ISO/TR 24578:2012

TECHNICAL ISO/TR
REPORT 24578
First edition
2012-05-15
Hydrometry — Acoustic Doppler
profiler — Method and application for
measurement of flow in open channels
Hydrométrie — Profils Doppler acoustiques — Méthode et application
pour le mesurage du débit en conduites ouvertes
Reference number
©
ISO 2012
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ii © ISO 2012 – All rights reserved

Contents  Page
Foreword .iv
1 Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Principles of operation . 3
4.1  General . 3
4.2  Doppler principle applied to moving objects . 4
4.3  Acoustic Doppler operating techniques . 6
4.4  Movement monitoring techniques .12
5  Principles of methods of measurement .13
5.1  Data retrieval modes .13
5.2  Maintenance .13
5.3  Training .13
5.4  Flow determination using a vertically mounted ADCP .13
5.5  Discharge measurement process .16
5.6  Section-by-section method .26
5.7  Ancillary equipment .26
6  Site selection for the use of vertically mounted ADCPs .27
6.1  General .27
6.2  Additional site-selection criteria .27
7  Computation of measurement .28
7.1  Vertically mounted ADCPs .28
7.2  Measurement review .29
8  Uncertainty .30
8.1  General .30
8.2  Definition of uncertainty .30
8.3  Uncertainties in ADCP measurements  .
General considerations .31
8.4  Sources of uncertainty .31
8.5  Minimizing uncertainties .32
Annex A (informative) Velocity distribution theory and the extrapolation of velocity profiles .33
Annex B (informative) Determination of discharge between banks and the area of
measured discharge .35
Annex C (informative) Example of an equipment check list .38
Annex D (informative) Example of ADCP gauging field sheets .39
Annex E (informative) Beam alignment test .42
Bibliography .44
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.
In exceptional circumstances, 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), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no longer
valid or useful.
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/TR 24578 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity
area methods.
iv © ISO 2012 – All rights reserved

TECHNICAL REPORT ISO/TR 24578:2012(E)
Hydrometry — Acoustic Doppler profiler — Method and
application for measurement of flow in open channels
1 Scope
This Technical Report deals with the use of boat-mounted acoustic Doppler current profilers (ADCPs) for
determining flow in open channels without ice cover. It describes a number of methods of deploying ADCPs to
determine flow. Although, in some cases, these measurements are intended to determine the stage-discharge
relationship of a gauging station, this Technical Report deals only with single determination of discharge.
The term ADCP has been adopted as a generic term for a technology that is manufactured by various
companies worldwide. They are also called acoustic Doppler velocity profilers (ADVPs) or acoustic Doppler
profilers (ADPs). ADCPs can be used to measure a variety of parameters, such as current or stream flow, water
velocity fields, channel bathymetry and estimation of sediment concentration from acoustic backscatter. This
Technical Report is generic in form and contains no operational details specific to particular ADCP makes and
models. Accordingly, to use this document effectively, it is essential that users are familiar with the terminology
and functions of their own ADCP equipment.
2  Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 772, Hydrometry — Vocabulary and symbols
3  Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 772 and the following apply
3.1
ADCP depth
transducer depth
depth of the ADCP transducers below the water surface during deployment measured from the centre point of
the transducer to the water surface
NOTE The ADCP depth may be measured either manually or by using an automatic pressure transducer.
3.2
bin
depth cell
truncated cone-shaped volume of water at a known distance and orientation from the transducers
NOTE The ADCP determines an estimated velocity for each cell using a weighted averaging scheme, which takes
account of the water not only in the bin itself but also in the two adjacent bins.
3.3
blank
blanking distance
distance travelled by the signal when the vibration of the transducer during transmission prevents the transducer
from receiving echoes or return signals
NOTE 1 This is the distance immediately below the ACDP transducers in which no measurement is taken.
NOTE 2 The distance should be the minimum possible. However, care must be taken not to make the distance too short
in order to avoid contamination by ringing or bias by flow disturbance.
3.4
bottom tracking
method whereby the velocity of the bottom is measured together with the water velocity, allowing the system to
correct for the movement of the vessel
NOTE This acoustic method is used to measure boat speed and direction by computing the Doppler shift of sound
reflected from the stream bed relative to the ADCP.
3.5
data retrieval modes
real-time mode in which the ADCP can retrieve data
NOTE A self-contained mode can be used but is not normally recommended.
3.6
deploy
ADCP initialized to collect data and propel the instrument across the section to record data
NOTE A deployment typically includes several (pairs) of transects or traverses across a river or estuary.
3.7
deployment method
operating mode
technique to propel the ADCP across a watercourse
NOTE Three different deployment methods are used: a manned boat; a tethered boat; or a remote-controlled boat.
3.8
ensemble
profile
collection of pings
NOTE 1 A column of bins equivalent to a vertical (in conventional current meter gauging).
NOTE 2 An ensemble or profile may refer to a single measurement of the water column or an average of pings or profile
measurements.
3.9
ping
series of acoustic pulses, of a given frequency, transmitted by an acoustic Doppler current profiler
NOTE Sound pulses transmitted by the ADCP for a single measurement.
3.10
profiling mode
ADCP settings for type pattern of sound pulses
NOTE 1 Some types of equipment allow settings to be selected by the user.
NOTE 2 Different modes are suitable for different flow regimes, e.g. fast or slow, deep or shallow.
3.11
real-time mode
data retrieval mode in which the ADCP relays information to the operating computer as it gathers it.
NOTE The ADCP and computer are connected (physically or wireless) throughout the deployment.
2 © ISO 2012 – All rights reserved

3.12
self-contained mode
autonomous mode
data retrieval mode in which the ADCP stores the information it gathers within its own memory and then
downloaded to a computer after deployment.
NOTE This method is generally not used by majority of ADCP practitioners nor recommended by the majority of
hydrometric practitioners.
3.13
transect
pass
one sweep across the watercourse during an ADCP deployment
NOTE 1 In the self-contained mode, a deployment can consist of any number of transects.
NOTE 2 In the real-time mode, a deployment consists of one transect.
4  Principles of operation
4.1  General
The Acoustic Doppler Current Profiler (ADCP) is a device for measuring current velocity and direction, throughout
the water column, in an efficient and non-intrusive manner. It can produce an instantaneous velocity profile
down through the water column while disturbing only the top few decimetres. ADCPs nominally work using the
Doppler principle (see 4.2). An ADCP is usually a cylinder with a transducer head on the end (see Figure 1).
The transducer head is a ring of three or four acoustic transducers with their faces angled to the horizontal and
at specified angles to each other.
Key
1 forward
2 port
3 starboard
4 aft
Figure 1 — Sketch illustrating typical ADCP with four sensors
The instrument was originally developed for use in the study of ocean currents – tracking them and producing
velocity profiles – and other oceanographic work. It has since been developed for use in estuaries and rivers.
An ADCP can be mounted on a boat or a flotation collar or raft and propelled across a river (see Figure 2). The
route taken does not need to be straight or perpendicular to the bank. The instrument collects measurements
of velocity, depth and position as it goes. The ADCP can also be used to take measurements in fixed positions
across the measurement cross section. These fixed positions are similar to verticals in conventional current
meter gauging (see ISO 748). This is referred to as the “section-by-section method” (see 5.6).
4 2
Key
1 start
2 path of boat
3 path of boat on river bottom
4 flow velocity vectors
5 finish
Figure 2 — Sketch illustrating moving-boat ADCP deployment principles
4.2  Doppler principle applied to moving objects
The ADCP uses ultrasound to measure water velocity using a principle of physics discovered by Christian
Doppler. The reflection of sound-waves from a moving particle causes an apparent change in frequency to the
reflected sound wave. The difference in frequency between the transmitted and reflected sound wave is known
as the Doppler shift.
4 © ISO 2012 – All rights reserved

It should be noted that only components of velocity parallel to the direction of the sound wave produce a
Doppler shift. Thus, particles moving at right angles to the direction of the sound waves (i.e. with no velocity
components in the direction of the sound wave) will not produce a Doppler shift.
Figure 3 — Reflection of sound-waves by a moving particle results in an apparent change in the
frequency of those sound waves
Doppler’s principle relates the change in frequency to the relative velocities of the source (reflector) and the
observer. In the case of most ADCPs, the transmitted sound is reflected off particulates or air bubbles in
the water column and reflected back to the transducer. It is assumed that the particulates move at the same
velocity as the water and from this the frequency shift can be translated to a velocity magnitude and direction.
It should be noted, however, that excessive air bubbles can cause distortion in, or loss of, the returned signal.
Furthermore, air bubbles naturally rise and therefore are likely not to be travelling in a representative magnitude
and direction.
4.2.1  Speed of sound in water
The calculated velocity is directly related to the speed of sound in the water. The speed of sound varies
significantly with changes in pressure, water temperature, salinity and sediment concentration, but is most
sensitive to changes water temperature. Most manufacturers of ADCP systems measure water temperature
near the transducer faces and apply correction factors to allow for temperature related differences in the speed
of sound. ADCPs that do not have temperature compensation facilities should be avoided.
If the instrument is to be used in waters of varying salinity, the software used to collect data should have the
facility to correct for salinity.
Figure 4 — Sound speed as a function of temperature at different salinity levels (left panel) and
salinity at different temperature levels (right panel)
Figure 4 indicates the effect of temperature and salinity on the speed of sound. As a general rule,
— a temperature change of 5 °C results in a sound speed change of 1 %,
— a salinity change of 12 ppt (parts per thousand) results in a change in sound speed of 1 %; freshwater is
0 ppt and seawater is in the region of 30 to 35 ppt), and
— the full range of typical temperature and salinity levels (−2 to 40 °C and 0 to 40 ppt) gives a sound speed
range of 1 400 to 1 570 m/s (total change of 11 %).
4.3  Acoustic Doppler operating techniques
4.3.1  General
All ADCPs fit into one of three general categories, based upon the method by which the Doppler
measurements are made:
— pulse incoherent (including narrowband);
— pulse-to-pulse coherent;
— spread spectrum or broadband.
Reference should be made to the instrument manual to determine the type of instrument being used.
4.3.2  Pulse incoherent
An incoherent Doppler transmits a single, relatively long, pulse of sound and measures the Doppler shift, which
is used to calculate the velocity of the particles along the path of the acoustic beam. The velocity measurements
made using incoherent processing are very robust over a large velocity range, although they have a relatively
high short-term (single ping) uncertainty. To reduce the uncertainty, multiple pulses are transmitted over a short
time period (typically 9 to 20 per second), these are then averaged before reporting a velocity. “Narrowband” is
used in the industry to describe a pulse-to-pulse incoherent ADCP. In a narrowband ADCP, only one pulse is
transmitted into the water per beam per measurement (ping), and the resolution of the Doppler shift must take
place during the duration of the received pulse. The narrowband acoustic pulse is a simple monochromatic
wave and can be processed quickly.
4.3.3  Pulse-to-pulse coherent
Coherent Doppler systems are the most accurate of the three, although they have significant range limitations.
Coherent systems transmit one, relatively short, pulse, record the return signal, then transmit a second
short pulse when the return from the first pulse is no longer detectable. The instrument measures the phase
difference between the two returns and uses this to calculate the Doppler shift. Velocity measurements made
using coherent processing are very precise (low short-term uncertainties), but they have significant limitations.
Coherent processing will work only in limited depth ranges and with a significantly limited maximum velocity.
If these limitations are exceeded, velocity data from a coherent Doppler system are effectively meaningless.
4.3.4  Spread spectrum (broadband)
Like coherent systems, broadband Dopplers transmit two pulses and look at the phase change of the return
from successive pulses. However, with broadband systems, both acoustic pulses are within the profiling range
at the same time. The broadband acoustic pulse is complex; it has a code superimposed on the waveform. The
code is imposed on the wave form by reversing the phase and creating a pseudo-random code within the wave
form. This pseudo-random code allows a number of independent samples to be collected from a single ping.
Due to the complexity of the pulse, the processing is slower than in a narrowband system; however, multiple
independent samples are obtained from each ping.
The short-term uncertainty of velocity measurements using broadband processing is between that of incoherent
and coherent systems. Broadband systems are capable of measuring over a wider velocity range than coherent
6 © ISO 2012 – All rights reserved

systems; although, if this range is exceeded, the velocity data will be rendered meaningless. The accuracy and
maximum velocity range of a broadband system is a function of the precise processing configuration used.
Although it can provide highly accurate velocity data in certain situations, coherent processing is not a practical
tool for most current profiling applications. Incoherent and broadband processing are the primary processing
techniques used in ADCPs in field applications.
4.3.5  Operational considerations
Following the blanking distance, ADCPs subdivide the water column being sampled by each beam into depth
cells ranging from 0,01 m to 1 m or greater (Figure 5). A centre-weighted radial velocity is measured for each
depth cell in each beam. With these results and using trigonometric relations, a 3-dimensional water velocity is
computed and assigned to a given depth cell in the water column. Although this is analogous to a velocity profile
obtained from a point velocity meter, the entire measurable region of the water column is sampled by the ADCP.
Key
1 cell/ bin 1
2 cell/ bin 2
3 cell/ bin 3
4 cell/ bin n
5 blanking distance
Figure 5 — ADCP depth cells or bins
The bin/cell size and the blanking distance should be set to minimize measurement uncertainty. This is dependant
on water depth, velocity and time of measurement .The bin size and lag should be optimized accordingly. Long
lags improve measurements and large bins increase the signal-to-noise ratio of the scatters in the pulse. This
also reduces uncertainty (see Clause 8). The disadvantage of larger bins is that they may limit profiling in
shallow depths. Small bins with a long lag lead to a decreased signal-to-noise ratio, increasing uncertainty.
Generally, the larger the sum of bin size and duration of individual measurement, the lower is the uncertainty
of the velocity measurement within each bin. The greater the number of bins in the water column, the lower the
uncertainty in the overall velocity estimate for that ensemble. A smaller bin size reduces the unmeasured area
in the water column (see Figure 8).
Shallower streams or rivers require smaller depth cells. A minimum of two measured bins is recommended
at the edges. However, for the majority of the cross section, a minimum of three cells are required in each
ensemble in order to allow extension of the velocity profile into the unmeasured sections of the water column.
The range-gating technique used by ADCPs creates centre-weighted averages for each depth cell with an
overlap between bins (see Figure 6). A pulse pair (with an overlap length equal to a bin size) is emitted by the
ADCP transducer. As the pulse pair propagates down through the water column, reflected signals are received
from successive depth cells. The loudest signal is received from reflections occurring when the full (overlap)
length of the pulse pair is within the depth cell. Thus, a weight of 1 is achieved at the centre of the cell and
tapers to a zero weight one bin size from the centre. The neighbouring bins would overlap such that each
portion of the water would achieve a weight of 1.
Key
1 depth
2 depth cell
3 time after ping
4 velocity weighting
5 pulse pair
6 loudest signal
Figure 6 — Showing the effect of range-gating and bin size on velocity averaging as a pulse pair
propagates down through the water column
8 © ISO 2012 – All rights reserved

4.3.6  Near boundary data collection
The angle of the ADCP transducers varies depending on the manufacturer and the instrument. They typically
range between 20 and 30 degrees from the vertical. The ADCP cannot measure all the way to the streambed.
When acoustic transducers produce sound, most of the energy is transmitted in the main beam. However,
there are also side lobes that contain less energy that propagate from the transducer as well. These side lobes
do not pose a problem in most of the water column because they are of low energy. However, when the side
lobe strikes the streambed, the streambed being a good reflector of this acoustic energy, much of the energy
is reflected back to the transducer. Due to the slant of the beams, the acoustic energy in the main beam
reflects off scatters in the water column near the bed at the same time that a vertical side lobe reflects from
the streambed. The energy in the main beam reflected from these scatters in the water column is relatively
low compared to the energy sent out from the transducer and the energy in the side lobe returned from the
streambed is sufficient to contaminate the energy from the main beam near the bed. Therefore, there is an area
near the bottom that cannot be measured due to side-lobe interference. This distance is computed as:
[1-cos(system angle)] x 100 (1)
Thus, for a 20 degree system, it is 6 % of the range from the transducer. As the profile approaches the boundary,
interference occurs due to reflection of side-lobe energy taking a direct (shorter) path to the boundary (see Figure 7).
Key
1 side lobe
2 main beam
3 maximum slant range
4 draft
5 blanking distance
6 area of measured discharge
7 side-lobe interference
8 stream bed
Figure 7 — Diagram illustrating depth zones within the water column: blanking distance, area of
measured discharge and zone subject to side-lobe interference
To ensure that there is no bias in the velocity estimate, the ADCP and its software should ignore that portion
of the water column affected by side-lobe contamination near the bed. This is undertaken automatically by the
instruments in current use. The user manual should provide information on this.
To avoid velocity bias, the mean velocity at depth should only be accepted if all beams are able to measure to
the same water depth. Data from shorter path lengths (maybe due to boulders or other channel undulations)
should not be used.
As illustrated in Figure 8, the instrument is unable to make velocity measurements in three areas:
— near the surface (due to the depth at which the instrument is located in the water and, added to this, the
instrument blanking distance);
10 © ISO 2012 – All rights reserved

— near the bed (due to sidelobe interference, channel undulations and acoustic reflections caused at the bed);
— near the channel edges(due to a lack of sufficient water depth or to acoustic interference from signals
returned from the bank).
The first two can be estimated by the ADCP using an appropriate velocity distribution extrapolation method
such as the 1/6th power law (see Annex A). In order to estimate the edge discharges, it is necessary to
measure the distance from the position where the first or last good data are obtained for the transect. This
distance is then used to assist with determination of discharge in the unmeasured portions close to the edges.
One technique is described in Annex B. The total discharge can then be estimated thus:
QQ=+QQ+ (2)
t adcplbrb
where
QQ=+QQ+ (3)
adcpm tb
and where
Q is the total discharge;
t
Q is the discharge determined by ADCP, i.e. total discharge minus edge discharge;
adcp
Q is the discharge at the left bank edge;
lb
Q is the discharge at the right bank edge;
rb
Q is the discharge measured by the ADCP, i.e. the total discharge in the measured bins;
m
Q is the discharge in top portion determined by the ADCP by velocity profile extrapolation;
t
Q is the discharge in bottom portion determined by the ADCP by velocity profile extrapolation.
b
Key
1 measured area
2 top
3 bottom
4 edge
Figure 8 — The velocity is only measured in the central area, elsewhere it is estimated by
extrapolation
4.4  Movement monitoring techniques
4.4.1  Bottom tracking
ADCPs are also used to make discharge measurements from a moving boat. The instruments can use the
Doppler principle to track their movements across a channel using a technique called “bottom tracking”. Bottom-
tracking measurements are similar to water-velocity measurements, but separate pulses are used. Bottom-
tracking pings are longer than water pings. These pings are also used to measure the depth of water. The
sound pulses are reflected from the channel bed and used to calculate the velocity of the instrument relative
to the bed. The bed is then assumed to be stable and still as seen by the equipment. ADCPs may also have
an onboard compass and can combine this data with bottom-tracking data to determine direction and speed.
Key
1 direction of flow
2 boat velocity
3 water velocity
Figure 9 — Velocity measurements taken during an ADCP gauging
4.4.2  Differential Global Positioning System (DGPS)
A DGPS is also available as an attachment to ADCPs to provide movement data. This is used as an alternative
to bottom tracking when the bed is unstable or when bottom tracking is unable to accurately determine bed
level due, for example, to weed growth or heavy suspended sediments. It is only suitable if a sufficiently
accurate DGPS is available (see 5.5.10.1). When using a DGPS, it is necessary to properly calibrate the
internal compass of the ADCP and obtain an accurate estimate of the local magnetic variation.
4.4.3  Stationary operation
The instrument can be used in place of a current meter, e.g. cableway-mounted and its horizontal position
identified as for a conventional flow determination. If the system has a built-in compass, the instrument can be
used without introducing errors. If there is no system compass, then it is critical to ensure that the instrument
is deployed perpendicular to the cross section without any instrument movement during the measurement. If
this is not possible, the direction of the instrument relative to the direction of flow should be determined. This is
similar to the principles applicable to conventional current meter gauging from a suspension cable.
12 © ISO 2012 – All rights reserved

Even though a stationary operation is similar to conventional current meter gauging and the general principles
of current meter gauging should apply, there are a number of issues that are specific to the use of ADCPs.
5  Principles of methods of measurement
5.1  Data retrieval modes
ADCPs can be used in two ways.
a) The first method is to record data in real-time mode. The equipment stays in communication with the
computer throughout the gauging process and the data are processed and displayed on the computer
screen as they are recorded.
b) The second method is to set the ADCP to record data in the self-contained/autonomous mode. The
instrument records the measurements internally and the data are downloaded later (see 5.4.6). This
method is generally not used by the majority of ADCP practitioners and is not recommended. It should be
possible to use real-time mode for most, if not all, applications these days.
A separate portable power source may be necessary to power the laptop when running the ADCP in real-time
mode, as laptop batteries may not last a full day’s gauging.
5.2  Maintenance
Most ADCPs are capable of running built-in diagnostic checks. A combination of firmware and software can
be run to verify that various ADCP systems are functioning properly and the ADCP is responding. These
checks should be carried out invariably at the beginning and end of each field day, and preferably before each
discharge determination/measurement, or during site inspections in the case of permanent installations. Key
checks are made for CPU tests, DSP tests, beam operation, sensor tests, and battery condition,.
Manufacturers recommend that ADCPs should be serviced at regular intervals. If these services are not carried
out, faults may lie undetected resulting in erroneous measurements. In general, ADCPs used for river discharge
measurements do not need frequent service. For example, manufacturers recommend regular replacement of
O-ring seals. However, since ADCPs used for river discharge measurements are rarely submerged more than
30 cm to 40 cm, this is not usually necessary.
5.3  Training
At least one member of an ADCP gauging team should have received formal, detailed training in the operation
of the equipment and associated software being used. The other team members should be familiar with field
operation of the equipment and the general principles of ADCP gauging.
As ADCP technology is continually changing, it is recommended that users keep up-to-date with these changes.
Arrangements should be made with the equipment suppliers to provide regular updates of software changes,
bug fixes and improvements to the equipment and changes in recommended operation practices. Whenever
possible, practitioners and users should have access to suitable first-time and refresher training in field use, as
well as training for data analysis, processing and quality control.
5.4  Flow determination using a vertically mounted ADCP
An ADCP determines the velocity in each depth cell (see Figure 10). Knowing the depth cell size and distance
between successive profiles, the discharge for that cell can be computed. The velocities in the unmeasured
areas of the cross section are extrapolated from those of the depth cells. The discharge from each unmeasured
area is calculated and added to that through the measured area to produce a total discharge for each ensemble.
The discharge for the portion of the cross section where measurements are made is the sum of the ensemble
discharges. The discharge in the unmeasured portions between the start bank and the first ensemble and
between the last ensemble and the finish bank are determined using an appropriate algorithm. The discharge
in the unsampled portion is then added to the total ensemble discharge to estimate the total discharge in the
cross section.
Figure 10 illustrates how discharge is determined using ADCPs. The discharge in each individual cell
is computed and these are summated to determine the measured discharge. The discharges close to the
surface, bed and banks are computed using an appropriate extrapolation technique (see Annex A). This can
be represented mathematically thus:
Qq=+ q (4)
totale∑∑nj, stimate
where
q
are the incremental discharges through each measured depth cell in the cross section;
n,j
q are the extrapolated discharges through the unmeasured areas in the cross section;
estimate
n
is the cell number in the vertical;
j
is the profile number in the horizontal.
Key
1 flow
2 profile
3 cell size
Figure 10 — Showing the measured area of the channel cross section, divided into individual
profiles and bins
To produce a discharge estimate, the ADCP has to cross a river with its transducers submerged to a known
constant depth. This is best achieved by mounting the instrument on a boat or a flotation platform. Different
methods are described in 5.4.1 to 5.4.6. For the tethered deployments, the ADCP is mounted on a flotation
platform. Different manufacturers supply different platforms. It is important to ensure that the flotation platform
is suitable for the expected water velocity range for which measurement is about to be undertaken. Platforms
may capsize if the water velocity is too high.
5.4.1  Boat mounted
If the ADCP is fixed to a boat, the fittings should be of non-ferrous materials and designed so that the position
of the ADCP can be vertically adjusted, i.e. the boat fixings should allow the transducers to be fixed at different
depths relative to the water surface. They should allow the easy installation and fixing of the ADCP to the
boat. The ADCP need not be permanently fixed to the boat. The ADCP should be mounted forward of the
engine to reduce noise and propeller wash. It should also be positioned so that the ADCP measures velocities
undisturbed by the hull of the vessel. Thus, the instrument should be mounted at the bow of the vessel or if
14 © ISO 2012 – All rights reserved

mounted at the side easily moved from port to starboard depending on which side is upstream. If mounted at
the bow, it is important that the bow wave be minimized so as not to affect velocity measurements.
The ADCP can also be deployed on a small floating platform, which can be tethered to a boat for transport
across a river. This allows movement of the instrument for optimal positioning during a deployment. The boat
hull should not be upstream of the ADCP.
5.4.2  Tethered deployment on a tow rope
Use of a tethered boat and tow rope is the simplest and most efficient method for deploying the equipment at
many gauging sites. The equipment needed is simple – two ropes that will stretch across the section and the
flotation platform. One operator has to be able to cross the river with the end of a rope. It may even be possible
to set up a pulley system with a single loop of rope. If the ADCP is to be deployed from a bridge, it may be
possible, depending on site conditions, to use a single rope (see 5.4.4.).
However, towing the ADCP on a rope across a wide, navigable river may be impractical and cumbersome. If
there is no option but to use this method at a site, then one of the ropes should be substituted with a cable
which can be lowered to the bed to allow boat traffic to pass. One or both of the operators should have a
megaphone so that they can warn boat traffic about the presence of a rope and inform them from which side
of the equipment they may pass.
This method is suitable for smaller rivers or canals, and sites with lower velocities. Very high velocities may
cause the operators to be dragged into the water.
5.4.3  Tethered deployment from a cableway
Existing cableways normally used for conventional current meter flow measurement can be used to deploy the
ADCP. At these sites, it is a highly effective and efficient deployment method as no additional equipment is
needed other than the flotation platform. If this method is used, the suspension cable should be slack enough
to ensure the platform is resting on the water surface so that the transducers remain at constant depth. The
suspension weight used to maintain tension and to overcome the sag of the cableway should be kept clear of
the water surface to avoid turbulence around the ADCP.
5.4.4  Tethered deployment from a bridge
The ADCP can also be deployed from a bridge over the river using a rope/handline or, in a similar manner to
that of a conventional current meter, using a bridge-gauging derrick or an “A“-frame to position the ADCP. The
instrument should be deployed in its flotation collar to ensure the transducers remain at constant depth. If the
instrument is to be lowered by the “A“-frame rather than launched from the bank, the ”A“-frame should be able
to support both the ADCP and flotation platform safely.
5.4.5  Tethered deployment on a remote control craft
Deployment of the ADCP on a remote control platform is the preferred option where there is no cableway and
no way for the operator to cross the river. As an ADCP is a relatively expensive piece of equipment, some
practitioners find it advisable to attach a light line so that, in case of failure of the motors or motor control
device, the ADCP can be recovered. If a light line is used for this purpose, care should be taken to ensure that
it does not cause a drag and does not get fouled in the props.
5.4.6  Self-contained mode
The use of ADCPs in self-contained mode is not recommended. This was a technique used earlier when it was
not possible to operate in real-time mode. However, it has been included in this document for completeness,
in case the user experiences a problem that results in real-time communication with the ADCP not being
possible. As in real-time mode, flow determination will require several transects of the river. However, the data
will be recorded as one continuous set and it may be difficult to identify the end of a transect. Therefore, care
should be taken to note the time at either end of each transect. It is also useful to pause at the end of each
crossing for 30s to clearly identify the end of a transect, so that measurements taken during each transect can
be identified and distinguished from other transects and pause time. The ADCP should be synchronized with
the timing device used to record the transect start and finish time.
5.5  Discharge measurement process
5.5.1  Instrument tests
Each ADCP used should be tested:
— when the ADCP is first acquired;
— after factory repair and prior to any data collection;
— after firmware or hardware upgrades and prior to any data collection; and
— at some periodic interval (for example, annually).
The purpose of an instrument test is to verify that the ADCP is working properly for making accurate discharge
measurements. Various methods for testing ADCP accuracy include tow-tank tests, flume tests, and
comparison of ADCP discharge measurements with discharge measurements from some other source, such
as conventional current meters. Each of these methods has limitations as discussed by Oberg (2002).
5.5.1.1  Beam-alignment test
A common source of instrument bias is for the beams to be misaligned. The user can evaluate the potential bias
caused by beam misalignment by a simple field test for instruments which have an internal compass. The beam-
alignment test compares the straight-line distance (commonly called the distance “made good”) measured by
bottom tracking to that measured by GPS. Detailed procedures for the beam-alignment test are provided in
Annex D. Bottom tracking is known to have a small bias caused by terrain effects, but this bias typically is less
than 0,2 %. The USGS-recommended criterion for the Rio Grande ADCP beam alignment to be acceptable is
for the ratio of bottom track made good to be between 0,995 and 1,003. For other ADCPs, sufficient data have
not been c
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