Appendix 6 Weir Assessments

Transcription

1 Appendix 6 Weir Assessments Second edition, September 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 Knightsford bridge Ashford Mill Casemill Weir Horseshoe Bridge Weir Mill Street Weir Dinham Bridge Oakly Park Bromfield Forge bridge Leintwardine Parsons Pole bridge Lingen Bridge Teme Bridge (Knighton) Crug-Y_Byddar bridge Ddol Ddol

2 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 practice 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 Weir pool depth Where an apron is present, is the hydraulic jump on the apron? Existing fish pass? 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. 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). Y/N 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 Notch in weir Rock Ramp Check weirs Engineered Pass This is the most sustainable solution as it renaturalises the river and requires no maintenance. The notch serves to lower the head difference across the obstruction. A low cost solution applicable at low head weirs which requires little maintenance. 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. Preliminary check weirs can be placed downstream of the main obstruction in order to reduce the effective head of the barrier. 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. 1 Armstrong et al Environment Agency Fish Pass Manual: Guidance Notes on the Legislation, Selection and Approval of Fish Passes in England and Wales.

3 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 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 2 Knightsford Bridge Partial Retain, modify/mitigate, but long term: consider replacement with alternative gauging method. Retain- modify or mitigate 6 Mill Street Fail Weir impoundment & fish passage issues. 7 Dinham Bridge Fail Retain- modify or mitigate impoundment and fish passage issues. Long term: At end of hydropower license consider full range of options. Priority Action (km) order Leintwardine Fail Retain-modify/mitigate fish passage issues. 13 Lingen Bridge Fail Retain-modify/mitigate fish passage issues. 10 Forge Bridge Fail Retain-modify/mitigate fish passage issues. 14 Teme Bridge Fail Retain-modify/mitigate fish passage 21 7 (Knighton) issues 15 Crug- Y_Byddar Bridge Fail Remove weir Ddol 2 Fail Remove weir 0.3 n/a 17 Ddol 1 Fail Remove weir 2 n/a

4 1. Ddol Weir 1 (NGR , ) 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) 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

5 2.Ddol Weir 2 (NGR , ) 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. Recommendation: Remove

6 4. Crug-Y-Byddar Bridge Weir (NGR , ) 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 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: Remove

7 11. Teme Bridge Weir (Knighton) (NGR , ) 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 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: Retain-modify/mitigate fish passage issues

8 14. Lingen Bridge Weir (NGR , ) 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. Habitat gain = 9.4km Options Appraisal: Fish Tek Consulting completed a detailed feasibility and design study for this weir in September 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: Retain-modify/mitigate fish passage issues

9 15. Parsons Pole Bridge Weir (NGR , ) 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 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. Recommendation: Remove

10 16. Leintwardine Bridge Weir (NGR , ) Description of structure: The weir forms part of the foundations of the road bridge and is located on the downstream face. Passability assessment = Fail Approx head over weir = 0.3m at Q10, larger at low flows. 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. Note: This structure is approximately 100m downstream of the confluence with the Clun. Any implementation project will require permission from the Environment Agency, Natural Resources Wales and/or Natural England. As part of these procedures, a Habitats Regulations Assessment of individual projects will be required to ensure they do not have significant negative effects on the River Clun SAC either alone or in combination with other plans and projects. Recommendation: Retain-modify/mitigate fish passage issues.

11 19. Forge Bridge (NGR , ) Description of structure: This Grade 2 listed structure is a semi-circular labyrinth weir located immediately upstream of an imposing stone arch bridge. Passability assessment = Fail Approx head over weir = 1m Weir pool depth = 1m 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: Retain-modify/mitigate fish passage issues

12 21. Bromfield Weir (NGR , ) 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. Passability assessment = Pass Approx head over weir = 1.5m. 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: Retain but long term at end of hydropower license consider full range of options.

13 22. Oakly Park Weir (NGR , ) 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. Passability assessment = Pass Approx head over weir = 1.5m Existing fish pass = Yes Habitat gain = 0.8km Recommendation: Retain but long term at end of hydropower license consider full range of options.

14 24. Dinham Bridge Weir (NGR , ) 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. Passability assessment = Fail Approx head over weir = 1.5m Habitat gain = 3.7km Dinham Bridge Weir Casemill Weir Mill Street Weir Horseshoe Bridge Weir 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. Recommendation: Retain- modify or mitigate impoundment and fish passage issues. Long term: At end of hydropower license consider full range of options.

15 25. Mill Street Weir (Upstream) (NGR , ) 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. Passability assessment = Fail Approx head over weir = 1.32m Depth of weir pool = Unknown Hydraulic jump on apron = Yes Existing fish pass = No Habitat gain = 0.6km Dinham Bridge Weir Casemill Weir Mill Street Weir Horseshoe Bridge Weir Options Appraisal: A fish pass feasibility study was undertaken by Fishtek Consulting in September 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: Retain- modify or mitigate impoundment & fish passage issues

16 26. Horseshoe Bridge Weir (NGR , ) 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 Existing fish pass = Yes Habitat gain = 0.4km Dinham Bridge Weir Casemill Weir Mill Street Weir Horseshoe Bridge Weir Recommendation: Retain-consider modifying/mitigating impounding effects

17 27. Casemill Weir (NGR , ) Description of structure: This skewed weir with sluice gates on the left hand bank was restored by the Teme Rivers Trust in 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 Existing fish pass = Yes Habitat gain = 0.4km Photograph from Teme Rivers Trust website. Dinham Bridge Weir Casemill Weir Mill Street Weir Horseshoe Bridge Weir Recommendation : Retain-consider modifying/mitigating impounding effects

18 30. Ashford Mill Weir (NGR , ) 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. Passability assessment = Pass Approx head over weir = 1m 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 / modify

19 37. Knightsford Bridge Weir (NGR , ) Description of structure: A flat vee gauging weir, which was constructed in A cableway has been constructed upstream of the weir to allow easy flow measurement. Passability Assessment = Fail Approx head over weir = 0.5m 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: Retain, modify/mitigate, but long term: consider replacement with alternative gauging method

20 39. Powick Bridge Weir (NGR , ) Description of structure: The most downstream obstruction on the Teme. A fish pass has been added adjacent to the left hand bank. Passability assessment = Pass Approx head over weir = 2.1m 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: Remove/modify