ISO 19234:2024

Hydrometry - Low cost baffles to aid fish passage on triangular profile gauging weirs

This document specifies how to integrate baffles to aid the passage of fish on the downstream face of triangular profile weirs that conform to ISO 4360 (including Crump weirs) and ISO 4377 (flat-V weirs).

Hydrométrie — Chicanes à faible coût pour faciliter le passage des poissons par les déversoirs à profil triangulaire

General Information

• Status: Published
• Publication Date: 20-Mar-2024
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 2 - Flow measurement structures
• Drafting Committee: ISO/TC 113/SC 2 - Flow measurement structures
• Current Stage: 6060 - International Standard published
• Start Date: 21-Mar-2024
• Due Date: 29-Feb-2024
• Completion Date: 21-Mar-2024

Relations

• Revises: ISO/TR 19234:2016 - Hydrometry - Low cost baffle solution to aid fish passage at triangular profile weirs that conform to ISO 4360
• Effective Date: 23-Apr-2020

Overview

ISO 19234:2024 - Hydrometry - Low cost baffles to aid fish passage on triangular profile gauging weirs specifies how to integrate low‑cost baffles (LCBs) on the downstream face of triangular‑profile gauging weirs. It applies to weirs operated in modular flow (critical depth) and covers both Crump weirs (ISO 4360) and flat‑V weirs (ISO 4377). The first edition (2024) replaces the 2016 technical report and adds guidance for flat‑V weirs and spreadsheet design tools available from ISO.

Key topics and technical requirements

• Scope and applicability: Integration of baffles on triangular profile weirs used for open‑channel flow gauging under modular flow conditions.
• Principles of operation: Baffles are arranged in horizontal rows with staggered gaps forming oblique low‑velocity flow paths that:
  • reduce flow acceleration on the downstream slope,
  • maintain greater local depth,
  • spread energy dissipation and reduce intense hydraulic jumps,
  • provide routes exploitable by a range of fish species.
• Design and placement: Guidance on baffle geometry, spacing, gap alignment (including reflections), and the location of the first baffle for both Crump and flat‑V weirs. Spreadsheet tools are provided for layout calculations.
• Materials and construction: Requirements and recommendations for baffle materials and low‑cost construction methods appropriate for gauging weirs.
• Limitations and suitability: Criteria for site selection, species suitability (e.g., anguillid, salmonid, coarse fish), and operational limits (plunging vs streaming flow conditions).
• Maintenance: Considerations for inspection, debris management and long‑term performance.
• Normative references and terminology: Uses ISO 772 (vocabulary), ISO 4360 and ISO 4377 as normative references.

Note: The document indicates the coefficient of discharge normally remains unchanged, though recalibration is optional when baffles are fitted.

Practical applications and users

ISO 19234 is practical for:

• hydrologists and flow‑measurement engineers designing or retrofitting gauging structures;
• environmental and fisheries engineers seeking low‑cost fish passage mitigation at existing weirs;
• water resources managers and regulators balancing accurate flow measurement with aquatic habitat connectivity;
• contractors and maintenance teams implementing baffle installations.

Typical uses include retrofitting Crump or flat‑V gauging weirs to reduce fish migration barriers without compromising flow gauging accuracy.

Related standards

• ISO 4360 - triangular profile (Crump) weirs
• ISO 4377 - flat‑V weirs
• ISO 772 - hydrometry vocabulary and symbols

For design tools and the full standard text, consult ISO 19234:2024 and the spreadsheet resources at: https://standards.iso.org/iso/19234//ed-1/en/

Standards Content (Sample)

International
Standard
ISO 19234
First edition
Hydrometry — Low cost baffles
2024-03
to aid fish passage on triangular
profile gauging weirs
Hydrométrie — Chicanes à faible coût pour faciliter le passage
des poissons par les déversoirs à profil triangulaire
Reference number
ISO 19234:2024(en)
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO 19234:2024(en)
Contents  Page
Foreword .iv
Introduction .v
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Symbols . 3
5  Principles . 4
5.1 General principles .4
5.2 Crump weirs — Structures that conform to ISO 4360 .5
5.3 Flat-V weirs — Structures that conform to ISO 4377 .5
5.4 Suitability for fish species .6
6  Installation — General considerations . 8
6.1 Site selection and application .8
6.2 Baffle dimensions .9
6.3 Baffle material and construction .10
6.4 Limitations for baffle installations .11
7  Installation at Crump weir — Weirs that conform to ISO 4360 .12
7.1 General baffle arrangements for Crump weirs . 12
7.2 Location of the first baffle on Crump weirs .14
7.3 Limitations for baffle installations on Crump weirs . 15
8  Installation at flat-V weir — Weirs that conform to ISO 4377 .16
8.1 General baffle arrangements for flat-V weirs .16
8.2 Location of the first baffle on flat-V weirs . 20
8.3 Limitations for baffle installations on flat-V weirs . 20
9  Maintenance considerations .21
Bibliography .22

iii
ISO 19234:2024(en)
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow
measurement structures.
This first edition cancels and replaces (ISO/TR 19234:2016), which has been technically revised.
The main changes are as follows:
— this document has been restructured;
— low-cost baffles on flat-V weirs have been included.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO 19234:2024(en)
Introduction
Flow gauging structures such as triangular profile weirs are commonly used for the measurement of open
channel flows. This document applies to weirs operating under modular flow conditions, with flow passing
through critical depth. To operate under these conditions, such weirs require a sufficient head difference to
be generated between upstream and downstream. At structures operating in the modular flow range, flow
rate is solely a function of the upstream head.
In recent years, greater emphasis has been placed on environmental issues, including the free migration of
fish in watercourses. It is acknowledged that the head drop required to achieve modular flow can inhibit
the movement of fish. It has become important, therefore, to consider ways of aiding fish migration without
significantly affecting flow measurement accuracy.
Applied research has shown that baffles of suitable form and placement on the downstream face of triangular
profile weirs can partially mitigate fish passage impacts while retaining the gauging function.
NOTE The coefficient of discharge of the weir would normally remain the same although it is an option to
recalibrate the coefficient to take into account the placement of baffles.
[1]
The baffle system described in this document was adapted from an optimal solution for aiding fish passage
[2]
on non-gauging sloping weirs commonly used for other purposes (e.g. abstraction, flow diversion, power
generation, navigation).
1)
The following Excel spreadsheet tools can be used to design the layout of the baffles according to this
document:
— Crump weir spreadsheet (LCB placement sheet for Crump weirs 2023.xlsm);
— Flat-V weir spreadsheet (LCB placement sheet for flat-V weirs 2023.xlsx).
The spreadsheet tools are available at: https://standards.iso.org/iso/19234//ed-1/en/
1) Excel is the trademark of a product supplied by Microsoft. This information is given for the convenience of users of
this document and does not constitute an endorsement by ISO of the product named. Equivalent products may be used if
they can be shown to lead to the same results.

v
International Standard ISO 19234:2024(en)
Hydrometry — Low cost baffles to aid fish passage on
triangular profile gauging weirs
1  Scope
This document specifies how to integrate baffles to aid the passage of fish on the downstream face of
triangular profile weirs that conform to ISO 4360 (including Crump weirs) and ISO 4377 (flat-V weirs).
2  Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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
ISO 4360, Hydrometry — Open channel flow measurement using triangular profile weirs
ISO 4377, Hydrometric determinations — Flow measurement in open channels using structures — Flat-V weirs
3  Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
anguillid
eel and lamprey
long and cylindrical body-shaped species including eel (Anguilla anguilla) and lamprey (Lampetra fluviatillis
and Petromizon marinus)
3.2
non-migratory salmonid
fish of the family Salmonidae that migrates solely in freshwater including brown trout (Salmo trutta) and
grayling (Thymallus thymallus)
3.3
coarse fish
non-salmonid fish found in freshwater habitats
3.4
Crump weir
weir with a triangular profile in the streamwise direction and a horizontal crest in the transverse direction
used for gauging
Note 1 to entry: This weir was named after the inventor E.S. Crump. The upstream slope is 1:2 and downstream slope
is 1:5 (see ISO 4360).
ISO 19234:2024(en)
3.5
anadromous
living and migrating between the sea and freshwater
3.6
flat-V weir
triangular profile weir (3.15) with a transverse V-shaped crest used for gauging
Note 1 to entry: The upstream slope is 1:2 and downstream slope is 1:5 (parallel to the centreline). The cross-slopes
can be between 1:10 and 1:40 (see ISO 4377).
3.7
low-cost baffle
LCB
low-cost deflector attached to the downstream face of the structure to aid fish passage
Note 1 to entry: These are low-cost baffles in comparison to having to incorporate a formal fish pass in a gauging weir.
Note 2 to entry: These are perpendicular to the downstream slope of the weir. The geometry of the baffle is precisely
described in Figure 4.
3.8
migratory salmonid
fish of the family Salmonidae that migrates between the sea and fresh water including salmon (Salmo salar)
and sea trout (Salmo trutta)
3.9
modular flow
flow that is independent of variations in tailwater level
3.10
plunging flow
flow passing an obstruction that is directed towards the floor and defined as
H /H ≤ 0,50
2 1
where
H is the depth of water on the upstream side of baffle relative to the base of the baffle;
H is the depth of water on the downstream side of baffle relative to the base of the baffle.
Note 1 to entry: Unstable flow conditions can occur for ratios of H /H between 0,51 to 0,59.
2 1
3.11
potamodromous
living and migrating solely in freshwater
3.12
reflection
change in direction of the position of the gaps in the baffles
3.13
streaming flow
flow passing an obstruction that remains at or near the surface and defined as
H /H ≥ 0,60
2 1
where
ISO 19234:2024(en)
H is the depth of water on the upstream side of baffle relative to the base of the baffle;
H is the depth of water on the downstream side of baffle relative to the base of the baffle.
Note 1 to entry: Unstable flow conditions can occur for ratios of H /H between 0,51 to 0,59.
2 1
3.14
structural head difference
SHD
difference in elevation (in metres) between the invert (lowest level) of the crest of the triangular profile weir
(3.15) and the downstream water surface at a flow exceeded 95 % of the time
Note 1 to entry: See Figure 3 an illustration of structural head difference.
3.15
triangular profile weir
weir with a triangular profile in the streamwise direction
Note 1 to entry: This includes Crump weirs (3.6) and flat-V weirs (3.8).
3.16
V
full
flow that just fills the whole width of a flat-V weir at the crest
4  Symbols
a baffle width m
b breadth of the weir crest perpendicular to the flow direction m
c gap offset distance immediately downstream from the reflection (see Figure 6) m
d distance between baffles, centre to centre along the slope (see Figure 6) (a * suffix m
indicates the dimension in plan view)
d intermediate variable used in the Crump weir spreadsheet for calculating cutting —
L
lengths for the baffles – left-hand-side baffle
d intermediate variable used in Crump weir spreadsheet for calculating cutting lengths —
R
for the baffles – right-hand-side baffle
f offset distance between the position of gaps in successive baffles (see Figure 6) m
h gauged head relative to the crest elevation (upstream head is implied if no subscript is m
used); for flat-V weirs, the crest elevation is taken from the invert of the V
H total head, energy head, relative to crest elevation; for flat-V weirs, the crest elevation m
is taken from the invert of the V
H depth of water on the upstream side of baffle relative to the base of the baffle m
H depth of water on the downstream side of baffle relative to the base of the baffle m
L for Crump weirs, this is the distance from the crest to the base of the upstream face of m
the first baffle along the slope (in plane view);
for flat-V weirs, this distance is the smallest distance to the base of the upstream face
of the first baffle (see Figures 6 and 7 for clarity) (a * suffix indicates the dimension in
plan view)
ISO 19234:2024(en)
L distance from the crest to the centre of the first baffle along the slope (only relevant to m
Crump weirs)
L rounded up value of L (only relevant to Crump weirs) m
2 1
L maximum apron length m
a
p height of the weir crest above the upstream bed level m
q gap width m
Q flow that is exceeded for nn % of the time m /s
nn
R radius mm
T height of the first baffle m
T height of subsequent baffles used in the Crump weir spreadsheet m
s
V flow that fills the whole width of the flat-V weir at the crest m /s
full
z intermediate variable used in the Crump weir spreadsheet to determine local coordi- —
L
nates (left-hand-side) for determining the gap location
z intermediate variable used in the Crump weir spreadsheet to determine local coordi- —
R
nates (right-hand-side) for determining the gap location
Δ axis in the direction of the flow (perpendicular to the crest) used in the Crump weir —
x
spreadsheet
Δ axis in the direction perpendicular to the face of the crest (vertical upwards) used in —
y
the Crump weir spreadsheet
Δ axis in the direction along the crest used in the Crump weir spreadsheet —
z
5  Principles
5.1  General principles
Baffles are placed in horizontal parallel rows on the downstream sloping face of the weir. There is a gap
in each row of baffles that runs at an angle progressively across and down the weir face. This forms an
oblique flow path that can be reflected from side to side in narrower channels forming a V-shaped pattern in
plan view (as shown in Figures 1 and 2). The baffles retard flow, maintain a consistent depth of water, and
substantially reduce the acceleration of the water on the downstream face of the weir. The oblique flow path
formed by the gaps provides a passage route for fish with greater flow depth and lower velocities than over
the baffles. The baffles also spread the dissipation of flow energy over the length of the downstream slope of
the weir, creating a series of small hydraulic jumps and reducing the intensity of a final hydraulic jump at the
junction with the tailwater pool.
The solution creates conditions that fish can exploit to find passage over a wide range of flows. Fish can
exploit the low velocity flow path, or, when flow tops the baffles, they can swim straight up the slope, taking
advantage of the lower velocities created by the baffles. However, if the top baffle is too close to the weir
crest, it can affect gauging performance.
For the application of this document, users shall either apply ISO 4360 (including Crump weirs) or ISO 4377
(flat-V weirs).
ISO 19234:2024(en)
5.2  Crump weirs — Structures that conform to ISO 4360
Baffles are placed in rows that are parallel to the crest on the downstream sloping face of the weir. See
Figure 1.
a) Baffles in the dry (viewed from upstream b) Baffles in operation before maintenance has
during construction) been carried out (viewed from upstream)
NOTE The figure shows Jessops Weir on the River Asker, Dorset, United Kingdom.
Figure 1 — Crump weir
Although the weir in Figure 1 is a Crump weir, it no longer has a gauging function. Therefore, the first
baffle has been placed closer to the crest than this document allows. The downstream slope of this crest is
sufficiently long for more than one reflection and the flow path of the water where the gaps are located is
evident when the baffles are in operation.
The distance from the weir crest to the upstream side of the first baffle is of critical importance. The
distance to the first baffle is determined by the range of flow rates for which modular flow is required at the
gauge. The distance can be determined by using the low-cost baffle placement tool. The spreadsheet tool is
available at: https:// standards .iso .org/ iso/ 19234// ed -1/ en/
The baffle solution was tested in a laboratory with structures that operate up to a maximum head of 0,49 m
at field scale. The dimensions and location of the baffles are determined (see Figure 6) so that they do not
[3][4]
reduce the coefficient of discharge of the weir by more than 1 % .
5.3  Flat-V weirs — Structures that conform to ISO 4377
Baffles are placed in parallel rows on the downstream sloping face of the flat-V weir, along the contours of
the weir so that each baffle top is level in the horizontal plane. There are gaps in each row of baffles that run
at an angle progressively across and down the weir face. This oblique flow path can be reflected from side to
side (see Figure 2).
ISO 19234:2024(en)
a) Bird’s eye view (including installation of a  b) Downstream view of weir showing baffles in
most downstream baffle to take truncation into  operation
account)
NOTE Figure 2 a) shows Lea Bridge Weir, on the River Cuckmere, East Sussex, United Kingdom. Figure 2 b) shows
Isfield Gauging Station on the River Uck, East Sussex, United Kingdom.
Figure 2 — Flat-V weir
The distance between the weir crest and the base of the first baffle downstream of the crest is of critical
importance, and for flat-V weirs this is fixed (refer to Figure 7 and Table 5). It is fixed at 918 mm measured
downslope from the crest (900 mm measured in plan) for a flow range up to V .
full
The recommended arrangement of baffles has a single central gap in the most upstream baffle and
symmetrical gaps on either side of the weir centre in ensuing baffles. In the case of flat-V weirs, all gaps are
set at 250 mm except for the upstream central gap (see Figure 7 and Table 5).
The development of an appropriate baffle solution for these weirs was based on computational fluid dynamics,
laboratory investigations and field observations. Comparisons were made between the coefficients of
discharge obtained up to a maximum head of 0,3 m at field scale with various baffle arrangements and those
of the plain flat-V weir, for head over the weir up to the highest level of the V crest. The configuration (in
Figure 7 and Table 5) was shown to be suitable from both fish and hydrometric viewpoints. The spreadsheet
tool is available at: https:// standards .iso .org/ iso/ 19234// ed -1/ en/
The dimensions and location of the baffles are determined in such a manner so that they do not reduce the
coefficient of discharge of the flat-V weir by more than 1,6 %.
There are additional benefits of baffles on flat-V weirs. They prevent flow convergence on the weir by
straightening the flow, and they also mitigate the formation of large eddies that would otherwise be formed
downstream of the weir. These features, in themselves, can otherwise confuse fish migrating upstream and
significantly reduce or prevent their ability to pass such structures.
5.4  Suitability for fish species
The baffle system was initially developed for Crump-like weirs with the objective of providing an effective
aid to fish passage at sites where there are high velocities and shallow flows. These conditions are typically
found on sloping weirs, such as triangular profile style weirs, where they frequently compromise or prevent
passage by fish. For weirs where gauging is not required, the size of the first baffle is smaller and can be
placed such that the first and second baffle are level with the crest. Therefore, the distance of the first baffle
to the crest is closer than that being required in this document for active gauging weirs. The baffle concept
was further developed to include flat-V weirs.

ISO 19234:2024(en)
The baffles aid passage for a wide range of species and sizes of fish by attenuating velocity, increasing water
depth on the weir, and providing a low velocity streaming flow access route across and up the weir face for
smaller fish. Streaming flow is more conducive than plunging flow for fish passage. Evidence suggests that
many of the species of fish living in rivers can exploit the baffles to gain passage where otherwise it would
be difficult or impossible. They do this in different ways. Smaller species and individuals exploit the low
velocity flow path through the gaps in the baffles and larger fish tend to swim over the baffles when there is
sufficient depth of water.
Powerful swimming fish that include anadromous fish such as salmon (Salmo salar) and sea trout (Salmo
trutta), as well as weaker swimming potamodromous fish which include brown trout (Salmo trutta),
grayling (Thymallus thymallus) and coarse fish, have been shown to pass over weirs using this type of baffle
installation on Crump weirs. It is anticipated that similar hydraulic conditions generated on a flat-V weir will
also improve the passage of fish in the same manner.
Adult migratory salmonids, including sea trout as small as 290 mm and salmon as large as 1 200 mm, have
been shown by video observation to successfully pass over a non-gauging Crump-like weir (1:5 slope) 2,8 m
[5]
high that was previously impassable . In this case, because Jessop’s Weir (River Asker, Dorset, United
Kingdom) was a non-gauging weir, the first two baffles are level with the crest. The first baffle is 120 mm in
height and 600 mm from the crest and the second baffle is 200 mm in height and 1 000 mm from the crest.
At the other end of the fish size scale, brown trout of 80 mm to 298 mm have demonstrated passage
efficiencies of 67 % in one year and 82 % in the next year over a 1,60 m high, 1:4,2 sloping weir that was
[6]
previously impassable . A 50 % probability of passage (P50) was attained by brown trout at a length of
113 mm, and a 90 % probability of passage (P90) by fish at a length of 222 mm on this non-gauging Crump-
like weir retrofitted with low-cost baffles. At the site (Swanside Beck, Yorkshire, United Kingdom), the first
and second baffle are level with the crest. These efficiencies were also attained despite the last baffle and
the downstream end of the weir being elevated above the downstream water level which would make it
more difficult for fish to pass.
Trials at Brimpton Gauging Weir (River Enborne, Berkshire, United Kingdom) with potamodromous coarse
fish have indicated that a range of species and sizes of fish have successfully used baffles retrofitted
[7]
to Crump gauging weirs. At this site which has a 0,7 m head drop over a 1:5 sloping Crump weir , chub
(213 mm to 489 mm), dace (Leuciscus leuciscus, 198 mm to 206 mm) and roach (Rutilus rutilus, 240 mm to
244 mm) successfully passed low-cost baffles at efficiencies of 54 %, 33 % and 50 %, respectively. At Trent
at Stoke-on-Trent Gauging Station (River Trent, Staffordshire, United Kingdom) with a head drop of 0,56 m,
[8]
1:5 slope, and also retrofitted with baffles , chub (Leuciscus cephalus, 225 mm to 400 mm), dace (145 mm to
280 mm) and roach (145 mm to 290 mm) passed at efficiencies of 56 %, 57 % and 66 %, respectively. At the
same site, brown trout (210 mm to 400 mm) passed at an efficiency of 81 %. It should be noted that the near
crest baffle was a smaller baffle (120 mm) and further from the crest (1 220 mm) than this document would
require.
[9]
At Eshton Beck (Yorkshire, United Kingdom), a thin plate gauging station with a compound sloping weir
face (3,09 m at 1:9, 4,05 m at 1:51) and with a head drop of 0,59 m, the use of an LCB significantly improved
passage efficiency of brown trout (156 mm – 269 mm) by at least 27 % from 64 % to 91 %. It also significantly
reduced delay at and time of passage over the obstruction, and it increased the range of flows over which
fish passed the weir. The smallest fish that passed (156 mm) ascended on ten occasions, demonstrating the
ease of passage provided by the baffles.
The expectation is that most species of coarse fish living in rivers, and certainly most of those above 200 mm
in length, are capable of taking advantage of retrofitted baffles to facilitate passage. An exception is possibly
perch (Perca fluviatillis), for which the limited evidence at Brimpton and Trent at Stoke-on-Trent Gauging
Stations sites suggests that they were not successful.
There is currently no evidence to show that smaller species and/or weak swimmers such as loach (Misgurnus
anguillicaudatus), minnows (Phoxinus phoxinus), bullhead (Cottus gobio), brook lamprey (Lampetra planeri)
and small eels (Anguilla anguilla) (<300 mm) can make use of an LCB modified weir. Additional facilities can
cater for these species but are not addressed in this document.
Fish need to make a considerable effort to pass obstructions and can be at risk of developing an oxygen debt
at structures such as sloping weirs, even with baffles to aid passage. Successful passage also depends on the
...