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IS0
FOR S TA N DARD IZATl O N
I N T ERN AT I O N A L O R G A N I2 AT 1 ON
I SO R EC O M M EN DATI O N
R 1100
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
ESTABLISHMENT AND OPERATION OF A GAUGING STATION
AND DETERMINATION OF THE STAGE-DISCHARGE RELATION
1 st E DIT ION
July 1969
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 1 100, Liquid flow measurement in open channels - Establishment and
operation of a gauging station and determination of the stage-discharge relation, was drawn up by Technical
Committee ISO/TC 113,Measurement of liquid flow in open channels, the Secretariat of which is held by the
Indian Standards Institution (ISI).
Work on this question led to the adoption of a Draft IS0 Recommendation.
'y
In December 1966, this Draft IS0 Recommendation (No. 1071) was circulated to all the IS0 Member
Bodies for enquiry. It was approved, subject to a few modifications of an editorial nature, by the following
Member Bodies :
Australia Ireland South Africa, Rep. of
Switzerland
Belgium Israel
Italy Thailand
Brazil
Chile Japan Turkey
Czechoslovakia
Korea, Rep. of United Kingdom
France Netherlands U.S.A.
Germany Portugal
India Romania
One Member Body opposed the approval of the Draft
Canada
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council, which decided,
in July 1969, to accept it as an IS0 RECOMMENDATION.
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ISO/R 1100-1969 (E)
CONTENTS
Page
1. Scope . 7
3. Definitions 7
..............................
3. Units of measurement . 7
4. Principle of the method of measurement . 8
5. Choice of site . 8
5.1 Preliminary survey . 8
5.2 Selection of site . 8
6. Design and construction of a gauging station . 9
6.1 General. . 9
6.2 Devices for measurement of stage . 9
6.3 Procedure for observation of gauges . 11
7. Survey of station - General requirements . 11
8. Calibration of station . 11
8.1 General. . 11
8.3 Construction of stage-discharge curve and rating table . 12
8.3 Verification of the stage-discharge relationship . 13
9. Operation of gauging station and compilation of records . 14
9.1 Operation of gauging station . 14
9.3 Compilation of records . 14
9.3 Extrapolation of the stage-discharge curve . 14
IO. Observation error and reliability of the stage-discharge curve . 14
10.1 Standard error of the stage-discharge curve . 15
10.3 “Acceptance limits” of the stage-discharge observations . 15
10.3 95 confidence limits of the mean (the stage-discharge curve) . 16
10.4 Alternative method of determining “acceptance limits of observations”
16
and “confidence limits of the stage-discharge curve” .
ANNEX A - Design, construction and operation of a gauging station . 17
A.l Preliminary survey . 17
A.2 Design of station . 17
.........................
A.3 Construction of station 18
A.4 Definitive survey . 19
..........................
A.5 Stage-discharge curve 19
..........................
A.5.1 Triai curves 19
A.5.2 Rejection of freak observations or incorrect measurements . 20
A.5.3 Determination of number of observations necessary for establishing a
reliable stage-discharge relation . 20
A.5.4 Stable channels . 21
A.5.5 Unstable channels . 2 1
A.5.6 Tests for absence from bias and goodness of fit . 22
A.5.7 Smoothness of curve . 24
A.5.8 Methods for locating shift in control . 24
A.5.9 Check on subsequent shifts in control . 25
A.5.10 Fitting of a mathematical curve . 25
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ISO/R 1100-1969 (E)
Page
A.6 Extrapolation of stage-discharge curve . 27
A.7 Operation of station . 28
29
A.8 Compilation of records .
31
ANNEX B -~ Correction for discharge in unsteady flow .
32
ANNEX C ~~ Family of curves giving the stage-discharge relation .
...............................
C.l General 32
C.2 Constant fall method . 32
C.3 Normal fall method . 33
FIGURES
L
1 . Determination of number of observations necessary for establishing a reliable
stage-discharge relationship . 34
35
2 . Testing of stage-discharge curves .
36
3 .
. Determination of Go . Gauge reading for zero discharge
37
.................
4 . Relationship Qn = f (zo) for unit fall H, = 1
Q H
..................... 38
5 . Relationship between - and -
Qn Hn
39
6 . Relation of stage to adjusted discharge for reference fall .
40
..................
7 . Normal fall method . Simple rating curve
8 . Relation between normal fall and height . 41
...................... 42
9 . Measured discharge-fall relation
List of symbols used . 43
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ISO/R 1100-1969 (E)
~~
IS0 Recommendation R 1100 July 1969
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
ESTABLISHMENT AND OPERATION OF A GAUGING STATION
OF THE STAGE-DISCHARGE RELATION
AND DETERMINATION
1. SCOPE
This IS0 Recommendation deals with the continuous measurement of water level, the determination of the
stage-discharge relation by correlating water levels to discharges and the compilation of records of flow at a
gauging station. It also deals with the establishment and operation of the gauging station on a river or open
channel.
Single-discharge measurements may be made by any of the accepted methods of measurement of liquid flow
in open channels in accordance with the relevant IS0 Recommendations*. This IS0 Recommendation covers
only such additional requirements as are necessitated by its wider scope.
Open channels have been grouped into stable and unstable types as the characteristic stage-discharge curves of
the two are different.
This IS0 Recommendation does not cover cases such as those encountered
during floods when flow conditions are suddenly or rapidly varying due to abrupt flood waves;
(a)
during periods when flow conditions are significantly impeded by the formation or presence of ice.
(b)
An Annex is included covering design and practice to assist compliance with this IS0 Recommendation
(Annex A); it also gives relevant statistical tests and procedures for construction of stage-discharge curves.
Further Annexes deal with the correction for discharge in unsteady flow (Annex B) and the family of curves
giving the stage-discharge relation (Annex C).
2. DEFINITIONS
For the purposes of this IS0 Recommendation, the definitions given in IS0 Recommendation R 772, Vocabulary
of terms and symbols used in connection with the measurement of liquid flow with a free surface, apply.
3. UNITS OF MEASUREMENT
The units of measurement used in this IS0 Recommendation are seconds, and metres or feet.
See the following IS0 Recommendations :
- IS0 Recommendation R 555, Liquidflow measurement in open channels - Dilution methods for measurement of
steady flow - Part I - Constant rate injection method;
- IS0 Recommendation R 1411, Liquid flow measurement in open channels by velocity area methods;
- IS0 Recommendation R . . ., Liquid flow measurement in open channels using thin plate weir and Venturi flumes (at
present, Draft IS0 Recommendation No. 1438).
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ISO/R 1100-1969 (
4. PRINCIPLE OF THE METHOD OF MEASUREMENT
The principle of the method is to establish a unique relation between the discharge of a channel and either the
water level in the section of a stream or the readings of water levels at each end of a reach. Knowledge of this
stagedischarge relation will enable the discharge to be determined by simple measurement of the level, during
the operating period of the station.
To establish this relation, it is necessary to carry out at the selected site a sufficient number of measurements
of the discharge and the corresponding stage simultaneously. Each discharge measurement should be made by
one of the accepted methods.
5. CHOICE OF SITE
5.1 Preliminary survey
A preliminary survey should be made to ensure that the physical and hydraulic features of the proposed
site conform to the requirements for the application of the methoàs of flow measurement which it is
intended to use.
5.2 Selection of site
The site selected should be such that it is possible to measure the whole range and all types of flow
which may be encountered or which it is required to measure. The whole range of measurement, referred
to one reference gauge, may be made at a single section, or, for certain ranges of discharge, at two or
more sections. Similarly, different methods of measurement may be employed for separate parts of the
range, the particular conditions relative to each of the methods of measurement being specified in the
relevant IS0 Recommendations for the measurement of liquid flow in open channels.*
Either single or twin gauge stations may be employed, depending upon conditions, but the former
should be preferred.
The operation of a single gauge station depends upon the assumption that the elevation of the free
surface is a substantially unique function of the discharge. In the case of stations affected by hysteresis,
the rise and fall should be calibrated separately by discharge measurement.
5.2.1 At a single gauge station
It is desirable to select a site where the relationship between stage and discharge is substantially
(a)
consistent and stable. However, this may not be possible on all alluvial rivers. For such rivers,
the stage-discharge relation is generally applicable only for the period for which it has been
determined.
There should not be any variable backwater effect.
(b)
5.2.2 At any gauging station (with single or twin gauges)
The site should be sensitive, i.e. a significant change in discharge, even for the lowest discharges,
(a)
should be accompanied by
- a significant change in stage in the case of a single gauge station;
- a significant change in stage (at one or other of the gauges) and in fall (between the
two gauges) in the case of a twin gauge station.
Otherwise, small errors in stage readings during calibration in a non-sensitive station can result
in large errors in the discharges indicated by the curve.
A comparison should be made between the change in discharge and the corresponding
minimum change in stage to ensure that the sensitivity of the station is sufficient for the
purpose for which the measurements are required.
*
IS0 Recommendations R 555 and R 148, and Draft IS0 Recommendation No. 1438.
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ISO/R 1100-1969 (E)
Sites where weed growth is prevalent should be avoided.
(b)
There should be no vortices, dead water or other abnormalities in flow.
(c)
Access to the site at all stages and at all times should be available as far as possible.
(d)
6. DESIGN AND CONSTRUCTION OF A GAUGING STATION
6.1 General
A gauging station consists of one or more natural or artificial measuring cross-sections (weir or flume),
and a reference gauge.
In cases where a general system of river gauges is in existence, it is not absolutely necessary that the
position of the measuring cross-section should coincide with the position of the gauge, provided that
the discharge is equal in both places for all stages. In all other cases, it is advisable to install the gauge in,
or very near to, the chosen cross-section.
Where a desirable site satisfies some but not all of the requirements stated in the relevant IS0 Recom-
mendations dealing with single-discharge measurement*, and in clause 5.2 of this IS0 Recommen-
dation, it may be made to do so by appropriate modifications (see Annex A). In general this will only
be feasible for smaller rivers.
In the case of a twin gauge station, the length of the reach should be sufficient to make any observational
error negligible relative to the fall of level between the two gauges. Further. there should be no additions
to the discharge, or withdrawals from it, between the two gauges.
6.2 Devices for measurement of stage
6.2.1 Reference gauge. The reference gauge should be a vertical gauge or an inclined gauge. The markings
should be clear and sufficiently accurate for the purpose for which the measurements are required.
The lowest marking and the highest marking on the reference gauge should be respectively below
and above the lowest and highest anticipated water levels.
The reference gauge should be securely futed to an immovable and rigid support in the stream and
should be correlated to a futed bench-mark by precise levelling to the national datum. It should
have a stilling arrangement, wherever necessary, so that the water level can be read accurately.
Where a continuous record of water levels is required, a water-level recorder should be installed. It is
essential, however, to combine such a recorder always with a normal river gauge, placed near the
point of measurement of the recorder. in other cases a normal river gauge will suffice.
NOTE. - When possible, an estimate of extreme values should be made by statistical analysis. Care should be taken
to incorporate such extreme values as may be required for the purpose of the station. Possible deepening of the bed
should also be taken into account.
6.2.1.1 VERTICAL GAUGE. This staff gauge should be truly vertical. It should be of such a shape as
not to cause any noticeable heading up of flow.
6.2.1.2 INCLINED GAUGE. The inclined gauge should fit closely and be solidly anchored to the slope
of the natural bank of the water-course. It may be calibrated on the site by precise levelling.
6.2.2 Continuous liquid level recorder. This may consist of a recorder operated by a float in a stilling well
communicating with the channel or a pneumatic recorder or other device. It is essential, however,
to combine such a recorder always with a reference gauge, placed near the point of measurement of
the recorder. In the case of the float-actuated recorder, an additional gauge should be installed inside
the float-well to serve as a check.
*
See IS0 Recommendations R 555 and R 748. and Draft IS0 Recommendation No. 1438.
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ISOlR 1100-1969 (E
STILLING WELL. The stilling well for the accommodation of the float of the float-operated
6.2.2.1
recorder should meet the following requirements.
(a) It should be vertical and have sufficient height and depth to allow the float to rise and
fall over the full range of water levels.
(b) In water courses with widely fluctuating silt content (i.e. densities), inlet pipes should
be provided at various stages.
(c) Joints with any inlet pipes should be watertight.
(d) The dimensions of the inlet pipe(s) or of the channel should be large enough for the
water level in the well to follow the rise and fail of stage without delay, and also to
prevent clogging due to sediment.
If the stage cannot be read on the chart with sufficient accuracy because of short
(e)
period wave effect, a constriction should be fitted in the inlet pipe to damp out
oscillation.
6.2.2.2 THE PNEUMATIC RECORDER
This recorder measures the stage by means of the pressure exerted on a take-off which
(a)
is immersed and solidly fmed at a known elevation.
The recorder should have a source of compressed gas (air or nitrogen) and a device for
(b)
adjusting the air flow in the form “bubble to bubble”.
A device for driving away the gas should be provided to discharge the pressure take-off,
(c)
if necessary.
The pipe connecting the manometer to the pressure take-off should have a length less
(d)
than the limit fixed by the manufacturer. It should not have low points running the risk
of accumulating the condensates of the gas.
The device for measuring the pressure should be sufficiently sensitive and accurate. In
(e)
case a manometer with a liquid stopper is used, the density of this liquid at various
temperatures should be eventually taken into account and the manufacturer should
indicate the value of the errors.
6.2.2.3 REQUIREMENTS OF THE RECORDER. The recorder either should provide a continuous graphical
record of the changing water level or should register the water levels digitally at suitably close
intervals of time.
The recorded results will depend directly upon the changing water levels. If they are affected by
other phenomena (damping of the stilling well of the float-operated recorder, loss of head due
to the gas flow or the liquid stopper of the manometer, in a pneumatic recorder) the user should
know the effects so that he may correct them if necessary.
in the case of a recorder giving a graphical chart, the chart sheet should be positively located
on the drum. The scales chosen for time and stage will depend on the characteristics of the
river, and should be such that readings can be made with a degree of accuracy sufficient to
show the different phases of the hydrograph.
Any mechanical linkage connecting parts of the recorder should be as short and direct as
possible and there should be no contact between any moving part of the mechanism and any
fmed part of the structure.
The clock mechanism should maintain accurate time.
The recorder should be placed out of danger of flooding and be protected against the
elements and interference by unauthorized persons.
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ISO/R 1100-1969 (E)
Precautions should be taken against the occurrence of errors in records due to
lag of the stylus behind the movement of the float;
(a)
change in the submergence of the float in the water;
(b)
submergence of the counterweight and the float line.
(c)
6.3 Procedure for observation of gauges
The gauge should be read from such a position as to avoid all parallax errors. The gauge should be
observed continuously for a minimum period of 2 minutes or the period of complete oscillation,
whichever is longer, and the maximum and minimum readings taken and averaged.
7. SURVEY OF STATION - GENERAL REQUIREMENTS
After a gauging station has been constructed a definitive survey should be made
The definitive survey should include the accurate determination of the elevations and relative positions of
reference gauges and inlet zeros, the survey of inlet pipes or channel inverts, base of stilling well in the case of a
float-operated recorder, the futation of the pressure take-off and tightness of the gas pipes and of all parts under
pressure in the case of a pneumatic recorder, cross-section markers and any other key points or significant
features of the site.
A periodic check, at least once a year, and on every occasion the gauge is changed, should also be made of the
elevations and relative positions taken at the time of the definitive survey.
8. CALIBRATION OF STATION
8.1 General
The object of a gauging station is to obtain a knowledge of the discharge of a river or open channel.
Once the stage-discharge curve is available, the discharge will be known by reading the gauge or
recorder. The operations necessary to obtain this relationship are called the calibration (rating) of the
station.
8.1.1 Measurement of discharge for calibration (rating). The discharge should be measured in accordance
with the relevant IS0 Recommendations*. Errors may also be estimated on the basis of those IS0
Recommendations.
The survey of the site should be carried out in greater detail than in the case of single discharge
measurements. Special attention should be given to the likelihood of changes in the channel that
may affect the stage-discharge curve.
NOTES
1. Very few rivers have absolutely stable characteristics. The calibration, therefore, cannot be carried out once
and for all, but has to be repeated as frequently as required by the rate of change in the stage-discharge curve.
be made as often
It is recommended that measurements at the stages which do not frequently occur should
as possible.
2. In the case of an unstable river the stage-discharge relationship changes more frequently, especially following
floods. Discharge observations have therefore to be made at more frequent intervals during these periods.
8.1.2 Notches, weirs and flumes. In the case of notches, weirs and flumes the operation recommended
above would not normally be necessary because the stage-discharge relationship may have been
determined by laboratory experiments. However, their operation should be checked periodically
by discharge measurements to ensure that the assumed rating applies and that they actually
(a)
account for all the flow passing the section;
by direct measurement of dimensions to verify stability.
(6)
*
See IS0 Recommendations R 555 and R 748, and Draft IS0 Recommendation No. 1438.
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ISO/R 1100-1969(E)
8.2 Consîruction of stage-discharge curve and rating table
8.2.1 Single gauge station
8.2.1.1 STAGE-DISCHARGE CURVE - STABLE CHANNELS. Discharges should be plotted as abscissae
against the corresponding stages as ordinates and the stage-discharge curve should be drawn
smoothly through the points as plotted. The stage used in plotting should be the mean stage
during the period of discharge measurement.
The curve should be defined by a sufficient number of measurements suitably distributed
throughout the whole range of water levels, each preferably made at steady stage. The
number of measurements to define the curve will depend on the range to be covered, the
shape of the stage-discharge curve and the desired accuracy (see clause A.5.3 of Annex A).
The spacing of the measurements should be closer at the lower end of the range and the
drawing of the curve should be checked by one of the methods described in Annex A. Where
observations are made at rising and falling stages, they should be indicated by suitable symbols,
and there should be about the same number of each at corresponding steps in order to
establish the suitable mean curve.
In order to have at least one measurement at or near the peak discharge, it is desirable to
increase the frequency of measurements when the gradient of the hydrograph is becoming
significantly flatter at or near the peak.
8.2.1.2 STAGE-DISCHARGE CURVE - UNSTABLE CHANNELS. In the case of unstable channels, the
stagedischarge relationship does not remain stable and frequent changes in “control” occur
during and after flood periods. Also in the case of unstable channels, the particular stage-
discharge relations prevailing over the different periods are required for the estimation of
discharges from stage records for the periods for which no discharge measurements are
available. The discharges for the water year should be plotted as abscissae against the correç-
ponding stages as ordinates and each point labelled in chronological order. The lie of the
points should be examined for shifts in control with reference to their chronological order.
Smooth curves should be drawn separately for each period having no shift in control. Thus,
there may be more than one curve within the rising and falling phases of the same water year
for an unstable channel subject to silting and scouring.
8.2.1.3
METHODS OF TESTING STAGE-DISCHARGE CURVES. The curves should invariably express the
stage-discharge relation objectively and should therefore be tested for absence from bias and
goodness of fit in the periods between shifts of control, and for the shifts in control. Methods
for locating shifts in control in the stage-discharge curves due either to physical changes in the
channel features or to changes occurring in course of time are also indicated in clause A.5.8 of
Annex A.
For stable channels, where the control is uniform and remains unchanged, it may be possible
to fit a mathematical curve. This may be done as explained in clause AS .IO of Annex A.
More frequently, even in a stable channel, where the curve has to be drawn by visual estimation
for example, when the section is not uniform, the tests described for unstable channels become
equally necessary.
In the case of natural unstable channels, different controls come into operation at different
stages in different years, so that not only are the curves for the rising and falling stages
different from each other and from year to year, but there are also inflexions and discon-
tinuities due to shifts in control within a stage. The inordinate labour involved in fitting the
high-degree composite curves rules them out in practice. The best-fitting rising and falling
curves have, therefore, to be drawn by visual estimation and should, therefore, be tested for
absence from bias and for goodness of fit, separately for the individual portions between shifts
in control.
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ISO/R 1100-1969 (E)
For absence from bias, there are two tests. In one, the curve is tested to satisfy the basic
requirement that a significantly equal number of points are expected to lie above and below
an unbiassed curve so that the deviations are not more than those due to chance fluctuations
(Test 1, clause A.5.6.1). In the other, the condition that the algebraic sum of the percentage
deviations of the observed discharges from an unbiassed curve should not be significantly
different from zero is tested by comparing the mean of the percentage deviations with the
standard error (Test 3, clause A.5.6.3).
For goodness of fit, a test is applied to check that a sign change in deviations (i.e. observed
value minus expected value from the curve) is as likely as a non-change of sign. This test also
helps in detecting shifts in control at different stages (Test 2, clause A.5.6.2).
The above-mentioned tests are described in detail in clause A.5.6 of Annex A with examples
illustrating their application.
8.2.1.4 RATING TABLE. A rating table should be prepared from the stage-discharge curve, or from
the equation of the curve, showing the discharges corresponding to stages in ascending order,
and with intervals corresponding to the desired accuracy of reading.
8.2.2 Twin gauge stations
STAGE-DISCHARGE CURVE. The establishment of a single gauge station is impossible in all
8.2.2.1
cases where the flow in the reach of the river is controlled by conditions existing in another
reach (upstream or downstream), if the latter vary owing to natural causes (rise in water level
of tributary, obstruction, etc.), or owing to artificial causes (moving flood gates of navigational
dams, hydro-electric schemes, etc.). It is, however, possible to obtain a measurement of flow
with two gauges placed in the same reach by the slope discharge method. For all pairs of values
of levels z,, and z read off each of the scales, there is a single corresponding value of discharge
Q, provided that the topography and roughness of the bed have not changed in the reach
between the scales.
The plotting of the stage-discharge observations with the value of fall against each observation
will reveal whether the relationship is affected by variable slope at all stages, or is affected only
when the fall reduces below a particular value. In the absence of any channel control, the
discharge would be affected by the fall at all times, and the correction is applied as indicated in
the constant fall method covered in Annex C. When the discharge is affected only when the fall
reduces below a particular value, the normal fall method is applied. which is also given in
Annex C.
8.2.2.2 RATING TABLE. The complexity of the relationship generally makes it difficult to prepare a
rating table and it is recommended that the values be obtained from the relevant graphs.
8.3 Verification of the stage-discharge relationship
If the procedure recommended in clause A.5.1 is followed,no actual verification is needed. When, after
one or more years, a sufficient number of observations is available for the whole range of discharges,
the curve should be drawn. By repeating the procedure it will be possible after a certain period to
draw a new curve. When shift is thus detected in an unstable channel, a new curve is drawn which
serves as a norm during the period until another shift is detected. Each curve is the calibration curve
which serves as the norm during the next period of observations. By comparing successive curves and
noting the
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