ISO 1100-1:1981

Liquid flow measurement in open channels - Part 1: Establishment and operation of a gauging station

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

General Information

• Status: Withdrawn
• Publication Date: 30-Sep-1981
• Withdrawal Date: 30-Sep-1981
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113 - Hydrometry
• Drafting Committee: ISO/TC 113 - Hydrometry
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 08-Feb-1996
• Completion Date: 13-Dec-2025

Relations

• Revises: ISO 1100:1973 - Withdrawal of ISO 1100-1973
• Effective Date: 15-Apr-2008
• Revised: ISO 1100-1:1996 - Measurement of liquid flow in open channels - Part 1: Establishment and operation of a gauging station
• Effective Date: 15-Apr-2008

Standards Content (Sample)

International Standard @ 1100/1
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION*ME~YHAPOI1HAR OPrAHHJAUHR fl0 CTAHL\APTHJAUHH.ORGANISATlON INTERNATIONALE DE NORMALISATION
Liquid flow measurement in open channels -
\ L Part I : Establishment and operation of a gauging station
Mesure de débit des liquides dans les canaux découverts - Partie 1 : Établissement et exploitation d'une station de jaugeage
Second edition - 1981-10-15
L-
I
e UDC 532.g :532.543 :627.133 Ref. No. IS0 1100/1-1981 (E)
Descriptors : liquid flow, water flow, open channel flow, flow measurement, gauging station, boats, velocity measurement.
c
3 7
c
O
Price based on 21 pages
E?
Foreword
IS0 (the International Organization for Standardization) is a worldwide federation of
national standards institutes (IS0 member bodies). The work of developing Inter-
national Standards is carried out through IS0 technical committees. Every member
body interested in a subject for which a technical committee has been set up 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.
Draft International Standards adopted by the technical committees are circulated to
the member bodies for approval before their acceptance as International Standards by
the IS0 Council.
International Standard IS0 1100/1 was developed by Technical Committee
ISO/TC 113, Measurement of liquid flow in open channels, and was circulated to the
member bodies in November 1979.
It has been approved by the member bodies of the following countries :
Canada Norway Switzerland
France Philippines United Kingdom
Germany, F. R. Romania USA
India South Africa, Rep. of
Netherlands Spain
The member body of the following country expressed disapproval of the document on
technical grounds :
Australia
This second edition cancels and replaces the first edition (i.e. IS0 1100-1973).
0 International Organization for Standardization, 1981 O
Printed in Switzerland
INTERNATIONAL STANDARD IS0 1100/1-1981 (E)
Liquid flow measurement in open channels -
Part 1 : Establishment and operation of a gauging station
Scope and field of application IS0 748, Liquid flow measurement in open channels - Velo-
city area methods.
W
IS0 772, Liquid flow measurement in open channels -
1 .I This International Standard deals with the establishment
and operation of a gauging station on a lake, reservoir, river or Vocabulary and symbols.
artificial open channel for the measurement of stage or
discharge or both. It is generally applicable to the measurement IS0 1O00, SI units and recommendations for the use of their
methods described in the International Standards which are multiples and of certain other units.
noted in clause 2 and it covers only such additional re-
quirements as are necessitated by its wider scope. IS0 1070, Liquid flow meausrement in open channels - Slope-
area method.
IS0 1088, Liquid flow measurement in open channels -
1.2 The requirements for a stage measuring station are set
Velocity area methods - Collection of data for determination
out in clause 5. The requirements for a discharge measuring
of errors in measurement.
station are classified under two headings :
IS0 110012, Liquid flow measurement in open channels -
a) Individual measurements.
Part 2 : Determination of the stage-discharge relation. 1)
These include methods suitable for a single measurement of
IS0 143811. Water flow measurement in open channels using
discharge or a limited number of measurements often used
weirs and Venturi flumes - Part 1 : Thin-plate weirs.
to calibrate a station.
IS0 2425, Measurement of flow in tidal channels.
b) Regular measurements.
L.
IS0 2537, Liquid flow measurement in open channels - Cup-
These include methods suitable for relatively frequent
type and propeller-type current meters.
measurements often made over many years.
IS0 3454, Liquid flow measurement in open channels - Soun-
ding and suspension equipment.
2 References IS0 3455, Liquid flow measurement in open channels -
Calibration of rotating element current meters in straight open
IS0 31, Quantities, units and symbols. tanks.
IS0 5551 1, Liquid flow measurement in open channels - Dilu-
IS0 3716, Liquid flow measurement in open channels - Func-
tion methods for measurement of steady flow - Part 1 : Cons- tional requirements and characteristics of suspended load
tant rate injection method.
samplers.
IS0 55512, Liquid flow measurement in open channels - Dilu- IS0 3846, Liquid flow measurement in open channels by weirs
tion methods for measurement of steady flow - Part 2 : ln- and flumes - Free overfall weirs of finite crest width Irec-
tegration (sudden injection) method.
tangular broad-crested weirs).
1) At present at the stage of draft.
IS0 1100/1-1981 (El
IS0 3847, Liquid flow measurement in open channels by weirs 5 Water level (stage) gauging station
- End-depth method for estimation of flow in rec-
and flumes
tangular channels with a free overfall.
5.1 Principle
IS0 4359, Liquid flow measurement in open channels using
5.f.1 The stage of a stream or lake is the height of the water
flumes. 1
surface above an established datum plane. Water levels of
rivers, lakes and reservoirs are used directly in hydrological
IS0 4360, Liquid flow measurement in open channels by weirs
forecasting, in delineating flood hazard areas, and in the design
and flumes - Triangular profile weirs.
of structures in, or near, water bodies. When correlated with
discharge of streams, or with the volume of storage in reser-
IS0 4363, Liquid flow measurement in open channels -
voirs and lakes, water levels are used as the basis for compu-
Methods of measurement of suspended sediment.
tation of records of discharge, or changes in storage. Records
of water level are obtained by systematic observations on a
IS0 4364, Liquid flow measurement in open channels - Bed
reference gauge, or from a water level recorder.
material sampling.
IS0 4366, Echo sounders for water depth measurements.
5.2 Preliminary survey
IS0 4369, Measurement of liquid flow in open channels -
Mo ving-boat method. 5.2.1 A preliminary survey should be made of the physical and
-
hydraulic features of the proposed site to ensure that it con-
forms to the requirements necessary for the measurement of
IS0 4373, Measurement of liquid flow in open channels -
Water level measuring devices. water level as specified in IS0 4373.
IS0 4375, Measurement of liquid flow in open channels -
5.3 Selection of site
Cableway system for stream gauging.
IS0 4377, Measurement of liquid flow in open channels -
5.3.1 The site selected for observation of stage should be
Flat- V weirs. 1)
governed by the purpose for which the records are collected,
the accessibility of the site, and the availability of an observer if
IS0 5168, Measurement of fluid flow - Estimation of uncer-
the gauge is non-recording. Gauges on lakes and reservoirs are
tainty of a flow-rate measurement.
normally located near the outlet, but upstream from the zone
where an increase in velocity causes a drawdown in water level.
ISOITR 7178, Measurement of liquid flow in open channels -
Gauges on large bodies of water should also be located so as to
Investigation of the total error in measurement of flow by
reduce the fetch of strong winds which may cause damage or
velocity area methods.')
misleading data. Hydraulic conditions are an important factor in
site selection on channels, particularly where water levels are
WMO (World Meteorological Organisationl - No. 168, Guide
used to compute discharge records.
to Hydrometeorological Practices.
5.3.2 Gauging stations should conform to the requirements of
WMO (World Meteorological Organisation) - No. 49,
the relevant sections of IS0 4373 which includes recommenda- w
Technical Regulations.
tions for the design of the reference gauge, recorder, and still-
ing well.
3 Definitions
5.4 Design and construction
For the purpose of this International Standard, the definitions
and symbols given in IS0 772 apply.
A water level gauging station shall consist essentially of
5.4.1
a reference gauge or gauges. Where a continuous record of
stage is not required see 7.1.7.2. It is usual however for a con-
4 Units of measurement
tinuous record of stage to be required and for this purpose a
The units of measurement used in this International Standard water level recorder is installed in addition to the reference
are SI Units in accordance with IS0 31 and IS0 1OOO. gauge (see 7.1.7.3).
~
1) At present at the stage of draft.
IS0 1100/1-1981 (E)
5.4.2 Reference gauge (see IS0 4373) e) The markings of the subdivisions shall be accurate to
f 0,5 mm, and the cumulative error in length shall not exceed
0,l % or 0.5 mm, whichever is greater.
Reference gauges may be either the direct or the indirect type,
with measuring instruments of the fixed or movable type, such
as vertical and inclined gauges, needle gauges, float gauges,
5.4.2.1.4 Installation and use
and wire weight gauges classified as direct reading in-
struments. The signifiant feature of this group of water-level in-
dicators is that the reading may be made directly in units of 5.4.2.1.4.1 General
length, without any intervening influences. Indirect water-level
indication devices include those gauging systems which con-
The gauge should preferably be placed near the bank so that a
vert a pressure or electrical signal to an output which is propor-
direct reading of water level may be made. If this is imprac-
tional to the water level. Of the indirect devices available, those
ticable because of excessive turbulence, wind effect, or inac-
in most common usage are the pressure type, such as the
cessibility, the measurement may be made in a suitable perma-
servo-manometer and the servo-beam balance.
nent stilling bay or stilling well in which the wave actions are
damped and the level of the water surface closely follows the
fluctuations of the water level in the channel. To ensure this, in-
5.4.2.1 Vertical and inclined gauges
takes to stilling wells should be properly designed and located.
Such gauges comprise a scale marked on or attached to a
The gauge should be located as closely as possible to the
suitable surface.
measuring section without affecting the flow conditions at this
L
point. It should not be placed where the water is disturbed by
5.4.2.1.1 Functional requirements
turbulence, or there is danger of damage by drift. Bridge
abutments or piers are generally unsuitable locations.
These gauges should meet the following functional re-
Wherever the gauge is situated, it must be readily and conve-
quirements :
niently accessible so that an observer may take readings as
nearly as possible at eyelevel. Where necessary, the con-
a) they should be accurate and clearly marked;
struction of a flight of steps to give convenient access is rec-
commended. The gauge board or plate should be securely fixed
they should be durable and easy to maintain;
b)
to the backing but provision must be made for removing the
gauge board or plate for maintenance or adjustment. The
c) they should be simple to install and use.
edges of the gauge board should be protected.
5.4.2.1.2 Material
5.4.2.1.4.2 Vertical gauges
The material of which a gauge is constructed should be
A suitable backing for a vertical gauge is provided by the sur-
durable, particularly in alternating wet and dry conditions and
face of a wall having a vertical or nearly vertical face parallel to
also in respect to the resistance to wear or fading of the mark-
the direction of flow. The gauge board or backing plate should
ings. The material should have a low coefficient of expansion
be attached to the surface so as to present a truly vertical face
with respect to temperature or wetting effects.
to receive the graduations. The gauge board and backing plate
should be securely fastened to the wall. Gauges may be fixed to
~ 5.4.2.1.3 Graduation
piles, either driven firmly into the bed or banks, or set in con-
crete so as to be free from sinking, tilting, or being washed
a) The graduations of a vertical gauge must be clearly and
away. In either case the anchorage should extend below the
permanently marked directly on a smooth surface or on a gauge
ground surface to a level free of disturbance by frost. In order
board. The numerals shall be distinct and placed so that the
to avoid velocity effects which may hinder accurate reading, a
lower edge of the numeral is close to the graduation to which it
pile may be shaped to be streamlined upstream and
refers.
downstream, or the gauge may be situated in a bay where it will
not be exposed to the force of the current. Where the range of
b) The graduations of an inclined gauge may be directly
water levels exceeds the capacity of a single vertical gauge, ad-
marked on a smooth surface or on a gauge board as described
ditional sections may be installed on the line of the cross sec-
in a) above, or may be carried on manufactured gauge plates
tion normal to the direction of flow. The scales on such a series
designed to be set for particular slopes. Except where use is
of stepped gauges should have adequate overlap. These
made of manufactured gauge plates designed to be set to a
stepped gauges should be installed at such intervals as to en-
specified slope, an inclined gauge should be calibrated in-situ
sure the measurement of water level at all stages of flow.
by precise levelling from the station bench mark.
5.4.2.1.4.3 Inclined gauges
Gauge plates should be manufactured in suitable lengths
c)
with the face of the scale not less than 50 mm in width.
An inclined gauge should be installed in such a manner as to
The marking on the gauge should be made to read in closely follow the contour of the river bank. The profile of the
d)
multiples of millimetres. bank may be such that a gauge of a single slope may be in-
stalled; frequently however, it may be necessary to construct
The smallest graduation shall depend on the accuracy required, the gauge in several sections, each with a different slope. The
but may normally be 10 mm. general installation requirements given in 5.4.2.1 apply.
IS0 1100/1-1981 (E)
5.4.2.2 Needle gauges accessibility, a suitable permanent stilling bay or stilling well
should be installed.
5.4.2.2.1 General
b) The location of the hook or point gauge should be as close
as possible to the measuring section and should be conve-
A needle water-level gauge consists of a point and some means
niently accessible to the observer.
of determining its exact vertical position relative to a datum. It
is mainly used for checking or calibrating other gauges or
cl The gauging point shall not be installed in a location where
recorders. The two types of needle gauges are :
the water surface is disturbed by turbulence, wind effect, or af-
flux. The vicinity of bridge abutments or piers is generally un-
a) the point gauge whose tip approaches the free surface
suitable.
from above, and
d) Where more than one datum plate or bracket is provided at
b) the hook gauge which is hook-shaped, and whose tip is
different levels, it is preferable that all should lie on the line of a
immersed and approaches the free surface from below.
single cross section normal to the direction of flow in the chan-
nel; if this is not practicable and it is necessary to stagger the
The vertical position may be determined by a graduated scale, a
points, all should lie within a distance of 1 m on either side of
tape with some vernier arrangement, or a digital indicator. The
the cross-section line.
scale is movable and graduated to read downward from top to
bottom. Application of needle gauges consists of positioning
Datum plates and brackets should be mounted on a secure
e)
the needle of the gauge near the water surface and detecting
-
foundation which extends below the frost line.
the moment the tip just touches the free surface, apparently
trying to pierce its skin. Setting a point exactly at the water sur-
fi The elevation of the datum plates, with reference to which
face may be facilitated by electrical means.
the level of the free surface is determined, should be estab-
lished with great care. This elevation should be checked from
The advantage of water-level needle gauges is their high
the station bench mark (see 5.5.4) at least annually. The uncer-
measuring accuracy, whereas their disadvantage is their small
tainty on the transfer of level from the station bench mark to
measuring range, usually about 1 m. However, this disadvan-
each datum plate shall not exceed ? 1,0 mm.
tage can be overcome by installing a series of datum plates at
different levels.
5.4.2.3 Float gauge
5.4.2.2.2 Functional requirements
5.4.2.3.1 General
a) A needle gauge installation should permit measurement of
stage to be made at all levels from below the lowest to above
The float gauge is used chiefly as an inside reference gauge in
the highest level anticipated.
water stage measurements. The typical float gauge consists of
a float operating in a stilling well, a graduated steel tape, a
b) There must be good illumination of the place where the tip
counterweight, a pulley, and a pointer. The float pulley is
meets the free liquid surface.
grooved on the circumference to accommodate the tape, and
mounted on a support. The tape is fastened to the upper side of
c) The hook or point should be made of metal sufficiently
the float and runs slip-free over the pulley in the gauge shed
strong to resist deformation in transport and under field condi-
W
above the well. The tape is kept tight by a counterweight at the
tions of use. The tip should be tapered to a point having an in-
free end or by a spring. In this way stage fluctuations are
cluded angle of approximately 60° and the point shall be round-
sensed by the float which positions the tape with respect to the
ed to a radius of approximately 0,25 mm.
pointer.
5.4.2.2.3 Material
5.4.2.3.2 Functional requirements
A hook or point gauge and auxiliary parts should be made
a) A float gauge installation should permit measurement of
throughout with durable corrosion-resistant materials.
stage to be made at all levels from below the lowest to above
the highest level anticipated.
5.4.2.2.4 Graduation
b) Float and counterweight dimensions and the quality of the
The graduations of a hook or point gauge shall be in millimetres
elements of the mechanical device for remote indication should
and shall be clearly and accurately marked. A vernier or
be selected so that there is a sufficiently high indication ac-
micrometer head may be provided which allows reading to
curacy and working reliability.
0,l mm, however, such a reading accuracy is normally only re-
quired for laboratory measurements.
c) The float should be made of durable corrosion-resistant
and antifouling material. It should be leak-proof and function in
5.4.2.2.5 Installation and use a truly vertical direction. Its density should not change
significantly.
a) A hook or point gauge may be mounted over an open
water surface at the edge of a channel if conditions permit. If d) The float should float properly and the tape or wire should
this is not practicable because of turbulence, wind effect, or in- be free of twists or kinks.
IS0 1100/1-1981 (E)
These gauges are locally made to different designs. Basically
5.4.2.3.3 Graduation
they may be a tube of about 50 mm internal diameter down the
The graduations of the float gauge shall be in millimetres and centre of which runs a rod. The tube is perforated to permit ris-
ing water to enter, the perforations being located to prevent
shall be clearly and accurately marked.
drawdown or velocity head from affecting the static water
level. The top of the tube must be closed to prevent the entry of
5.4.2.4 Wire-weight gauge rain, but it should have an air vent to permit water to rise up the
tube without significant delay. Powdered cork in the bottom of
the tube floats on the surface of the floodwater being deposited
5.4.2.4.1 General
on the centre rod as the water recedes. Alternatively the centre
rod is coated with a paint whose colour is permanently affected
A typical wire-weight gauge consists of a drum wound with a
by water.
single layer of cable, a weight attached to the end of the cable,
a graduated disc, and a counter, all housed within a cast-
5.4.2.6 Pressure gauges
aluminium box. The disc is graduated and is permanently con-
nected to the counter and the shaft of the drum. The cable is
Pressure gauges are frequently used at sites where it would be
guided to its position on the drum by a threading sheave. The
too expensive to install stilling wells. They are also used on
reel is equipped with a pawl and ratchet for holding the weight
sand-channel streams because the intake line can be extended
at any desired elevation. The gauge is set so that when the bot-
to follow a stream channel that shifts its location, and if the
tom of the weight is at the water surface, the gauge height is
-\.-
gas-purge technique is used, the gas flow tends to keep the
indicated by the combined readings of the counter and the
orifice from becoming plugged with sand.
graduated disc.
5.4.2.6.1 General
5.4.2.4.2 Functional requirements
One widely-used method of measuring water level is to
a) A wire-weight gauge installation should permit measure-
measure the. height of a column of water with respect to some
ment of stage to be made at all levels from below the lowest to
datum plane. This can be accomplished indirectly by sensing
above the highest level anticipated.
the water pressure at a fixed point below the water surface, and
then utilizing the hydrostatic principle that the pressure of a li-
The tape or wire should be free of twists or kinks.
b)
quid is proportional to the depth.
The weight should be made of durable corrosion-resistant
c)
5.4.2.6.2 Functional requirements
material.
The range of the instrument must be adequate to accom-
d) The installation should be provided with a horizontal
modate any anticipated ranges in water level and the response
checking bar for checking the calibration datum of the
must be sufficiently rapid to follow any expected rate of change
graduated disc or counter.
in water level.
The method of transmitting pressure from the water column to
5.4.2.4.3 Installation and use
the sensor may be direct or indirect. When the sensor is located
below the point in the water column at which the pressure is to
a) The wire-weight gauge is used as an outside reference
be measured, the water pressure may be transmitted directly to
gauge where other outside gauges are difficult to maintain. The
the sensor. However, if the sensor is located above the water
wire-weight gauge is normally mounted where there is a bridge,
column, the direct method is usually not satisfactory because
deck, or other structure over the water.
gases entrained in the water can create air locks in the line.
Also, if the water is highly corrosive, it is undesirable to bring it
b) The gauging point should not be installed in a location
into direct contact with the sensor.
where the water surface is disturbed by turbulence, wind ef-
fects, or afflux. The vicinity of bridge abutments or piers is
generally unsuitable.
5.4.2.6.3 Gas-purge (bubbler) technique
c) The check bar elevation of the wire-weight gauge should
The most successful and widely used method of transmitting
be read frequently to ensure reliability of correct base elevation.
pressure is the gas-purge technique. This technique may be
used regardless of the elevation of the pressure device with
respect to the water column; and because the water does not
5.4.2.5 Crest stage gauge
come into direct contact with the pressure sensor, it is suitable
for use in highly corrosive waters.
The crest stage gauge is used to obtain a record of the peak
level reached during a flood when other methods of recording In the gas-purge technique, a small discharge of non-corrosive
levels cannot be used. Peak discharges may be calculated from gas or compressed air is allowed to bleed into a tube, the free
the water levels at two gauges installed some distance apart in end of which has been lowered into the water and fixed at an
a stretch of channel, provided that the time lag between
elevation below the water column to be measured. For exam-
measurements is negligible. These gauges do not meet the ac- ple, dry nitrogen is frequently used. The sensor, which is
curacy requirements of 5.6.
located at the opposite end, detects the pressure of the gas re-

IS0 1100/1-1981 (E)
quired to displace the liquid in the tube, this pressure is directly 5.4.2.6.5 Servo-manometer and servo-beam balance
proportional to the head of liquid above the orifice.
Both the servo-manometer and the servo-beam balance are
pressure sensors that convert the pressure detected to a rota-
When using the gas-purge technique there are certain installa-
tion and operation requirements that should be observed. The tional shaft position proportional to the height of the column of
principal ones are listed below. water. The shaft rotation is used for driving a recorder and a
water-level indicator. As the name implies, the servo-
manometer is essentially a manometer with a servo-system
a) An adequate supply of gas or compressed air must be
provided. A continuous flow of gas to the tube is necessary detecting and following the liquid differential within the
manometer. The servo-beam balance is a beam balance with a
to prevent the liquid from entering it when the water level is
rising. A particular rate of gas supplied will cause the pressure balance on one side of the beam and a weight on the
other. Here the servo-system positions the weight so that the
pressure in the system to rise at the same rate as the head. If
gas is supplied at a lower rate, liquid will enter the tube; and beam is in balance and detects this position.
conversely, a higher rate will provide a continuous
discharge of gas from the opening in the bottom of the
5.4.2.6.6 Water density compensation
tube. The gas is usually supplied from a cylinder or by an air
compressor. In either case, the supply must have a delivery
Since the density of the water in which the sensor operates will
pressure in excess of the range to be measured.
vary with temperature and also with chemical and silt content,
either automatic or manual means of compensating for these
b) A pressure reducing valve must be provided so that a
changes should be provided. v
pressure safely in excess of that of the maximum range can
be set. A flow control valve and some form of visual flow-
5.4.2.6.7 Changes in gas weight
rate indicator is necessary, so that the discharge of gas sup-
plied to the system can be properly adjusted. The pressure
If one of the gas techniques is used to transmit pressure, provi-
should be set to prevent water from entering the tube, even
sions should be made for compensating for changes in the den-
under the most rapid rates of change expected.
sity of the gas, as all gases vary in volume with temperature and
pressure changes.
c) Incorrect readings due to the friction of the gas moving
through the tube should be minimised. Long lengths of tube
or very small diameter tubing aggravate the friction prob-
5.4.2.6.8 Miscellaneous pressure sensors
lem. This problem is frequently solved by running two tubes
to a junction very near to the orifice with one tube serving as
There are numerous pressure sensors available commercially
a gas-supply line and the other as a pressure-detection line.
operating on numerous principles. Most of these have an elec-
Under this arrangement the movement of gas in the
trical output which is proportional to the pressure sensed. They
pressure detection line is kept to a minimum, thereby reduc-
are occasionally used for the detection of water level. The pro-
ing the friction to a minimum.
per selection is dependent upon the particular application in-
volved.
a continuous down
d) The tubing should be installed with
slope to the orifice.
Their application is generally restricted to limited ranges
because the accuracy requirements set out in 5.6 are difficult to
meet over extended ranges.
5.4.2.6.4 Pressure bulb system
5.4.3 Stilling well
Where there is no gas supply available, a pressure bulb system
is sometimes used to transmit pressure to the sensor. This
device, frequently referred to as an elastic pressure bulb, is
5.4.3.1 Functional requirements of stilling well
usually made of casting in the form of a short hollow cylinder
with one open end. The open end is sealed with a slack, highly
5.4.3.1.1 General
flexible diaphragm, and the cylinder is connected by means of
tubing to the pressure sensor. The whole unit forms a closed
The function of the stilling well is
gas system with pressure initially equal to atmospheric
pressure. The cylinder is lowered into the water and fixed at an
a) to accommodate the instrument and protect the float
elevation below the water column to be measured. The slack
system;
diaphragm permits water pressure to compress the gas in the
cylinder until the pressure within the system is proportional to
to provide within the well an accurate representation of
b)
the height of the water column above. One of the major disad-
the water level in the channel;
vantages of this device is that ultimately an excessive amount
of gas will escape from the system with a resultant stretching of
ci to damp cut oscillations of the water surface.
the diaphragm. When this occurs the pressure within the
system will no longer be equal to the pressure head. This disad-
5.4.3.1.2 Specific
vantage can be overcome by periodic renewal of the gas in the
system by opening and re-sealing under atmospheric pressure
a) The well may be circular, oval, square or rectangular in plan
and checking the calibration. It is difficult to maintain the ac-
and may be made of any suitable material.
curacy stipulated under 5.6 with this device.
IS0 1100/1-1981 (E)
b) The well may be placed in the bank of a channel or directly b) The dimensions of the intakes shall be large enough to
in the stream when attached to a bridge pier or abutment. It allow the water level in the well to follow the rise and fall of river
should not, however, be located directly in the channel where stage without appreciable delay.
flow conditions would lead to separation and stagnation ef-
The dimensions of the intakes shall be small enough to
fects. When placed in the bank, the well shall be connected to c)
the channel by intake pipe(s1. When placed directly in the chan- damp oscillations caused by wave action or surges.
nel, the intakes may take the form of holes or slots cut in the
d) Two or more intakes may be installed, one vertically above
well itself.
the other, to ensure operation of the system if the lowest pipe
becomes blocked.
c) The well shall not interfere with the flow pattern in the ap-
proach channel and if set in relation to a control, it shall be
e) For a pipe well set into a bank the lowest intake shall be at
located far enough upstream to be outside the area affected by
the control. least 150 mm below the lowest anticipated stage and shall enter
the pipe well at least 300 mm above the well bottom.
d) The well shall be firmly founded when placed in the bank
and firmly anchored when standing in the stream so that it shall f) Intake pipes shall be laid at a constant gradient on a suitable
remain stable at all times. foundation which will not subside.
e) The well and all construction joints of well and intake pipes g) The intake shall be so oriented in the stream that it will
shall be watertight so that water can enter or leave only by the sense the true water level. When velocities in the stream at the
intake itself. point of measurement are sufficiently large, that the dynamic
a
pressure is of sensible magnitude, the intake shall incorporate
static pressure sensing device (wall piezometer, piezometer in a
f) The well shall be vertical within acceptable limits and have
sufficient height and depth to allow the float to travel freely the plate, surface parallel to flow, static tubes etc.).
full range of water levels.
h) It is desirable that intake pipes more than 20 m in length
shall be provided with an intermediate manhole fitted with in-
g) The dimensions of the well shall be such as to allow
unrestricted operation of all equipment installed in it. Clearance ternal baffles to act as a silt trap and provide access for clean-
ing.
between walls and float shall be at least 75 mm and where two
or more floats are used within the well, clearance between
them shall be at least 150 mm. In silt laden rivers, it is an advan- j) Means of cleaning the intakes shall be provided, either by a
tage to have the well large enough to be entered and cleaned. flushing system where water under head can be applied to the
stilling well end of the intake, by pumping water through the in-
take or by hand cleaning with collapsible draining rods.
h) When placed in the bank of the stream the stilling well shall
have a sealed bottom to prevent seepage into or leakage out of
the chamber. k) Where velocity past the river end of the intake is high,
drawdown of the water level in the well may occur. This can be
reduced by attaching a capped and perforated static tube to the
j) In wells with sealed bottoms the bottom of the well shall be
at least 300 mm below the invert of the lowest intake to provide river end of the intake.
space for sediment storage and to avoid the danger of the float
grounding at times of low flow.
\-
5.4.3.3 Protection under ice conditions
5.4.3.2 Functional requirements of intakes For precautions to be taken under ice conditions see clause 9.
5.4.3.2.1 General
5.4.4 Water level recorders (see IS0 43733
The function of the intakes to the stilling well is :
5.4.4.1 Mechanical recorders
a) to allow water to enter or leave the stilling well so that
the water in the well is maintained at the same elevation as Mechanical recorders can be classified as either analog or
that in the stream under all conditions of flow; .digital, depending upon the mode used in recording the rota-
tional position of the input shaft. The analog recorder produces
b) to permit some form of control to limit lag and a graphic record of the rise and fall of parameter values with
oscillating effects within the well. respect to time, while the digital recorder punches coded
parameter values on paper tape at preselected time intervals.
Anolog recorders can further be classified into two types, those
5.4.3.2.2 Specific
which record continuously or at intervals for a fixed period of
time (daily, weekly, monthly, etc).
a) Intakes may take the form of one or more pipes connecting
the well to the river when the well is set back into the bank or a
series of holes or slots cut into the well itself when it is set 5.4.4.2 Pressure-actuated recorders
directly into the river. In rivers with a permanent high silt con-
tent, a well set in the stream may have a hopper-shaped bottom Pressure-actuated recorders are simple in design and relatively
to serve as an intake and also as a means of self-cleansing.
rugged in construction. They are, however, susceptible to er-
IS0 1100/1-1981 (E)
rors and, consequently, are not frequently used in recording in a concrete block or similar mounting that extends below the
water level. ground surface to a level free from disturbance, such as frost. It
should be correlated with a national survey datum by accurate
They usually consist of a pressure-sensing element, a stylus levelling. To facilitate accurate levelling between the station
linked to the pressure element by a lever arrangement, and a bench mark and the gauge zero, the bench mark should be
chart driven by a clock. located in a position such that the transfer of the level may be
carried out by reciprocal levelling or with equally balanced
foresights and backsights on the setting of the level. Where it is
5.4.4.3 Electronic recorders
not feasible to correlate the bench mark with the national
survey datum more than one (preferably three) station bench
The electronic recorder consists of a case, an operating
marks shall be established in significantly different locations.
mechanism, a stylus, and a chart transport mechanism. The
function of the operating mechanism is to position the stylus at
the proper location on the chart in response to a signal. 5.6 Accuracy
Although there is an extremely wide assortment of electronic
recorders available, there are basically only two types, those For the measurement of stage, in certain installations an uncer-
with a direct-acting operating mechanism and those with a tainty of f 10 mm may be satisfactory; in others an uncertain-
servo-operated mechanism. The low torque and relatively ty of f 3 mm or better may be required; however, in no case
limited output motion of a direct acting mechanism will position should be uncertainty be worse than f 10 mm or f 0,l % of
only a light, low friction stylus over a limited range, whereas the
the range, whichever is greater.
sturdy servo-operated mechanism has ample power and a
much wider motion capability.
The operating mechanism is the heart of the electronic 6 Station for the measurement of
recorder. Its quality and the techniques employed in its design discharge : individual measurements
determine the characteristics of the entire recorder.
The methods which follow are most suitable for a single
measurement, a limited number of measurements or infrequent
5.4.4.4 Regardless of the type of recorder used it should
measurements of discharge.
satisfy the requirements enumerated in IS0 4373.
6.1 Velocity area method
5.5 Gauge datum
6.1.1
Principle of method of measurement
5.5.1 The datum of the gauge may be a recognised datum,
such as mean sea level, or an arbitrary datum plane selected for
The principle of the method is to measure the velocity and area
the convenience of using gauge readings of relatively low
of cross section of flow in an open channel, the product of
numbers. It is generally desirable to avoid negative values for
which is the discharge. The recommendations which follow
these readings, therefore, the datum selected for operating
restrict the application of the method to the case in which
purposes should be below the elevation of zero flow on the
velocity is measured using current meters or floats.
control. At sites that are subject to severe scour care must be
taken to select a datum that is sufficiently low.
6.1.2 Preliminary survey U
5.5.2 If an arbitrary datum plane is used, it should be referred
A preliminary survey should be made to ensure that the
to a bench mark of known elevation by accurate levelling so
physical and hydraulic features of the proposed site conform to
that the arbitrary datum may be recovered if the gauge and
the requirements of IS0 748. Several such surveys under dif-
reference marks are destroyed. A permanent datum must be
ferent flow conditions may be required to ensure that the site is
maintained so that only one datum for the stage record is used
not subject to, for example, standing waves at high flow, weed
for the life of the station .
conditions, adverse ice conditions, and so forth.
5.5.3 Gauge zero
6.1.3 Selection of site
The zero of the gauge should be correlated with a national
6.1.3.1 The site selected should be such that it is possible to
datum through a station bench mark. The relation between the
gauge zero and the station bench mark shall be checked at least measure the whole range and all types of flow which may be
it is required to measure.
annually. The relation between the gauge zero and other gauge encountered or which
sections shall be checked from time to time. As far as possible
the gauge zero shall be kept the same. The uncertainty in the
6.1.3.2 Particular attention should be paid to the following
transfer of the level from the station bench mark to the gauge
features :
shall not exceed I 1,0 mm.
a) sites where weed growth is prevalent should be
5.5.4 Station bench mark avoided, if possible;
The station bench mark shall be set in a position offering max- b) there should be no vortices, dead water or other abnor-
imum security against disturbance. It should be securely fixed malities of flow;
IS0 1100/1-1981 (E)
sites where poor ice conditions are prevalent should be often be avoided by constructing flood banks to confine the
c)
flow in a defined floodway.
avoided, if possible;
access to the site should be feasible under most condi- b) Minor irregularities in the bank or bed causing local ed-
d)
dies may be eliminated by trimming the bank to a regular
tions.
line and a stable slope and by removing from the bed any
large stones or boulders.
6.1.4 Survey for a permanent gauging station
ci Unstable banks should be protected wherever possible.
6.1.4.1 After a preliminary survey, a topographical survey Such protection should extend upstream and downstream
should be made when selecting a permanent site for a suitable of a measuring section for a distance equal to at least one
quarter of the bankfull width of the channel in each direc-
measuring section. This should include a plan of the site in-
dicating the width of the water surface at a stated stage, the tion. In the case of float gauging the whole of the measuring
edges of the natural banks of the channelk), the line of any reach should be protected.
definite discontinuity of the slope of these banks and the toe
and crest of any artificial flood bank.
6.1.5.5 Where in the normal measuring section there is insuf-
ficient depth to comply with the requirements of IS0 748, or
6.1.4.2 The detailed survey of the reach should be extended
where there is excessively low velocity at low stages, these
through the floodway to an elevation well above the highest an-
discharges may often be measured in the same reach of the
ticipated flood level. The spacing of levels or soundings should
channel at another section which is more suitable under these
be close enough to reveal any abrupt change in the contour of
conditions but which may not be satisfactory for higher flows.
the channel.
6.1.5.6 Trees obstructing the clear view of the measuring sec-
6.1.4.3 The bed of the reach should be examined carefully for
tion or measuring reach should be trimmed or removed. The
the presence of rocks or boulders, particularly in the vicinity of
field of view of a measuring section should extend sufficiently
the measuring section.
upstrean to enable floating debris, which might damage any
measuring instrument, to be seen in sufficient time to permit
6.1.4.4 Where velocities are to be measured by current meter,
the removal of the instrument from the stream.
exploratory measurements of velocities should be made in the
proposed measuring section and in the cross-section im-
mediately upstream and downstream. When possible, the
6.1.5.7 Where not already existing, a suitable access to the
method of velocity distribution described in IS0 748 should be
site should be constructed, where possible, to provide safe
used for these measurements to determine the feasibility of us-
passage at all stages of flow and in all weathers for personnel
ing reduced point methods.
and for any vehicles used for the conveyance of instruments
and equipment.
6.1.4.5 When floats are to be used for velocity measurements
trial runs of floats should be closely spread across the width of
6.1.5.8 All key points at the site should be permanently mark-
the channel.
ed on the ground by markers sunk to such a depth below the
surface as will secure them against movement. Cross-section
6.1.5 Design and construction
markers should be set on the line of the cross-section to
facilitate
...