1. introduction - severn rivers trust 6 - weir assessment and... · the one exception to this is...

1. Introduction

This section of work assesses the connectivity of the River Teme with respect to fish passage. A

number of barriers have been removed along the River Teme, however seventeen remain which

could present a barrier to fish movement. The Teme supports a wide variety of finfish species,

including Brown Trout Salmo trutta, coarse fish, Twaite Shad Alosa falax and Atlantic Salmon Salmo

salar.

The River Teme has been divided into three zones for the purpose of this restoration project

Barbel zone: From Tenbury Wells downstream to confluence

Grayling zone: from Knighton to Tenbury Wells.

Trout zone: upstream of Knighton

Table 1 shows the 17 remaining obstructions on the River Teme ranked by proximity to the

confluence with the River Severn (i.e. No 1 is the closest to the River Severn, No 17 is in the

headwaters). Weirs 1 and 2 are in the barbel zone, weirs 3 to 13 are in the grayling zone and weirs

14 to 17 are in the trout zone.

Table 1. Existing obstructions in the River Teme

Weir No. Name Easting Northing

1 Powick Bridge 383190 252416

2 Knightsford bridge 373476 255726

3 Ashford Mill 352129 271050

4 Ludford Mill 351790 274290

5 Ludford Bridge DS 351420 274214

6 Ludford Bridge US 351047 274212

7 Dinham Bridge 350693 274562

8 Oakly Park 348797 276484

9 Bromfield 348134 276661

10 Forge bridge 345402 274978

11 Leintwardine 340403 273824

12 Parsons Pole bridge 337540 273075

13 Lingen Bridge 335865 272923

14 Teme Bridge (Knighton) 328948 272383

15 Crug-Y_Byddar bridge 316420 282093

16 Ddol 2 312479 283128

17 Ddol 1 312394 283352

This section of work forms a catchment scale overview of structures limiting longitudinal migration.

High level recommendations on type, placement of any proposed measure are presented where

necessary based on experience and information contained within the published best practice1.

1 Armstrong et al. 2010. Environment Agency Fish Pass Manual: Guidance Notes on the Legislation, Selection and Approval of

Fish Passes in England and Wales.

2. Methodology

Each of the obstructions on the Teme was assessed for passability. This high level assessment was

based on criteria either for new build fish passes or for given in Armstrong et al. (2010). The table

below highlights the criteria used.

Approximate head over weir

0.1m is used for coarse fish, 0.2 for brown trout and 0.3m for salmon (based on criteria given for pool passes in Armstrong et al (2010). The different thresholds will be used in the different fisheries zones as identified in section 1.

Weir pool depth The depth of the weir pool needs to be greater than twice the height of the obstruction for the obstruction to be considered passable (based on criteria given in Armstrong et al (2010).

Where an apron is present, is the hydraulic jump on the apron?

Y/N

Existing fish pass? Y/N If there is an existing fish pass present at the site, we have assumed that it facilitates appropriate passage.

Where one or more of the listed criteria are not met, the weir is deemed impassable. If an obstruction

was considered not to be passable, solutions were considered in the following order:

First Last

Remove weir This is the most sustainable solution as it renaturalises the river and requires no maintenance.

Notch in weir The notch serves to lower the head difference across the obstruction. A low cost solution applicable at low head weirs which requires little maintenance.

Rock Ramp A ramp of heterogenous material is located on the downstream side of an existing weir to create a semi natural bed of reasonable gradient. A relatively low cost solution.

Check weirs Preliminary check weirs can be placed downstream of the main obstruction in order to reduce the effective head of the barrier.

Engineered Pass A number of engineered pass solutions are possible, these are generally more costly than the above options.

It should be noted at this point that any recommendations associated with implementation of

passage measures are made based on an a largely unconstrained site i.e. issues such as

landowner approval, local planning authority approval and presence of underground services

are not considered. A fuller assessment of feasibility is essential before undertaking any

further design work or implementing any proposed solution.

3. Summary of Results

Each weir has been assessed on an individual basis

Seven of the weirs already have a fish pass in place, either as a result of weir restoration projects or

hydropower schemes: Powick Bridge, Ashford Mill, Ludford Mill, Ludford Bridge DS, Oakly Park,

Bromfield and Parsons Pole. In the absence of a detailed, comparative assessment of passage,

where a fish pass is present it is assumed that the pass functions well and so the weir is passable.

The one exception to this is Powick Bridge weir where Tony Bostock (Chief Executive of the Severn

Rivers Trust) noted in an email that Shad had not been observed upstream of the weir for many years

– in this case it will be worth redesigning the existing pass to facilitate the passage of shad also.

Of the remaining eight weirs, four have had a fish pass feasibility study or detailed design study

conducted on them: Ludford Bridge US, Dinham Bridge, Leintwardine and Lingen Bridge. In each

case the recommendation from this study is to follow the findings of the detailed study. At each other

locations solutions for fish passage easement have been suggested.

The weirs have been ranked into a ‘priority action order’ depending on the habitat gain, location in the

system, type of measure proposed and work conducted to date. The potential habitat gain from

improving fish passage has been calculated by measuring the distance upstream from the structure of

interest to the next obstruction. This analysis shows that upstream of Knightsford Bridge there is

45km of potential habitat gain. Therefore this structure has the highest priority of any in the system.

Priority 2 to 5 are the four weirs where significant spending has already taken place on feasibility and

design studies, the furthest downstream having the highest priorities.

The remaining structures were also ranked with the highest priority given to the obstruction furthest

downstream. The two remaining key structures on the Teme are Forge Bridge and Teme Bridge.

Considerable habitat gains are possible if the passability of the weirs could be improved. There are

complicating factors at each of these sites, with Forge Bridge a Grade II listed building and Teme

Bridge weir very close to a road bridge. In each case potential solutions have been presented,

however, further detailed studies are recommended.

Table 2. Obstructions which restrict access on the River Teme.

Weir No. Name Passability Unconstrained Measure Habitat Gain (km)

Priority Action order

2 Knightsford Bridge Fail Retain weir - Low cost

baffles 45.3 1

6 Ludford Bridge US Fail Retain weir 0.6 2

7 Dinham Bridge Fail Remove weir 3.7 3

11 Leintwardine Fail Retain weir 4.1 4

13 Lingen Bridge Fail Retain weir 9.4 5

10 Forge Bridge Fail Retain weir and existing

bypass channel 12.2 6

14 Teme Bridge (Knighton) Fail Retain weir - Notch against

LHB 21 7

15 Crug-Y_Byddar Bridge Fail Remove weir 4.7 8

16 Ddol 2 Fail Remove weir 0.3 n/a

17 Ddol 1 Fail Remove weir 2 n/a

1. Ddol Weir 1 (NGR 312 394, 283 352)

Description: The weir consists of a

small step in the bed in the very upper reaches of the River Teme. The Teme in this area is approximately 2m wide and has a mixture of gravel/cobble and bedrock bed. The weir pool downstream of the obstruction is approximately 1m deep.

Fish Passability assessment = Fail

Approx head over weir = 0.3m Pool depth = 1m Hydraulic jump = N/A Habitat gain = 2km (to source)

Contains Ordnance Survey data © Crown copyright and database rights 2013.

Options Appraisal: The River Teme in this area is small. It is unlikely that brown trout will need to

access the very upper reaches of the river. Access to this location is via steeply sloping pasture fields and so access for plant equipment would be difficult. The step in the bed could possibly be broken manually. This would be cheap and feasible however the habitat gained would be minimal. An engineered fish pass is unlikely to feasible due to the high costs and small habitat gains. In terms of in channel morphology, upstream and downstream morphology would be effected if the barrier was removed (bank stability) and some re-profiling would be required to remove sediment trapped behind the barrier

Recommendation: Remove

2.Ddol Weir 2 (NGR 312 479, 283 128)

Description: The weir consists of a

metal bar which runs across a pipe as protection. The barrier could be old farm machinery or a service the pipe (this is unknown). The Teme in this area is approximately 2m wide and has a mixture of gravel/cobble and bedrock bed.

Fish Passability assessment =

Fail

Approx head over weir = 0.2m Pool depth = 0.3m Hydraulic jump = n/a Habitat gain = 0.3km


Options Appraisal: The head over the obstruction combined with the shallow weir pool means

that the obstruction is likely to be a barrier to fish movement, particularly at low flows. It may be possible for the structure to be removed, however, confirmation of the presence and the nature of any services in the carry pipe will obviously need to be established first. Furthermore, access to the weir may also be an issue with the weir surrounded by steeply sloping pasture fields. As with Ddol 1 an engineered fish pass or rock ramp is unlikely to be feasible due to the high costs and small habitat gains.


4. Crug-Y-Byddar Bridge Weir (NGR 316 420, 282 093)

Description: This shallow structure is located

downstream of Crug-Y-Byddar bridge. The weir exhibits a shallow ‘v’ shape as it is lower in the middle than at the banks.

Fish Passability assessment =

Marginal

Approx head over weir = 0.15m Pool depth = Unknown Hydraulic jump on apron = No apron

Habitat gain = 4.7km


Options Appraisal: This structure has a relatively small head in the centre of the structure, which

should be passable by brown trout, assuming sufficient depth in the weir pool. Therefore, there is no need for an engineered fish pass at this location. Access for fish could be eased by cutting a notch in the centre of the weir or by removing the weir. From a hydromorphologial perspective the barrier has very little influence, as the structure is small. The channel is slightly incised at this point, suggesting a larger weir once existed here.

Recommendation: Weir removal.

11. Teme Bridge Weir (Knighton) – (NGR 328 948, 272 383)

Description of structure: Teme Bridge

weir is located 5 to 10m upstream of Teme Bridge in Knighton. The bridge is skewed so the right hand bank of the weir is much closer to the bridge than the left hand bank.

Fish Passability assessment = Fail

Approx head over weir = 0.5m Weir pool depth = Unknown Hydraulic jump on apron = No Habitat ‘gain’ = 21km


Options Appraisal: Due to the close proximity of the weir and the bridge, before any fish passage

solution is designed, it is recommended that an engineering assessment is conducted to determine whether the weir is structurally tied into the bridge. Any such existing structural tie would likely make any works on the weir more expensive, if not unfeasible due to the impact onthe adjacent road. Weir removal is likely to be unfeasible given the proximity of the bridge and associated impacts on scour. Given the skew of the bridge, it would be sensible to locate any fish pass measure on the left hand bank. Access to the left hand bank seems reasonable via a field. Any works adjacent to the left hand bank should give consideration to the condition of the wall which forms the left hand bank and part of an adjacent property.

Recommendation: Notch adjacent to left hand bank, following further engineering studies on the

weir, bridge and adjacent wall.

14. Lingen Bridge Weir (NGR 335 865, 272 923)

Description of structure: Lingen weir

was an old stone ford, on top of which a bridge was built. The creation of the stone ford led to erosion of the river bed downstream, leading to a significant head drop.

Passability assessment = Fail

Approx head over weir = 1.78m Weir pool depth = Unknown Hydraulic jump on apron = No. Habitat ‘gain’ = 9.4km


Options Appraisal: Fish Tek Consulting completed a detailed feasibility and design study for this

weir in September 2012. They have designed a Larinier super active baffle pass, which has been approved by the National Fish Pass Panel. Upstream of the bridge a small anastomosed system has developed due to the impounding effect of the weir which provides a good habitat. Incision has occurred downstream of the bridge, however, options for weir removal are limited as the bridge sits on top of the weir structure.

Recommendation: Implement existing study recommendations.

15. Parsons Pole Bridge Weir (NGR 337 540, 273 075)

Description of structure: This

substantial weir creates a head difference of around 2m. The structure has stonework wingwalls and a long apron. Historically the weir was used to raise water levels to divert flows down a mill leat. There is an existing fish pass on the right hand bank around the weir.

Passability assessment = Pass

Approx head over weir = 2m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’ = 2km


Options Appraisal: The weir impounds water upstream and has incised downstream. There is

also evidence to suggest that the channel has been straightened downstream of the weir. The mill leat still appears to be functional, however its current purpose is unknown. Removal would require upstream and downstream channel works as a large amount of sediment will be trapped behind the weir.


16. Leintwardine Bridge Weir (NGR 340 403, 273 824)

Description of structure: The weir

forms part of the foundations of the road bridge and is located on the downstream face.


Approx head over weir = 0.3m at Q10, larger at low flows. Weir pool depth = Unknown Hydraulic jump on apron = No Habitat ‘gain’ = 4.1km


Options Appraisal: A feasibility and detailed design study has been undertaken by Fishtek

consulting for the Severn Rivers Trust. Their preferred option was a rock ramp fish pass. As the weir and bridge are linked removal won’t be possible at the location.

Recommendation: Implement the findings from the detailed study.

19. Forge Bridge (NGR 345 402, 274 978)

#

Description of structure: This Grade 2

listed structure is a semi-circular labyrinth weir located immediately upstream of an imposing stone arch bridge.


Approx head over weir = 1m Weir pool depth = 1m Hydraulic jump on apron = No Habitat ‘gain’ = 12.2km

Options Appraisal: As this structure is

listed and tied into the bridge, notching and weir removal are not feasible. An engineered solution should be located in the middle of the weir, although access for construction and maintenance would be difficult. There is an existing bypass channel on the left hand bank and a mill leat on the right hand bank. Either of those two features could be formalised to provide fish passage. In each case the entrance to the pass would need to be some distance downstream of the toe of the weir. Care would have to be taken during design to ensure sufficient attraction flow in the pass.


Recommendation: The listed status of this structure is likely to dictate utilisation of the bypass

channel on left bank or mill leat on right hand bank and also means the option of weir removal is not possible at this location.

21. Bromfield Weir (NGR 348 134, 276 661)

Description of structure: This listed

structure was rebuilt in 2010 in association with a hydropower development. Fish passage was accounted for as part of the hydropower development.


Approx head over weir = 1.5m Weir pool depth = Unknown. Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’ = 3.9km


Options apprasial: The listed nature and close proximity of the barrier to the bridge means that

removal is not an option at this location.

Recommendation: Do Nothing.

22. Oakly Park Weir (NGR 348 797, 276 484)

Description of structure: A

hydropower scheme is currently being installed at this weir, taking advantage of the significant head over the structure. As part of this development a new fish pass has been included.


Approx head over weir = 1.5m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’ = 0.8km


Recommendation: Do Nothing (However, in the long term options for removal should be

investigated from a hydromorphological perspective)

24. Dinham Bridge Weir (NGR 350 693, 274 562)

Description: Dinham Weir is located to the

west of Ludlow below Ludlow Castle. The weir has a raised crest about 50m wide, with raised flanking crests to either side. The right hand flanking crest is approximately 21m long.


Approx head over weir = 1.5m Weir pool depth = Unknown Hydraulic jump on apron = No Habitat ‘gain’ = 3.7km


Options Appraisal M&M Beach Consultants have produced a fish pass feasibility report in

November 2011, which recommended the installation of a Larinier Pass on the left hand bank. Following production of that report, a period of low flows resulted in large numbers of salmonids being stranded below the weir. Emergency work led to a notch being cut into the weir crest and a baulk added to decrease effective head. This was considered a temporary measure. In the course of these temporary works, the weir was found to be considerably more undercut than previously thought, raising doubts as to whether it could support a Lariner. A bypass channel is now the preferred option. Removal of the weir should be considered to return the channel back to its natural form.


25. Ludford Bridge Weir (Upstream) (NGR 351 047, 274 212)

Description: This skewed weir (also known as

Mill Street weir) and its associated sluice gates was restored in 2003 by the Teme Rivers Trust. The weir is 134m long and skewed across the river with the sluice gates located on the left hand bank.


Approx head over weir = 1.32m Depth of weir pool = Unknown Hydraulic jump on apron = Yes Existing fish pass = No Habitat ‘gain’ = 0.6km


Options Appraisal: A fish pass feasibility study was undertaken by Fishtek Consulting in

September 2012. A Larinier super-active baffle pass was chosen as the preferred option. Removal of the weir should be investigated. The proximity of the weir to the bridge would need to be evaluated.

Recommendation: Use findings from report to build a fish pass. Removal should be investigated

in the long term.

26. Ludford Bridge Weir (Downstream) (NGR 351 420, 274 214)

Description of structure: This weir was

restored in September 2002 by the Teme Rivers Trust. This weir is listed. As part of the restoration a Larinier pass was added to the middle of the weir.

Passability Assessment = Pass

Approx head over weir = 1m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’= 0.4km


Recommendation: Do Nothing

27. Ludford Mill Weir (NGR 351 790, 274 290)

Photograph from Teme Rivers Trust website.

Description of structure: This skewed weir

with sluice gates on the left hand bank was restored by the Teme Rivers Trust in 2005. As part of this restoration a fish pass was added to the weir. This weir is listed.

Passability Assessment = Pass

Approx head over weir = 1m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’= 0.4km


Recommendation : Do nothing

30. Ashford Mill Weir (NGR 352 129, 271 050)

Description of structure: This

labyrinth weir was recently restored. As part of the restoration a fish pass was added to the weir adjacent to the mill building.


Approx head over weir = 1m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’ = 4.4km


Options appraisal: Options exist for the removal of this weir in the long term. Investigations would

be required in to the stability of the bridge near by as upstream and downstream channel modifications would be required.

Recommendation: Remove in long term

37. Knightsford Bridge Weir (NGR 373 476, 255 726)

Description of structure: A flat vee

gauging weir, which was constructed in 1998. A cableway has been constructed upstream of the weir to allow easy flow measurement.

Passability Assessment = Fail

Approx head over weir = 0.5m Weir pool depth = Unknown Hydraulic jump on apron = in places Habitat ‘gain’ = 45.3km


Options Appraisal: As the weir is a gauging weir significant modifications to the weir are unlikely

to be allowed by the Environment Agency. Therefore weir removal, notching or an engineered solution are not thought to be feasible. Low cost baffles have been piloted at one gauging station in Thames region. This solution involves fixing baffles to the downstream face of the weir. Sensibly placed, the baffles should have little impact on the rating curve at the site. For installation purposes access to the site should be simple, using the existing track to the gauging station. It is recommended that a detailed feasibility study including consultation with key stakeholders should be undertaken.

Recommendation: Install low cost baffles.

39. Powick Bridge Weir (NGR 383 190, 252 416)

Description of structure: The most

downstream obstruction on the Teme. A fish pass has been added adjacent to the left hand bank.


Approx head over weir = 2.1m Weir pool depth = Unknown Hydraulic jump on apron = No Existing fish pass = Yes Habitat ‘gain’ = 16.1km


Options appraisal: Options should be investigated for the removal of the weir, however, the

heritage nature of the structure will need to be considered.

Recommendation: Personal communication with the Severn Rivers Trust suggest they are

concerned with the fact that Shad haven’t been observed upstream of Powick weir for many years. A review of the hydraulic operation of the existing fish pass should be undertaken, with a full re-design recommended if the current pass is found to be inefficient.

1. introduction - severn rivers trust 6 - weir assessment and... · the one exception to this is...

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