EN ISO 772:2011

SLOVENSKI STANDARD
oSIST prEN ISO 772:2007
01-december-2007
+LGURPHWULþQHGRORþEH6ORYDULQVLPEROL,62',6
Hydrometric determinations - Vocabulary and symbols (ISO/DIS 772:2007)
Déterminations hydrométriques - Vocabulaire et symboles (ISO/DIS 772:2007)
Ta slovenski standard je istoveten z: prEN ISO 772
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
oSIST prEN ISO 772:2007 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
DRAFT
prEN ISO 772
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2007
ICS 01.040.17; 17.120.20 Will supersede EN ISO 772:2000
English Version
Hydrometric determinations - Vocabulary and symbols (ISO/DIS
772:2007)
Déterminations hydrométriques - Vocabulaire et symboles
(ISO/DIS 772:2007)
This draft European Standard is submitted to CEN members for parallel enquiry. It has been drawn up by the Technical Committee
CEN/TC 318.
If this draft becomes a European Standard, 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.
This draft European Standard was established by CEN 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 Management Centre has the
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CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.
: This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
Warning
shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN ISO 772:2007: E
worldwide for CEN national Members.

prEN ISO 772:2007 (E)
Contents Page
Foreword.3

prEN ISO 772:2007 (E)
Foreword
This document (prEN ISO 772:2007) has been prepared by Technical Committee ISO/TC 113 "Hydrometric
determinations" in collaboration with Technical Committee CEN/TC 318 “Hydrometry” the secretariat of which
is held by BSI.
This document is currently submitted to the parallel Enquiry.
This document will supersede EN ISO 772:2000.
Endorsement notice
The text of ISO/DIS 772:2007 has been approved by CEN as a prEN ISO 772:2007 without any modification.

DRAFT INTERNATIONAL STANDARD ISO/DIS 772
ISO/TC 113/SC 3 Secretariat: BSI
Voting begins on: Voting terminates on:
2007-10-11 2008-03-11
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION • МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ • ORGANISATION INTERNATIONALE DE NORMALISATION
Hydrometry — Vocabulary and symbols
Hydrométrie — Vocabulaire et symboles
[Revision of ISO 772:1996, ISO 772:1996/Amd 1:2002 and ISO 772:1996/Amd 2:2004]
ICS 01.040.17; 17.120.20
ISO/CEN PARALLEL ENQUIRY
The CEN Secretary-General has advised the ISO Secretary-General that this ISO/DIS covers a subject
of interest to European standardization. In accordance with the ISO-lead mode of collaboration as
defined in the Vienna Agreement, consultation on this ISO/DIS has the same effect for CEN
members as would a CEN enquiry on a draft European Standard. Should this draft be accepted, a
final draft, established on the basis of comments received, will be submitted to a parallel two-month FDIS
vote in ISO and formal vote in CEN.
In accordance with the provisions of Council Resolution 15/1993 this document is circulated in
the English language only.
Conformément aux dispositions de la Résolution du Conseil 15/1993, ce document est distribué
en version anglaise seulement.
To expedite distribution, this document is circulated as received from the committee secretariat.
ISO Central Secretariat work of editing and text composition will be undertaken at publication
stage.
Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
secrétariat du comité. Le travail de rédaction et de composition de texte sera effectué au
Secrétariat central de l'ISO au stade de publication.
THIS DOCUMENT IS A DRAFT CIRCULATED FOR COMMENT AND APPROVAL. IT IS THEREFORE SUBJECT TO CHANGE AND MAY NOT BE
REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT, WITH THEIR COMMENTS, NOTIFICATION OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPORTING DOCUMENTATION.
©
International Organization for Standardization, 2007

ISO/DIS 772
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ii ISO 2007 – All rights reserved

ISO/DIS 772: 2006
Contents Page
Foreword.iv
Introduction .v
1 Scope.1
2 Velocity-area methods.21
3 Flow measurement structures.31
4 Dilution methods.44
5 Instruments and equipment.51
6 Sediment transport .64
7 Uncertainties in hydrometric determinations .72
8 Groundwater.79
Annex A (normative) Symbols used in hydrometry.90
Annex B (informative) Bibliography .93
Annex C (informative) Classification of flow measurement structures .94

ISO/DIS 772: 2006
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives,
Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
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.
ISO 772 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 3,
Terminology and symbols.
The 4th Edition of ISO 772 was brought out in 1996. Thereafter two amendments FDAM1 and FDAM2
were issued and some comments were received from the members. In view of the above, during the
last ISO/TC 113/SC3 held on 20th May, 2004 at Tsukuba, Japan, it was decided to revise ISO
772:1996. The Subcommittee also decided to prepare the revised document in English only.
This fifth edition cancels and replaces the fourth edition (ISO 772:1996) which has been technically
revised.
Annex A forms an integral part of this international standard. Annex B and C are for information only.
iv © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
Introduction
In the preparation of this International Standard, the following three 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 by different people,
or by 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.
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 better understanding of the practice of hydrometric
determinations.
DRAFT INTERNATIONAL STANDARD ISO/DIS 772: 2006

Hydrometry — Vocabulary and symbols
1 Scope
This International Standards gives terms, definitions and symbols in English and used in the field of
hydrometric determinations.
Structure of the Vocabulary
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 international Standard. Annex B refers the user to ISO 3454 for statistical terminology,
and to ISO 5168 for more extensive information about the evaluation of uncertainties. An alphabetical
index is included at the end.
The structure of each entry is in accordance with ISO 10241, International terminology standards –
Preparation and layout. Country codes are in accordance with ISO 3166, Codes for the representation
of names of countries.
General terms
1.1
liquid flow
movement of a volume of a substance that is neither a solid nor a gas, that is practically
incompressible, that offers insignificant resistance to change of shape and that flows freely
Example:
Water or water with sediment.
1.2
flow regime
state of flow in alluvial streams characterized by a bed configuration of ripples, dunes (lower regime),
plane bed (transition), standing waves and antidunes (upper regime)
NOTE The lower regime flow is sub-critical; the upper regime flow is supercritical.
1.3
steady flow
condition in which the discharge does not change in magnitude with respect to time
1.4
unsteady flow
condition in which the discharge changes in magnitude with respect to time
1.5
uniform flow
flow, in an open channel, in which the depth and velocity remain constant along the open channel
NOTE For uniform flow, the velocity vector is constant along every stream line. Uniform flow is possible only
in an open channel of constant cross-section.
ISO/DIS 772: 2006
1.6
critical flow
flow in an open channel, in which the specific energy is a minimum for a given discharge
NOTE Under this condition the Froude number is equal to unity and small surface disturbances cannot
travel upstream.
1.7
subcritical flow
flow in an open channel at less than critical velocity, that has a Froude number of less than unity, and
in which small surface disturbances can travel upstream
1.8
supercritical flow
flow in an open channel at more than critical velocity, that has a Froude number of greater than unity,
and in which small surface disturbances cannot travel upstream
1.9
transverse flow
flow horizontally perpendicular to the main direction of flow parallel to the axis of the open channel(s)
NOTE 1 Transverse flow is frequently associated with secondary flow.
NOTE 2 Transverse flow in open channel(s) with a curved plan form causes super elevation of the water
surface at the outside of the bend.
1.10
stratification of flow
state of a fluid that consists of two or more layers arranged according to their density, the lightest
layer being on top and the heaviest at the bottom
1.11
critical depth
depth of flow at which critical flow occurs
1.12
critical velocity
velocity at critical flow
1.13
channel
deep part of a river or other waterway
NOTE The term can be qualified adjectivally to describe a particular type of channel, such as a low-water
channel, a main channel, or an artificial channel.
1.14
open channel
longitudinal boundary surface consisting of the bed and banks or sides within which the liquid flows
with a free surface
1.15
canal
man-made channel, usually of regular cross-sectional shape

2 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.16
stable channel
open channel in which the bed and the sides remain essentially stable over a substantial period of
time in the control reach, and in which the scour and deposition during the rising and falling stages
are negligible
1.17
unstable channel
open channel with a control reach that changes frequently and significantly
1.18
tidal channel
open channel in which the flow is subject to tidal action
1.19
tidal waterway
one or more tidal channels together with the shallows and the banks or sides by which the water at
high tide is bounded
1.20
estuary
partially enclosed body of water in the lower reaches of a river that is freely connected with the sea
and which receives fresh water supplies from upland drainage areas
1.21
stream
water flowing in an open channel (see1.23)
NOTE By extension: moving water and the channel containing it.
1.22
current
liquid flow in a discernable general direction
1.23
river
stream of water in a natural open channel
1.24
alluvial river
river which flows through alluvium formed from its own deposits
NOTE The sediment carried by an alluvial river, except for the wash load, is similar to that in the bed.
1.25
incised river
river which has cut its channel down through part of the valley floor (see also 1.26)
1.26
incised river
river which has formed its channel by a process of degradation (see also 1.25)
NOTE The sediment carried by an incised river generally is dissimilar to that in the bed.

ISO/DIS 772: 2006
1.27
braided river
river characterized by a wide and shallow open channel in which flow passes through a number of
small interlaced channels separated by shoals
NOTE 1 Frequently there is little or no erosion of the main banks of a braided river
NOTE 2 Generally there is little or no meandering of the main channel of a braided river, but meandering in
the minor channels is usual.
1.28
reach
length of open channel between two defined cross-sections
1.29
meandering channel
channel following a sinuous path, characterized by curved flow leading to bank erosion alternating
with shoaling
1.30
dune
large bed form having a triangular profile, a gentle upstream slope and a steep downstream slope
NOTE Dunes are formed in quiet 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.
1.31
antidunes
bed form of a curved symmetrically-shaped sand wave that may move upstream, remain stationary or
move downstream
NOTE Antidunes are curved in wave train but they are in phase with, and interact strongly with, gravity
water surface waves.
1.32
ripple
small triangular-shaped bed form similar to a dune
NOTE Ripples have much smaller and more uniform amplitudes and lengths than dune. Ripple wavelengths
are less than 0.6 m and wave heights are less than 0.06 m.
1.33
thalweg
line in plan joining the deepest points of stream bed, a channel or a valley (see also 1.34)
1.34
thalweg
line of greatest depth, and thus the lowest water thread, along the stream channel (see also 1.33)
1.35
transition
crossover
inflection reach between two meander loops in which the main flow crosses from one side of the
channel to the other
NOTE The depth of flow in a transition is usually reduced from normal depth and is more uniform than in the
curved reach.
4 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.36
node
nodal point
inflection point
point in a transition at which the sinuous path crosses the mean axis of the meander system
NOTE In a meandering stable channel, the node migrates downstream with the meander loops. Migration
can be prevented by the creation of a natural or artificial obstruction in the channel.
1.37
discharge
volume of liquid flowing through cross-section in a unit time
NOTE This term is not synonymous with flow.
1.38
unit discharge
discharge per unit width
discharge through a unit width of a section at a given vertical
1.39
specific discharge
discharge per unit area of catchment (see also 1.40)
1.40
specific discharge
discharge corresponding to a specific stage or to a specific gauge height (see also 1.39)
1.41
stream gauging
all of the operations necessary for the measurement of discharge of a stream
1.42
discharge measurement
process of measuring the discharge of liquid in an open channel
1.43
gauge
device installed at a gauging station for measuring the level of the surface of the liquid relative to a
datum
1.44
velocity
sped of flow past a point in a specified direction
1.45
speed
(of flow) ratio of the distance covered by a body of water, moving in a specified direction, to the time
taken to cover that distance
1.46
left bank
bank to the left of an observer looking downstream
1.47
right bank
bank to the right of an observer looking downstream
ISO/DIS 772: 2006
1.48
invert
lowest part of the cross-section of a natural or an artificial channel
1.49
bed slope
bottom slope
friction slope US
difference in elevation of the bed per unit horizontal distance measured in the direction of flow
NOTE The slope is usually mathematically positive in the direction of flow.
1.50
bed profile
shape of the bed in a vertical plane
NOTE The shape of the bed may be considered longitudinally or transversely; this should be stated.
1.51
side slope
ratio of the horizontal to the vertical components of the bank slope, unless stated otherwise
1.52
surface slope
difference in elevation of the surface of the stream per unit distance, measured horizontally in the
direction of flow
1.53
surface drawdown
local lowering of the water surface in an approach channel, caused by acceleration of the flow
passing over an obstacle or through a control
1.54
fall
difference in elevation of the water surface between the extremities of a defined reach at a given
instant of time, for example as recorded at a twin-gauge station
1.55
top width
width of the open channel measured across the stream at the water surface
1.56
wetted perimeter
extent of wetted contact between a stream of flowing water and its containing open channel,
measured in a direction normal to the flow (see also 1.57)
1.57
wetted perimeter
wetted boundary of an open channel at a specified section (see also 1.56)
1.58
cross-section
(of a stream) section normal to the mean direction of flow bounded by the free surface and wetted
perimeter of the stream
6 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.59
gauging section
measuring section
section in which discharge measurements are taken
1.60
measuring reach
reach of open channel selected for measurement of hydraulic parameters
1.61
converging reach
reach in which the cross-sectional area gradually decreases in the direction of flow
1.62
expanding reach
reach in which the cross-sectional area gradually increases in the direction of flow
1.63
flood mark
trash line
debris line
traces of any kind left on the banks or obstacles or flood plain by a flood (see 6.6)
NOTE The flood mark may be used to determine the highest level attained by the water surface during a
flood.
1.64
normal velocity distribution
velocity distribution in a straight open channel of uniform cross-section which is of sufficient length to
develop uniform resistance-controlled flow
1.65
surface velocity
velocity of a liquid at its surface at a given point
1.66
mean velocity depth
depth below the surface at which the mean velocity on a vertical occurs
1.67
mean velocity
(at a cross-section) velocity at a given cross-section of a stream, obtained by dividing the discharge
by the cross-sectional area of the stream at that section
1.68
mean velocity
(of a reach) velocity calculated by dividing the discharge by the average cross-sectional area of the
stream along the reach
1.69
velocity of approach
approach velocity
mean velocity at a cross-section at a specified distance upstream of a measuring device

ISO/DIS 772: 2006
1.70
velocity head
head due to velocity, equal to the vertical height, through which a fall under the influence of gravity
alone would give the liquid a velocity equal to the actual velocity, expressed as the square of the
velocity divided by twice the acceleration due to gravity
NOTE The velocity head is the kinetic energy of the flow (total head line).
1.71
gauged head
level of the water surface or stage related to the invert of the flume or weir crest level as datum
1.72
piezometric head
elevation of the free surface plus the pressure head; at any cross-section, it is the total head above
the datum minus the velocity head at that cross-section
1.73
total head
energy head
sum of the elevation of the free surface above the horizontal datum of a section plus the velocity head
based on the mean velocity at that section (see 2.78 and Figure 6)
NOTE The total head, H, is given by the following expression:
v
Hh=+α
2g
where
h is the head of liquid level;
v is the mean velocity of the liquid;
α is the Coriolis coefficient;
g is the acceleration due to gravity.
The Coriolis coefficient (α ≥ 1) takes into account the non-uniform velocity distribution. In many cases,
α is assumed to equal unity.
1.74
total head line
energy head line
plot of the total head in the direction of flow(See Figure 6)
1.75
total head level
sum of the elevation of the free surface plus, the velocity head based on the mean velocity at the
section
1.76
energy gradient
difference in total head per unit horizontal distance measured in the direction of flow

8 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.77
energy loss
head loss
difference in total head between two cross-sections in the direction of flow (see also 1.78)
1.78
energy loss
head loss
decrease in total head expressed units of height due to energy dissipation (see also 1.77)
1.79
specific energy
sum of the elevation of the free surface above the bed and the velocity head based on the mean
velocity at that section
cf. total head, energy head (1.73)
1.80
stage
gauge height
liquid level
elevation of the free surface of a stream, lake or reservoir relative to a specified datum
cf. gauge datum (1.117)
1.81
gauge height of zero flow
highest point on the thalweg downstream from the gauge in a natural or artificial channel
1.82
gauge height of zero flow line
line on a shift diagram where the sum of the stage plus the shift adjustment is equal to the gauge
height at zero flow for the rating
1.83
stage-discharge relation
curve, equation or table that expresses the relation between the stage and the discharge in an open
channel at a given cross-section, for a given condition of steady, rising or falling stage (See Figure 1)

ISO/DIS 772: 2006
Y
100 200 300
X
Key
X  discharge, m /s
Y stage, m
Figure 1 — Stage-discharge relation

1.84
stage hydrograph
graphical representation of changes in stage with respect to time
1.85
discharge hydrograph
graphical representation of changes in discharge with respect to time
1.86
shift adjustment
correction made to the recorded stage to compensate for vertical movement of the bed or for shifting
of the control reach
1.87
cumulative volume curve
mass discharge curve
curve in which the cumulative volume of flow or flow mass is plotted against time
10 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.88
gauging station
site selected on a stream, river or open channel at which systematic measurements of water level or
discharge, or both, are made
1.89
single-gauge station
gauging station at which stage records from a single gauge are adequate to establish a stage-
discharge relation
1.90
twin-gauge station
gauging station at which two water-level gauges define a reach for measurement of water-surface
slope as an essential parameter for establishing a stage-discharge relation
1.91
control
physical properties of a cross-section or a reach of an open channel, either natural or artificial that
govern the relation between stage and discharge at a location in the open channel
1.92
rating
relation between discharge and other variables, or the taking of observations and making of
calculations needed to establish the relation
1.93
calibration
rating
(of a station) establishment of a discharge relation with the measured variable(s)
1.94
unit-fall rating
relation between stage and discharge when the fall is equal to 1 m
1.95
discontinuous rating
rating that exhibits a change in shape resulting from a change from lower to upper flow regime in all or
part of the control reach
NOTE The change in shape is usually abrupt.
1.96
shift diagram
curve or set of curves expressing the relation between stage and shift adjustment for a given rating
1.97
afflux
rise in liquid level immediately upstream of and due to an obstruction
1.98
backwater
afflux upstream from a given location on an open channel resulting from impedance offered to flow
NOTE Backwater may be caused by momentary storage in a channel
1.99
backwater curve
profile of water surface, concave upwards, along an open channel, from the raised surface at an
obstruction or confluence to the point upstream at which the flow is at normal depth
ISO/DIS 772: 2006
NOTE 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.
1.100
drawdown curve
profile of the liquid surface when its surface slope exceeds the bed slope
NOTE From the point at which the bed slope increases, or drops abruptly, to the point at which normal
depth occurs, the profile along an open channel is convex upwards in an upstream direction and concave
upwards in a downstream direction.
1.101
depth
linear dimension measured in the vertical direction from the water surface to the bed
1.102
normal depth
depth from the water surface to the bottom grade line of a channel, for uniform flow
NOTE Normal depth is a function of the geometry, slope and roughness of the channel, and of the rate of
discharge.
1.103
length
linear dimension measured in the direction of the stream flow
1.104
width
breadth
linear dimension measured perpendicularly to the direction of the stream
1.105
frazil ice
fine spicules, plates or discoids of ice suspended in water that are generally formed by the super
cooling of turbulent water
NOTE Frazil ice may float or accumulate under ice cover or adhere to the stream bed as anchor ice.
1.106
anchor ice
submerged ice found attached to the bed, irrespective of the nature of its formation
1.107
rime ice
white mass of tiny ice crystals or granular ice tufts formed, on exposed objects, due to atmospheric
moisture
1.108
surface ice
ice cover
ice sheet
layer of ice formed on the surface of a lake or river
1.109
slush ice
mass of loosely packed anchor ice that is released from the bottom, or frazil ice that float or
accumulates under surface ice
12 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
1.110
peak stage
maximum instantaneous stage during a given period
cf. flood mark (1.43) and crest stage gauge (5.6)
1.111
friction
drag
boundary shear resistance that opposes the flow of a liquid
1.112
conveyance
K (ISO)
Conveyance factor (deprecated) carrying capacity of a channel:
−1/ 2
KQ= S
where
Q  is the total discharge;
S   is the bed slope
1.113
hydraulic jump
sudden transition from supercritical flow to sub-critical flow
NOTE Immediately upstream of the hydraulic jump, the velocity and the depth 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.
1.114
hydraulic mean depth
mean depth
area of the cross-section of water flowing in an open channel divided by the width of the open channel
at the water surface
1.115
hydraulic radius
cross-sectional area of water flowing in an open channel divided by the length of the wetted perimeter
at that cross-section
1.116
stage gauge
device installed for measuring the level of the surface of the liquid relative to a gauge datum
1.117
gauge datum
elevation of the zero of the gauge to which the level of the liquid surface is referred
NOTE The gauge datum is related to a benchmark
1.118
benchmark
permanent mark, the elevation of which should be related where practicable to a national datum
ISO/DIS 772: 2006
1.119
gauge well
stilling well
chamber open to the atmosphere and connected with the stream in such a way as to permit the
measurement of the stage in relatively still water
1.120
stilling tube
tube connected with the stream in such a way as to permit the measurement of the stage in relatively
still liquid
1.121
float well
stilling well in which a float device is used
1.122
rugosity coefficient
roughness coefficient
coefficient that characterizes the roughness of the wetted perimeter and which is taken into account
when computing the resistance to flow
1.123
friction coefficient
coefficient used to calculate the energy gradient caused by friction
1.124
Froude number
Fr
Mean velocity divided by the square root of the product of the mean depth and the acceleration due to
gravity:
v
Fr=
1/ 2
()gD
where
v
is the mean velocity of the liquid;
g is the acceleration due to gravity;
D
is the mean depth of the cross-section.
NOTE The Froude number is dimensionless.
1.125
Reynolds number
Re
Ratio of the forces of inertia to forces of viscosity
For open channels;
vD
Re=
ν
where
14 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
v
is the mean velocity of the liquid;
D
is the mean depth of the cross-section;
v is the kinematic viscosity of the liquid.
NOTE The Reynolds number is dimensionless.
1.126
Weber number
We
Ratio of the forces of inertia to surface tension forces
For open channels:
ρvD
We=
σ
where
ρ is the density of the liquid;
v
is the mean velocity of the liquid;
D
is the mean depth of the cross-section;
σ is the surface tension of the liquid.
NOTE The Weber number is dimensionless.
1.127
telemetry
data or information acquisition system in which the measurement facility is sufficiently remote from the
location of data presentation that a system of data transmission is necessary
1.128
remote sensing
(hydrometry) acquisition of data or information on some property of an object or phenomenon by a
sensor which is significantly remote from the object or phenomenon
NOTE 1 Common usage of this term usually implies that the sensor is mounted in an aircraft or in a space
vehicle.
NOTE 2 It is recommended that the term not be applied when the sensor merely is not in contact with the
object or phenomenon.
1.129
remote telemetry station
all the facilities necessary to accept or to acquire measured data and to transmit the data from a
hydrometric station without human intervention
1.130
remote telemetry unit
set of equipment which acquires the input of signals from sensors and status indicators, and performs
all the processes required to present a data message to a communication link
ISO/DIS 772: 2006
1.131
redundancy
employment of extra facilities, each capable of performing the same function in order to improve the
reliability of a particular function
1.132
encoding
process of converting data to a specific code
1.133
parity check
addition of an extra bit so that the total number of bits in a sample is either always even or always odd
1.134
code
set of rules which specifies the format in which data may be represented
1.135
system
set of elements organized to perform a set of designated functions in order to achieve the desired
results
1.136
data
raw data
output resulting directly from the measurement of variables
1.137
information
result of analyzing or interpreting data
1.138
energy
quantity characterizing, the ability of a system to do work
cf. power (1.139)
1.139
power
time-rate of transferring energy or of transforming energy, or of doing work (see 1.140)
1.140
work
transfer of energy expressed as the product of a force and the distance through which its point of
application moves in the direction of the force
1.41
hardware
tangible equipment associated with a system
1.142
software
intangible element of a system which when applied to the hardware enables the system to perform in
the desired manner
1.143
firmware
element of hardware in which its associated software is integrated as a stage in its manufacture
16 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
NOTE In operation, the hardware and the software act together as a fixed entity.
1.144
real time
pertaining to the processing of data by a computer in connection with another process outside the
computer according to time requirements imposed by the outside process
1.145
free surface flow
flow within a closed conduit, under gravity, and normally having a free surface
NOTE 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 normally do not
affect the overall concept of free surface flow in closed conduits.
1.146
hydrometry
science of the measurement of water including the methods, techniques and instrumentation used
NOTE The adjective is “hydrometric”.
1.147
hydrological cycle
constant movement of water above, on and below the earth’s surface
1.148
hydrogeology
study of subsurface water in its geological context
1.149
hydraulic gradient
change in static head per unit distance in a given direction
1.150
static head
height, relative to an arbitrary reference level, of a column of water that can be supported by the static
pressure at a given point
1.151
creek
(river) small river, often a tributary to a larger river
1.152
creek
(sea coast) recessed inlet on a sea coast or estuary
1.153
hydrograph
relation in graphical, equational or tabular form between time and flow variables such as depth,
discharge, stage and velocity
NOTE Typically, stage and discharge hydrographs are used for open channel flows.
1.154
gradually-varied unsteady flow
generally nonuniform flow in which there are no abrupt changes in depth along the longitudinal axis of
a channel and in which depth, together with discharge and velocity changes with time
ISO/DIS 772: 2006
1.155
live storage
reservoir storage which can be drawn off for users downstream
1.156
total storage
reservoir storage between the lowest bed level and the top water level
1.157
flood storage
volume of water temporarily held above the top water level of a reservoir during a flood event
NOTE Flood storage is not retained in the reservoir but is discharged through an overflow until the normal
top water level is reached.
1.158
boundary condition
condition to be satisfied by a dependent variable of a differential equation along the boundary of a
model domain
1.159
courant condition
condition for the numerical stability of the explicit formulation of a numerical scheme which requires
that the ratio (C ) of the propagation speed of a physical disturbance to that of a numerical signal
r
should not exceed unity, i.e. C < 1
r
NOTE The condition is a requirement for an explicit-finite difference formulation applied to a hyperbolic
partial differential equation
1.160
explicit finite-difference numerical scheme
scheme which converts either the characteristic equation or the governing equation into an equation
from which any unknown may be evaluated directly (explicitly) without an iterative computation
NOTE 1 Dependent variables on the advanced time level are determined one point at a time from known
values and conditions at the present or previous time levels.
NOTE 2 The stability of an explicit scheme is conditional upon an error being a function of the time and
distance finite-difference step sizes which may result in an error growing as the solution progresses.
NOTE 3 When the Courant condition is met, resulting in limitations in the maximum time and distance steps
which can be used, generally an explicit scheme is stable, but there can be instances of instability.
NOTE 4 If the converted equation is linear and algebraic, an iterative computation is not needed.
1.161
implicit finite-difference numerical scheme
scheme which converts either the characteristic equation or the governing equation into a nonlinear
algebraic equation from which n unknown may be evaluated iteratively
NOTE 1 All of the unknown within the model domain are determined simultaneously
NOTE 2 Generally an implicit scheme is stable
NOTE 3 Although complex algorithms are required generally an implicit scheme is computationally sufficient
1.162
initial condition
description of the discharge, depth of flow or other dynamic condition at the beginning of a simulation
period for unsteady flow models
18 © ISO 2007 – All rights reserved

ISO/DIS 772: 2006
NOTE For subsequent times, the state of the system is described by the governing equations and the
boundary conditions.
1.163
method of characteristics
mathematical approach for solving boundary values by transforming the original partial differential
equations representing the physical system into corresponding characteristic equations
NOTE Characteristic equations are ordinary differential equations and generally are more amenable to
numerical solution than are the partial differential equations.
1.164
momentum coefficient
Boussinesq coefficient
quantification of the deviation of the velocity at any point in a cross-section from a uniform velocity
distribution in the same cross-section
NOTE Values of the coefficient:
a) unity indicates that a uniform velocity distribution is present in the cross-section.
b) 1.01 to 1.12 indicates a fairly straight prismatic channel;
c) 1.0 indicates large or deep channel
1.165
standing wave
stationary wave
curved symmetrically-shaped wave on the water surface, and on the channel bed, that are virtually
stationary
NOTE When standing waves form, the water surface and the bed surfaces are roughly parallel and in phase.
1.166
isotropic
having the same properties in all directions
1.167
photomultiplier
electronic device for amplifying and converting light pulses into measurable electrical signals
1.168
water course
a channel taking off water from a branch canal, distributary or a minor to a delineated block of land. It
may be built at Government expenses or by the beneficiary agriculturists depending upon the local
irrigation practice and the area irrigated
1.169
tributary
surface or underground stream which contributes its water, continuously or intermittently, to another
large or larger stream
1.170
brook
small shallow stream, usually continuous in its discharge, which flows in somewhat turbulent manner;
its channels are usually irregular
...