ISO 26906:2015

INTERNATIONAL ISO
STANDARD 26906
Second edition
2015-10-15
Hydrometry — Fishpasses at flow
measurement structures
Hydrométrie — Échelles à poissons auprès des structures mesurant le
débit
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 1
4  Symbols . 2
5  Principle . 4
6  Installation . 4
6.1 Requirements for gauging structure and fishpass installations . 4
6.1.1 Selection of site . 4
6.1.2 Installation conditions . 5
6.1.3 Upstream channel . 6
6.1.4 Separate channel . 6
6.1.5 Downstream channel . 7
6.1.6 Flow measurement and fishpass structures . 7
6.1.7 Maintenance . 7
6.1.8 Measurement of head . 8
6.2 Requirements specific to the fishpass. 8
6.2.1 General. 8
6.2.2 Guidelines for basic parameters of fishpasses . 9
6.2.3 Location and attraction flows . 9
6.2.4 Downstream entry/exit to fishpass . 9
6.2.5 Upstream exit/entry from fishpass .10
7 Fishpass performance .10
7.1 General .10
7.1.1 Types .10
7.1.2 Fishpasses with interconnected pools .10
7.1.3 Fishpasses with continuous energy dissipation .10
7.1.4 Calibration and discharge coefficient .11
7.2 Larinier super-active baffle fishpass with baffle sizes between 75 mm and 150 mm .11
7.2.1 Description . . .11
7.2.2 The following features apply to the range of baffle sizes from 75 mm to
150 mm .12
7.2.3 Limitations .13
7.2.4 Modular flow calibration .14
7.2.5 Modular limit .15
7.3 Pool-type fishpass with V-shaped overfalls.15
7.3.1 Description . . .15
7.3.2 Determination of discharge under free flow conditions .18
7.3.3 Modular limit .21
7.3.4 Determination of discharge under submerged flow conditions .22
7.3.5 Limitations .23
7.3.6 Scaling up to standard design .23
7.4 Dutch pool and orifice fishpass .23
7.4.1 Description . . .23
7.4.2 Determination of discharge .25
7.4.3 Scaling up the standard design .27
8 Computation of discharge .27
8.1 Principles .27
8.2 Details .28
9  Uncertainties in flow determination .28
9.1 General .28
9.2 Combining uncertainties .29
9.3 Uncertainty in the discharge coefficient u*(C ) for the fish-pass .30
De 68
9.4 Uncertainty in the effective head .30
9.5 Uncertainty budget.30
9.5.1 General.30
9.5.2 Compound structures – sources of uncertainty .31
10  Example .31
10.1 Installation .31
10.2 Flow conditions .32
10.3 Computation of discharge .32
10.3.1 Modularity .32
10.3.2 Flow over the flat-V weir .32
10.3.3 Flow through the fish-pass.33
10.3.4 Total flow .33
10.3.5 Uncertainty in flow through the fish-pass .33
10.3.6 Uncertainty in flow over the flat-V weir .34
10.3.7 Uncertainty in total flow .34
10.3.8 Reduction in overall uncertainty if second head gauge were to be installed
upstream of the fish-pass .34
Annex A (informative) Introduction to measurement uncertainty .35
Annex B (informative) Performance guide for hydrometric equipment for use in
technical standards.44
Bibliography .48
iv © ISO 2015 – All rights reserved

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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow
measurement structures.
This second edition cancels and replaces the first edition (ISO 26906:2009), which has been
technically revised.
Introduction
Flow gauging structures are commonly used for the determination of open channel flows. To operate
satisfactorily, these structures require a head difference to be generated between the upstream and
downstream water levels. At structures designed to operate in the modular flow range, an upstream
head measurement is used to interpret flow rates. At structures designed to operate in both the
modular and drowned flow ranges, the upstream head measurement is augmented by a second
measurement which senses tailwater conditions. The former type tends to require higher head losses
over the structure.
In recent years, greater emphasis has been placed on environmental issues, including the free migration
of fish in watercourses. It is acknowledged that flow measurement structures, with their requirement
for a head loss between upstream and downstream conditions, that create high velocities may inhibit
the movement of fish. It has become important, therefore, to consider ways of aiding fish migration
without significantly affecting flow measurement accuracy.
vi © ISO 2015 – All rights reserved

INTERNATIONAL STANDARD ISO 26906:2015(E)
Hydrometry — Fishpasses at flow measurement structures
1 Scope
This International Standard specifies requirements for the integration of fishpasses with flow
measurement structures. It identifies those fishpasses which have satisfactory hydrometric calibration
data and gives methods for computing combined flows and uncertainties.
NOTE Flow measurement structures and fishpasses have inherently different hydraulic performance
criteria. Flow measurement structures perform better with uniform flow patterns; conversely, fish passage
performance is improved by the variability of the flow conditions that allow fish and other aquatic inhabitants
to select the passage conditions that best meet their mode of movement. This International Standard does not
suggest that the fishpasses discussed are the preferred methods of fish passage or that they are good enough
that passage performance can be sacrificed to obtain a single structure that meets both the fish passage and flow
measurement requirements.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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
ISO 14139, Hydrometric determinations — Flow measurements in open channels using structures —
Compound gauging structures
3  Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 772 and the following apply.
3.1
fish pass design range
range of flows within which fish require passage
Note 1 to entry: This may be defined on an annual or a seasonal basis, e.g. for spawning.
Note 2 to entry: In some countries the design range is determined in terms of flow statistics. For example, the
design range for coarse fish in the UK is usually between the estimated mean daily flow that is exceeded 95 % of
the time and the mean daily flow that is exceeded 20 % of the time. While for migratory salmonids including sea
trout and salmon, it is 95 % and 90 % to 10 % respectively.
4  Symbols
Where a symbol applies to a particular type of fishpass, it is indicated as follows.
[L] indicates applicable to the Larinier super-active baffle fishpass (see 7.2)
[PT] indicates applicable to the pool type fishpass with V-shaped overfalls (see 7.3)
[PO] indicates applicable to the Dutch pool and orifice fishpass (see 7.4)
[D] indicates distance between baffles in a Larinier Fish Pass
[S] indicates the Slope of the Larinier Fish Pass in degrees
Symbol Term Unit
a height of baffle [L] m
a proportion of total flow through fish-pass [L]
fp
b orifice width [PO] m
b crest breadth measured at transverse section of upstream baffle [L] m
b proportion of total flow through flow measuring structure [L]
fms
B total width of fishpass channel [L] m
B pool width [PT and PO] m
B width of the non-aerated nappe [PT] m
C characteristic discharge coefficient [PO]
C dimensionless coefficient of discharge [L]
de
C characteristic discharge coefficient [PT]
D
C coefficient for the approach velocity [PT]
V
D longitudinal spacing of transverse baffles [L] m
D pipe diameter [PO] m
g acceleration due to gravity [All] m/s
h maximum head m
max
h orifice height [PO] m
v
h upstream gauged head relative to transverse section of upstream baffle [L] m
h upstream head [PT] m
h downstream head [PT] m
H upstream total head relative to transverse section of upstream baffle [L] m
H upstream total head [PT] m
H effective upstream total head relative to transverse section of upstream baffle m
1e
[L]
2 © ISO 2015 – All rights reserved

Symbol Term Unit
k head correction factor taking into account fluid property effects [L] m
h
l pool length [PT] m
L crest length [PT] m
L pool length [PO] m
L width of one unit of Larinier fishpass [L] m
n number of partitions [PO]
n length scale factor [PO]
l
n flow velocity scale [PO]
V
n discharge scale [PO]
Q
n scale factor for length dimensions [PT]
ℓ
P height of the top baffle above the upstream bed [L] m
P pool depth [PT] m
Q discharge [All] m /s
Q design discharge [PT and PO] m /s
d
S bed slope of fishpass [PO and L]
U burst velocity of fish [PO] m/s
v flow velocity [PT and PO] m/s
v velocity of approach at tapping location [L] m/s
mean flow velocity [PO] m/s
V
W upstream water level [PO] m
L1
W downstream water level [PO] m
L2
X distance to h measurement section [PT] m
u*C standard uncertainty in discharge coefficient C [L, PT and PO] %
De68 D
u* uncertainty in the gauging structure breadth measurement [L, PT and PO] %
b
u(E) absolute uncertainty in gauge/head zero [L, PT and PO] m or mm
u* uncertainty in the total head measurement [L, PT and PO] %
H1e
u absolute uncertainty in the measured upstream head [L, PT and PO] m or mm
h1
U* overall uncertainty in fish pass flow [L, PT and PO] %
fp
U* overall uncertainty in flow measurement structure [L, PT and PO] %
fms
Symbol Term Unit
U* combined overall uncertainty in fish pass and flow measurement structure %
c
flows [L, PT and PO]
Y downstream water depth, related to upstream bed level [PO] m
d
Y upstream water depth, related to upstream bed level [PO] m
α angle of V-shape [PT] deg
δ error in measurement of h [PT] m
h 1
Δh drop over the fishpass for modular flow [PT] m
head drop per pool [PO] m
Δh
design head drop per pool [PO] m
Δh
d
Δt pool drop [PT] m
5  Principle
The discharge over a flow measurement structure is a function of the upstream head (plus a measure
of the downstream head in the case of those structures designed to operate in the non-modular flow
range). When a fishpass is placed alongside a flow gauging structure, an additional flow path is created.
In certain circumstances, where the fishpass has a well-defined hydrometric calibration, total flows and
uncertainties may be calculated. Thus the fishpass becomes an integral part of the flow measurement
system. This International Standard provides the necessary design and performance information for
this type of arrangement.
6  Installation
NOTE General requirements of combined flow measurement structure and fishpass installations are given
in the following clauses.
6.1  Requirements for gauging structure and fishpass installations
NOTE Requirements for the installation of measuring structures are given in the appropriate International
Standard (see Clause 2 and the Bibliography). There is much in common between the different structures and
the requirements, which can also be applied to flow measurement structure and fishpass installations, and are
summarized in the following clauses.
6.1.1  Selection of site
6.1.1.1 A preliminary survey shall be made of the physical and hydraulic features of the proposed
site, to check that it conforms (or may be made to conform) to the requirements necessary for flow
determination by a structure, or combination of structures.
6.1.1.2 Particular attention shall be paid to the following features when investigating a site:
— at existing measurement locations the type(s), state of repair and hydraulic performance of existing
structures shall be assessed;
— availability of an adequate length of channel of regular cross-section;
4 © ISO 2015 – All rights reserved

— availability of an adequate width of cross sectional area outside the channel to install a bypass
channel if required. The banks of the river need to be low enough to install a bypass channel that is
not overdeep;
— the existing velocity distribution;
— the avoidance of a steep channel, if possible;
— the effects of any increased upstream water level due to the measuring structure;
— conditions downstream, including such influences as tides, confluences with other streams, sluice
gates, mill dams and other controlling features which might cause submerged flow;
— the impermeability of the ground on which the structure is to be founded, and the necessity for
piling, grouting or other sealing, in river installations;
— the necessity for the use of flood banks to confine the maximum discharge to the channel;
— the stability of the banks and the necessity for trimming and/or revetment in natural channels;
— the clearance of rocks or boulders from the bed of the approach channel;
— the effect of wind; wind can have a considerable effect on the flow in a river or over a weir, especially
when these are wide and the head is small and when the prevailing wind is in a transverse direction.
6.1.1.3 If the site does not possess the characteristics necessary for satisfactory measurement, the site
shall be rejected unless suitable improvements are practicable.
6.1.1.4 If an inspection of the stream shows that the existing velocity distribution is uniform, then it may
be assumed that the velocity distribution will remain satisfactory after the construction of a structure.
6.1.1.5 If the existing velocity distribution is irregular and no other site for a measurement is feasible,
due consideration shall be given to checking the distribution after the installation of the weir and to
improving it if necessary.
6.1.1.6 Several methods are available for obtaining a more precise indication of non-uniform velocity
distribution. Velocity rods, floats or concentrations of dye can be used in small channels, the latter being
useful in checking conditions at the bottom of the channel. A complete and quantitative assessment of
velocity distribution may be made by means of a current meter or acoustic doppler current profiler.
6.1.2  Installation conditions
6.1.2.1 The complete installation consists of an approach channel, the flow measurement and fishpass
structures and a downstream channel. The conditions of each of these three components affect the
overall accuracy of the measurements.
6.1.2.2 Installation requirements include such features as the quality of the structures, the cross-
sectional shape of channel, channel roughness and the influence of control devices upstream or
downstream of the structures.
6.1.2.3 The distribution and direction of velocity, determined by the features outlined in 6.1.1, have an
important influence on the performance of the flow measurement structure and the fishpass.
6.1.2.4 Once an installation has been constructed, the user shall prevent any change which could affect
the flow characteristics, particularly the accumulation of sediment or debris within the fishpass.
6.1.3  Upstream channel
6.1.3.1 At all installations the flow in the upstream channel shall be smooth, free from disturbance and
shall have a velocity distribution as uniform as possible over the cross-sectional area. This can usually be
verified by inspection or measurement. In the case of natural streams or rivers, this can only be attained
by having a long, straight upstream channel free from projections either at the side or on the bottom. It
is recommended that wherever possible the approach shall be straight for a distance of five times the
channel width upstream of the head measuring section. Unless otherwise specified in the appropriate
clauses, the following general 6.1.3.2 to 6.1.3.8 shall be complied with.
6.1.3.2 The altered flow-conditions due to the construction of the structure(s) might have the effect
of building up shoals of debris and sediment upstream of the structure, which in time might affect the
flow conditions. Changes in upstream bed level at the head measuring section may result in significant
changes in the distance between the bed elevation and the crest of the structure (P value) which can
affect the stage-discharge relationship due to the resultant variations in the velocity head.
6.1.3.3 In an artificial channel, the cross-section shall be uniform and the channel shall be straight for a
length equal to at least five times its breadth upstream of the head monitoring point. This also applies to
the approach to the fishpass.
6.1.3.4 In a natural stream or river, the cross-section shall be reasonably uniform and the channel shall
be straight for a length equal to at least five times its breadth upstream of the head monitoring point
6.1.3.5 If the entry to the upstream channel is through a bend or if the flow is discharged into the
channel through a conduit submerged entrance of a smaller cross-section, or at an angle, then a longer
length (say 10 channel widths) of straight approach channel may be required to achieve a regular velocity
distribution.
6.1.3.6 There shall be no baffles in the upstream channel, which are nearer than five times the
maximum head to the point of head measurement.
6.1.3.7 Under certain conditions, a standing wave may occur upstream of the installation, for
example if the approach channel is steep. Provided this wave is at a distance of not less than 30 times
the maximum head upstream, flow measurement will be feasible, subject to confirmation that a regular
velocity distribution exists at the structure.
6.1.3.8 Stop log slots, and/or fish observation camera housings shall be designed so as not to create
disturbances and irregular velocity distributions in the approach channel to the flow measuring
structure, or in specific components thereof, e.g. fish pass in separate channel. For example, where
cameras may be fixed in slots/compartments in the approach channel walls then the slot/compartment
should be protected with a transparent cover set flush with the channel wall.
6.1.4  Separate channel
6.1.4.1 For some fish passes, or at some sites, separate channels may be required to facilitate the
movement of fish into or out of the fish pass, These will help minimize the operational and maintenance
requirements such as the removal of debris and create a more favourable hydraulic conditions for flow
determination purposes. For example, the performance of Larinier fish passes as flow measurement
structures is very dependent on the crests being kept clear of trash/debris and this should be considered
when developing the design. A separate channel with a deflector/boom or submerged entrance should
be considered. It is recommended that the upstream exit of the separate fishpass channel is set laterally
to the line of the river to help reduce trash ingress.
6.1.4.2 Where a separate channel is required, an additional head measurement device will be required
in the approach channel to the fish pass at the recommended distance upstream for the type of structure
6 © ISO 2015 – All rights reserved

or fish pass concerned. The water level sensing device and the supporting arrangement shall be installed
in accordance with the appropriate hydrometric standards and the manufacturer’s guidance.
6.1.5  Downstream channel
6.1.5.1 The channel downstream of the structure is of no importance to flow measurement if the
measuring structure or gauging structure installation has been so designed that the flow is modular
under all operating conditions. A downstream gauge shall be provided to measure tailwater levels to
determine if and when submerged flow occurs. The downstream conditions will affect the location of the
downstream entrance to the fishpass and its performance. See 6.2.3.
6.1.5.2 In the event of the possibility of scouring downstream, which may lead to the instability of the
structure, particular measures to prevent this happening may be necessary.
6.1.5.3 A downstream water level sensor shall be installed if there is a possibility that the structure
may become non-modular in the future or if it is required to assess the modular limit.
6.1.5.4 The circumstances described in 6.1.4.3 may arise if the altered flow conditions, due to the
construction of the structure, have the effect of building up shoals of debris immediately downstream of
the structure or if river works are carried out downstream at a later date.
6.1.5.5 For optimum fishpass performance, the jet of water issuing into the downstream channel shall
be discernable to the fish amongst all the other competing flows and from as far away as possible. Care
shall be taken to avoid the jet being masked by cross-flows or turbulence in the receiving water. Further
details, which specifically relate to the fishpass, are given in 6.2.3.
6.1.5.6 Prior to the design of a fishpass at an existing flow gauge, it is recommended that a downstream
water level sensor is installed for a period of time in order to gain a record of water levels across the
appropriate fish migration window. This is important because the downstream level record is essential
to ensure the correct design of the fish pass.
6.1.6  Flow measurement and fishpass structures
6.1.6.1 The flow measurement structure(s) should comply with the requirements given in the
appropriate International Standard (see Bibliography).
6.1.6.2 The fishpass shall conform to with the requirements of Clause 7.
6.1.7 Maintenance
6.1.7.1 Maintenance of the flow measurement structure, the fishpass and the approach channel is
important to secure accurate continuous measurements of discharge.
6.1.7.2 It is essential that the approach channel to flow measurement structure and fishpass
installation need to be kept clean and free from silt, debris and vegetation. The float well and the entry
from the upstream channel shall also be kept clean and free from deposits.
6.1.7.3 The flow measurement structure and fishpass shall be kept clean and free from clinging debris
and care shall be taken in the process of cleaning to avoid damage to the weir or fishpass.
6.1.7.4 The provision of remote surveillance cameras is recommended in order to reduce manpower
operating resource requirements.
6.1.8 Measurement of head
6.1.8.1 When a fishpass is set alongside a flow measurement structure, an additional flow path is
created and the fishpass flow needs to be evaluated with a similar precision to that of the measuring
structure itself. The following are the two ways of doing this.
6.1.8.2 Head gauges are placed at both the fishpass and the gauging structure, and the two flows
are determined separately and then combined to give the total river flow. This method requires more
computation and instrumentation but is reliable, particularly where the upstream entry to the fishpass is
remote from the gauging structure.
6.1.8.3 Head gauges are placed only at the gauging structure and the flow at the fishpass is determined
by transferring the single measured head to the fishpass using the established principles which apply to
compound weirs. This method is more economical and is particularly useful where the upstream entry to
the fishpass is close to the gauging structure. This method should not be used when the fish pass channel
is totally separated and has an orifice or boom at the upstream end. Checks shall be undertaken to ensure
that there is no significant water level difference between the main structure head measuring point and
the level upstream of the fish pass. If the level difference is consistently greater than 3 mm, consideration
should be given to installing a separate water level sensor in the fish pass.
6.1.8.4 Head gauges shall be designed and installed in accordance with the relevant International
Standard (see Bibliography for gauging structures and this International Standard for fishpasses).
6.1.8.5 Head gauges shall be zeroed to the crest of horizontal flow measurement weirs or to the invert
level of flumes and v-shaped weirs Accuracy in zeroing gauges is very important, particularly for low
flow determination. The gauge zero should be established to within no greater than 2 mm of the weir
crest or structure invert level. See 7.2 for details of the gauge zero of a Larinier fishpass.
6.2  Requirements specific to the fishpass
6.2.1  General
The swimming performance of fish depends on many factors, including the following:
— species;
— individual size and ability;
— water temperature;
— water depth;
— water velocity;
— water quality;
— turbulence;
— motivation;
— migration period.
It is thus a complex subject with many variations. The data available are variable in both quantity and
quality, and are complex to interpret. Furthermore, the effectiveness of a fishpass in terms of ease of
passage depends on a suitable match between the type of fishpass, the specific hydraulic conditions
within the fishpass and the particular species of fish wishing to migrate. It is not within the scope of
this International Standard to cover this complex subject in detail. Instead, basic requirements which
8 © ISO 2015 – All rights reserved

apply to a range of species of fish and a range of types of fishpass are identified to help those designing
flow measurement structure/fishpass installations.
6.2.2  Guidelines for basic parameters of fishpasses
Guidelines for maximum water velocities within, head drops across and lengths of fishpasses are
given in Table 1.
Table 1 — Guidelines for maximum water velocities within, head drops across
and lengths of fishpasses
Species
Pass parameters
Coarse fish Brown trout Sea trout Salmon
−1
Max. velocity (ms ) 1,4 to 2,0 1,7 to 2,4 2,4 to 3,0 3,0 to 3,4
Pool pass
Max. head drop (m) 0,1 to 0,2 0,15 to 0,3 0,3 to 0,45 0,45 to 0,6
−1
Max. velocity (ms ) 1,1 to 1,3 1,2 to 1,6 1,3 to 2,0 1,3 to 2,0
Baffled pass
Length of pass (m) 8 to 10 8 to 10 10 to 12 10 to 12
6.2.3  Location and attraction flows
6.2.3.1  General
In many respects, the most significant problem in fish pass design is creating both upstream and
downstream conditions, to attract fish into the fish pass.
6.2.3.2 Location
For those fish travelling upstream, the entrance to the fishpass in the downstream reach shall be
located as far upstream as possible and shall be near one of the banks wherever practicable since this is
the preferred migration route for many species. This location facilitates monitoring and maintenance.
See also 6.2.4 and 6.2.5.
6.2.3.3  Attraction flows
The jet of water issuing from the fishpass shall be discernible to the fish. Exit velocities shall be in
−1 −1
excess of 0,75 ms and preferably in excess of 1,5 ms for salmonids.
The discharge through the fishpass shall be large enough to attract fish towards the downstream
entrance. There are various criteria for this including the following:
— 5 % to 10 % of the competing river flow across the fish migration window;
— a starting flow of 5 % to 10 % of the annual daily flow of the river in the fish pass at a river flow
exceeding 95 % of the time;
— a starting flow equal to the river flow which is exceeded 97 % of the time.
The discharge through the fishpass and the velocity of the outflow shall be determined in relation to
the specific circumstances, and the specific species and size of fish which need to be conveyed.
6.2.4  Downstream entry/exit to fishpass
Fish normally find their way to the most upstream point. The downstream entrance to the fishpass shall
therefore be located at the most upstream position which is easily accessible to the fish, for example
close to the downstream truncation of a gauging structure. The downstream entry to the fishpass shall
not be in areas of either re-circulating flows or highly turbulent flows. A vertical slot entry shall be
installed such that a significant jet of water flows from the fishpass over a range of river flows.
6.2.5  Upstream exit/entry from fishpass
The upstream exit from the fishpass shall not be located where there is a danger of fish being immediately
swept back downstream. A submerged orifice exit will help to minimize the ingress of floating and
underwater trash. The size of the orifice shall be large enough to avoid significant head losses which
would complicate flow measurement. The edges of the orifice shall be rounded to minimize head losses.
Velocities in the upstream channel exit from the fish pass shall not exceed the upper limit of velocity
for the target fish species. The following velocities should be used as a guideline unless other specific
limitations are required:
−1
— the mean approach velocity in the exit must be no more than 1,0 ms for migratory salmonids,
−1 −1
0,70 ms for grayling and brown trout, and 0,5 ms for coarse fish.
7 Fishpass performance
7.1  General
7.1.1 Types
There are many different types of fishpass. Generally, they form variations on the themes of steps,
slopes or lifts.
The step approach involves splitting the height to be passed into a series of small drops with various
forms of transverse separating resting pools. The slope approach involves spilling water down
relatively steep slopes where various forms of baffles are used to dissipate energy and slow down the
water velocity. To these can be added fish lifts, diversion or by-pass channels that may vary from the
totally artificial to the “natural stream-mimicking” type and many adaptations to ease the passage of
fish, including adaptations to the flow measurement structure itself.
7.1.2  Fishpasses with interconnected pools
Fishpasses with interconnected pools are perhaps the oldest type of pass in use. They are generally
applicable for most fish species, are extensively used throughout the world and in most cases require low
maintenance. They can frequently change direction, even very sharply, and therefore may be integrated
into some locations much more easily than some other types of pass. The connection between the pools
may take one of several forms including simple over-falls, a variety of notches, vertical slots, or orifices.
There may also be a combination of these.
Constraints on the use of these fishpasses include the height between each pool and the need for little
turbulence within the pools to provide the fish with resting areas during transit.
This International Standard includes two types of fishpass which fit into the interconnected pools
category:
— the pool-type fishpass with V-shaped overfalls;
— the Dutch pool and orifice fishpass.
7.1.3  Fishpasses with continuous energy dissipation
7.1.3.1  General
The two best known fishpasses with continuous energy dissipation al
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