Measurement of liquid flow in open channels — Part 1: Establishment and operation of a gauging station

Deals with the establishment and operation of a gauging station for the measurement of stage oder discharge, or both, of a lake, reservoir, river or artificial open channel. Replaces the first edition.

Mesurage de débit des liquides dans les canaux découverts — Partie 1: Établissement et exploitation d'une station hydrométrique

General Information

Status
Withdrawn
Publication Date
07-Feb-1996
Withdrawal Date
07-Feb-1996
Current Stage
9599 - Withdrawal of International Standard
Start Date
06-Dec-2013
Completion Date
06-Dec-2013
Ref Project

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ISO 1100-1:1996 - Measurement of liquid flow in open channels
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INTERNATIONAL IS0
STANDARD 1100-1
Second edition
1996-02-I 5
Measurement of liquid flow in open
channels -
Part 1:
Establishment and operation of a gauging
station
Mesurage de d6bit des liquides dans /es canaux dkouverts -
Partie I: &ablissement et exploitation d’une station hydrom&ique
Reference number
IS0 1100-l :I 996(E)
---------------------- Page: 1 ----------------------
IS0 1100=1:1996(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(I EC) on all matters of electrotechnical standardization.
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.
International Standard IS0 1100-I was prepared by Technical Committee
lSO/TC 113, Hydrometric determinations, Subcommittee SC 1, Velocity
area methods.
This second edition cancels and replaces the first edition
which has been technically revised.
(IS0 1100-I :I 981)
IS0 1100 consists of the following parts, under the general title Meas-
urement of liquid flow in open channels:
- Part 7: Establishment and operation of a gauging station
- Part 2: Determination of the stage-discharge relation
Annex A of this part of IS0 1100 is for information only.
0 IS0 1996

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced

or utilized in any form or by any means, electronic or mechanical, including photocopying and

microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
---------------------- Page: 2 ----------------------
IS0 1100=1:1996(E)
INTERNATIONAL STANDARD Q ISO
Measurement of liquid flow in open channels -
Part 1: .
aauaina station
Establishment and operation of a
IS0 1000: 1992, SI units and recommendations for the
1 Scope
use of their multiples and of certain other units.
1.1 This part of IS0 1100 deals with the establish-
IS0 1070:1992, Liquid flow measurement in open
ment and operation of a gauging station for the
channels - Slope-area method.
measurement of stage or discharge, or both, of a lake,

reservoir, river or artificial open channel. IS0 1 IOO-2:1982, Liquid flow measurement in open

channels - Part 2: Determination of the stage-
discharge rela Con.
1.2 Requirements are specified for stage and for
stage-discharge stations in natural channels and
IS0 3454:1983, Liquid flow measurement in open
stations with artificial structures, for direct discharge
channels - Direct depth sounding and suspension
measurement and for measurement under ice condi-
equlpmen t.
tions.
IS0 3846:1989, Liquid flow measurement in open
channels by weirs and flumes - Rectangular broad-
2 Normative references
crested weirs.
The following standards contain provisions which,
IS0 3847:1977, Liquid flow measurement in open
through reference in this text, constitute provisions
channels by weirs and flumes - End-depth method
of this part of IS0 II 00. At the time of publication, the
for estimation of flow in rectangular channels with a
editions indicated were valid. All standards are subject
free 0 verfall.
to revision, and parties to agreements based on this
part of IS0 1100 are encouraged to investigate the
IS0 4359:1983, Liquid flow measurement in open
possibility of applying the most recent editions of the
channels - Rectangular, trapezoidal and U-shaped
standards indicated below. Members of IEC and IS0
flumes.
maintain registers of currently valid International
Standards.
IS0 4360:1984, Liquid flow measurement in open
channels by weirs and flumes - Triangular profile
IS0 31 :I992 Quantities and units, all parts.
weirs.
I SO 748:---I), Measurement of liquid flow in open
IS0 4369:1979, Measurement of liquid flow in open
channels - Velocity-area methods.
channels - Moving-boat method.
Measurement of liquid flow in open
I SO 772:-J,
channels - Vocabulary and symbols.
1) To be published. (Revision of IS0 748:1979)
2) To be published. (Revision of IS0 772:1988)
---------------------- Page: 3 ----------------------
0 IS0
IS0 1100=1:1996(E)
IS0 437311995, Measurement of liquid flow in open
5 Stage-discharge gauging stations
channels - Water--level measuring devices.
(natural channels)
IS0 4375:1979, Measurement of liquid flow in open
channels - Cableway system for stream gauging.
5.1 Principle
IS0 4377:1990, Liquid flow measurement in open
Water levels (stages) of rivers, lakes or reservoirs are
- Flat-V weirs.
channels
used in delineating flood hazard areas, and in the de-
sign of structures in or near rivers, lakes or reservoirs.
IS0 6416:1992, Measurement of liquid flow in open
channels - Measurement of discharge by the ultra-
The stage or water level of a stream or lake is the
sonic (acoustic) method.
height of the water surface above an established da-
tum plane.
IS0 8368: 1985, Liquid flow measurement in open
channels - Guidelines for the selection of flow
Water-level records are obtained by systematic ob-
gauging structures.
servations of a reference gauge, or from a water-level
sensor.
lSO/TR 9123:1986, Liquid flow measurements in
open channels - Stage-fall-discharge relations.
When records of water level in streams are to be
used as a basis for computation of discharge, the re-
IS0 9196: 1992, Liquid flow measurement in open
lation between stage and discharge should be deter-
channels - Flow measurements under ice
mined.
conditions.
In a stable channel with appropriate control of the
IS0 9213:1992, Measurement of total discharge in
downstream water level, a single relation may exist
Electromagnetic method using a
open channels -
between stage and discharge. In this case, the re-
full-channel-width coil.
lation shall be determined from discharge measure-
ments at selected stages throughout the range of
IS0 9555-l :I 994, Measurement of liquid flow in open
stage experienced at the station.
channels - Tracer dilution methods for the measure-
ment of steady flow - Part 1: General.
Discharges can be determined using either velocity-
area methods according to IS0 748, tracer dilution
IS0 9555-2: 1992, Measurement of liquid flow in open
methods according to IS0 9555-1, IS0 9555-2,
channels - Tracer dilution methods for the measure-
IS0 9555-3 or IS0 9555-4, or ultrasonic methods us-
ment of steady flow - Part 2: Radioactive tracers.
ing a temporary installation (see IS0 6416).
IS0 9555-3:1992, Measurement of liquid flow in open
In the case where no single relation exists between
Tracer dilution methods for the measure-
channels -
stage and discharge, a relation may exist between the
Part 3: Chemical tracers.
ment of steady flow -
surface slope or fall, stage and discharge which may
require the establishment of a second stage (slope)
IS0 9555-4: 1992, Measurement of liquid flow in open
gauge. Relations shall then be established on the ba-
channels - Tracer dilution methods for the measure-
sis of these three factors, and discharge shall be de-
ment of steady flow - Part 4: Fluorescent tracers.
termined from measurements of surface slope and
stage (see IS0 1070 or lSO/TR 9123 as appropriate).
3 Definitions
5.2 Main elements of a gauging station
For the purposes of this part of IS0 1100, the defi-
The main elements required for establishing the his-
nitions and symbols given in IS0 772 apply.
torical records of discharge in a stream from water-
level records are as follows:
- choice of control section or reach (see 5.2.1);
4 Units of measurement
- stage-measuring device (see 5.2.2);
The units of measurement used in this part of
- stage-sensing and -recording device (see 5.2.3 and
IS0 1100 are SI units in accordance with the appro-
5.2.5);
priate parts of IS0 31 and IS0 1000.
---------------------- Page: 4 ----------------------
0 IS0 IS0 11004:1996(E)
section (or reach) (see 5.2.2.1 Vertical and inclined staff gauges
- discharge-measuring
5.2.6).
Vertical and inclined staff gauges shall meet the
For a stage-measuring station, the aim of which is to
functional requirements described in IS0 4373, which
establish stage records only, see 5.2.2 to 5.2.4.
may be briefly summarized as follows.
The graduations of a staff gauge shall be clearly,
5.2.1 Control section (or reach)
accurately and permanently marked directly on a
smooth surface. The numerals shall be distinct
A control section or control reach of a channel is a
and placed so that an ambiguous reading is not
natural or artificial section or reach whose physical
likely.
characteristics are measured and used to determine
the relation between stage (or stage and slope) and
b) The gauges shall be durable and easy to maintain.
discharge.
The material shall have a low coefficient of ex-
A control section is one in which any change in the
pansion, and shall be resistant to alternating wet
downstream stage does not affect the upstream
and dry conditions and to wear or fading of the
in the control section,
stage. Whatever the discharge markings.
recorded. In a control
a critical stage can always be

reach no critical stage can be recorded. A control c) The gauges shall be placed near the bank, in an

section may be natural (e.g. a rock outcrop or sandbar) easily accessible position, so that water level can

or artificial (e.g. a weir, flume or culvert). be read from the shortest possible distance.

The sensitivity of a control section or reach shall be
d) The gauges shall be simple to install and use.
such that any significant change in discharge shall re-
sult in either
e) The gauges shall be placed in a calm area, as
close as possible to, and preferably in the same
- a measurable change in stage (for control
cross-section as, the stage sensor, without how-
sections), or
ever affecting stage at this level. When the am-
plitude of variation of stage can exceed the
- a measurable change in stage at one extremity of
capacity of a staff gauge, other additional ele-
the control reach, and a measurable change in
ments may be installed in the same cross-
surface slope between the two extremities.
sectional area, normal to the direction of flow.
In order to establish a stable stage-discharge relation,
the control section or reach shall be stable, i.e. no
5.2.2.2 Other devices
change shall occur over time in its physical character-
istics.
In some cases, a reference-point gauge with a device
for locating the water level with respect to the refer-
Several control sections may be considered for dis-
ence may be substituted for a vertical or inclined
charge measurement for one gauging station.
gauge.
Under given discharge conditions, the presence of a
Needle gauges may be used when level variation is
downstream weir may create a water level which
small (I m max.) and when the water surface is sta-
submerges an upstream weir used as a control sec-
ble .
tion. This downstream weir may then be considered
as the new control section.
When it is not possible to install vertical or inclined
gauges, wire-weight gauges may be used if a struc-
ture exists permitting their installation over the water.
5.2.2 Stage-measuring devices
The functional requirements and conditions of instal-
Stage-sensing and -measuring devices are the basic
lation of these gauges are specified in IS0 4373.
elements of the equipment for measuring and re-
cording stage. They shall be stable.
5.2.2.3 Gauge-zero elevation (see IS0 4373)
near
A vertical or inclined staff g auge shall be located
the stage sensor, to act as the refere nce gauge.
The establishment of the gauge zero shall be chosen

The water level indicated by the stage sensor should so as to avoid negative readings. To ensure that the

gauge zero remains the same over the duration of the
follow the water level indicated by the reference
station operation, care shall be taken to ensure that it
gauge.
---------------------- Page: 5 ----------------------
0 IS0
IS0 1100=1:1996(E)
expensive or impractical. The principle of the sensor
is fixed low enough, especially for sites where scour
consists of discharging a small flow of compressed
is severe.
gas into a tube, the free end (orifice) of which has
The zero of the gauge shall be correlated with a na-
been placed in the water and fixed at an elevation
tional datum through a station benchmark. The gauge
below the level to be measured. The sensor at the
zero and the other gauge divisions should be checked
opposite end of the tube detects the pressure of the
annually with respect to this benchmark.
gas, which is proportional to the head of water above
the orifice. Servomanometer and servo beam balance
This procedure allows replacement of the gauge in
devices are some of the mechanisms employed in
case of destruction and maintenance of the same
which pressure is detected, and a strip chart recording
stage-discharge relation provided the control section
or electrical signal is obtained through a servometer.
is not modified. At least two independent station
Pressure transducers of appropriate range and accu-
benchmarks should be established so that the gauge
racy based on a piezoresistive principle, quartz crystal
zero can be conveniently recovered if one of the
or other type may also be used to produce an elec-
benchmarks is lost or destroyed.
trical signal proportional to the water head. The func-
tional requirements and conditions of installation of
5.2.3 Stage-sensing devices
these devices are described in IS0 4373.
When variations in stage are small, stage records may
5.2.3.3 Diaphragm pressure sensor
be established by direct readings of the reference
gauge by an observer. However, when stage varies
This is a differential pressure transducer which senses
rapidly, the station should preferably be equipped with
the difference between hydrostatic and atmospheric
a sensor and recorder (see also 5.4.5).
pressures. The body of the sensor is fixed in the
channel at an elevation below the lowest stage to be
The stage sensor converts a change in stage into a
measured. It is fitted with a diaphragm, one face of
proportional quantity of shaft rotation or electrical sig-
which is in contact with the water and the other face
nal, which is then recorded. The stage sensor may be
is subjected to atmospheric pressure through a
a mechanical, pressure, electronic or acoustic device.
capillary tube which is vented to the atmosphere
The recorder associated with a stage sensor may be
above water level.
a graphic (analog), digital, magnetic tape or electronic
device.
The deformation of the diaphragm under hydrostatic
pressure is converted into an electrical signal which
5.2.3.1 Float system
is proportional to the head of water above the sensor.
The typical float system consists of a float operating
The use of such sensors is generally limited to re-
in a stilling well, a graduated steel tape or wire, a
stricted measuring ranges because of the difficulty in
counterweight, a pulley and a pointer. The stage fluc-
meeting the accuracy requirements defined in 5.2.4
tuations are sensed by the float and converted into
over extended measuring ranges. The installation re-
an angular moment of the pulley-bearing shaft.
quirements for these sensors are similar to those for
pneumatic sensors.
The dimensions of the float and counterweight deter-
mine the sensor sensitivity and the driving torque on

the output shaft. The functional requirements of still- 5.2.3.4 Downward-looking ultrasonic device

ing wells are given in IS0 4373 and are summarized
This device is located above the water surface, away
as follows:
from the influence of the banks. The time is measured
- to provide, within the well, an accurate represen-
for pulses of ultrasound to travel from the device to
tation of the water level in the channel;
the water surface and back. The speed of sound in
air is assessed either from a measurement of air
- to damp out oscillations of the water surface;
temperature, or by direct measurement using a target
placed at a fixed distance between the device and the
- to accommodate the recording instrument and
water surface.
protect the float system.
The manufacturer’s recommendations for minimum
distances between the device and the banks and wa-
Pneumatic pressure sensor
5.2.3.2
ter surface should be followed. In the presence of
waves on the water surface, the device tends to de-
This type of stage-sensing device is frequently used
termine the elevation of the wave crests. The range
where the installation of a stilling well would be too
---------------------- Page: 6 ----------------------
@a IS0 IS0 1100=1:1996(E)

of these devices (meeting the accuracy requirements surface of the flood water, and is deposited on the

of 5.2.4) is limited to about 2 m. centre rod as the water recedes, thus indicating the

maximum stage. Alternatively, the centre rod may be
coated with a paint whose colour is permanently af-
5.2.3.5 Upward-looking ultrasonic device
fected by water.
A transducer is located below the water surface away

from the banks, and connected to an electronic unit. 5.2.4 Accuracy of stage measurements

The time is measured for pulses of ultrasound to
For the measurement of stage, in certain installations
travel from the transducer to the water surface and
an uncertainty of + 10 mm may be satisfactory; in
back. The speed of sound in water is assessed either
others, an uncertainty of + 3 mm or better may be
by direct measurement, using a target placed at a
required; however, in no case should the uncertainty
fixed distance above the transducer, or from data de-
be greater than + 10 mm, or + 0,l % of the range of
rived from an ultrasonic flowmeter (see IS0 6416) of - -
the measuring device, whichever is greater (see
which the level sensor is an integral part.
IS0 4373).
The manufacturer’s recommendations for the mini-
mum distances between the transducer and the
5.2.5 Water-level recorders
banks and water surface should be followed. In the
presence of waves on the water surface, the device
5.2.5.1 Analog recorders
tends to determine the elevation of the wave troughs.
Analog recorders produce a continuous graphic record
A common configuration consists of a vertical tube
on a paper chart of the rise and fall of the stream with
extending above the water surface, with the
respect to time, as measured by the stage sensor.
transducer and target at the lower end.
Graphic recorders may be mechanical, with a shaft
rotation as input signal delivered directly by the level
5.2.3.6 Other stage sensors
sensor, or electronic (e.g. potentiometric recorders).
Other types of sensors exist which operate according
Regardless of their type, graphic recorders shall meet
to mechanical principles (buoyancy), electrical prin-
the requirements of IS0 4373.
ciples (capacitance or resistance sensors), or optical
principles.
5.2.5.2 Digital paper tape recorders
However, they are not covered in this part of
IS0 1100. Digital paper tape recorders punch or inscribe coded
instantaneous or discrete values on paper tape at
preselected time intervals.
5.2.3.7 Maximum-stage gauge

A maximum-stage gauge may be used to obtain a re- 5.2.5.3 Magnetic tape recorders

cord of the peak level reached during a flood when
Magnetic type recorders record coded values of a
other methods of recording levels cannot be used.
variable on a magnetic tape at preselected time inter-
Peak discharges may be calculated from the water
vals. Coding may be incremental, i.e. only level vari-
levels at two gauges installed some distance apart in
ations between two measurements are recorded over
a stretch of channel, provided that the time lag be-
the time interval, or discrete values may be recorded.
tween measurements is negligible (see IS0 1070).
In the latter case, the integer value is generally re-
These gauges do not meet the accuracy requirements
corded in binary form. These recorders are coupled to
of 5.2.4.
stage sensors via encoders, such as a rotational shaft
Maximum-stage gauges are locally made to different
movement or an electronic encoder delivering elec-
designs. Basically they may consist of a vertically in- trical signals.
stalled tube of approximately 50 mm internal diam-
eter, down the centre of which runs a rod. The tube
5.2.5.4 Electronic memory (solid state) recorders
is perforated to permit rising water to enter, the per-

forations being located to prevent drawdown or vel- These recorders store coded values in an electronic

ocity head from affecting the static water level. The memory. Like magnetic tape recorders, they are cou-

top of the tube shall be closed to prevent the entry pled to stage sensors via digital coders suited to the

of rain, but it should have an air vent to permit water signal delivered. Stored values may be retrieved on-

to rise up the tube without significant delay. Pow- site or remotely consulted, using an appropriate de-

dered cork in the bottom of the tube floats on the vice.
---------------------- Page: 7 ----------------------
0 IS0
IS0 1100=1:1996(E)

5.2.6 Discharge-measuring section or reach removed. The field of view of a measuring section

(gauging section) shall extend sufficiently upstream to enable float-
ing debris, which might damage a measuring in-
The establishment of the stage-discharge relation at
strument, to be seen in sufficient time to permit

a gauging station is carried out by direct measurement the removal of the instrument from the stream.

of discharge using the methods described in the ap-

propriate International Standard. c) Suitable access to the site shall be constructed

where possible, to provide safe passage at all
In a permanent gauging station, the measurement
stages of flow and in all weather for personnel
section should be clearly identified and suitably
and for any vehicles used for the conveyance of
equipped to provide satisfactory performance.
instruments and equipment.
Regardless of the measuring method, the discharge
5.2.8 Discharge measurement by the dil ution
through the discharge-measuring section or reach
method
shall be the same as the discharge normal to the
reference-stage gauge, over the entire range of dis-
This method consists of injecting a tracer solution of
charge rates.
known concentration into the stream and sampling
the tracer concentration at a point further down-
At a gauging station, different measuring sections or
stream, where turbulence has mixed the tracer uni-
different methods may be used to cover the discharge
formly throughout the cross-section.
range.
The stream discharge is computed from a comparison
The various methods which can be used for calibrat-
between the concentrations of the injected solution
ing a gauging station are:
and of the samples taken downstream.
- the velocity-area method using a current-meter
The physical and hydraulic characteristics of the dis-
(see IS0 748), the moving boat method (see
charge measuring reach shall meet the requirements
IS0 4369) or floats (see IS0 748);
of the appropriate part of IS0 9555. The method relies
on there being good mixing of the water and tracer
- the dilution method using a tracer (see
throughout the entire cross-section. Adequate length
IS0 9555-1, IS0 9555-2, IS0 9555-3 and
of channel shall be used between the injection and
I so 9555-4);
sampling points.
- the ultrasonic method using a temporary instal-
lation in a self-calibrating mode (IS0 6416).
5.3 Preliminary survey and selection criteria
5.2.7 Discharge measurement by the
The site selected for observation of stage should be
velocity-area method
determined by the purpose for which the records are
collected, the accessibility of the site, and the avail-
The principle of the method is to measure the mean
ability of an observer if the gauge is nonrecording.
velocity and the area of cross-section of flow, the
Gauges on lakes and reservoirs are normally located
product of which is the discharge.
near the outlet, but upstream from the zone where
an increase in velocity causes a drawdown in water
The physical and hydraulic characteristics of the dis-
level. Gauges on large bodies of water should also be
charge measuring section shall meet the require-
located so as to reduce the fetch of strong winds,
ments of IS0 748 for the method to be implemented.
which may cause damage or misleading data. Hy-
draulic conditions are an important factor in site se-
Where the site does not offer the main requirements
lection on channels, particularly where water levels
for a gauging according to the specifications, condi-
are used to compute discharge records.
tions shall be improved as described below.
5.3.1 Preliminary survey
a) Minor irregularities in the bank or bed causing lo-
cal eddies shall be eliminated by trimming the
Detailed examination of a large-scale map is required
bank to a regular line and a stable slope, and by
in the first instance. A low-altitude aerial survey (using
removing from the bed any large stones or boul-
a plane or helicopter) may be made if the basin is
ders.
large and not readily accessible by road vehicles. This

Trees obst rutting the clear view of the measuring procedure gives a better view than ground surveys.

section or measuring reac h shall be trimmed or Aerial views can be used as a basis for selecting po-

---------------------- Page: 8 ----------------------
0 IS0 IS0 1100=1:1996(E)
nonsensitive station can result in large errors in the
tential sites, which can then be evaluated and studied

more precisely by ground reconnaissance. Ground re- discharges indicated by the stage-discharge relation.

connaissance will include a detailed visual examin-
A comparison shall be made between the change in
ation of the site and enquiry among competent
discharge and the corresponding minimum change in
services to determine whether hydraulic work pro-
stage to ensure that the sensitivity of the station is
jects exist which could modify the stream bed regime.
sufficient for the purpose for which the measure-
This enquiry should include an investigation of past
ments are required.
flow history, low water, high water, overflow areas,
floods and bed instability.
5.3.2.4 Scale readability
5.3.2 Selection criteria
It is essential that the reference gauge is easily read-
able and accurate at all values of discharge, because
with their
A list of surveyed sites shall be establi shed
stage readings are the basis of discharge measure-
blishment
advantages and drawbacks as to the esta
ments.
of a gauging station. Selection shall then be made
according to the following criteria.
The water surface shall be calm to ensure that
readings shall correspond to the indication of the
stage sensor. The gauge and the sensor shall there-
5.3.2.1 Measurement range
fore be installed close to each other in a low-velocity
area.
The site selected shall be such that it is possible to
measure the entire range and all types of flow which
5.3.2.5 Accessibility
may be encountered or which are required to be
measured.
Ideally the station should be accessible, or made ac-
The entire range of measurement may be referred to
cessible, at all seasons regardless of the discharge
one reference gauge, or certain ranges of discharge
conditions.
may be referred to different gauges. Different meth-
ods of calibration may be employed for separate parts
5.3.2.6 Silting
of the range, the particular conditions relative to each

of the methods of calibration being specified in the When the stream carries a high sediment load, silting

relevant International Standard (see clause 2 and an- of the gauge may occur, especially in low-velocity

nex A). protected areas. Access to any stilling well should
therefore be provided to permit quick and easy
cleaning.
5.3.2.2 Stability
5.3.2.7 Flood protection
The operation of a gauging station is based on the
assumption of a relation between stage and dis-
Site inspection shall be carried out under low-water
charge.
and high-water conditions to study currents and

It is therefore desirable that this relation is stable at eddies. The sensor shall be placed out of reach of any

the selected site. This’ condition is met if the control
floating debris to avoid accidental damage, and the

section or reach is stable and not subject to variable recorder should be set at an elevation to avoid being

backwater. flooded under high-water conditions.
shall be

Sites where weed growth prevalent Public records shall be consulted, the vegetation shall

avoided, if possible. be observed and the population questioned to this
end.
There shall be no vortices, dead water or other
abnormalities in flow. Sites where difficult ice condi-
5.3.2.8 Discharge measurements
tions are prevalent shall be avoided, if possible.
During preliminary surveys, the possibility of using
one method of discharge measurement for the whole
5.3.2.3 Sensitivity
discharge range shall be considered. It is preferable,
The site shall be sensitive
...

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