Available online at www.sciencedirect.com

Journal of Hydro-environment Research 8 (2014) 43e49www.elsevier.com/locate/jher

Technical communications

Restoration of a gauging weir to aid fish passage

Sohan Ghimire a,*, Gavin Jones b,1

a The James Hutton Institute, Aberdeen AB15 8QH, Scotland, UKbHalcrow Group Limited, One Kingsway, Cardiff CF10 3AN, UK

Received 4 June 2012; revised 14 June 2013; accepted 24 June 2013

Abstract

This paper presents a case study of restoration of a flow gauging station namely Pontneddfechan located in South West Wales, UK. The weirwas found to be affected by significant deterioration due to prolonged use and also created obstacles to fish migrating upstream. The mainobjective of this paper is to examine the hydraulic conditions of the weir considering the opportunities for fish passage while still retaining theaccuracy of flow measurement. A one-dimensional hydrodynamic model called ISIS was used to estimate hydraulic parameters under differentflow conditions. Results indicated that the weir did not fulfil the fishery requirements in the original condition. The stilling basin was modified asa way of improving the fish pass. While the modified weir was favourable for strong swimmers like salmon and trout, it was not adequate forweaker swimmers such as coarse fish. A rock ramp structure has been designed and assessed to aid fish passage across the weir.� 2013 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rightsreserved.

Keywords: Crump weir; Fish passage; Gauging weir; Hydraulic modelling; Hydrometry

1. Background

Hydrometric structures such as weirs and flumes dominatethe hydrometric network in the UK because they offer a numberof advantages over other available gauging methods. Theyprovide a relatively accurate and reliable means of measuringflow on a continuous basis with minimal calibration and littleday-to-day maintenance (Zaidman et al., 2005). Crump and FlatV weirs are the most common flow gauging structures in theUK (Environment Agency, 2010a). While these gauging weirsare important assets for water resources management, suchtypes of intrusive structural barriers create problems for migrantfish species and result in direct influence on the stream habitatconditions, particularly riverine fish populations (Fjeldstadet al., 2011; Mouton et al., 2007; Poulet, 2007; Turnpennyet al., 2002; Mills, 1989). Obstructions created by in-channel

* Corresponding author. Tel.: þ44 (0) 844 928 5428.

E-mail addresses: sohan.ghimire@hutton.ac.uk (S. Ghimire), JonesGT@

halcrow.com (G. Jones).1 Tel.: þ44 (0) 29 2072 0920.

1570-6443/$ - see front matter � 2013 International Association for Hydro-environment Engine

http://dx.doi.org/10.1016/j.jher.2013.06.001

structures result in changes in flow regimes (Armstrong et al.,2003), and can impact upon fisheries by changing wettedperimeter (habitat availability), velocity, flow depth and theability of fish to undertake upstream and downstream migration(Solomon et al., 1999; Clough and Turnpenny, 2001).

Upstream fish pass structures are an integral and growingcomponent of projects designed to restore river connectivity andthus facilitate the upstream or downstream migration of fish(Bunt et al., 2011; FAO, 2002). The effectiveness of differenttypes of fish pass structures has been extensively studied overthe past few decades (eg. Beach, 1984; Yagci, 2010; Rhodes andServais, 2008; Kim, 2001). However, reconciling fish passage atgauging weirs has become an important issue as many weirs areaffected by significant deterioration due to prolonged useleading to reduced confidence in the accuracy of flow mea-surement data. The rehabilitation of a weir provides the op-portunity to improve upon the original design and to include fishpassage measures that may not have been considered originally(Rickard et al., 2003).

As part of its strategic policy of monitoring the environmentand managing flood risk and water resources, Environment

ering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.

44 S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

Agency Wales considered restoration options for some existinggauging weirs in South West Wales which were in poor condi-tion. Restoration of the weirs was necessary to improve thehydrometric data quality as well as offer the opportunity toenhance fish passage, thereby improving access to upstreamhabitats. In this context, the aim of this studywas to examine thehydraulic conditions of a gauging weir considering the oppor-tunities for fish passage whilst still retaining the required ac-curacy for flow measurement.

2. Location and characteristics of the gauging weir

Pontneddfechan Gauging Station is located northeast ofGlynneath on a steep reach of the River Mellte (catchmentarea circa 66 km2), approximately 1.6 km upstream of itsconfluence with the River Neath (Fig. 1). The weir has been incontinuous use for over 40 years as important assets for waterresources and flood risk management and provides flow datafor the National River Flow Archive held by the Centre forEcology and Hydrology (http://www.ceh.ac.uk/). The weir isalso used to calibrate high flow estimates for local develop-ment control (Flood Consequences Assessments) and strategicflood risk planning such as Catchment Flood ManagementPlans (CFMP’s). In addition, the data are used for variouspurposes such as resource assessment for abstraction licensing,water resources strategic planning together with WaterFramework Directive, Habitats Directive and Drought Man-agement Planning (Halcrow, 2007).

Pontneddfechan Gauging Station consists of a flat V-shapedCrump weir approximately 15 m wide and 16 m long (Fig. 2,Table 1). A Crump weir is a gauging weir with triangularcross-section, typically with a 1:2 upslope and 1:5 downslopeface and is suitable for measuring flow in low and high flowconditions. Crump weirs are used as measuring structures inopen channels and have the advantage that the coefficient ofdischarge is predictable and that they operate over a widemodular limit. A diving survey, undertaken in 2007 to assessthe condition of the weir, concluded that significant erosion

Fig. 1. Location map.

had occurred across the weir’s crest, stilling basin and sillwhich were made of reinforced concrete. Significant under-cutting of the sill toe had also occurred, compromising thestructure’s integrity (Halcrow, 2007).

3. Methodology

3.1. Fish passage requirements and design criteria

The Environment Agency Fish Pass Manual (EnvironmentAgency, 2010b) stipulates that refurbished gauging stationweirs should be modified to ensure the guideline parametersare adhered to, e.g. reduced afflux, improved approach con-ditions, or that a fish pass is constructed to form a compoundgauging and fish pass structure. The design guidelines devel-oped by the Environment Agency Joint Hydrometry andFisheries Fish Passage Group (Environment Agency, 2010b)are useful for assessing the fisheries requirements of a gaugingweir and are adopted in this study. This study considers mainlytwo factors which are directly relevant to the passage of fish:the head difference between the crest level and downstreamtail water level, and the velocity of the flow. The head dif-ference governs the height that fish will be required to ascend.The velocity of flow governs the swimming speed that a fishmust be able to maintain to be able to pass over a structure.

The high velocities and thin flows created at Flat V weirscan be very problematic as fish are often attracted by the noisyturbulence of the hydraulic jump, which they then have toovercome before negotiating the long downstream face of theweir (Zaidman et al., 2005). In the design of any fish passfacility, it is important to consider the swimming capability ofthe fish. There are generally three levels of speed. “Cruising”is the speed that can be maintained for a long period of time(hours). “Sustained” speed can be maintained for minutes.“Burst” speed is the maximum speed that can be maintainedby a fish for less than 20 s. The burst and sustained swimmingperformance of fish governs their ability to ascend weirs andfish passes and it varies between species and between in-dividuals of the same species (Clough and Turnpenny, 2001).

The fish pass must be designed in such a way that the waterdepths that fish need for ascending are respected and that thepermissible flow velocities are not exceeded for the designflow conditions. Three design criteria have been considered inthis study which relates to the position of weir crest level, flowvelocity in the stilling basin and the depth of flow. In accor-dance with the design guidelines of the Environment AgencyJoint Hydrometry and Fisheries Fish Passage Group, thefollowing criteria have been used (see Fig. 2 for notations):

(1) The maximum difference between the crest level and thedownstream tail water level (h) should be less than 0.3 mfor a Flat V weir.

(2) The mean approach velocity in the stilling basin (v) shouldnot be greater than 0.7 m/s for migratory salmon (includingtrout) or 0.3 m/s for coarse fish.

(3) The minimum flow depth (d) in the stilling basin should be300 mm.

Fig. 2. A longitudinal profile of Pontneddfechan weir (The notations are described in Methodology, Section 3.1).

Table 1

Weir geometric parameters.

Weir parameters

Type Flat V Crump

Elevation of crest, mAOL 92.80

Breadth of crest, m 15.20

Height of weir crest above bed, u/s, m 0.41

Height of weir crest above bed, d/s, m 1.27

Slope (V slope) 1:20

Channel side slope Vertical

45S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

3.2. Fish species and design flow ranges

The main species of fish present at the study site includesalmon, trout (brown trout and sea trout) and coarse fish, andare considered in this study. Consideration of the fish specieshas been based on a feasibility study of the site in consultationwith Environment Agency fish pass officers (Halcrow, 2007).

The guidelines also stipulate that facilities for the upstreammigration of fish should consider a specified flow range depend-ing upon species present (Table 2). Salmon and sea trout havesimilar lifecycles. Typically they migrate into smaller spawningstreams on elevated flows following rainfall in the autumn. Afterspawning in October to December the adult fish return seawardsover a period of up to several months (FAO, 2002).

3.3. Model development

A one-dimensional hydrodynamic model called ISIS wasused in the study. The model includes full solution modellingof open channels, floodplains, embankments and hydraulicstructures. The model solves full unsteady flow equations withthe inclusion of hydraulic control structures. Details about themodel can be found in Halcrow (2010). A one-dimensionalnetwork of river channels and associated regulating struc-tures was developed using the model (Fig. 3). The upstream

Table 2

Design flow range for different fish species.

Species Migration

period

Design flow

range

Design flow

range (m3/s)

Salmon ApreDec Q90eQ10 0.45e7.85Brown trout SepeNov Q95eQ10 0.37e7.85

Sea trout ApreDec Q95eQ10 0.37e7.85

Coarse fish MareJun Q50eQ20 1.50e5.00

boundary is defined by an inflow hydrograph using the Revi-talised Flood Estimation Method (ReFH). The ReFH boundaryis a rainfall-runoff model using procedures developed by theCentre for Ecology and Hydrology (CEH) to update the FloodStudies Report (FSR)/Flood Estimation Handbook (FEH)rainfall runoff method, in response to concerns that the FSR/FEH design model tended to overestimate design floods. Fulldetails of the ReFH method can be found in Kjeldsen (2007).

Data on daily flow and water levels were obtained fromEnvironment Agency Wales gauging records. A topographicsurvey was undertaken to map the river cross section profilesupstream and downstream of the weir. River profiles were sur-veyed at an interval of 250m as the river was fairly uniform overthe surveyed reach. The Manning’s roughness coefficient n wasestimated based on the condition of the channel at the time of thesurvey. An n value of 0.035 was selected as the channels werefairly straight with bed composed of gravels and fine sediment.

The Flat V weir is characterised by a triangular longitudinalprofile and a transverse symmetrical V-shaped crest havingsmall side slopes. Thy hydraulic characteristics of the Flat V

Fig. 3. Model network development in ISIS.

46 S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

weir are extensively studied (e.g. White, 1971; Bos, 1989).The weir conforms to BSI and ISO standards. Details aboutthe discharge measurement through the Flat V weir are givenin British Standard Institutions (1986). The general form of thedischarge relationship for the Flat V weir is given as (Fig. 4):

Q¼ 0:8CdCdr

ffiffiffig

pmZHe5=2

where:

Cd is the discharge coefficient;Cdr is a drowned flow reduction factor;m is the slope of the V (1 vertical: m horizontal);Z is a shape function dependent on the channel side slope;its value is unity when the flow is wholly within the V;when the water surface lies above the top of the V,

Z ¼ 1��1� n

m

��1� h0

He

�5=2

h0 is the depth of the V (¼b/(2m)),m;He is the total effective upstream head which is given byh1 þ v2/2g where h1 is the upstream gauged head, m abovecrest;b is the breadth of the weir crest (at the top of the V),m;v is the upstream velocity, m/s;

3.4. Model calibration and sensitivity test

The hydraulic model was calibrated against the measured dataprior to its application to assess the flow conditions. Sensitivitytests were undertaken to assess the sensitivity of some key hy-draulic parameters. The roughness coefficients assigned to thechannel bed and side banks were not significantly sensitive interms of flow discharge and water levels over theweir. Sensitivityof two types of downstream boundary conditions was tested-normal depth and critical depth, however neither of these werefound to be significant.

A number of design events were tested and applied duringthe calibration. The design flood hydrograph developed usingthe ReFH method was used as the upstream boundary condi-tion. Model-predicted water levels were compared withmeasured values. Results indicated a very good match betweenthe model-predicted and measured water levels at the weirwhen calibration coefficient of 1.3 was adopted. Longitudinal

Fig. 4. Flat V weir.

profile of the weir with typical free surface water levels for Q10

and Q90 flows are shown in Fig. 5.Despite inherent discrepancies and uncertainty in data and the

model itself, it appears that the model is fit for purpose as pre-dicted parameters correlate well with the observed values. How-ever, the model has some limitations. It can not simulate the flowin the transition zone accurately where supercritical flow from theweir face changes into sub-critical flow in the stilling basin toform a hydraulic jump. From the perspectives of fish passage, ahydraulic jump should be formed on the downstream face of theweir which was evident during the field observations. Assessmentof flow within the hydraulic jump is therefore not included in thisstudy.

4. Results and discussion

4.1. Assessment of flow condition for fish passage

White and Woods-Ballard (2003) have outlined three maincategories of potential solutions to the fish passage problem.First, construction of a bypass channel which could be muchlonger than the gauging structure in the direction of flow.Second, providing a fish pass which can be combined with thegauging structure to form compound units. Third, adopting aneasement approach which involves adaptation of gaugingstructure in the form of fish “aids” on the downstream face ofthe weir. The first two options are not appropriate for thePontneddfechan weir, mainly because of topography, and arenot considered in the study. The third option of an easementapproach has been considered and the aim was to assess thehydraulic conditions of the weir based upon appropriate weirgeometry modification to aid fish passage. Geometry of boththe main weir structure and the stilling basin will be consid-ered to assess this solution.

The weir structure is also a problem for fish passage atPontneddfechan weir. An additional obstacle to fish passagewas posed by the stilling basin sill. Due to on-going erosion ofthe river bed, a significant vertical drop (approximately1.80 m) immediately downstream of the sill had been created,which adversely affects upstream fish passage. The weir andthe stilling basin sill were therefore separately considered forassessing fish passage.

4.1.1. Assessment of fish passage at weirThe hydraulic parameters for the original Pontneddfechan

weir under the different flow conditions relating to the threespecies of fish are shown in Table 3. The difference in crestlevel and the downstream tail water level is much higher thanthe recommended limit of 0.30 m under the low flow (Q95 andQ90) conditions. Although the weir fulfils other requirementsrelating to velocity and flow depth, it does not fulfil the con-ditions for the passage of salmon and trout because of theexcessive head difference especially during the low flowcondition. Similarly, the weir is not favourable for the passageof coarse fish as the difference in crest level and downstreamwater level is significantly higher than the recommended limitof 0.30 m particularly for the Q50 flow. In addition, the flow

Fig. 5. Typical water levels at Pontneddfechan weir.

Table 3

Hydraulic parameters of the original weir (values in parenthesis correspond to the modified weir and values higher than limit are indicated in bold).

Hydraulic parameters Salmon Brown trout/sea trout Coarse fish

Limit Predicted Limit Predicted Limit Predicted

Q90 Q10 Q95 Q10 Q50 Q20

Crest level-d/s tail water level difference, m <0.30 0.63 (0.40) 0.25 (0.05) <0.30 0.64 (0.44) 0.25 (0.05) <0.30 0.55 (0.34) 0.37 (0.17)

Mean approach velocity, m/s <0.70 0.15 (0.08) 0.80 (0.66) <0.70 0.14 (0.07) 0.80 (0.66) <0.30 0.33 (0.22) 0.64 (0.50)

Stilling basin flow depth, m >0.30 0.38 (0.56) 0.84 (0.97) >0.30 0.37 (0.55) 0.84 (0.97) >0.30 0.50 (0.65) 0.71 (0.85)

47S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

velocity during the high flow condition (Q20) is much higherthan the limit of 0.30 m/s. Based on these findings it isconsidered that the weir does not sufficiently fulfil the flowconditions required for fish passage. Fish passage is likely tobe affected by the significant difference in the crest level andthe downstream tail water level, particularly during low-flowconditions as well as the average flow condition. In addition,during the high flow condition it is likely that fish passage willbe hindered by relatively high approach velocity within thestilling basin. Therefore, modification to the weir geometry(mainly the stilling basin) was proposed such that this issuecan be overcome. This was done by maintaining the weir crestat the existing level and raising the stilling basin sill level toensure that the increase in downstream tail water level willfacilitate upstream fish passage (Fig. 6).

The hydraulic parameters for the weir under the modifiedgeometry show that the difference in crest level and tail waterlevel is decreased compared to the original condition despitebeing slightly more than the limits (Table 3). The approach flowvelocity is generally below the limits, however for coarse fishthe increase in flow velocity in the stilling basin could be anissue especially during the high flow condition. It is thereforeconsidered that the modified weir geometry is favourable forfish pass of salmon and trout but not very effective for coarsefish.

Fig. 6. Sketch showing the modification to the Pontneddf

4.1.2. Assessment of fish passage at stilling basin sillAs discussed above, there was a vertical drop approximately

1.80 m in height immediately downstream of the weir sill. In theoriginal condition the difference in the sill level and the downstreamwater level ismuch higher than the recommended limits under eachflow condition (Table 4). It can also be seen that the flow velocity atthe downstream end of the sill is much higher than the recom-mended limit. The stilling basin sill therefore does not fulfil the fishpassage requirements for anyof thefish species considered. In orderto overcome this issue, a rock ramp fish pass was proposeddownstream of theweir. Rock ramp fish passes can be cheaper thantechnical fishways, and can provide passage for fish of a variety ofspecies and sizes (Harris et al., 1998). The ramp at Pontneddfechanis comprised of arched blockstones infilledwith appropriately sizedriprap material (Fig. 6). The ramp creates pools and small falls tomimic stream riffles. A range of rock ramp slopes varying from 1%to 12% were assessed in terms of the flow velocity appropriate forfish passage. The aim was to determine the optimum design slope,and hence length of the ramp to improve fish passage.

The fish pass manual suggests different ranges of flow ve-locity for different fish species. As a guide, for a pool type fishpass, the maximum flow velocity ranges between 3.0 and 3.4 m/s for fast swimmers like salmon. A velocity of 1.7e2.4 m/s isadequate for brown trout and 2.4e3.0 m/s for sea trout. A lowerlimit of 1.4e2.0 m/s is considered appropriate for slow

echan weir with a rock ramp fish pass (not to scale).

Table 4

Hydraulic parameters of the stilling basin sill under original condition (values higher than limit are indicated in bold).

Hydraulic parameters Salmon Brown trout/sea trout Coarse fish

Limit Predicted Limit Predicted Limit Predicted

Q90 Q10 Q95 Q10 Q50 Q20

Sill level-d/s tail water level difference, m <0.30 1.06 0.55 <0.30 1.08 0.55 <0.30 0.87 0.70

Mean approach velocity, m/s <0.70 1.24 1.43 <0.70 1.06 1.43 <0.30 1.34 1.40

Stilling basin flow depth, m >0.30 0.54 1.05 >0.30 0.52 1.05 >0.30 0.73 0.90

Table 5

Modes of flow under different flow conditions.

Weir

condition

Flow conditions

Q10 Qav Q95

Original Modular (out of V) Modular (within V) Modular (within V)

Modified Modular (out of V) Modular (within V) Modular (within V)

48 S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

swimmers like coarse fish. In this study, lower ranges of flowvelocities have been considered. For salmon, a limit of 2.5m/s isconsidered appropriate. Similarly, limits of 1.7 m/s and 1.4 m/sare considered appropriate for trout and coarse fish respectively.

Theflowvelocityon the rampwasestimatedby incorporating itsgeometry (i.e. length and slope gradient) in the model. The up-stream boundary is controlled by the flow and head over the stillingbasin sill which is the starting point of the ramp while downstreamflowboundary is controlledby theflowdepth at the endof the ramp.Themodel then computes flowvelocities for a range of ramp slopesalong the channel (Fig. 7). For salmon the maximum permissiblevelocity is 2.5m/s. Thiswill be attained by a slope of 5%during thehigh flow condition (Q10). The slope of 5% is therefore themaximum slope favourable for the passage of salmon. Similarly, a1.5% ramp slope will be favourable for Trouts and Coarse fish.

It is important to note that a ramp of a given slope isrequired to tie in with the existing river bed preferably in astable condition. The design slopes and required lengths willneed to be evaluated based on the existing topography of theriver bed. The length required to create a slope of 5% that isfavourable for the passage of salmon is 36 m. Similarly, thelength required for trout and coarse fish is 120 m. From thisanalysis it is clear that in order to ensure that all of the fishspecies including the slow swimmers like coarse fish pass thestructure, a ramp with a gentler slope of 1.5% will be required.

4.2. Assessment of flow condition for hydrometricrequirements

The gauging structure must be set at a level not much higherthan the head of the fish pass to minimise obstruction as far asfish are concerned. On the other hand, the gauging structure

Fig. 7. Flow velocity and rock ramp slopes under different flow conditions.

must be high enough to ensure that modular flow conditions, andhence gauging accuracy, are maintained up to a defined flowrate. Gauging structures are therefore often designed to ensurethat they operate in the modular flow condition up to certainflow percentiles or a certain multiple of the dry weather flow.

The modular limit of a gauging structure is an importantfactor because it determines how readily the structure becomessubmerged. A Crump weir does not start to submerge until thedownstream total head reaches 75 per cent of the upstream totalhead (White and Woods-Ballard, 2003). Under these conditionsdownstream water levels are normally well above crest level. Inthe case of a Flat V weir, modular limit values generally liebetween 0.60 and 0.80 depending on the upstream head over theweir crest and the dimensions of the shallow V crest. A modularlimit of 0.70 has been used in this study.

The model shows that the flow is modular in the existingarrangement up to the 10% exceedence flow (Table 5). Also, theflow is modular in the modified arrangement up to the 10% ex-ceedence flow condition. This demonstrates that the weir in boththe original and modified arrangement fulfils the hydrometricrequirements for accurate flow gauging in modular operation.

In natural rivers where it is necessary to measure a wide rangeof discharges, a triangular control structure has the advantage ofproviding a wide opening at high flows so that it causes noexcessive backwater effects, whilst at low flows its opening isreduced so that the sensitivity of the structure remains acceptable.Details on triangular flat v weir can be found in Measurement ofliquid flow in open channelse Part 4B: Triangular profile weirs,BS 3680-4B (1986). Hence, the basis of the modifications toPontneddfechanweirwas to retain its flatV shapewith amodifiedgeometry to the stilling basin to aid fish passage. The modifiedform of Pontneddfechan weir will better fulfil the combinedhydrometry and fishery requirements.

5. Conclusions

This study describes the application of hydrodynamic mo-delling to assess the hydraulic conditions of the Pontneddfechanweir from the perspective of fish passage, mainly for salmon,brown trout, sea trout and coarse fish. Results indicated that the

49S. Ghimire, G. Jones / Journal of Hydro-environment Research 8 (2014) 43e49

existing weir does not adequately fulfil the fish pass requirementsfor any of the species considered. Modification to the weir ge-ometry will enable improved fish passage by reducing the affluxacross the weir. The modified weir will fulfil the fish pass re-quirements for strong swimmers such as salmon and trout, how-ever it is not favourable for weaker swimmers such as coarse fish.In addition, flow conditions were assessed over the stilling basinsill with a vertical dropwhichwas an obstacle for fish passage. Asa means of providing a solution, a rock ramp fish pass was tested.Hydraulicmodelling of the fishpass indicated that a 5% rampwillbe favourable for the passage of salmon. However, in order toensure that all of thefish species including the slow swimmers likecoarse fish pass the structure, a 1.5% ramp slopewill be required.

The hydraulic modelling represents a useful method for de-signing gauging weirs and assessing the influence on fishmigration. The design criteria set out by the JointHydrometry andFisheries Fish Passage Group are explicit and easy to measurewhich can be directly applied in hydraulicmodelling. Themethodadopted in this study would be beneficial for improving fishpassage because of the large numbers of existing flow measure-ment structures in the UK and elsewhere.

Acknowledgement

We are grateful to the Environment Agency Wales for pro-viding hydrometric data used in this study. The help and supportprovided by the James Hutton Institute, Scotland during thepreparation of the paper is gratefully acknowledged.

References

Armstrong, J.D., Kemp, P.S., Kennedy, G.D.A., Ladle, M., Milner, N.J., 2003.

Habitat requirements of Atlantic salmon and brown trout in rivers and

streams. Fisheries Research 62 (2), 143e170.Beach, M.H., 1984. Fish Pass Design: Criteria for the Design and Approval of

Fish Passes and Other Structures to Facilitate the Passage of Migratory

Fish in Rivers. Ministry of Agriculture, Fisheries, and Food, Directorate of

Fisheries Research, Lowestoft, UK. Fisheries Research Technical Report.

Bos, M.G., 1989. Discharge Measurement Structures. ILRI Publication 20,

Netherlands.

British Standards Institution, 1986. Measurement of Liquid Flow in Open

Channels e Part 4B: Triangular Profile Weirs. BS 3680e4B.Bunt, C.M., Castro-Santos, T., Haro, A., 2011. Performance of fish passage

structures at upstream barriers to migration. River Research and Application.

<http://onlinelibrary.wiley.com/doi/10.1002/rra.1565/pdf> (21.08.2011).

Clough, S.C., Turnpenny, A.W.H., 2001. Swimming Speeds of Freshwater

Fish. Phase 1. R&D Technical Report No. W2-026/TR1 & 2. Environment

Agency, Bristol, UK.

Environment Agency, 2010a. Our River Habitats: River Habitats in England

and Wales-current State and Changes since 1995e96. Environment

Agency, Bristol, UK. <http://publications.environment-agency.gov.uk/

PDF/GEHO0910BSZV-E-E.pdf> (11.08.2011).

Environment Agency, 2010b. Environment Agency Fish Pass Manual. Docu-

mente GEHO 0910 BTBP-E-E.<http://publications.environment-agency.

gov.uk/PDF/GEHO0910BTBP-E-E.pdf> (09.08.2011).

FAO, 2002. Fish Passes-design, Dimensions and Monitoring. FAO, Rome. P1.

Fjeldstad, H.P., Barlaup, B.T., Stickler, M., Gabrielsen, S.E., Alfredsen, K.,

2011. Removal of weirs and the influence on physical habitat for salmonids

in a Norwegian river. River Research and Applications. <http://

onlinelibrary.wiley.com/doi/10.1002/rra.1529/pdf>.

Halcrow, 2007. South West Weirs Improvements, Business Justification, Envi-

ronment Agency Wales. Business Justification Report no. IMWA000522.

Halcrow, 2010. ISIS by Halcrow. <http://www.halcrow.com/isis/> (15.08.2011).

Harris, J.H., Thrncraft, G., Wem, P., 1998. Evaluation of rock-ramp fishways

in Australia. In: Jungwirth, M., Schmutz, S., Weiss, S. (Eds.), Fish

Migration and Fish Bypasses. Fishing News Books, Vienna, pp. 331e347.

Kim, J.H., 2001. Hydraulic characteristics by weir type in a pool-weir fishway.

Ecological Engineering 16, 425e433.

Kjeldsen, T.R., 2007. The Revitalised FSR/FEH Rainfall-runoff Method,

Flood Estimation Handbook. Supplementary Report No. 1. NERC/Centre

for Ecology & Hydrology, Wallingford, UK.

Mills, D.M., 1989. Ecology and Management of Atlantic Salmon. Chapman

and Hall, London.

Mouton, A.M., Schneider, M., Depestele, J., Goethals, P.L.M., Pauw, N.D.,

2007. Fish habitat modelling as a tool for river management. Ecological

Engineering 29, 305e315.

Poulet, N., 2007. Impact of weirs on fish communities in a piedmont stream.

River Research and Applications 23, 1038e1047.

Rhodes, D.G., Servais, S.A., 2008. Low-cost Modifications of the Crump Weir

to Improve Fish Passage. Science Report SC010027. Environment Agency,

Bristol, UK.

Rickard, C., Day, R., Purseglove, J., 2003. River Weirs e Good Practice

Guide. Science Report W5B-023/HQP. Environment Agency, Bristol, UK.

Solomon, D.J., Sambrook, H.T., Broad, K.J., 1999. Salmon Migration and

River Flows. RD publication 4, Environment Agency, Bristol, UK.

Turnpenny, A., Lawton, K., Clough, S., Hanson, K., Ramsay, R., 2002. Fish

Passage at Flow Gauging Stations in England and Wales. Stage 1: Liter-

ature Review and Regional Survey. Science Report W6-029/SR1. Envi-

ronment Agency, Bristol, UK.

White, W.R., Woods-Ballard, B.A., 2003. The Investigation and Specification

of Flow Measurement Structure Design Features that Aid the Migration of

Fish without Significantly Compromising Flow Measurement Accuracy

with the Potential to Influence the Production of Suitable British Stan-

dards. Science Report W6-084/TR1. Environment Agency, Bristol, UK.

White, W.R., 1971. Flat V weirs in alluvial channels. In: Proceedings of the

American Society of Civil Engineers, 97, HY3, March, pp. 395e408.

Yagci, O., 2010. Hydraulic aspects of pool-weir fishways as ecologically

friendly water structure. Ecological Engineering 36, 36e46.

Zaidman, R.L., Mawdsley, J., Lawless, M.R., Archer, D.R., Melching, C.S.,

2005. Non-invasive Techniques for River Flow Measurement. Science

Report SC030230/SR. Environment Agency, Bristol, UK.