Hydrometric determinations — Unstable channels and ephemeral streams

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

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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
Reference number
ISO/TR 11332:1998(E)
---------------------- Page: 1 ----------------------
ISO/TR 11332:1998(E)
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 iso@iso.ch
Printed in Switzerland
---------------------- Page: 2 ----------------------
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
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ISO/TR 11332:1998(E) ISO
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.
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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.

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)
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ISO
ISO/TR 11332:1998(E)

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.

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ISO
ISO/TR 11332:1998(E)
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.).

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ISO
ISO/TR 11332:1998(E)
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.

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ISO
ISO/TR 11332:1998(E)

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.
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ISO
ISO/TR 11332:1998(E)
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.
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ISO
ISO/TR 11332:1998(E)
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
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ISO
ISO/TR 11332:1998(E)

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 o

...

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