-, 36-, and
| Average Head. 48-inch Cippoletti
Ou ; 348 saci ore 2 PTF FO weirs, having a trans-

verse slope of 0.01,

DISCHARGE ERROR - PERCENT



SPECIAL JACK SUPPORTS RADIAL GATES
(Suggestion No. R2-57-94)

In changing the hoist cables of the radial check gates on the
Delta~Mendota Canal of the Central Valley Project, it is necessary to
secure the gate in an open position while the old cables are removed,
and the new ones installed. Since the cables must be changed when
there is water in the canal, this can be a slow and also hazardous under-
taking using the previous method of putting a sling and chains around the
operating deck to support the gate. A special jack built by Robert H.
Vouch, Tracy Operations Field Branch, is light, and makes it possible
to do the work required in a much easier, safer, and less costly manner.

In addition to the photographs of the jack above, a sketch of the
parts is shown on the following page. Two jacks are used, one on each
side of the gate, during the cable replacement.

OK OK
16


drawdown, extends some distance upstream, The exact distance is de-
pendent upon local conditions. The head of the weir must be measured
beyond the effect of the drawdown, In the development of the basic weir
formulas, the head was observed at distances upstream from the weir
notch varying from about 4 to 9 times the maximum head over the weir.
Therefore, many authorities have accepted a minimum distance of four
times the maximum head to be measured, However, one authority says
the distance should be at least 2.5 times the maximum head, Experi-
ménts by others have shown that there is some effect of drawdown to a
distance of some six times the head on the weir. The influence at this
greater distance is minor, Within the practical limits of the gages used
at weir installations in irrigation systems, it appears that a distance up-
stream of four times the maximum head is quite adequate providing other
criteria such as height of weir, width of weir pool, etc., are complied
with,

Unpublished results of brief studies conducted in the Hydraulic
Laboratory of the Bureau of Reclamation show that it is extremely diffi-
cult to detect differences of head on enameled staff gages located 2, 4,
and 10 times the head on the weir, These same studies did show that
positioning the enamel staff gage on the weir bulkhead, a practice some-
times followed in irrigation measurements, as shown in the photograph
below, may result inerrors, Certain positions on the bulkhead, with re-
spect to the weir notch, gave a higher reading for certain flows than a
gage correctly posi-
tioned, At other flows,
the reading was less.
As the gage on the
bulkhead was moved
away from the weir
notch more consistent
results were evident.
It was found, for the
flow conditions tested,
that when the gage was
placed on the weir
bulkhead at a minimum
distance of twice the
maximum recommended
head for the weir, the
difference in the heads
read on this gage and
one correctly placed
upstream was within
the limits of visual
observation,

When the velocity of approach is high and the irrigation channel
has a high loss coefficient, there is a danger of placing the gage so far
upstream from the weir crest that an error will prevail unless a correc-
tion is made for the loss of head due to channel friction between the point
of measurement and the weir,


A close-up view of one of the forms is shown below. A bar
bending machine, set up in the building, is used to bend the reinforc-
ing bars, and to make U-bolts as needed.

The type of structure that can be constructed with the blocks
is quite impressive. There was no leakage of water through the joints
between the blocks of the structures examined, and the walls appeared
to be rigid and sturdy. There is, of course, a limit to the height of
wingwalls and headwalls that can be built without the use of struts to
support the pressure from earth or water. The project utilizes an air
entraining agent in the concrete used in the blocks and the aggregate
is carefully measured. Blocks are cured with curing compound, It is
understood that two sets of steel forms were built for the project by a
steel fabrication company at a cost of about $1,700. However, it seems
probable that less expensive wood forms might be utilized if a less ex-
tensive program were being contemplated.

In the event that there should be questions relative to the de-
sign of the blocks or the methods of construction, contact H. F.
Bahmeier, Project Manager, U. S. Bureau of Reclamation, P. O. Box
780, Grand Junction, Colorado.


- LAST COPY - DO NOT REMOVE

LAST COPY - DO NOT RENOVE

Department of the Interior

Bureau of Reclamation

OPERATION AND MAINTENANCE

EQUIPMENT AND PROCEDURES
RELEASE NO. 23

January, February and March 1958

CONTENTS
Common Errors in Water Measurement
Use of Stainless Steel Wire Ropes for Hoists
Special Jack Supports Radial Gates
Precast Concrete Blocks for Minor Structures

Farm Gates Replace Cattleguard
Cables Supplement Guardrail Protection

LAST COPY - BO NOT REMOVE

LAST COPY - 00 NOT REMOVE


This error is introduced in the field by improper maintenance
and cleaning of the weir pool. As the pool fills the ratio of H over P in-
creases and the error increases.

Numerous instances have been noted in the field where weirs
have been placed in channels having relatively high gradients. It is very
difficult to hold a properly proportioned weir pool under these conditions
and obtain smooth flow
through the weir notch,
Obviously, the in-
creased velocity of
approach and turbu-
lence will cause
errors in measure-
ment, Channel cur-
vature and conse-
quently poor velocity
distribution over the
weir crest will also
cause excessive
errors which are not
easily evaluated,
Laboratory experi-
ments have shown
that the extreme dif-
ference in discharge
over a weir fora
constant head, but with the upstream velocity distribution varied,
amounted to 26 percent. A weir with very poor approach conditions
is shown in the photograph above.

Discharge Error Due to Turbulence and Surges

Turbulence and surges occur in approach channels to weirs and
other types of measuring devices. The cause is usually high velocity of
approach but may be from gates or valves, sudden changes in section,
or others, Such disturbances are usually evidenced by erratic results
in measurements, The disturbances on the surface rarely follow a true
sine wave pattern. Hence, the average reading of the head may cause
appreciable error, Since the pattern is very complex, corrections are
not readily applied to the calculations. Corrective measures to quiet
the flow provide the best solution, This may not be an easy task,

Weir Blade Sloping Upstream or Downstream

In constructing a weir, it is necessary to have the plane of the
upstream face of the weir vertical if accurate measurements are to be
obtained, Experiments with sloping weirs show that the coefficient
changes if the weir blade is tilted in an upstream or downstream direc-
tion; that is, when the face of the weir blade is not plumb, This change
is slight, and the weir face may be out of plumb a few degrees before
the accuracy of the measurement is seriously affected,

9


TABLE II

Discharge Error
Resulting from Failure to Correct
for Velocity of Approach

Velocity of Error in Percent
Approach Observed Head Over Weir - (Feet)
(ft, /sec.) | O27 0.2% | 0.6 | 0.8 | 1.0

0.5 2.7 1.3 0.9 0.6 0.6
1.0 9.8 5.1 3.4 | 2.7 2,2
1.5 20.8 |10.9 7.5 5.7 | 4.7
2.0 33.5 18.1 |12.6 9.7 7.9
2.5 48.0 |26.6 {18.7 114.5 |11.9
3.0 63.7 [36.1 |25.6 {19.9 {16.5

Table IV gives the percentage error in discharge that will occur
if regular weir tables are used instead of correcting for the reduced
height of weir by use of the Rehbock formula. The table is divided into
two parts: The first part shows a constant head of 0, 2-foot over a weir.
The value of the ratio of H over P is varied and the error shown, The
second part of the table is calculated for a constant head of 0.5 foot and
is handled in a manner similar to the first part of the table.

TABLE IZ
ERROR IN DISCHARGE FOR
CHANGES IN HEIGHT OF WEIR

% ERROR
High 4 COEFFICIENT

Head = 0.2 foot
0.5 0.4 3.49 5.6
1.0 0.2 3.41 2.7
2.0 0.1 3.37 15
3.0 0.07 3.35 0.9
t) ° 3.32 °

Head = 0.5 foot
0.5 1.0 3.70 13.1
1.0 0.5 3.48 6.4
2.0 0.25 3.38 3.4
3.0 0.17 3.34 2.1
00 fe) 3.27 °



are shown in this figure. It can be noted that the error in general is
approximately the same as for misreading the head an equal amount
when weirs are considered,

The figure below shows the error in measurement resulting
from an error of
GATE OPENING - 1.45 FEET. 0. 01-foot in reading
CONSTANT HEAD ORIFIGE—% . the head on 8-, 12-,
__ TURNOUT | YL and 18-inch screw-
2 RW lift gates. The meter
. gate has a circular
leaf and the screw-
| lift gate has a rectan-
gular leaf. Included
in the figure is the
error caused by 0.01-
foot incorrect head
reading for the con-
stant-head orifice
METERGATES AND turnout. It should be
SCREWLIFT GATE noted that the percent-
porey OPEN: | | age error in discharge

Discharge Error for

0.01-Foot incorrect resulting from mis-

Head Reading on
ou Gates. reading the head on an
15 a s-67 8810 20304080 6680100 .
DISCHARGE ~ CFS orifice is in general
less than the same

misreading on a weir.

GATE OPENING -0.2 FOOT.

ew a yavd

“8h
PON SS
ni

ber
Buoe

wat
4
\
ont-SLt
pwors

aN

3.
+

DISCHARGE ERROR ~ PERCENT

Error in Discharge Measurement Caused by Incorrect Zero Setting

The error for incorrect zero setting of the head gage is of the
same magnitude as the error for misreading the head an equal amount.
Improper positioning of the gage used to read the head is probably the
most common error found, In the field it is difficult to reference the
exact zero of the gage to the crest of a weir, a submerged orifice, or
to a turnout gate. Extreme care should be exercised in setting the
gages since incorrect setting cause errors in all flows, as shown in the
figures on this and the preceding page.

Errors Resulting from Improper Gage Location

Proper location of the gage for obtaining head on measuring
devices is important if errors are to be avoided. In most instances,
flow relationships have been determined empirically with a particular
type of gaging device placed in a specific location. Hence, there is
included in the overall calibration a secondary effect of calibration of
the gaging system employed to obtain the head. Because of changes in
the flow pattern of the stream as it passes through the measuring sec-
tion, minor deviations from the standard in gage design and location
may appreciably affect the quality of the measurements,

In the case of a weir, there is a downward curve of the water
surface as the flow passes through the notch, This curved surface, or

5



precast blocks for approximately 60 to 65 percent of the cost of a simi-
lar drop on which the concrete is placed on the job during subfreezing

temperatures.

Other advantages in the use of the blocks are:

1. Blocks may be cast and properly cured during
warm weather, or easily protected from frost

during cold weather.

2. There is no waste of aggregate or cement.

3. Use of the blocks eliminates transportation
of a mixer for mixing concrete in cold weather;
and protecting concrete from freezing.

4. A drop of the dimensions given above can be
placed in about 1/3 the time required to place

concrete on the job.

5. No damage results if inclement weather occurs
during construction, and it becomes necessary

to suspend work,

Construction

Only two men are required to fabricate and stockpile the prefab-
ricated blocks. The forms are bolted to permanent trestles at about av-
erage waist height, as shown in the photograph below, for convenience in
assembling the forms, troweling the concrete, and moving the finished
blocks from the forms to the stockpile.

19

The blocks are
made with dove-tailed
joints, and are also
joined together hori-
zontally by the use of
bolts and strap iron.
Vertically they are
joined together by
placing 8-inch lengths
of 5/8-inch reinforc-
ing steel in the holes
provided in their manu-
facture. Design details
of the blocks and block
forms are shown in the
drawings on the following
pages.


W)

QO



COMMON ERRORS IN WATER MEASUREMENT

Many of the measurements made of water flowing in our irriga-
tion system may not be as accurate as assumed or as would be expected
from the type of device or structure being employed, It is probable that
absolute accuracy cannot be obtained in the measurement of water in all
instances. However, the reduction of errors to a minimum may be pos-
sible if all factors are taken into consideration,

Some of the reasons for errors and deviations in water meas-
urement are discussed in this article adapted for the Bulletin from a
paper presented by Mr. C. W. Thomas, Hydraulic Engineer, Bureau
of Reclamation, Denver, Colorado at a meeting in Denver of the ASCE
Irrigation and Drainage Division, September 8-10, 1955, The paper,
published in the Journal of the Irrigation and Drainage Division of the
American Society of Civil Engineers as Paper 1362, September 1957,
"Common Errors in Measurement of Irrigation Water," is available and
includes references to the source of data used in the analyses. For the
sake of brevity, these are omitted in this article. However, some equa-
tions used by Mr. Thomas in the discussions herein are given as notes
at the end of this article.

The design of essentially all of the open channel irrigation sys-
tems is such that fluctuations of the water levels both upstream and
downstream of the measuring device are tolerated. The measuring de-
vices have been developed to accommodate this design procedure, In
effect, any change in water level upstream is reflected as a change in
discharge through the measuring device. When the device is submerged,
changes in downstream levels also affect the discharge.

The generally accepted approach to the solution of the problem
of fluctuating water levels is the standardization and calibration of the
measuring equipment. The result is a device, which, when built and in-
stalled in accordance with the established standards, will pass a range
of known discharges for a range of upstream and downstream water
levels. The exact instantaneous discharge can be determined by observ-
ing the upstream and downstream heads, by means of suitable gages,
correctly referenced, and entering charts, graphs, or tables which have
been developed by prior calibration of the device under carefully con-
trolled conditions,

Such a procedure assumes that the field installation is a suit-
able replica of the installation which was calibrated, usually in a hy-
draulic laboratory. Further assumptions are that conditions of flow,
especially in regard to velocity distribution patterns, are similar and
that the heads, and other necessary measurements, can be determined
with a comparable degree of accuracy.

Since the discharge tables, charts, or graphs are the results
of calibrations, they are based on empirical relationships and not on a
rational analysis in all instances. Therefore, they are not necessarily
susceptible to accurate extrapolation beyond the range of observations
from which they were developed.



Other Factors Affecting the Accuracy of Discharge Measurements Over
Weirs

There are factors, other than those covered separately above,
which may cause errors in discharge measurements made with weirs,
Many of them apply equally as well to other types of structures and de-
vices.

Obstructions in the measuring section cause errors proportion-
ate to the magnitude of such an obstruction. In irrigation systems float-
ing detritus, weeds,
moss, etc., may ob-
struct the water pas-
sage, as shown in the
photograph at left.
Frequent and close in-
spection accompanied
by remedial measures
will relieve this con-
dition,

Changes in
viscosity and surface
tension of the fluid are
known to alter the flow
coefficient. However,
the effect of these two
factors are considered
negligible in irrigation
systems where the flow
media is water, and wide variations of temperatures are not encountered
and, further, provided the restrictions on high and low heads over the
weir are complied with.

At very low heads, flow over a weir may become quite unstable
and errors and inconsistencies in the measurements will result. Be-
cause of viscous drag and the tendency of the nappe to adhere to the weir
crest there is general agreement among experimenters that heads of less
than 0, 2-foot will not produce reliable results when the usual discharge
tables or formulas are used,

The results of many experiments on weirs show that the formu-
las developed for rectangular weirs do not hold when the head exceeds
about one-third of the length. There are indications that the discharge
formula for the Cippoletti weir, in lengths over 1-foot, is slightly in
error at heads less than one-third the length. Possibly the rule should
be that the head should not exceed one-fourth of the length if errors are
to be reduced to a minimum,

As previously stated, the flow formulas for weirs have been de-
veloped empirically and are not necessarily susceptible to extrapolation,
Most of the data have been derived for heads up to 2.0 feet. Although
some data are available for higher heads, authorities generally agree

12


USE OF STAINLESS STEEL WIRE ROPES FOR HOISTS

The use of stainless steel hoisting rope for radial gates has
been gaining favor among operating supervisors during the last few years
because it has the desirable effect of increasing rope life and reducing
the hazard of rope failure due to corrosion. This practice has been,
and will continue to be encouraged where it appears to be economically
advantageous.

A word of caution is appropriate, however, as pointed up by a
recent failure of stainless steel hoisting ropes after about six months
of service at a loading of only about one-eighth of breaking strength.
The failure occured on an automatic gate and was due to hunting over
a range of 2 or 3 inches.

The major break shown above was a few inches above the point
of attachment to the gate, where the rope was straight and not subject
to bending; but breakage of single wires had occurred all through the rope
structure as shown on the more detail photographs on the following page,
and particularly where the rope was flexing as it ran on and off the drum.

Stainless steel is very susceptible to work hardening. Military
Specifications No. MIL-C-5424 allow a 50-percent reduction in tensile
strength after 50,000 bending reversals of 1/4-inch 6 by 9 stainless
steel rope. Fewer reversals would be required to produce a similar
weakening of larger ropes. Individual wires of a rope are subject to
torsional, as well as tensile, stresses when the rope is under straight
tension; and each wire twists and untwists if the rope pull is alternately
increased and decreased. This explains the failure of the ropes ata

“ point of straight tension.

Stainless steel wire rope should not be used on gates which are

likely to hunt and the need for lubrication of stainless ropes should not
be overlooked. Keystone wire cable grease is recommended.

14




FARM GATES REPLACE CATTLEGUARD
(Suggestion No. R2-57-54)

Where a bridge is used for transferring cattle across the Friant-
Kern Canal, Central Valley Project, California, use is being made of 16-
foot metal farm gates instead of constructing or replacing cattleguards.
The gates serve a dual purpose. The large double gates ‘shown in the
first photograph below can be opened and swung into position shown in
the lower photograph
to close off the canal
roadway leaving an
open runway for the
cattle to cross the
canal bridge.

Other similar
installations are to be
made in the Central
Valley Project to re-
place numerous cattle-
guards. It has been
estimated that in re-
placing eight old
cattleguards on the
Orange Cove Unit of
the Project, use of
Canal Superintendent
E. I. Curran's sug-
gestion has saved up-
wards to $500 per in-
stallation over the
cost of the conven-
tional cattleguards.

There are
a great many places
where the movement
of cattle across a
canal is done only
occasionally, and
this suggestion,
rather than the in-
stallation of cattle-
guards, will serve
to solve the prob-
lem very nicely.

OK OK Ok Ok

23


OPERATION AND MAINTENANCE

EQUIPMENT AND PROCEDURES

Release No. 23

January, February and March 1958

INTRODUCTION

In order to insure proper recognition to those individuals whose
suggestions are published in this and subsequent bulletins, the suggestion
number as well as the person's name will be shown. All offices are also
reminded to notify their Suggestion Awards Committee when a suggestion
is adopted for Bureau operation.

This bulletin, published quarterly, is circulated for the benefit
of irrigation project operation and maintenance people. Its principal pur-
pose is to serve as a medium of exchanging operation and maintenance
information, Reference to a trade name does not constitute the endorse-
ment of a particular product, and omission of any commercially available
item does not imply discrimination against any manufacturer. It is hoped
that the labor-saving devices or less costly equipment developed by the
resourceful water users will be a step toward commercial development
of equipment for use on irrigation projects in a continued effort to reduce
costs and increase operating efficiency.

FO OK OK

Division of Irrigation Operations
Commissioner's Office
Denver, Colorado


Roughness of Upstream Face of Weir and Bulkhead

For consistent and accurate flow measurements the upstream
face of the bulkhead and weir blade must be smooth, Offsets, protruding
bolt heads, and surface roughness must be avoided upon installation.
Maintenance is necessary to retain a smooth surface, Sufficient work
has not been done to provide an exact evaluation of the errors resulting
from the many possibilities of roughness. It was found from one series
of experiments that the percentage increase in discharge, due to chang-
ing the roughness of the upstream face of the weir bulkhead from that of
a polished brass plate to that of a coarse file for a distance of 12 inches
below the crest, is shown to range from about 2 percent for 0. 50-foot
head to about 1 percent for 1.35-foot head. Other experiments showed
that when the upstream face of the weir was roughened, to the crest,
with coarse sand (retained on No, 8 standard sieve and passing No, 4)
that the increase in discharge ranged from 6.5 percent at a 0.2-foot
head to 4,7 percent for a 0.5-foot head. The larger projections caused
by the addition of nuts and pieces of metal on the bulkhead below the
crest in these same experiments caused about the same increase in dis-
charge,

Rounding of Sharp Edge at Crest of Weir

In irrigation practice many of the older weirs were constructed
of wood, In this type of construction the original sharp edge of the crest
soon becomes rounded. Rust and corrosion also produce a rounding ef-
fect on metal weir blades, The effect of this rounding is to cause an

increase in the flow rate for a given head when compared to a sharp
crested weir. Considerable experimentation has been done to evaluate
the effect of the round-
ing of the crest. The
results show that the
percentage increase in
discharge due to the
rounding, decreases as
the head increases,
For a head of 0. 5-foot,
an increase of some 2,
3, 5-1/2, 11, and
13-1/2 percent may be
expected for roundings
having radii of 1/24,
1/8, 1/4, 1/2 and 3/4
inch, respectively.
There is a deficiency
of data for the higher
heads with the longer
radius roundings.
However, with radii
smaller than those given above, the increases become consistently
smaller as the head increases, As an example, the increase in dis-
charge of 2 percent, given above for the 1/24-inch rounding at 0, 5-foot

10


that a 2,0-foot head should not be exceeded for any length weir if good
quality results are desired,

It has been previously pointed out that the percentage of error
in discharge resulting from a given error in measuring the head will de-
crease as the head increases, Therefore, the minimum error and,
hence, the greater accuracy can be expected if the discharge occurs
under the maximum head commensurate with the above limitations,

Careful visual inspections made at regular intervals will remove
many of the sources of error mentioned above. These inspections should
also disclose other sources of errors such as leaks around the measur-
ing structure, through weir bulkheads, or from drains in the structure,

CONCLUSIONS

The charts, tables, and discussions presented in this article
are not intended to point out all the possible errors in all the devices
and structures used in measuring irrigation water. However, from the
examples cited, the following conclusions may be drawn.

To obtain accurate measurements of irrigation water it is nec-
essary to make a careful study for the selection of a proper device to fit
the conditions pertaining at the site. Even with careful planning and se-
lection of an excellent primary measuring device, it is probable that
errors may be introduced into the measurements unless due care is ex-
ercised in fabrication, installation, operation and proper maintenance
of the devices or structures, The magnitude of these errors can be
appreciable and the value of a well planned measuring program may be
reduced considerably by failure to anticipate and remove the cause of
the errors,

The possible errors cited are both negative and positive and
may tend to cancel each other, However, more careful scrutiny shows,
especially in the case of weirs, the probability is that there is a pre-
dominance of negative errors, This means then that usually more water
is being delivered than is apparent from the measurements,

Equation "A"
Q = cLH3/2
Equation "B" °

@ = 4wul. 522w0- 026

KR Rk

13