Measurement of liquid flow in open channels — Stage-fall-discharge relationships

Mesure de débit des liquides dans les canaux découverts — Relations hauteur-chute-débit

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Status
Withdrawn
Publication Date
21-Nov-2001
Withdrawal Date
21-Nov-2001
Current Stage
9599 - Withdrawal of International Standard
Start Date
30-Oct-2017
Completion Date
30-Oct-2017
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ISO 9123:2001 - Measurement of liquid flow in open channels -- Stage-fall-discharge relationships
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INTERNATIONAL ISO
STANDARD 9123
First edition
2001-11-15
Measurement of liquid flow in open
channels — Stage-fall-discharge
relationships
Mesure de débit des liquides dans les canaux découverts — Relations
hauteur-chute-débit
Reference number
ISO 9123:2001(E)
© ISO 2001
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ISO 9123:2001(E)
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© ISO 2001

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ii ISO 2001 – All rights reserved
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ISO 9123:2001(E)
Contents Page

1 Scope ............................................................................................................................................................... 1

2 Normative references ....................................................................................................................................... 1

3 Terms and definitions ........................................................................................................................................ 1

4 Units of measurement ...................................................................................................................................... 1

5 General considerations ..................................................................................................................................... 1

6 Unit-fall method ................................................................................................................................................ 3

7 Constant-fall method ........................................................................................................................................ 5

8 Variable-fall method .......................................................................................................................................... 9

9 Rating curves and tables ................................................................................................................................ 14

10 Method of computation ................................................................................................................................. 14

11 Periodic checking of stage-fall-discharge ratings .......................................................................................... 14

12 Extrapolations ............................................................................................................................................... 14

13 Uncertainties ................................................................................................................................................ 14

ISO 2001 – All rights reserved iii
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ISO 9123:2001(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO

member bodies). The work of preparing International Standards is normally carried out through ISO technical

committees. Each member body interested in a subject for which a technical committee has been established has

the right to be represented on that committee. International organizations, governmental and non-governmental, in

liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical

Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

International Standard ISO 9123 was prepared by Technical Committee ISO/TC 113, Hydrometric determinations,

Subcommittee SC 1, Velocity area methods.

This first edition of ISO 9123 cancels and replaces Technical Report ISO/TR 9123:1986, which has been technically

revised.
iv ISO 2001 – All rights reserved
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INTERNATIONAL STANDARD ISO 9123:2001(E)
Measurement of liquid flow in open channels — Stage-fall-
discharge relationships
1 Scope

This International Standard specifies methods for determining stage-fall-discharge relationships for a stream reach

where variable backwater occurs either intermittently or continuously. Two gauging stations, a base reference gauge

and an auxiliary gauge are required for gauge height measurements. A number of discharge measurements are re-

quired in order to calibrate the rating to the accuracy required by this International Standard.

The preparation of rating curves is not described in detail in this International Standard.

NOTE For a more detailed description of preparing rating curves, see the methods described in ISO 1100-2

2 Normative references

The following normative documents contain provisions which, through reference in this text, constitute provisions of

this International Standard. For dated references, subsequent amendments to, or revisions of, any of these publica-

tions do not apply. However, parties to agreements based on this International Standard are encouraged to investi-

gate the possibility of applying the most recent editions of the normative documents indicated below. For undated

references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain reg-

isters of currently valid International Standards.
ISO 772, Hydrometric determinations — Vocabulary and symbols.

ISO 1000, SI units and recommendations for the use of their multiples and of certain other units.

ISO 1100-2, Measurement of liquid flow in open channels — Part 2: Determination of the stage-discharge relation.

3 Terms and definitions

For the purposes of this International Standard, the terms and definitions and symbols given in ISO 772 apply. Note,

however that the application of the definition of backwater given in ISO 772 to the determination of discharge under

intermittent or continuous backwater conditions should take into account that a higher gauge height would prevail for

a given discharge than would be the case if the variable backwater was not present.

4 Units of measurement

The International System of Units (SI System) is used in this International Standard in accordance with ISO 1000.

5 General considerations
5.1 Importance of backwater

Most programmes for collecting records of discharge of streams are based on the fact that a relatively simple

relationship exists between gauge height and discharge so that, by simply recording gauge height and developing

the stage-discharge relationship, a continuous record of discharge can be computed. Several factors, however, can

cause scatter of discharge measurements about the stage-discharge relationship at some stations. Backwater is one

ISO 2001 – All rights reserved 1
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ISO 9123:2001(E)

of these factors and is defined as a condition whereby the flow is retarded so that a higher gauge height is necessary

to maintain a given discharge than would be necessary if the backwater were not present.

5.2 Backwater conditions

Constant backwater, as caused by section controls for instance, will not adversely affect the stage-discharge

relationship. The presence of variable backwater, on the other hand, does not allow the use of simple stage-

discharge relationships for accurate determination of discharge. Regulated streams may have variable backwater

virtually all of the time, while other streams will have only occasional backwater from downstream tributaries, vegetal

growth, or from the return of overbank flow.
5.3 Gauging requirements

Many of these sites can be operated as stage-fall-discharge stations by using a reference gauge at which gauge

height is measured continuously and current-meter measurements of discharge are made occasionally. An auxiliary

gauge some distance downstream from the reference gauge is operated to measure gauge height continuously.

When the two gauges are set to the same datum, the difference between the two gauge height records is the water-

surface fall and provides a measure of water-surface slope. The shorter the slope reach, the closer the relationship

between measured fall and water-surface slope. On the other hand, the longer the slope reach, the smaller the

percentage of error in the recorded fall.

Precise time synchronization between reference and auxiliary gauges is very important when gauge height changes

rapidly, or when fall is small. Reliable discharge records can usually be computed when fall exceeds about 0,1 m.

Timing and gauge-height errors that are trivial at high discharges become significant at very low flow.

5.4 Types of stage-fall-discharge relationships

5.4.1 Under conditions of variable backwater, the fall as measured between the reference gauge and the auxiliary

gauge is used as a third parameter, and the rating becomes a stage-fall-discharge relationship. Stage-fall-discharge

methods fall into the following two broad categories:
a) constant-fall method, of which the unit-fall method is a special case;
b) variable-fall method.

The applicable method for a stream reach depends to a large degree on whether the backwater is intermittent or

always present.

5.4.2 The constant-fall method works best when backwater is always present at all gauge heights, but can

sometimes be adapted to intermittent backwater conditions.

5.4.3 The unit-fall method is the simplest and requires the least amount of data for calibration. The unit-fall method

should be used as a starting point before attempting more complex methods.

5.4.4 Variable-fall methods are the most complex and require the most data for calibration. The variable-fall method

works best for the intermittent backwater condition.

NOTE The unit-fall method, the constant-fall method and the variable-fall method, are also referred to in this International

Standard as unit-fall rating, constant-fall rating and variable-fall rating.
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ISO 9123:2001(E)
6 Unit-fall method
6.1 General

The unit-fall method is a special case of the constant-fall method, where the constant fall is unity (1m). The unit-fall

method is used with the assumption that the relationship between the discharge ratio (Q=Q ) and the fall ratio

(h=h ) is exactly a square root relationship, as given by the following formulae:

0,5 0,5 0,5
Q=Q =(h=h ) =(h=1) =h
c c
0,5 0,5
Q =Q (h ) orQ =Q=(h )
c c
where
Q is the measured discharge, expressed in cubic metres per second;
h is the measured fall, expressed in metres;

Q is the discharge, expressed in cubic metres per second, from the rating curve corresponding to the constant

fall and the reference gauge height;
h 1m
is the constant fall, expressed in metres ( for the unit-fall method).
6.2 Method of analysis

The unit-fall rating shall be developed by plotting each measured discharge divided by the square root of the

measured fall against the reference gauge height for the discharge measurement. The rating curve shall then be

fitted to these plotted points.
6.3 Computation of discharge

The rating shall be used to compute discharge by determining the value of from the rating for a given reference

gauge height, and multiplying this discharge by the square root of the measured fall. This type of rating will usually be

satisfactory when backwater is always present, fall is greater than about 0,1 m, and the datums of the two gauges are

within about 0,01 m.

If backwater is intermittent, it is also necessary to develop a free-fall rating or rating where backwater is not present.

The free-fall rating shall be used at all times except during periods when backwater is suspected, during which times

discharge should be computed from both the free-fall and unit-fall ratings. The lower of the two discharges shall be

considered to be the true value.
6.4 Example of unit-fall method

Figure 1 and Table 1 illustrate the unit-fall rating for a site with high backwater from a power dam. The backwater

exists at all gauge heights and at all times.
ISO 2001 – All rights reserved 3
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ISO 9123:2001(E)
Table 1 — Unit-fall calibration measurements
Measurement No. Gauge height Difference
hQ Q
Q h
mm m =s%
m =s
327 5,907 1,917 1 160 838 840 −0,2
328 7,105 2,182 1 520 1 030 1 030 0
332 5,026 1,597 889 703 700 0,4
373 7,013 2,225 1 490 1 000 1 000 0
384 11,558 2,880 2 830 1 670 1 700 −1,8
385 8,108 1,920 1 640 1 180 1 190 −0,8
386 8,638 2,652 1 990 1 220 1 260 −3,3
387 3,139 0,808 399 444 410 7,7
391 2,755 0,701 317 379 360 5,0
398 2,963 0,616 289 368 388 −5,4
400 2,359 0,204 156 345 300 13,0
401 2,286 0,290 145 269 290 −7,8
404 3,206 0,927 411 427 426 0,2
428 2,036 0,058 39,9 166 255 −53,6
429 2,012 0,061 66,0 267 250 6,4
Fall,h = 1m
NOTE The numbers on the plot refer to the measurement number (see Table 1).
Figure 1 — Unit-fall rating
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ISO 9123:2001(E)
7 Constant-fall method
7.1 General

The constant-fall method is more complex than the unit-fall method in that it uses two relationship curves. In addition,

it does not require that the constant fall be equal to unity, but can be any selected value. The constant fall is usually

selected to be equal to the average fall in the gauging reach. The constant-fall method requires the use of the

following two curves:

a) the relationship between gauge height and discharge for a constant fall of some specified value;

b) the relationship between measured fall,h, and the discharge ratio,Q=Q .

A unique feature of the constant-fall method is that the reference gauge and auxiliary gauge need not be at the same

datum.
7.2 Method of a
...

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