ISO/TR 11332:1998

Hydrometric determinations - Unstable channels and ephemeral streams

Déterminations hydrométriques — Canaux non stables et cours d'eau éphémères

General Information

• Status: Withdrawn
• Publication Date: 24-Jun-1998
• Withdrawal Date: 24-Jun-1998
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 1 - Velocity area methods
• Drafting Committee: ISO/TC 113/SC 1 - Velocity area methods
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 10-Dec-2013
• Completion Date: 13-Dec-2025

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 11332
First edition
1998-07-01
Hydrometric determinations — Unstable
channels and ephemeral streams
Déterminations hydrométriques — Canaux non stables et cours d'eau
éphémères
A
Reference number
Contents Page
1  Scope . 1
2  Normative references . 1
3  Definitions . 2
4  Units of measurement . 3
5  Location of water level (stage) gauge . 4
6  Stage measurements . 5
7  Discharge measurements . 10
8  Controls . 20
9  Stage-discharge relation . 26
10  Site information . 31
11  Discharge records . 35
12  Uncertainties . 38
©  ISO 1998
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 • CH-1211 Genève 20 • Switzerland
Internet [e-mail]
Printed in Switzerland
ii
©
ISO ISO/TR 11332:1998(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.
The main task of technical committees is to prepare International Standards, but in exceptional circumstances a
technical committee may propose the publication of a Technical Report of one of the following types:
— type 1, when the required support cannot be obtained for the publication of an International Standard, despite
repeated efforts;
— type 2, when the subject is still under technical development or where for any other reason there is the future
but not immediate possibility of an agreement on an International Standard;
— type 3, when a technical committee has collected data of a different kind from that which is normally published
as an International Standard ("state of the art", for example).
Technical Reports of types 1 and 2 are subject to review within three years of publication, to decide whether they
can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be
reviewed until the data they provide are considered to be no longer valid or useful.
ISO/TR 11332, which is a Technical Report of type 2, was prepared by Technical Committee ISO/TC 113,
, Subcommittee SC 1, .
Hydrometric determinations Velocity area methods
iii
©
Introduction
This Technical Report presents methods that are particularly applicable to the gauging of streamflow in unstable
channels and ephemeral streams. In this report, unstable channel refers to channels whose boundary condition of
bed and banks frequently or continuously move so as to result in a progressively changing stage-discharge relation.
This does not include instabilities resulting from aquatic growth. Reference is often made to sand-channel streams
or to alluvial streams in this report. Many, but not all, unstable channels are of these types.
This Technical Report is not a substitute for other manuals of general procedures for gauging streams. Rather, it is
a source of information, not generally included in stream-gauging manuals, that specifically addresses unstable
channels and ephemeral streams.
The gauging of streamflow in unstable channels is considered, to some degree, an art and the techniques
presented herein have been used successfully at specific stream-gauging sites. The good judgement of the
technician is important when selecting techniques and procedures for gauging streams, because of the highly
variable hydraulic characteristics along unstable channels and ephemeral streams.
Many channels, particularly in materials of small particle size, continually change configuration in response to flow.
Because of the frequent and significant changing of the control, these channels are considered unstable (see 1.17
of ISO 772). The control changes are the result of scour and fill, changes in the configuration of the channel bed
due to ripples, dunes, standing waves, antidunes and plane-bed formation, and channel braiding. The configuration
of unstable channels can change appreciably in a short period of time, changes of bedform can occur in a few
seconds. Changes in the control resulting from bed forms can be cyclic and vary with increasing and decreasing
discharges. During high flow, multiple bed configurations across a channel are common. Dune beds alternating with
plane beds along a channel have been observed moving down a channel.
The changing channel configuration and sediment deposition affects the sensing of stage at gauging stations. Stage
sensors become isolated from the stream when channels migrate or scour and when sediment is deposited
between the sensor and the flow. Sensors in contact with the flow wash away because of the difficulty of securing
sensors in these unstable channels. Stilling wells fill with sediment and bubble-gauge orifices become covered with
sediment. For convenience of access and construction considerations, sensors are often located on bridges and
rock banks at constrictions where hydraulic conditions are not suitable for obtaining reliable records of stage or
discharge.
Control changes resulting from causes such as the variation of energy gradient on rapidly rising and falling flood
waves and from aquatic growth are not included in this report. Conditions such as these are also common in more
stable streams.
A discussion of debris flows and translatory waves also are not included in detail in this report. Methods of
computing discharge, such as the slope-area method (see ISO 1070), and the use of stage-discharge ratings do not
directly apply to debris flows that are highly viscous, acting as a non-Newtonian fluid, nor to translatory waves.
Debris flows and translatory waves do occasionally occur in ephemeral streams with unstable channels but the
recording of stage and computation of discharge for those types of flows are beyond the scope of this Technical
Report.
iv
©
TECHNICAL REPORT  ISO ISO/TR 11332:1998(E)
Hydrometric determinations — Unstable channels and
ephemeral streams
1 Scope
This Technical Report deals with the measurement of stage and discharge and the establishment and operation of a
gauging station on an unstable channel and/or ephemeral stream. It covers additional requirements and general
considerations specifically related to sand-channel streams that are described in the measurement methods in the
International Standards noted in clause 2.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
Technical Report. At the time of publication, the editions indicated were valid. All standards are subject to revision,
and parties to agreements based on this Technical Report are encouraged to investigate the possibility of applying
the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently
valid International Standards.
ISO 748:1997, .
Measurement of liquid flow in open channels — Velocity-area methods
ISO 772:1996, Hydrometric determinations — Vocabulary and symbols.
ISO 9555-1:1994, Measurement of liquid flow in open channels — Dilution methods for measurement of steady flow —
Tracer dilution methods of steady flow — Part 1: General.
ISO 9555-2:1992, Measurement of liquid flow in open channels — Dilution methods for measurement of steady flow —
Tracer dilution methods of steady flow — Part 2: Radioactive tracers.
ISO 9555-3:1992, Measurement of liquid flow in open channels — Dilution methods for measurement of steady flow —
Tracer dilution methods of steady flow — Part 3: Chemical tracers.
ISO 9555-4:1992, Measurement of liquid flow in open channels — Dilution methods for measurement of steady flow —
Tracer dilution methods of steady flow — Part 4: Fluorescent tracers.
ISO 1070:1992, Liquid flow measurement in open channels — Slope area method.
ISO 1088:1985, Liquid flow measurement in open channels — Velocity area methods — Collection and processing
of data for determination of errors in measurement.
ISO 1100-1:1996, Measurement of liquid flow in open channels— Part 1: Establishment and operation of a gauging
station.
1)
ISO 1100-2:— , Liquid flow measurement in open channels — Part 2: Determination of the stage-discharge relation.

1)  To be published. (Revision of ISO 1100-2:1982)
©
ISO
ISO 1438-1:1980, Water flow measurement in open channels using weirs and venturi flumes — Part 1: Thin-plate
weirs.
ISO 2537:1988, Liquid flow measurement in open channels — Rotating element current-meters.
ISO 3454:1983, Liquid flow measurement in open channels — Direct depth sounding and suspension equipment.
ISO 3846:1989,
Liquid flow measurement in open channels by weirs and flumes — Free overfall weirs of finite crest
width (rectangular broad-crested weirs).
ISO 3847:1977, Liquid flow measurement in open channels by weirs and flumes — End-depth method for
estimation of flow in rectangular channels with a free overfall.
ISO 4359:1983, Liquid flow measurement in open channels — Rectangular, trapezoidal and U-shaped flumes.
ISO 4360:1984, Liquid flow measurement in open channels by weirs and flumes — Triangular profile weirs.
ISO 4366:1979, Echo sounders for water depth measurements.
ISO 4369:1979, Measurement of liquid flow in open channels — Moving-boat method.
ISO 4373:1995, Measurement of liquid flow in open channels — Water level measuring devices.
ISO 4374:1990, Liquid flow measurement in open channels — Round-nose horizontal crest weirs.
ISO 4375:1979, Measurement of liquid flow in open channels — Cableway system for stream gauging.
ISO 4377:1990, Liquid flow measurement in open channels — Flat-V weirs.
ISO/TR 7178:1983, Liquid flow measurement in open channels — Velocity-area methods — Investigation of the
total error.
3 Definitions
For the purposes of this Technical Report, the definitions given in ISO 772 and the following definitions apply.
3.1 General terms for liquid flow measurement in open channels
3.1.1
gauge height of zero flow
GZF
highest point on the thalweg downstream from the gauge on a natural or artificial channel, relative to a gauge datum
3.1.2
thalweg
line of greatest depth and thus the lowest water thread, along the stream channel
3.2 General terms for the computation of discharge in unstable channels and ephemeral
streams
3.2.1
antidune
bed form of curved symmetrically-shaped sand waves that may move upstream
NOTE Antidunes occur in trains that are in phase with and strongly interact with gravity water-surface waves.
©
ISO
3.2.2
discontinuous rating
rating that has a change in shape, commonly an abrupt change, that is the result of a change from lower to upper
flow regime in all or part of the length of river acting as the control
3.2.3
dune
large bed form having a triangular profile, a gentle upstream slope, and a steep downstream slope
NOTE Dunes form in tranquil flow, and thus are out of phase with any water-surface disturbance that they may produce.
They travel slowly downstream as sand is moved across their comparatively gentle, upstream slopes and deposited on their
steeper downstream slopes.
3.2.4
flow regime
state of flow in sand-channel streams characterized by bed configuration of ripples, dunes, plane bed, standing
waves and antidunes
NOTE Lower-regime flow is subcritical and upper-regime flow is super-critical (ISO 772:1996, 1.2).
3.2.5
GZF line
line on a shift diagram where the sum of the stage and the shift adjustment is equal to the gauge height of zero flow
(GZF) for the rating
3.2.6
ripple
small triangular-shaped bed form that is similar to a dune but has a much smaller and more uniform amplitude and
length
NOTE Ripple wavelengths are less than about 0,6 m and heights are less than about 0,06 m.
3.2.7
sand point
pipe with a well screen, underlying or adjacent to a stream, in which a gas-purge orifice is installed
NOTE The system usually has a device for flushing the sand point.
3.2.8
shift adjustment
correction made to the recorded stage that compensates for vertical movement or shifting of the control
3.2.9
shift diagram
curve or curves that expresses the relation between stage and shift adjustment for a given rating
3.2.10
standing waves
curved symmetrically shaped waves on the water surface and on the channel bottom that are virtually stationary
NOTE When standing waves form, the water and bed surfaces are roughly parallel and in phase.
4 Units of measurement
The units of measurement used in this Technical Report are those of the International System (S.I.).
©
ISO
5 Location of water level (stage) gauge
See 5.1 to 5.3 of ISO 1100-1:1996 for general principles and site characteristics.
5.1 Principles
For a stream with mobile boundaries, as with one having rigid boundaries, the best site for a stream-gauging station
is in a long length of channel of uniform shape, slope and rugosity. Where the channel is the control, the gauge is
located in the control reach of the channel and the site for high-water discharge measurements should be located
near the gauge. This will permit the use of high-water current-meter measurements to define the characteristics of
the stage-discharge relation. If an artificial control is installed, the gauge is located a short distance upstream from
the control. If the channel in the vicinity of the gauge is suitable for the determination of peak discharge by the
slope-area method (see ISO 1070), high-water current-meter measurements can be used to verify computed peak
discharges.
In terms of a few years, or the life of many stream gauges, it is unlikely that the channel of many alluvial rivers will
be stable because a precise balance is not maintained between their flow, sediment discharge, slope, meander
pattern, channel cross-section and rugosity. For example, minor fluctuations in meteorological conditions over a few
years can alter the flow of sediment in the drainage basin. During dry years, sediment can accumulate in stream
channels and during subsequent wet years, the sediment is flushed from the basin. Both uniform and nonuniform
parts of the stream channel may appear to be aggrading or degrading. Thus, there is no assurance that any length
of channel on some alluvial rivers will remain stable over a period of a few years.
At a constriction on a sand-channel stream, the rating will be unstable because the constricted section will
experience maximum streambed scour and fill. Except for channels with only a minor contraction, contracting
stretches of a sand-channel stream are undesirable for use as a gauging-station site because of probable unstable
hydraulic conditions. An opposite effect occurs, however, at a constriction on a stream with rigid boundaries
because the control tends to be sensitive and stable. Often gauges on streams with unstable channels have been
located at constrictions because of construction or access considerations; the ratings are unstable and may behave
in a manner that seems to be unpredictable.
The gauging of streamflow may be particularly difficult where a channel is expanding, in a stretch with braided
channels and where a channel is very wide and flat. The controls for these channels generally are insensitive and
tend to be unstable. AIso, records of water level for low flow are difficult to obtain because a low-water channel may
move laterally across the wide stream channel leaving, the sensor isolated from low flows.
5.2 Water-level considerations
5.2.1 General
The continuous sensing of stream stage in unstable channels is often difficult mostly because (1) the flow may
move laterally or vertically away from the sensor, (2) the sensor cannot be adequately secured and is easily-
washed out, (3) the sensor may become inoperative because of sediment accumulation, and (4) the amount of
surge the sensor is exposed to in an antidune or standing-wave environment may be very large. The cross-
sectional shape of unstable channels is continuously changing and the stream can move away from a sensor at a
fixed location; multiple sensors may be needed to monitor stream stage at these sites reliably. At streams with
erodible banks, sensors may periodically need to be re-secured. Sediment accumulation around a sensor such as a
pressure-gauge orifice can cause erroneous readings of stage and the stilling well can fill with mud. Sensors directly
located in an upper-flow regime where there are standing waves or antidunes will be subject to violent surge;
mechanical or electronic damping of the sensor signal may be required to obtain a readable record of water level.
5.2.2 Channels with stable banks
Bedrock outcrops on banks toward which the flow is directed by upstream conditions are good sites for sensors only
from the standpoint that the sensor has a good chance of being in constant communication with low flows. Other
factors such as pileup or drawdown in the sensor area or the generally unstable hydraulic conditions may outweigh
the benefit of having the water in continuous contact at all stages with the sensor.
©
ISO
Generally, for streams with stable banks, a good location for a water-level sensor is on the outside or concave side,
when viewed from within the channel, of very gradual bends of uniform channels. The thalweg of alluvial channels
tends to be along the outside of bends and thus the sensor will be in contact with low flows and a wide range of
stage can be sensed. During high flows, pileup may occur in the vicinity of the sensor, causing undesirably high
recording of water level.
5.2.3 Channels with unstable banks
Straight uniform channels generally are good sites for sensors but some record may be lost during low flows when
the water's edge moves away from the sensor and the sensor becomes disconnected from the stream. If the banks
erode easily, the most secure locations are on the inside or convex side of gradual bends where a continuous
streamward relocation of the sensor may be needed to keep in contact with the stream.
5.3 Discharge considerations
If a gauge must be located on a stream with an unstable channel, the effect may be lessened if the gauge is in a
single uniform channel. A flat-floored vertically walled channel that resembles a rectangular laboratory flume may
serve as a good gauging site because the results of research in such flumes, reported by several investigators,
might assist the hydrographer in defining the rating. If the channel is relatively narrow the rating will tend to be
sensitive and with a single-bed form across the channel rather than a less sensitive, more complex rating with
multiple-bed configurations that are common across wide channels.
Steep-smooth channels where the Froude number exceeds 0,5 should be avoided if possible (ISO 1100-1:1996,
5.4.6.2). At Froude numbers of 0,5, the transition from dunes to rapid flow starts and the stage-discharge relation
can be discontinuous and very unstable. For many sand-channel ephemeral streams, it is difficult to avoid high
Froude numbers in part or all of the cross-section.
See clauses 5 and 6 of ISO 1100-1:1996 for methods that are suitable for measurements of discharge.
6 Stage measurements
See ISO 4373 for general requirements of stage-sensing devices.
6.1 Stilling wells
6.1.1 General
A stilling-well gauge consists of a float in a stilling well to sense stage. Stilling wells are located in the bank of a
stream or are located directly in the stream and attached to bedrock banks, bridge piers, bridge abutments and
other stable structures. For stilling wells in the bank of a stream, the water enters and leaves the well through a
length of pipe (intake) connecting the well and the stream. The in-bank installation can be installed away from the
higher floodflow velocities because the well and intake may be subject to filling and sealing from sediment
accumulation, especially for ephemeral streams with unstable channels. Flushing systems to unclog intakes that
apply water under a metre or more of head at the well end of the intake are often ineffective and difficult to use,
particularly if the stream is dry and water for flushing must be transported to the gauge.
The most common and effective stilling well installation for unstable channels is achieved by locating the stilling well
in the stream in direct contact with the flow. Intake holes should be normal to the flow as holes facing into the flow
will create a higher stage in the well than in the stream; holes facing downstream will create drawdown and stages
in the well will be lower than in the stream. If possible, the well should be located to avoid direct impact with large
fast-moving debris and to avoid the lodging of drift and fibrous debris against the well. The bottom of the well should
be more than 0,3 m below the maximum anticipated scour of the low-flow bed of the stream. Wells in direct contact
with the stream can be serviced from outside the well using access doors at convenient intervals along the length of
the well. Because the well can be serviced (sediment removal and inspection of floodmarks for example) through
the access doors from outside the well, relatively small diameter wells can be used. Water enters and leaves the
stilling well through holes in the side and/or bottom of the well.
©
ISO
6.1.2 Sediment deposition in wells
A problem common to all stilling wells on alluvial-channel streams with a large sediment load is sediment deposition
in the well. For wells with a single intake, sediment-laden water enters the well when the stage is rising; the rises
include general increases in stage and momentary increase of surges. The low velocities in the well allow the
sediment to deposit in the bottom of the well. For wells with multiple intakes, additional deposition of sediment in the
bottom of the well can result from eddy currents in the well induced by head difference at the intakes. The
circulation of water laden with sediment between multiple intakes can bring large amounts of sediment into a well,
with rapid deposition in the well.
Systems to flush sediment from intakes automatically and to prevent sediment-laden water from entering the well
have been used. For example, on rising stages, sediment-free water from an external source can be automatically
injected into the well using a system of valves and sensors. Also, an external source of water that is free of
sediment, can be used to automatically flush intakes at regular intervals or during floodflow.
6.1.3 Sediment traps
For in-bank installations, stilling wells often fill with sediment, particularly those located in arid or semiarid regions on
unstable channels. If a well is located on a stream carrying heavy sediment loads, it must be cleaned often to
maintain a continuous record of stage. In those locations, sediment traps are helpful in reducing the frequency and
labour of sediment removal.
A sediment trap is a large boxlike structure that occupies a gap in the lower intake line, streamward from the stilling
well. The bottom of the sediment trap is usually about 1 m below the elevation of the intake. Inside the trap are one
or more baffles to cause suspended sediment to settle in the trap, rather than pass into the well. A removable top to
the trap provides access to the interior of the trap for periodic removal of trapped sediment.
6.1.4 Open-bottom wells
Wells located directly in the stream often have a bottom that serves as the intake. The bottom of the well is covered
with some sturdy screen-like material that prevents the float from leaving the well. Some wells have a cone-shaped
hopper bottom that serves as an intake. Open-bottom wells can be self cleaning if the bed of the stream scours
below the well bottom during high flows.
Excessive surge in the stilling well can be reduced by reducing the number and size of the holes in the side and
bottom of the well. A trial and error adjustment of intake holes can be used to achieve minimum surge, minimum
sediment deposition in the well, self cleaning of the well, and sufficient flow of water into and out of the well to follow
the rise and fall of stage without significant delay.
6.2 Gas-purge systems
6.2.1 General
A gas-purge system (bubble gauge) transmits the pressure head of water at an orifice in the stream to a manometer, or
pressure transducer, and recording device in a shelter. A gas, usually nitrogen, is fed through a tube and bubbled freely
into the stream through an orifice at a fixed location in the stream [figure 1 a)]. The servo-manometer, or pressure
transducer, and water-stage recorder converts the pressure signal to water stage. A major advantage of bubble
gauges in unstable channels is that the orifice is small and relatively easy and inexpensive to relocate in the event the
stream channel moves away from the sensor. See 6.2.3 for a discussion of manifold orifices.
Another advantage is that the orifice can be installed in a “muffler” or sand point under or adjacent to the stream in
permeable material [figure 1 b)]. This installation avoids direct contact of the orifice with flow and eliminates the
transverse effects of velocity head on the static head readings.
A disadvantage of the gas-purge systems is that the orifice can become covered with silt or fine sand and effectively
sealed off from the head in the stream. Another disadvantage is that the system, particularly the servo-manometer
is more complex than a stilling-well system. A bubble gauge can require more time to service and maintain and
requires specialized training of operating personnel. See ISO 4373:1995, 8.1 for additional discussion of pressure
gauges.
©
ISO
6.2.2 Anchoring of the orifice
The anchoring of the orifice and keeping the orifice in contact with the water in the stream are difficult at many sites
with unstable channels. The intake pipe should be at right angles to the flow (see 6.1) and should be level or sloping
downward from the manometer, or pressure transducer, to avoid accumulation of moisture in the pipe above the
water level. The orifice shouId be anchored securely to avoid movement during high flow and it should be below the
lowest stage to be recorded. For ephemeral streams with a high silt-clay load, the orifice should be installed above
the channel bed to avoid covering and sealing of the orifice with silt and clay.
a)  Orifice above the stream bed
b)  Orifice in sand point below the stream bed
Key 6 Anchors
1 Bubble tube 7 Orifice
2 To pressure sensor 8 Sand or gravel streambed
3 Soft bank 9 Water surface
4 Pipe, 30 mm-50 mm diameter 10 Flushing riser for adding and extracting liquid
5 Flexible joint 11 Sand point with well screen
Figure 1 — Orifice installation in soft banks
©
ISO
An example of an orifice installation that can be adjusted to follow a streambed that scours and fills is shown in
figure 2. The mounting brackets are loosened and the pipe, orifice, and bubble tube are raised or lowered to follow
the streambed. When the elevation of the orifice of the bubble tube is changed, the new elevation must be
determined and appropriate corrections made to the recorder or the data. This type of installation can be
successfully used where the streambed scours or fills as a result of large floods.
Key
1 Gas line 6 Water surface
2 Flexible joint 7 Pipe with orifice can be raised or lowered to follow
3 Pipe, 50 mm diameter changing elevation of streambed
4 Adjustable mounting brackets 8 Silt-clay streambed
5 Concrete wall or rock bank 9 Orifice end cap
Figure 2 — Adjustable orifice installation
6.2.3 Manifold orifices
At streams that move laterally, a series of orifices can be installed across the channel, at bridge piers for example,
and only the orifice that is in contact with the water is operated. The bubble tubes for each orifice are connected to a
manifold for easy switching from one orifice to another. A single line connects the manifold to the manometer, or
pressure transducer. Only one orifice is operated at a time and orifices can be activated and deactivated to follow
the movement of the unstable channel. For many streams a manifold multi-orifice system can be much easier to
operate than a single orifice that is manually moved to follow the stream.
6.2.4 Sand points and precautions
In sand and gravel channels, the orifice can be installed in a sand point beneath the stream bed. The orifice should
be installed below the depth of maximum anticipated scour to avoid destruction when the bed scours during high
flow. For streams where the ground-water level is higher than the stream surface, the head at a buried orifice will be
slightly greater than the water surface of the stream and for a stream where the ground-water level is lower than the
stream surface and with a saturated-flow connection between the stream and aquifer, the head at the orifice will be
slightly less than the water surface of the stream. The head difference for an orifice located only 1 m or 2 m below
the streambed in saturated sand and gravel will be insignificant for most gauges.
©
ISO
For normally dry streams perched above an aquifer, the initial flow by the buried orifice will be unsaturated and the
head at the orifice will not be the same as the water-surface elevation of the stream. Until saturated conditions are
achieved at the orifice, the head at the orifice cannot be accurately related to the stream stage. The entire stage
hydrograph for large flash floods may not be recorded by a buried orifice located in an unsaturated flow
environment.
Sand points do not perform well in streams with high concentrations of silt, clay, and fine sand, because the well
screen becomes plugged with sediment. For streams with small concentrations of silt and clay, the well screen can
be flushed with water during field inspections as shown in figure 1 b) or it can be temporarily cleaned by purging
with gas. The orifice and well screen can be purged with gas at 1 MPa during field inspections or an automatic
purge system can be used to purge at preset intervals. For high concentrations of silt, clay and fine sand, purging
and flushing normally are ineffective and the orifice becomes sealed from the stream. Even with fine sands, the
passage of water pressure from the river to the orifice can be so impeded as to cause a lag in the recorded stage.
Thus, sand points are not recommended for streams with high concentrations of silt, clay, and/or fine sand.
Sand points require periodic servicing and cleaning to ensure satisfactory operation. The sand point should be
removed from beneath the streambed and the well screen cleaned or replaced at least every two years. More
frequent servicing and cleaning will be needed for sand points in many streams. In general, the more silt and clay in
the stream and the more chemical reaction of the well-screen material with substances in the water, the more
frequently will servicing be needed. The main advantage of orifice sand-point installations is that the sensor is
relatively inexpensive and can be installed under the streambed free from damage by vandals and flood flow. Also,
the stream can move laterally and vertically one or more metres without affecting the reliability of the record. These
advantages can be offset by clogging of the well screens and the frequent maintenance needed to keep the
equipment operating.
6.3 Problems with water-sediment densities
The density of the water and sediment mixture may increase as the result of increased suspended-sediment
concentration. Because the manometer senses pressure at the orifice, the pressure or head will be affected by the
change in density of the fluid. To a lesser degree for most streams, the density of water will also change due to
variation of water temperature and chemical content. With the possible exception for large fluctuations in stage and
large changes in suspended-sediment concentration, the density correction can be ignored. For high-head
installations, the effects of temperature can be compensated for by using a temperature-compensated manometer.
If the density of water consistently increases linearly with stage, the manometer can be adjusted to compensate for
the effect (see ISO 4373:1995, 8.1.2.1).
6.4 Acoustic systems
Acoustic distance meters are installed above the stream to sense stream stage continuously. The non-contact
sensor generally is within 10 m of the water surface, and an average stage over a period of a few seconds is
obtained. The sensor shall be rigidly mounted over the stream. Because the speed of sound varies with air
temperature, temperature compensating meters are recommended for most sites. Acoustic distance meters with
monthly calibration can provide a record of stage reliable to within 30 mm.
6.5 Wire-weight gauges
A commonly used wire-weight gauge consists of a drum wound with a single layer of cable, a bronze weight attached
to the end of the cable, a graduated disc attached to the drum shaft, and a counter. The gauge is mounted on a bridge
handrail, parapet wall, pier or some other rigid structure over the stream for use as an outside gauge. The bronze
weight is raised or lowered by turning the drum. The gauge is set so that when the bottom of the weight is at the water
surface, the gauge height is indicated by the combined readings of the counter and the graduated disc.
Reliable readings of stream stage are obtained with a wire-weight gauge where there is little surface disturbance
and the velocities are not great. For high velocities with turbulent surges or where there are dunes or antidunes, it is
difficult to determine the mean stage because the weight is carried downstream and the water surface is undulating
too rapidly to obtain reliable readings of maximum and minimum stage. Reliable measurements of stage on steep
streams with unstable channels generally cannot be obtained with a wire-weight gauge (see ISO 4373:1995,
7.4.5.2).
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ISO
6.6 Staff gauges
A vertical or inclined staff gauge normally is used as a reference (base) gauge at recording-gauging stations (see
ISO 4373:1995, 7.1). The staff gauge should not be located where there is pileup, drawdown, or large amounts of
surge. It is often difficult to avoid excessive surge because that is a common characteristic of high flows. A mean
stage can be obtained by observing the stage on the staff gauge at the peaks and troughs of waves or surges and
computing the mean of the observations.
It is common to have a low-flow staff gauge in the main channel with one or more staff gauges in the cross-section
at different shoreward locations for higher stages. The scales of the stepped staff gauges should overlap and the
staffs may be vertical or inclined. Staff gauges located in the main channel of alluvial streams may be washed away
due to local scour at the gauge and/or by the lodging of debris on the gauge. It is preferable to install staff gauges
flush on channel banks to avoid lodging of debris; inclined staff gauges that hug a sloping bank can be used to
avoid the debris problem (see ISO 4373:1995, 7.1.4.3).
At sites that are particularly unstable with soft banks, it can be impractical to install a low-flow staff gauge in the
main channel. The stream may move or a low-flow gauge may wash away during high flows. For these adverse
conditions the water surface for low flows can be referenced to a shoreward staff gauge or a vertical control
reference mark by hand levelling or with an engineer's level.
7 Discharge measurements
7.1 General
Specialized supplemental methods discussed here are for single measurements of discharge that are used primarily
for the definition of the stage-discharge relation. Thus, the special methods to determine both discharge and gauge
height for a discharge measurement are presented.
The turbidity of floodflows can be great and the discharge measured can include a large amount of silt in addition to
water. For most streams and discharges, the amount of silt load is less than 1 % by volume, but some flows can
have much larger amounts of silt (see 7.2.7.4 for an example). A correction for the silt load is not normally made.
7.2 Velocity-area methods
7.2.1 Measurement characteristics
The velocity-area method consists of the measurement of velocity and area at a cross-section. A complete
measurement consists of a representative gauge height and the discharge that is the product of the velocity and
area. For ephemeral streams, storm runoff often is flashy, and stages and discharges change rapidly. For unstable
channels, the cross-section geometry also can change greatly during short intervals of time.
Because discharge, stage, and cross-sectional area and shape change with time, it is necessary to obtain
measurements of discharge in unstable ephemeral streams in short intervals of time.
7.2.2 Selection of site
7.2.2.1 Ideal site
Discharge measurements made in the channel near the control or downstream from the gauge can be used to
greater advantage for rating development than discharge measurements made at other locations. If the control is
the channel, then the hydraulic characteristics of measurements of discharge made at a representative cross-
section (channel geometry and roughness) can be used to develop the rating curve. Any changes in channel shape
in the control area also will be documented by the measurements if the water surface at the measurement section is
referenced to the datum of the gauge.
The requirements of a good discharge measuring site for alluvial channels are a firm channel bed and banks,
uniform distribution of velocity across the channel, uniform channel shape, and a straight, uniform stretch of river.
Bends or banks with large irregularities should be avoided because of potential scour holes or a soft stream bed at
the measurement cross-section.
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ISO
To measure high discharge using a current meter, it is generally more suitable to suspend the meter from a
cableway rather than a bridge (see ISO 4375). There usually is scour at abutments and piers of bridges and the
amount of scour changes with time; significant changes in scour and fill have been observed in short periods of
time. A cableway should be located in a straight, uniform stretch of river with a good view upstream for the
observation of oncoming debris; nearly submerged large trees are common in some streams during large floods.
7.2.2.2 Low flows
Current-meter measurements should represent the amount of flow passing the control of the gauge as streamflow.
Where there is seepage of water into or out of alluvium in the gauge-control area, the amount of flow at the control
may be significantly different from the amount of flow at the gauge or at other locations along the channel.
If measurements of discharge are made at various locations along such an influent (inflow to the river) or effluent
(discharge out of the river) stretch of river, then the departure of the measurements from the stage-discharge
relation will be the result of where a particular measurement was made and not the result of control change. Thus,
for consistency of the streamflow record, measurements during low-flow periods when the amount of seepage is
significant should be made at (or very near) the low-water control. For many sand-channel streams, the
measurements of discharge should be made at the gauge because the low-water control is a short length of
channel starting at the gauge. See clause 8 for a discussion of controls.
7.2.2.3 Median and high flows
In many unstable alluvial channels, the stage-discharge relation changes abruptly during high flows and a single,
rather stable stage-discharge relation may apply for high flows following the abrupt change. During these rather
stable periods, measurements of discharge needed to define the corresponding stage-discharge relation are often
difficult to obtain. If the gauging station is ideally located in a straight uniform stretch, a considerable amount of
additional information to define the rating is obtained if the discharge measurement is made at, or a short distance
downstream from, the gauge.
For example, if a current-meter measurement at a gauge is made at a gauge height of 1,85 m with a discharge of
3 2
1,27 m /s (area = 1,9 m , velocity = 0,67 m/s, wetted perimeter = 4,6 m) and the channel shape is uniform and the
hydraulic gradient is constant between gauge heights of 1,5 m and 2,3 m, then a rating for the temporary channel
condition represented by the measurement can be computed (see figures 3 and 4). From the Manning equation with a
constant roughness and hydraulic gradient for 1,5 m to 2,3 m stage.
Key
1 Range of uniform flow
2 Gauge height of velocity-area measurement
3 Streambed
Figure 3 — Cross-section for a velocity-area measurement at a gauging station
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ISO
Key
1 Discharge, m /s
2 Area, m
3 Wetted perimeter, m
Figure 4 — Discharge, area and wetted perimeter versus stage relation for cross-section
at gauging station
2/3
Q = C AR . . . (1)
1 h
where
Q is the discharge, in cubic metres per second;
A is the area of cross-section, in square metres;
R is the hydraulic radius, calculated from A/P, in metres;
h
P is the wetted perimeter, in metres;
C is a constant.
Using the discharge, area, and wetted perimeter for the measurement to solve for C:
Q
C=
23/ . . . (2)
AR()
h
12, 7
C = = 1,205
23/
19,
 
19,
 
46,
 
and
2/3
Q = 1,205 A(R ) . . . (3)
h
for 1,5 m to 2,3 m stage.
The stage-discharge relation computed from this equation is shown in figure 4. Thus. by making the discharge
measurement at a representative part of the channel that acts as the control for the gauge, a rating for th
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