SIST EN ISO 772:2022

SLOVENSKI STANDARD
01-maj-2022
Nadomešča:
SIST EN ISO 772:2011
Hidrometrija - Slovar in simboli (ISO 772:2022)
Hydrometry - Vocabulary and symbols (ISO 772:2022)
Hydrometrie - Begriffe und Symbole (ISO 772:2022)
Hydrométrie - Vocabulaire et symboles (ISO 772:2022)
Ta slovenski standard je istoveten z: EN ISO 772:2022
ICS:
01.040.17 Meroslovje in merjenje. Metrology and measurement.
Fizikalni pojavi (Slovarji) Physical phenomena
(Vocabularies)
17.120.20 Pretok v odprtih kanalih Flow in open channels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 772
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2022
EUROPÄISCHE NORM
ICS 01.040.17; 17.120.20 Supersedes EN ISO 772:2011
English Version
Hydrometry - Vocabulary and symbols (ISO 772:2022)
Hydrom?rie - Vocabulaire et symboles (ISO 772:2022) Hydrometrie - Begriffe und Symbole (ISO 772:2022)
This European Standard was approved by CEN on 5 February 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 772:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 772:2022) has been prepared by Technical Committee ISO/TC 113
"Hydrometry" in collaboration with Technical Committee CEN/TC 318 “Hydrometry” the secretariat of
which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2022, and conflicting national standards
shall be withdrawn at the latest by September 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 772:2011.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 772:2022 has been approved by CEN as EN ISO 772:2022 without any modification.

INTERNATIONAL ISO
STANDARD 772
Sixth edition
2022-02
Hydrometry — Vocabulary and
symbols
Hydrométrie — Vocabulaire et symboles
Reference number
ISO 772:2022(E)
ISO 772:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 772:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Terms related to velocity-area methods .13
5 Terms related to flow measurement structures .17
6 Terms related to dilution method .30
7 Terms related to instruments and equipment .32
8 Terms related to sediment transport .38
9 Terms related to precipitation . 44
10 Terms related to snow .45
11 Terms related to groundwater .51
12 Terms related to uncertainties in hydrometric determinations .61
Annex A (informative) Symbols used in hydrometry .70
Bibliography .73
Index .74
iii
ISO 772:2022(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 procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, in collaboration with
the European Committee for Standardization (CEN) Technical Committee CEN/TC 318, Hydrometry, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This sixth edition cancels and replaces the fifth edition (ISO 772:2011) which has been technically
revised. The main changes compared with the previous edition are as follows:
— terms related to precipitation have been added in a new Clause 9;
— additional terms have been added in Clause 10;
— Figures 1, 3, 4, 5, 6, 9, 11 and 12 have been modified and updated.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
ISO 772:2022(E)
Introduction
In the preparation of this document, the following principles were adopted wherever possible:
a) to standardize suitable terms and symbols without perpetuating unsuitable ones;
b) to discard any term or symbol with differing meanings in different countries, or for different
people, or for the same person at different times, and to replace that term or symbol by one which
has an unequivocal meaning;
c) to exclude terms which are self-evident.
Terms in existing International Standards have been included as much as possible; however, these
terms can be subject to future amendments.
NOTE Similar or identical terms can have separate definitions under the different categories.
It is recognized that it is not possible to produce a complete set of definitions which will be universally
acceptable, but it is hoped that the definitions provided and the symbols used will find widespread
acceptance and that their use will lead to a better understanding of hydrometric practices.
The terminology entries are presented in systematic order, grouped into sections according to
particular methods of determination or in relation to particular subjects. Annex A lists the symbols
used in this document.
The structure of each entry is in accordance with the ISO 10241 series. Country codes are in accordance
with ISO 3166-1.
v
INTERNATIONAL STANDARD ISO 772:2022(E)
Hydrometry — Vocabulary and symbols
1 Scope
This document defines terms and symbols used in standards in the field of hydrometry.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
hydrometry
science and practice of measuring the components of the hydrological cycle (3.92), including rainfall
(9.10), water level (3.64), flow and sediment transport (8.2) of surface waters, and groundwater (11.1)
characteristics
3.2
hydrology
science that deals with the waters above and below the land surfaces of the Earth, their occurrence,
circulation and distribution, their properties and their reaction with the environment
3.3
flow
water flowing on or below the land surface under gravitational influence
3.4
runoff
volume of water flowing through a given channel cross-section related to a given drainage basin (3.103)
in a defined period of time
3.5
discharge
Q
volume of water flowing through a given channel cross-section in unit time
3.6
current
directed movement of water
3.7
steady flow
flow (3.3) in which parameters [such as velocity (3.113), pressure, density and temperature] are
constant with respect to time
ISO 772:2022(E)
3.8
unsteady flow
flow (3.3) in which one or more parameters [such as velocity (3.113), pressure, density and temperature]
change with respect to time
3.9
uniform flow
flow (3.3) in which the magnitude and direction of flow at a given moment are constant with respect to
distance
Note 1 to entry: For uniform flow, the velocity vector is constant along every stream line. Uniform flow is possible
only in an open channel (3.19) of constant slope and cross-section.
3.10
non-uniform flow
flow (3.3) in which the magnitude and direction of flow at a given moment are changing with respect to
distance
3.11
critical flow
flow (3.3) in an open channel (3.19) in which the specific energy is a minimum for
a given discharge (3.5)
Note 1 to entry: Under this condition, the Froude number (3.89) is equal to unity and small surface disturbances
cannot travel upstream.
3.12
subcritical flow
flow (3.3) in an open channel (3.19) at less than critical velocity (3.17), which has a Froude number (3.89)
of less than unity and in which small surface disturbances can travel upstream
3.13
supercritical flow
flow (3.3) in an open channel (3.19) at more than critical velocity (3.17), which has a Froude number
(3.89) of greater than unity and in which small surface disturbances cannot travel upstream
3.14
transverse flow
lateral flow
flow (3.3) horizontally perpendicular to the main direction of flow
Note 1 to entry: Transverse (lateral) flow is frequently associated with secondary flow.
Note 2 to entry: Transverse (lateral) flow in open channels (3.19) with a curved plan form causes superelevation
of the water surface at the outside of the bend.
3.15
stratification
state of a water body that consists of two or more layers arranged according to their density, the lightest
layer being on top and the heaviest at the bottom
3.16
critical depth
depth (3.78) of flow (3.3) at which critical flow (3.11) occurs
3.17
critical velocity
velocity (3.113) of flow (3.3) that has minimum specific energy for a given discharge (3.5) or has unit
Froude number (3.89)
ISO 772:2022(E)
3.18
channel
course of a river (3.27), stream (3.26) or other watercourse
3.19
open channel
longitudinal boundary surface consisting of the bed and banks or sides within which water flows with
a free surface
3.20
canal
man-made channel (3.18), usually of regular cross-sectional shape
3.21
stable channel
open channel (3.19) in which the bed and the sides remain essentially stable over a substantial period
of time in the reach (3.34) under consideration, and in which the scour and deposition (10.5) during the
rising and falling stages are negligible
3.22
unstable channel
open channel (3.19) that changes frequently and significantly in its plan form and/or cross-sectional
form for the reach (3.34) under consideration
3.23
tidal channel
open channel (3.19) in which the flow (3.3) is subject to tidal influence
3.24
tide
periodic rise and fall of water due principally to the gravitational attraction of the sun and the moon
3.25
estuary
lower tidal reaches (3.34) of a river (3.27) that is freely connected with the sea which receives fresh
water supplies from upland drainage areas
3.26
stream
water course, water flowing in an open channel (3.19)
3.27
river
large natural water course
3.28
large river
major river
large natural water course that generally flows into the sea
3.29
creek
brook
small natural water course
3.30
torrent
small natural water course that is characterized by steep slopes and significant rapid changes in
discharge (3.5) and that can transport considerable volumes of solid material
ISO 772:2022(E)
3.31
alluvial river
river (3.27) which flows through alluvium formed from its own deposits
3.32
incised river
river (3.27) which has formed its channel (3.18) by a process of erosion
3.33
braided river
river (3.27) characterized by a wide and shallow open channel (3.19) in which flow (3.3) passes through
a number of small interlaced channels (3.18)
3.34
reach
length of open channel (3.19) between two defined cross-sections
3.35
meandering channel
water course formed by natural flow processes and movement of sediments following generally an
alternating regular sinuous path
3.36
thalweg
line joining the lowest points of successive cross-sections of a water course
3.37
unit discharge
discharge per unit width
q
u
discharge (3.5) through a unit width of a given vertical section
3.38
yield specific discharge
q
discharge (3.5) per unit area of catchment or aquifer (11.15)
3.39
stream gauging
discharge measurement
flow measurement
stream flow measurement
river gauging
all of the operations necessary for the measurement of discharge (3.5) of a stream (3.26)
3.40
gauge
device installed at a gauging station for measuring the level of the surface of water relative to a datum
3.41
left bank
bank to the left of an observer looking downstream
3.42
right bank
bank to the right of an observer looking downstream
ISO 772:2022(E)
3.43
channel bed
invert
stream bed
stream bottom
channel bottom
lower part of the stream channel situated between the banks
3.44
bed slope
bottom slope
S
o
difference in elevation of the bed per unit horizontal distance, measured in the direction of flow (3.3)
Note 1 to entry: The slope is usually mathematically negative in the direction of flow.
3.45
bed profile
shape of the bed in a longitudinal vertical plane
3.46
side slope
difference in elevation between the bottom and top of a bank per unit horizontal distance
3.47
surface slope
S
w
inclination of the surface of the stream (3.26) in a reach (3.34) measured in the direction of flow (3.3)
3.48
fall
difference in elevation of the water surface between the extremities of a defined reach (3.34) at a given
instant of time
EXAMPLE As recorded at a slope station (3.71).
3.49
top width
width of the open channel (3.19) measured across the stream (3.26) at the water surface normal to the
direction of flow (3.3)
3.50
wetted perimeter
P
w
contact length between a stream (3.26) of flowing water and its containing open channel (3.19),
measured in a direction normal to the flow (3.3)
3.51
wetted cross-section
section normal to the mean direction of flow (3.3) bounded by the free surface and wetted
perimeter (3.50)
3.52
gauging section
measuring section
section at which discharge (3.5) measurements are taken
3.53
high water mark
flood mark
mark left on a structure or any other object indicating exceptional stages of flood
ISO 772:2022(E)
3.54
debris line
trash line
traces of any kind left on the banks or obstacles or flood plain by a flood
Note 1 to entry: The debris line may be used to determine the highest level attained by the water surface during
a flood.
3.55
surface velocity
flow (3.3) velocity (3.113) at a given point on the surface
3.56
mean velocity
flow (3.3) velocity (3.113) at a given cross-section of a stream (3.26), obtained by
dividing the discharge (3.5) by the cross-sectional area
3.57
slush ice
mass of loosely packed anchor ice (3.105) that is released from the bottom, or frazil ice (3.104) that
floats or accumulates under surface ice (3.107)
3.58
velocity head
theoretical vertical height to which liquid particles can be elevated by kinetic energy
Note 1 to entry: It is expressed as the square of the velocity (3.113) divided by twice the acceleration due to
gravity.
3.59
gauged head
elevation of the free surface above the horizontal datum of a section
3.60
total head
energy head
H
sum of the elevation of the free surface above the horizontal datum of a section plus the velocity head
(3.58) based on the mean velocity (3.56) at that section
Note 1 to entry: The total head, H, is given by the following formula:
v
Hh=+α
2g
where
h is the gauged head of water level (3.64);
v
is the mean velocity of the water;
α is the Coriolis coefficient;
g is the acceleration due to gravity.
Note 2 to entry: The Coriolis coefficient (α ≥ 1), also known as “energy coefficient” or “energy correction factor”,
takes into account the non-uniform velocity distribution. In many cases, α is assumed to equal unity.
ISO 772:2022(E)
3.61
total head line
energy head line
plot of the total head (3.60) in the direction of flow (3.3)
3.62
energy gradient
difference in total head (3.60) per unit horizontal distance, measured in the direction of flow (3.3)
3.63
energy loss
head loss
difference in total head (3.60) between two cross-sections in the direction of flow (3.3)
3.64
water level
stage
gauge height
elevation of the free surface of a stream (3.26), lake or reservoir relative to a specified datum
3.65
reference gauge
stage gauge that discharge (3.5) is normally linked to
3.66
stage-discharge relation
rating curve
rating table
equation, curve or table that expresses the relation between the stage and the discharge (3.5) in an open
channel (3.19) at a given cross-section
3.67
hydrograph
graphical representation of changes of hydrometric parameters with respect to
time
Note 1 to entry: Typically, stage and discharge hydrographs are used for open channel flows.
3.68
cumulative volume curve
curve in which the cumulative value of a hydrometric parameter is plotted against time
Note 1 to entry: Integral of the hydrograph (3.67), such as cumulative discharge curve.
3.69
storage curve
table
curve depicting the volume of stored water plotted against water level (3.64)
3.70
gauging station
site on a stream (3.26), river (3.27) or lake at which systematic measurements of water level (3.64),
velocity (3.113) or discharge (3.5) or any combination of the three are made
3.71
slope station
twin-gauge station
gauging station (3.70) at which two water-level gauges (3.40) define a reach (3.34) for measurement of
water-surface slope as an essential parameter for establishing a stage-discharge relation (3.66)
ISO 772:2022(E)
3.72
control
physical properties of a cross-section or a reach (3.34) of an open channel (3.19), either natural or
artificial, that govern the relation between stage and discharge (3.5) at a location in the open channel
3.73
rating
relation between discharge (3.5) and other variables, or the taking of observations and making of
calculations needed to establish the relation
3.74
unit-fall rating
relation between stage and discharge (3.5) when the fall (3.48) is equal to one
3.75
afflux
rise in water level (3.64) immediately upstream of, and due to, an obstruction
3.76
backwater curve
profile of water surface, along an open channel (3.19), from the raised surface at an obstruction or
confluence to the point upstream at which the flow (3.3) is at normal depth (3.78)
Note 1 to entry: The term is also used to denote all liquid surface profiles that are non-uniform with respect to
distance upstream or downstream. However, this usage is deprecated.
3.77
drawdown curve
profile of the liquid surface when its surface slope (S ) (3.47) exceeds the bed slope (S ) (3.44)
w o
Note 1 to entry: From the point at which the bed slope increases, or bed level drops abruptly, to the point at which
normal depth (3.78) occurs, the profile along an open channel (3.19) is convex upwards in an upstream direction
and concave upwards in a downstream direction.
3.78
depth
D
linear dimension measured in the vertical direction from the water surface to the bed
3.79
peak stage
maximum instantaneous stage during a given period
3.80
friction
drag
boundary shear resistance that opposes the flow (3.3) of a liquid
3.81
conveyance
K
carrying capacity of a channel (3.18)
Note 1 to entry: The term “conveyance factor” is also used, e.g. in the formula:
−12/
KQ= S
where
K is the conveyance factor;
ISO 772:2022(E)
Q is the total discharge (3.5);
S is the energy gradient (3.62).
3.82
hydraulic jump
sudden transition from supercritical flow (3.13) to subcritical flow (3.12)
Note 1 to entry: Immediately upstream of the hydraulic jump, the velocity (3.113) and the depth (3.78) are
respectively greater and less than their critical values. Beyond the jump, the velocity and the depth are
respectively less and greater than their critical values.
3.83
hydraulic mean depth
mean depth
D
m
area of the cross-section of water flowing in an open channel (3.19) divided by the width of the open
channel at the water surface
3.84
hydraulic radius
r
h
wetted cross-sectional area of water flowing in an open channel (3.19) divided by the length of the
wetted perimeter (3.50) at that cross-section
3.85
gauge datum
elevation of the zero of the gauge (3.40) to which the level of the liquid surface is referred
Note 1 to entry: The gauge datum is related to a benchmark (3.86).
3.86
benchmark
permanent mark, the elevation of which should be related, where practicable, to a national datum
3.87
gauge/float well
stilling well/tube
chamber open to the atmosphere and connected with the stream (3.26) in such a way as to permit the
measurement of the water level (3.64) in relatively still water
3.88
roughness coefficient
coefficient that characterizes the roughness of the channel cross-section and which is taken into
account when computing the resistance to flow (3.3) or the energy gradient (3.62)
Note 1 to entry: The common types are the Manning’s/Strickler n, Chezy C or an element roughness height, k.
3.89
Froude number
Fr
mean velocity (3.56) divided by the square root of the product of the hydraulic mean depth (3.83) and the
acceleration due to gravity
v
Fr =
12/
gD
()
m
where
v
is the mean velocity of the liquid;
ISO 772:2022(E)
g is the acceleration due to gravity;
D is the hydraulic mean depth of the cross-section.
m
Note 1 to entry: The Froude number is dimensionless.
3.90
Reynolds number
Re
ratio of the forces of inertia to forces of viscosity (11.68)
Note 1 to entry: For open channels (3.19):
vr
h
Re=
η
where
v
is the mean velocity (3.56) of the liquid;
r is the hydraulic radius (3.84) of the cross-section;
h
η is the kinematic viscosity (11.69) of the liquid.
Note 2 to entry: The Reynolds number is dimensionless.
3.91
free surface flow
flow (3.3) within a closed or open conduit, under gravity and having a free surface
Note 1 to entry: Where the flow exceeds the free surface capacity of the conduit, the flow will become surcharged
with the consequent disappearance of the free surface. Instances of surcharging of short duration do not normally
affect the overall concept of free surface flow in closed conduits.
3.92
hydrological cycle
constant movement of water in all states of its form, above, on and below the Earth’s surface
3.93
hydrogeology
study of subsurface water in its geological context
3.94
live storage
reservoir storage which can be drawn off for users downstream
3.95
total storage
reservoir storage between the lowest bed level and the top water level (3.64)
3.96
flood storage
volume of water temporarily held above the top water level (3.64) of a reservoir during a flood event
Note 1 to entry: Flood storage is not retained in the reservoir but is discharged through an overflow until the
normal top water level is reached.
ISO 772:2022(E)
3.97
standing wave
stationary wave
curved symmetrically shaped wave on the water surface and on the channel bed (3.43) that is virtually
stationary
Note 1 to entry: When standing waves form, the water surface and the bed surfaces are roughly parallel and in
phase.
3.98
tributary
surface or underground stream (3.26) which contributes its water, continuously or intermittently, to
another stream
3.99
river delta
reach (3.34) of a river (3.27) when it approaches a body of quieter water with very gradual bed slope (S )
o
(3.44) and surface slope (S ) (3.47), and, at low velocity (3.113), deposits its sediment and divides out
w
into channels (3.18) on either side of the deposits, resulting in the formation of deltas
3.100
annual flood
highest momentary peak discharge (3.5), recorded at the respective point of observation, which is
equalled or exceeded once every year
3.101
annual storage within-the-year storage
difference between the maximum and minimum volumes in storage over a year of reservoir operation
3.102
base flow
sustained flow of a stream (3.26) resulting from outflow of groundwater (11.1) and from drainage of
large lakes and swamps
Note 1 to entry: Base flow includes water sustained in glaciers (10.9), snow and other sources, not a result of
direct runoff (3.4).
3.103
drainage basin
catchment area
part of the land area enclosed by a topographic divide from which direct surface runoff (3.4) from
precipitation (9.9) drains by gravity into a stream (3.26) or other water body
3.104
frazil ice
fine spicules, plates or discoids of ice suspended in water that are generally formed by the supercooling
of turbulent water
Note 1 to entry: Frazil ice may float or accumulate under surface ice (3.107) or adhere to the channel bed (3.43) as
anchor ice (3.105).
3.105
anchor ice
submerged ice found attached to the bed, irrespective of the nature of its formation
3.106
rime ice
white mass of tiny ice crystals or granular ice tufts formed on exposed objects due to atmospheric
moisture
ISO 772:2022(E)
3.107
surface ice
ice cover
ice sheet
layer of ice formed on the surface of a lake or river (3.27)
3.108
flow regime
state of flow (3.3) in alluvial streams (3.26) characterized by a bed configuration of ripples (3.111),
dunes (3.109) (lower regime), plane bed (transition), standing waves (3.97) and antidunes (3.110) (upper
regime)
Note 1 to entry: The lower-regime flow is subcritical. The upper-regime flow is supercritical.
3.109
dune
large bed form having a triangular profile, a gentle upstream slope and a steep downstream slope
Note 1 to entry: Dunes are formed in quiet flow and thus are out of phase with any possible water surface
disturbance they produce. They travel slowly downstream as sand is moved across their comparatively gentle
upstream slopes and deposited on their steeper downstream slopes.
3.110
antidune
bed form of a curved symmetrically shaped sand or gravel wave that may move upstream, remain
stationary or move downstream
Note 1 to entry: Antidunes are curved in a wave train but they are in phase and interact strongly with gravity
water surface waves.
3.111
ripple
small triangular-shaped bed form similar to a dune (3.109)
Note 1 to entry: Ripples have much smaller and more uniform amplitudes and lengths than dunes. Ripple
wavelengths are less than 0,6 m and wave heights are less than 0,06 m.
3.112
transition
crossover
inflection reach (3.34) between two meander loops in which the main flow (3.3) crosses from one side
of the channel (3.18) to the other
Note 1 to entry: The depth (3.78) of flow in a transition is usually reduced from normal depth and is more uniform
than in the curved reach.
3.113
velocity
speed of flow (3.3) past a point in a specified direction
3.114
gauge height of zero flow
highest point on the thalweg (3.36) downstream from the gauge (3.40) in a natural or artificial channel
(3.18)
3.115
shift adjustment
correction made to the recorded water level (3.64) to compensate for vertical movement of the bed or
for shifting of the control reach
ISO 772:2022(E)
3.116
shift diagram
curve or set of curves expressing the relation between water level (3.64) and shift adjustment (3.115) for
a given rating (3.73)
3.117
turbulent flow
flow (3.3) in which water particles move in irregular paths that is not laminar or streamline flow and
for which the Reynolds number (3.90) is greater than the critical Reynolds number
3.118
laminar flow
flow (3.3) of a fluid in which the viscous forces are predominant and in which, in channel flow, the fluid
particles move in approximately definite and relatively smooth paths with no significant transverse
mixing
Note 1 to entry: The Reynolds number (3.90) is smaller than 500 to 2 000 in flow channels and smaller than 1 to
10 in flow through porous media.
Note 2 to entry: Groundwater flow is laminar flow under natural conditions.
4 Terms related to velocity-area methods
4.1
current-meter
instrument for measuring water velocity (3.113)
4.2
drift
distance that a measuring boat travels during the time taken to make a stationary
velocity (3.113) observation/measurement
4.3
period of pulsation
average period of a cycle of pulsation during which the velocity (3.113) in the cross-section fluctuates
between limiting high and low values
4.4
sounding
operation of measuring the depth (3.78) from the free surface to the bed
4.5
air line correction
correction to the sounding line (7.16) measurement applied to that part of the sounding line above the
liquid surface
Note 1 to entry: See Figure 1.
ISO 772:2022(E)
Key
air line correction (4.5) B drift (4.2)
kl
a
wet line correction (4.6) Direction of flow.
kl
Figure 1 — Sounding line corrections
4.6
wet line correction
correction to the sounding line (7.16) measurement applied to that part of the sounding line below the
liquid surface
Note 1 to entry: See Figure 1.
4.7
standard current-meter
calibrated current-meter (4.1) used as a basis of comparison with other current-meters
4.8
velocity integration method
method of measuring the velocity (3.113) along a vertical (4.29), involving
the raising and lowering of a current-meter (4.1) at a constant rate through the entire depth (3.78) of the
vertical
4.9
point velocity method
method of measuring the velocity (3.113) along a vertical (4.29) by placing a current-meter (4.1) at a
number of designated points in the vertical
Note 1 to entry: The velocity is usually measured at one, two, three, five or six points on the vertical.
ISO 772:2022(E)
4.10
one-point method
method in which observations of velocity (3.113) are made in each vertical (4.29) at one point below the
surface
4.11
multi-point method
method in which observations of velocity (3.113) are made in each vertical (4.29) at two points or more
points (up to six) below the surface
4.12
moving boat method
method of measuring discharge (3.5) from a boat by traversing the stream (3.26) along the gauging
section (3.52) while continuously measuring velocity (3.113), depth (3.78) and distance travelled, and
angle of current velocity
4.13
cubature
numerical technique for computing discharge (3.5) in a tidal channel (3.23) at a cross-section from the
rates of change in volume of water up to the tidal limit, with algebraic allowance for the fresh water
discharges entering the channel
Note 1 to entry: The maximum volume is usually that occurring at high water of spring tide (3.24).
4.14
looped stage-discharge curve
hysteresis of the stage-discharge relation
effect on the stage-discharge relation (3.66) at a gauging station (3.70) where, for the same water level
(3.64), the discharge (3.5) on the rising stage is different from that on the falling stage
4.15
sensitivity of the stage-discharge relation
measure of the change in stage of a gauging station (3.70) due to a change in discharge (3.5)
Note 1 to entry: When a small change in discharge produces a relatively large change in stage, the relation is said
to be sensitive. When a large change in discharge produces a relatively small change in stage, the relation is said
to be non-sensitive.
4.16
stilling well lag
difference at a given instant between the channel stage and the stilling well stage
4.17
stage fall-discharge relation
slope-stage-discharge relation
family of curves that expresses the relationship between the free water surface slope (S ) stage and
w
discharge (3.5) in a given reach (3.34) of an open channel (3.19) subject to variable backwater
4.18
seiche
oscillation of the surface of a liquid caused mainly by winds and variations in atmospheric pressure
4.19
salt-water wedge
wedge-like intrusion of a large mass of salt water flowing in from the sea under the fresh water in a
tidal waterway
4.20
flood flow
flow (3.3) corresponding to or exceeding the natural bankfull stage (4.21)
ISO 772:2022(E)
4.21
bankfull stage
water level (3.64) at which an open watercourse just overflows its natural banks
4.22
tidal amplitude
one-half of the difference in height between consecutive high water and low water, hence half the tide
(3.24) range
4.23
tidal prism
volume of water flowing into a tidal channel (3.23) on high tide (3.24)
4.24
index velocity method
method used to compute the discharge (3.5) by multiplying mean channel velocity (3.113) and cross-
sectional area using outputs from index velocity rating and cross-sectional area, which are derived
from stage-area rating, respectively
4.25
rating change
any change in the stage-discharge relation (3.66) due to temporary or permanent, sudden or gradual
modifications of the controls (3.72)
4.26
rating shift
correction of the stage-discharge relation (3.66), usually the offsets of one or several controls (3.72) to
account for a rating change (4.25)
4.27
velocity-area method
method of determining discharge (3.5) from the area of the cross-section, bounded by the wetted
perimeter (3.50) and the free surface, while integrating the component velocities (3.113) in the cross-
section
4.28
slope-area method
indirect method of determining discharge (3.5) in a reach (3.34) which is based on the friction (energy)
slope (S), the reach roughness, the wetted perimeters (3.50) and the flow areas of the various wetted
cross-sections (3.51) in the reach
4.29
vertical
vertical line on which velocity (3.113) measurements or depth (3.78) measurements are made
4.30
vertical velocity curve
vertical velocity distribution
variation in velocity (3.113) in a stream (3.26) or river (3.27) between the surface and bed which can be
represented as a curve for that specific vertical (4.29)
4.31
vertical velocity coefficient
coefficient applied to a single velocity determination at any depth (3.78) on a vertical to infer the mean
velocity (3.56) on that vertical (4.29)
ISO 772:2022(E)
4.32
reference current-meter
current-meter (4.1) that is immersed at a fixed position in the cross-section during the discharge (3.5)
measurement
Note 1 to entry: For slight changes in discharge during the gauging operation, it is assumed that the change in
velocity (3.113) indicated by the reference current-meter is proportional to the change in discharge.
4.33
float gauging
measurement of velocity (3.113) of a stream (3.26) by means of a float (7.19) or floats
4.34
mean section segment
area bounded by two consecutive verticals (4.29) in a cross-section, the bed of the open channel (3.19)
and the water surface
Note 1 to entry: See Figure 2.
4.35
mid-section segment
area at a vertical (4.29) defined by the depth (3.78) at that vertical multiplied by one-half of the distance
between the preceding and succeeding verticals
Note 1 to entry: See Figure 2.

Key
1 stream panels 4 mean section segment (4.34)
2 vertical (4.29) 5 bed profile (3.45)
3 mid-section segment (4.35)
Figure 2 — Geometric definitions
5 Terms related to flow measurement structures
5.1
flow measurement structure
hydraulic structure [e.g. weir (5.2), flume (5.49), gate] installed in an open channel (3.19) where in most
cases the discharge (3.5) can be derived from the measured upstream water level (3.64)
ISO 772:2022(E)
5.2
weir
overflow structure that may be used for controlling the upstream water level (3.64) or for measuring
discharge (3.5) or both
Note 1 to entry: See Figure 3.
Key
1 weir crest 7 baffle (5.18)
2 weir block (5.8) H total head over crest
3 hydraulic jump (3.82) h head over crest
4 total head line (3.61) p height of weir (5.3)
a
5 control block (5.19) Direction of flow.
6 stilling basin (5.20)
Figure 3 — Weir
5.3
height of weir
apex height
height from the upstream bed to the lowest point of the crest
5.4
head over the weir
elevation of the water surface above the lowest point of the crest, measured at a point sufficiently
upstream to be unaffected by the drawdown curve (3.77)
...