Supply Chain - River Work

During the industrial revolution, pollution and weir constructions in Yorkshire resulted in the rapid decline of river standards and loss of once abundant species such as salmon, trout, and grayling. The rivers were wiped of ecological habitats and the number of species found in the river declined by 83%. In 1999 Yorkshire Water installed the Niagara Weir to bridge two gravity fed sewer pipes across the river Don in Sheffield. Recently, juvenile salmon have been found in Sheffield, for the first time in over 150 years. Niagara Weir is one of the final barriers for migrating fish such as salmon and trout. This fish pass is the gateway to greater spawning habitats upstream that will boost their population to a sustainable size.

Project driver & scope of work

In 2016 the left bank’s masonry retaining wall downstream of the weir collapsed. A downstream masonry weir also collapsed, altering the river levels and rendering the existing fish pass unsuitable.  Niagara Weir became undermined, compromising the two sewer pipes and threatening a pollution incident in the river. The work undertaken in this project included stabilising the weir to eliminate the pollution risk, rebuilding the collapsed left bank, and upgrading the fish pass to aid the migration of over half a dozen fish species of conservational importance.

Temporary works

During the design and construction phase, the two sewer pipes running through the weir were diverted into a temporary pumping station and pumped through pipework over the river. The temporary pipework was supported by a steel bridge, installed using a 220 ton crane. The original sewer pipes were isolated, eliminating the pollution risk. [caption id="" align="alignnone" width="1200"] (top) Collapsed left bank and masonry weir, (left) left hand cofferdam dewatered, exposing the extent of the undermining to the weir foundations, and (right) sewer pipes diverted through a temporary bridge (PortaDam and cofferdam installed on the left hand side of the river) - Courtesy of Ward & Burke[/caption]

Cofferdams

To stabilise the weir, cofferdams were constructed upstream and downstream of the weir. The weir was monitored daily and was only allowed to move 5mm before posing a risk of damaging the sewer pipes encased inside. A flood risk assessment determined the cofferdam could span half the width of the river and must be flooded when local river levels reached Q10+500mm. Construction began on the left bank, rebuilding the bank and stabilising the left half of the weir. A PortaDam was installed upstream of the river by OnSite Central Ltd. A cofferdam was installed downstream of the river by Ward & Burke. The cofferdam was made of PU18 sheet piles 6m long. An extra row of sheet piles were driven at each end of the cofferdam to form ‘wedges’ that were filled with puddle clay to seal the cofferdam to the bank and weir face. The top pile level was set so that the cofferdam would flood automatically when river levels reached Q10+500mm.

Flood prevention

Flooding was a major risk during construction. The risk was managed by continuous monitoring of upstream and downstream river gauges, weather predictions and surveys on site. The construction began June 2022 and completed April 2023. During this time to cofferdam flooded three times. Six temporary pumps were installed to dewater the cofferdam. The pumps discharged into settlement tanks. The settlement tanks discharged through silt socks and the effluent filtered through the grassy bank before returning to the river.

Weir stabilisation works

Once the left bank cofferdam was dewatered, the full extent of the undermining was assessed. Shotcrete was used to stabilise the fill material underneath the concrete weir. Then a 4.5m wide slab and 1.45m tall toe were constructed at the base of the weir. A similar 2.5m wide slab and toe was constructed along the left bank. The initial design involved a large mass concrete fill as the left bank retaining wall. Collaborative working during the design phase enabled Ward & Burke to query the function of this mass fill with the designers. An assessment was made and the design was altered, reducing the size of the excavation and replacing the mass fill with an RC wall. This reduced the embodied carbon by 40%. Furthermore, it improved the buildability, making the construction process safer. [caption id="" align="alignnone" width="1200"] (top left) Left bank foundation RC slab and wall built, excavator grab positioning reclaimed stone wall, (top right) left half of weir stabilised, shutter for right hand cofferdam installed, river bed graded ready for flooding, (bottom left) with the weir stabilisation complete the existing fish pass and part of the weir are demolished to make room for the new fish pass, and (bottom right) sewer pipes reconnected through the fish pass walls - Courtesy of Ward & Burke[/caption] During construction, large stones from the collapsed downstream weir and left bank were recovered from the river. The design was altered to reuse these stones and incorporate them into the structure, avoiding embodied carbon associated with the original proposed concrete structures and imported aggregate. The stones were used to rebuild the left bank, installed as scour protection on the right bank, and used to landscape canoe landings. The stones helped the structure blend harmoniously with the surrounding environment. Before flooding the left bank downstream cofferdam, a steel shutter was designed and installed on the new concrete slab to seal the proposed right bank cofferdam to the weir face. This enabled part of the right bank cofferdam to be constructed within the left bank cofferdam, ensuring a better seal and reducing risks associated with piling in the river. Once the left bank was complete and the left half of the weir stable, the cofferdams were flooded and removed. A similar process was followed on the right bank to stabilise the weir. Sheet pile cofferdams were installed upstream and downstream of the weir. Temporary pumps kept the cofferdams dewatered.

Niagara Weir Stabilisation: Supply chain - key participants

• Principal contractor: Ward & Burke
• Principle designer: JBA Consulting
• PortaDam: OnSite Central Ltd
• Cofferdam props: MGF Ltd
• Larinier baffle fabrication: C&D Engineering Services
• Handrail & access metalwork: Galliford Try Fabrications
• Penstock & stoplogs: Midland Valves
• Security fencing: Burn Fencing Ltd
• Concrete: Right Mix Concrete Ltd
• Concrete: Breedon Group

Fish pass

A new 1.8m wide 30m long Larinier fish pass was designed to replace the existing Alaskan A fish pass. The fish pass wall height varies from 4.1m to 1.28m, as the water level drops 2.9m over the weir. The fish pass consists of two 10m long Larinier baffled flights with a resting pool in the middle. Galvanised steel baffles are installed on the 8.53° flights. The baffles were fabricated in six pieces by C&D Engineering Services. The material was laser cut to ensure a tolerance of 1mm. It was critical that the shape, location, and height of the baffles were accurate to achieve the most efficient hydraulic flow for the fish. These baffle sections were slung and lifted into position with an accuracy of +/- 1mm. When all the baffles were in position and fixed to the base, grout was poured underneath to ensure they were fully sealed. [caption id="" align="alignnone" width="1200"] (top left) Propped cofferdam for upstream section of fish pass, preparing for blinding, (bottom left) fish pass wall rebar, upstream section, and (right) upstream baffles, penstock, and access metalwork installed in fish pass - Courtesy of Ward & Burke[/caption] Midland Valves supplied a penstock upstream and stop logs downstream to isolate the fish pass for maintenance. The penstock and stop log frames were cast into the fish pass walls, flush with the base of the fish pass. This avoids grit being caught in the frame, preventing a tight seal. The fish pass was constructed in two halves; upstream and downstream of the weir. The two sewer pipes were cast into the walls of the downstream section. The downstream section was constructed first so that the sewer pipes could be re-energised and the temporary pumping station decommissioned before the end of the whole project. To construct the downstream section of the fish pass, a section of the weir and sewer pipes were removed. Then the ground was formed and blinded with a 8.53° slope. The base steel was fixed and the base poured. Next the wall steel was tied, shutters erected and the walls poured. The walls were constructed with a tolerance of +-5mm to ensure the baffles could fit and there wasn’t excessive space for debris to get caught. For the construction of the walls, the formwork panels had to be stepped to tie in with the run of the slope. The on-site joiners made relevant angled shutter bases to integrate with the main wall shutters so each wall could be poured in one go. To construct the upstream section of the fish pass, a new cofferdam was installed. The cofferdam was initially propped with MGF frames. The blinding was designed strong enough so that the MGF frames could be removed once it reached strength. It was important that the MGF props were removed because they would have clashed with the fish pass wall, and limited lifting operations in the cofferdam. The construction process was repeated and the fish pass base and walls were poured. Surveys and inspections of the fish pass by Yorkshire Water and the Environment Agency confirmed that the overall design and construction of the structure was satisfactory and within the design specification. On confirmation of this, the dewatering pumps were decommissioned and the sheet piles removed from the river to allow flows through the new pass. A 160t crane was employed to remove the sheet piles upstream of the weir. The excavator that had driven them in could not reach over the new fish pass. [caption id="" align="alignnone" width="1200"] (left) right bank cofferdam, looking at the rebuilt left bank masonry wall (and right) 160 ton crane and vibro hammer removing the upstream cofferdam - Courtesy of Ward & Burke[/caption]

Legacy

Niagara Weir Fish Pass is located on public land owned by Sheffield Council, situated next to football pitches behind a housing estate. After the project was complete, the area was landscaped and returned to public use. Canoe portages were constructed upstream and downstream of the weir, using the reclaimed Yorkshire stones from the river.

Conclusion

The River Don has long been blocked by barriers and now opening them up is creating a wealth of virgin habitat that could boost the fish populations massively. The main species the fish pass is aimed at (salmon, trout, grayling) are under threat due to climate change. Indeed salmon are considered so threatened now that they could disappear in the next 20 years without help. The River Don is vital in safeguarding these fish against climate change due to its location in England and size of the estuary into which it flows. Niagara Weir Fish Pass was completed and opened in May 2023.

Dunside Reservoir consisted of two reservoirs named Dunside Upper and Dunside Lower. They were opened in 1884 to supply drinking water to the surrounding area. They ceased operations in 2009 when Scottish Water deemed that it was not economically viable to continue to maintain the two impounding dams. The dams required weekly leakage monitoring and had to be inspected regularly by the Scottish Water Reservoir Engineer and Reservoir Panel Engineer (ARPE) to assess the dam conditions as part of the Reservoir Act 1975. The act states that any impoundment of 25,000m³ or more should be part of the Act. Scottish Water assessed the viability and future cost of the dams remaining within the Act and with agreement with the ARPE and SEPA deemed that they should be breached.

Full abandonment/enabling works

Scottish Environmental Protection Agency (SEPA), the ARPE and Scottish Water decided that the breach would be a full abandonment. As a result, the reservoirs were drawn down to empty and the fish rescued and relocated to a place of safety under licence by the Clyde River Trust team by Spring 2023.

Design requirements

Mott McDonald was appointed as designers with the outline design scope being:

“Dunside Upper and Lower Reservoirs are to be removed so that they are no longer capable of storing water, and therefore no longer subject to the Reservoirs (Scotland) Act 2011. This will be achieved by the removal of both embankments, distributing the excavated material across the former reservoirs and profiling to match the existing ground, and forming a channel through the former embankments to reinstate the natural watercourse.”

[caption id="attachment_18268" align="alignnone" width="1200"] Dunside Reservoirs prior to breach - Courtesy of George Leslie Ltd[/caption]

Scope of works

George Leslie Ltd was contracted by Scottish Water to carry out the reservoir abandonment works. The works were undertaken between May 2023 and January 2024:

• Management of inflow to the reservoir, including temporary pumping to prevent refilling of the reservoirs during removal of each dam, and control of sediment to prevent discharges of water from the work site causing pollution of the receiving watercourses.
• Breach cut through the Dunside upper embankment (down to the original watercourse level). Work to be undertaken in a specified sequence.
• Breach cut through the Dunside lower embankment (down to the original watercourse level). Work to be undertaken in a specified sequence.
• Creation of a meandering channel through the base of each embankment, following the alignment of the original watercourse from historic drawings.
• Grouting of the outlet pipes through both embankments.
• Removal and disposal of valve towers/chambers, bridges, chambers, pipework.
• Infilling of the existing spillway channel.
• Hydroseeding of the breach cuts and material removed from the embankments.

[caption id="attachment_18266" align="alignnone" width="1200"] Water management plan layout - Courtesy of George Leslie Ltd[/caption]

Construction challenges

Flood risk: The earthworks required were not particularly unusual in that only some 60,000m³ of cohesive materials had to be cut and carted to designated on-site depositories for placing and compaction. To control the levels of the earthworks, Trimble GPS controlled bulldozers and excavators were used.

The excavations were carried in pre-agreed stages to set levels above AOD as a requirement of a Mott MacDonald study ‘Dunside Hydrology to Inform Construction Risk Review’ to negate theoretical risks arising of a flood event whilst works were in progress. In addition, a maximum legally permissible impounded water level at each stage was set by the ARPE. This required a minimum of eight 200mm pumps to be located on site throughout the construction period in order that they could be deployed at short notice in flood events.

Silt mitigation: In the 140 years the dams were in place, the natural downstream movement of sediments on the submerged watercourses had stopped, resulting in a substantial volume of silts in the deeper areas of the reservoirs.

George Leslie Ltd had to ensure that every practicable measure was utilised to minimise silt discharges to the watercourses and avoid harm to the ecosystems. These measures were required during the reservoir drawdown and throughout the construction period as the excavated and fill surfaces remained exposed to the elements.

A Water Management Plan, detailing an extensive regime of settlement lagoons and swales, filtration through matting and bags, and finally lagoon flocculation using QP33 was developed and implemented prior to any release of water to existing watercourses. QP33 is an organic coagulant, which is supplied and installed by Taytech Environmental Ltd. This coagulant is a primary chemical used in suspended solids settlement. At the time of writing (August 2024) QP33 was the only flocculant chemical approved by SEPA.

[caption id="attachment_18270" align="alignnone" width="1200"] (left) Dunside Reservoirs during construction July 2023, and (right) after breach - Courtesy of George Leslie Ltd[/caption]

Sediment filter bags from Murlac Land & Marine Environmental Solutions are made of a 90 micron non-woven high strength (400 gsm) geotextile, which catch silt, sediment and oil, whilst releasing clean water. The Murlac sacs were connected to the de-silters to catch any finer silt before discharge.

During the construction phase, turbidity levels throughout the site and at designated points were monitored using turbidity meters and Secchi tubes. An Environmental Clerk of Works visited site at least once a week and compiled a report with recommendations and actions if required, which was made available to SEPA.

The weather throughout July and August was generally cool and wet and as a consequence, all the earthmoving plant was required to stand down for long periods of time rather than generate turbid runoff. The water management, however, was required throughout the construction period and utilised a significant amount of resource and consequent cost.

Ecological constraints: Ecological constraints were present on site, such as nesting birds and otters. Exclusion zones were set up to protect ecological features, as part of George Leslie Ltd’s commitment to minimise their environmental impact.

Hydroseeding: George Leslie Ltd was required to carry out hydroseeding as soon as was practicable, and as early in the growing season due to hilly nature of the site, on any finished surfaces in order to encourage early growth to minimise the area of exposed bare earth.

Dunside Reservoirs Abandonment: Supply chain - key participants

• Designers: Mott MacDonald
• Main contractors: George Leslie Ltd
• Ecological surveys: JK Ecology
• Drone surveys: Whitehouse Studios
• Fish rescue: Clyde River Foundation
• Removal of tower: PMP Utilities
• Temporary bridges: Groundforce
• Pumping supply: Selwood
• Silt mitigation: TayTech Ltd
• Silt filter bags: Murlac Land & Marine Environmental Solutions
• Silt mitigation fences: Hy-Tex UK Ltd
• Hydroseeding: Aitchison
• Civils materials: Keyline Civils
• Tankering services: Enviroclean

[caption id="attachment_18269" align="alignnone" width="1200"] Reinstatement of the watercourse - Courtesy of George Leslie Ltd[/caption]

Reinstatements and formation of watercourses

Following the earthworks being completed, the formation of the watercourse was carried out. Hydro-morphologists from Mott McDonald and SEPA had consulted historical maps, from prior to the original dam construction.

George Leslie Ltd was tasked with reforming the watercourse as closely as possible to the original course to replicate the sinuosity to appear more natural and less engineered. Prior to flows being gradually re-introduced over a prolonged period, coir fabric was installed temporarily on the burn banks to minimise scouring whilst vegetation growth took place.

The Clyde River Trust carried out invertebrate surveys of the stream beds prior to works and then subsequently one year after completion.

Carbon savings

One of the main targets within the contract was to utilise initiatives consistent with The Scottish Water Net-Zero Strategy. Initiatives George Leslie Ltd implemented included using batteries within the site accommodation and welfare buildings, and recycling and using arisings from the works by processing, crushing and grading to meet specifications. Scottish Water are planning to undertake tree planting throughout the area.

Conclusion

This contract is one in a series of similar future planned works and as such the lessons learned during the works are to be used in the planning and design of future dam breaching works.

Llyn Bran is located in Denbighshire, north-east Wales and was a natural lake that was raised in 1899 by a concrete dam. The reservoir was used to supply water to the North Wales Hospital in Denbigh. The hospital closed in 1995 and Llyn Bran has not been used for drinking water since. As part of a separate discontinuance scheme at Llyn Anafon, Llyn Bran was identified as a ‘compensatory site’ for the loss of habitat at Anafon. One element of turning Bran into a compensation site was the removal of the dam, as well as other ecological enhancements. Through careful restoration work, Llyn Bran will eventually become a Special Area of Conservation.

The reason for Llyn Bran discontinuance

Llyn Bran Reservoir is required as a ‘compensatory site’ for the loss of habitat area that will be caused by discontinuing the dam at Llyn Anafon Reservoir, as required by the Habitats Directive. Llyn Anafon Reservoir was originally a natural lake which was raised by 1.4m in 1931 through the construction of an embankment dam. The use of this site for local water supply ended in 2002. A 2016 S10 inspection of Llyn Anafon resulted in a MITIOS (Measures in the Interests of Safety) to “develop a long-term solution to the poor and deteriorating condition of the dam”. In January 2019, it was agreed with relevant regulators that to provide a long-term solution, the reservoir would need to be decommissioned, with the dam structure removed and the lake lowered by 1.4m. [caption id="" align="alignnone" width="1200"] Llyn Bran before discontinuance - Courtesy of Welsh Water[/caption] Llyn Anafon is nationally and internationally protected for specific conservation features. This means prior to any project taking place, a full Habitats Regulation Assessment (HRA) had to be undertaken. The outcome of this assessment was that it was not possible to avoid adverse impacts to protected features and as such, the only route to proceed with the project was under the “Imperative Reasons of Overriding Public Interest (IROPI)” (Article 6.4 of the Habitats Directive) on the grounds of public safety. The Anafon Dam Discontinuance Scheme is the first IROPI case of its kind in the UK and has recently been approved by Welsh Government. To satisfy the IROPI case, Welsh Water was required to secure ‘compensatory measures’ for the loss of habitat at Anafon. Using a set of criteria agreed with National Resources Wales (NRW), Llyn Bran was identified as the most suitable site to provide the necessary compensation. A key element of the work was to ‘restore and maintain hydrological functioning’ i.e., removal of the dam to create a natural lake and restore connectivity to the downstream watercourse. Although the primary reason for discontinuing Llyn Bran was for compensation, it also had the added benefit of removing a dam which was no longer needed for operational purposes. [caption id="" align="alignnone" width="1200"] Llyn Bran dam before discontinuance - Courtesy of Welsh Water[/caption]

Scope of works

• Create temporary construction access using bog mats.
• Draw down the reservoir by means of siphon pipes and pumps.
• Remove silt from the area immediately upstream of the dam.
• Demolish the entire dam and remove arisings for disposal off-site.
• Restore the watercourse channel downstream of the retained natural lake, incorporating silt management and retention measures.
• Create new water vole habitat along the shore areas of the retained natural lake using coir matting.
• Demolish outbuildings and associated structures with all building waste removed for disposal off-site.
• Remove exposed sections of pipework. Buried pipework to be sealed and filled by grouting.
• Install new roadside fencing along the north end of the reservoir.
• Install rip-rap protection around the upstream end of the B4501 culvert.

[caption id="" align="alignnone" width="1200"] Ground conditions - Courtesy of Welsh Water[/caption] As with all discontinuance schemes the various surveys, permits, consents and negotiations in advance of the physical works were particularly complex, taking more than two years to complete. An overview of the consents and constraints required is below:

• Impoundment Licence and SSSI consent from Natural Resources Wales (NRW).
• Permitted Development Certificate from Denbighshire CC.
• Ordinary Watercourse Consent from Denbighshire CC.
• Protected species licence required for trapping and temporarily removing water voles (this was needed before permitted development could be granted).
• Annex 1 Habitats present around lake margins.
• Reservoir drawdown rate limited to 120 l/s.
• HRA level 1 & 2 required.
• Peat bog deposits along route of temporary access track.
• Bat surveys required prior to demolition of valve house & boat house.

Challenges & constraints

Site access With no formal access track to the dam, and the surrounding land being designated as Annex 1 Habitat due to the presence of peat, access was one of the main challenges. With 140 peat depths probes taken along the route of the proposed access track, and working closely with NRW, a methodology was developed for installing over 280m of bogmats to the dam without the need to excavate and remove the top layer of peat. This minimised the potential damage to the peat and made site reinstatement far easier. [caption id="" align="alignnone" width="1200"] (left) Bog mat access track and (right) amphibious excavator used to load barge with coir matting - Courtesy of Welsh Water[/caption] Peat The extent of peat encountered on site was far greater than expected once construction began. This resulted in an entire change in the contractors methodology for the coir matting. It was initially planned to use Bog Master excavator and tracked dumper to access the western shore of the reservoir, however it quickly became clear this was not practical. Thanks to the contractor being proactive, the methodology was quickly changed to using an amphibious excavator and barge, to transport the coir matting across the reservoir. Water voles Water voles were identified on site during the early site survey stages, meaning a licence was required to temporarily remove the voles during construction. Six water voles were captured by specialists and were transferred to a facility in Kent, where they were looked after while the work took place. A condition of the licence was that there must be 5-days with no water voles captured before it could be concluded that all voles were removed from site. Rather unluckily, there were two occasions where a vole was captured at the end of the fifth day, meaning the process took far longer than expected and the licence had to be extended. The voles are due to return home to Llyn Bran this summer. [caption id="" align="alignnone" width="1200"] Water voles were captured and transferred to a facility in Kent for the duration of the works - Courtesy of Welsh Water[/caption] Compensation flow A condition of the NRW impoundment licence was for a continuous compensation flow of at least 20 l/s for the duration of the works. This meant pumping to the watercourse, which had to be continually monitored. It was hoped to use a cloud-based telematics tool that makes it easy to monitor the pumps off-site but unfortunately, due to lack of signal, this did not work meaning William Hughes Civil Engineering needed to attend site over the weekends to check on the compensation flows. The summer of 2022 proved to be exceptionally dry, which crated a challenge in meeting compensation flows whist also not allowing the remaining lake to become too low. Flood risk The existing reservoir provided significant flood attenuation meaning the discontinuance increased the flood risk to a downstream highway culvert. Initially, there were concerns from the highways department that the culvert would need to be upsized, which would have added significant costs to the project. Following a detailed flood risk and stability assessment, Welsh Water were able to prove that the increased flood risk would not adversely affect the culvert and only minimal slope protection work around the culvert was required. [caption id="" align="alignnone" width="1200"] Amphibious excavator forming the new channel connecting to the downstream watercourse - Courtesy of Welsh Water[/caption]

Llyn Bran Reservoir Discontinuance Project

• Construction timeline: 16 May 2022 - 4 November 2022
• Project cost: £1.7m
• Drivers: Dam safety | Habitats Directive | Reducing costs | Ecological compensation measures

Supply chain - key participants

• Principal Contractor: William Hughes Civil Engineering
• Design Partner: Stillwater Associates
• Environmental Consultants: Ricardo Energy & Environment
• Ecology/water vole licensing: Wildwood Ecology Ltd
• Silt management: Salix River and Wetland Services
• Amphibious excavator hire: Land and Water Plant

Opportunities & Savings

Forestry culvert At the design stages there were concerns about the condition of a nearby downstream forestry culvert and its ability to accommodate the reservoir outflows during emptying. Expertise within Welsh Water was utilised by requesting a CCTV survey of the culvert via the wastewater team. The team provided a prompt and efficient service and enabled us to confirm the capacity and condition of the culvert. [caption id="" align="alignnone" width="1200"] Cores being drilled through concrete dam to allow any extreme storm flows to pass through - Courtesy of Welsh Water[/caption] Dam safety mitigation During detailed design, the Dam Safety Team suggested coring holes though the concrete dam once the reservoir was drawn down to the required level. This would mitigate the risk of the reservoir refilling during severe weather, before the dam was fully demolished. Fortunately, on this occasion the weather was favourable and the cores were not needed. This approach will be considered for any future concrete dam discontinuances. Waste removal It was agreed with NRW that the stone from the haul road and compound could be transferred to a nearby quarry, just down the road from site. This was done under a waste transfer notice, saving costs. The arisings from the dam were removed off site. The dam was demolished far easier and quicker than expected, making the removal of concrete straightforward. [caption id="" align="alignnone" width="1200"] Dam demolition in progress - Courtesy of Welsh Water[/caption] Coir matting Due to the silt being drier than anticipated, there was spare rolls of coir matting left over. Rather than this being wasted, it was moved to nearby Alwen WTW where it is being stored before being used for silt management on the upcoming Cilcain Discontinuance Scheme which is due to start this spring.

The future for Llyn Bran

The discontinuance will give significant biodiversity opportunities in relation to the marginal areas around the retained natural lake and with the restored connectivity to the watercourse. The retained lake will also be used as a receptor site for aquatic plants translocated from Llyn Anafon. Long term ecological monitoring of the lake is required to ensure the compensatory criteria are met. This includes water quality and macrophyte monitoring as well as botanical surveys to check the natural regeneration of exposed shore habitats. A condition of the water vole licence is that activity is monitored until 2034. Through careful management and monitoring of the site, it is hoped that Llyn Bran will become recognised as a Special Area of Conservation.

Starting on site in 2022, the conceived project was to construct a new fish passage around the Holme Sluice Gates on the River Trent, allowing bi-directional travel of all breeds of fish from salmon to coarse river fish. The existing gates were constructed in the 1950s and are currently operated by the Environment Agency to maintain navigable river levels within the Nottingham area.

This paper follows up on one published by Water Projects in 2022, and looks at the construction and commissioning elements of the project.

Background

When the Holme Sluice Gates structure was constructed, the River Trent was diverted from its original path around the Country Park through a newly excavated 80m wide cut channel. At the side of the sluice gates is the Holme Canal Lock with a further adjacent water structure of the Holme Pierrepont canoe slalom course; all of which create barriers for migratory fish back up the River Trent.

Project summary

As the sluice gates will remain in operation, the fish pass channel requires two bridges rated to 40t to be built to maintain access to that area. The channel is to be protected at the intake via a single floating boom and the water isolated from the channel by a combination of removable stop logs and an automated radial gate system.

[caption id="attachment_12348" align="alignnone" width="1200"] Aerial view of the completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]

The design of the fish pass has been provided by the Environment Agency’s designer, TEAM Van Oord (TVO), with Jackson Civil Engineering Ltd (JCE) undertaking design elements for the temporary works and the Mechanical, Electrical, Instrumentation, Control and Automation (MEICA) elements.

The fish pass design has a total flow rate of 4.4m³/s - 4.9m³/s with a varying water depth of 1.5m- 1.7m. Although the velocities within the vertical slots have a flow rate of 2m³/s, the pools created at each stanchion bay slows this to >1m³/s; allowing fish to rest as they travel through the pass.

Where we left off……

This paper follows on from an initial case study published by Water Projects in August 2022. In the time since, the piling works (which started in April 2022) were completed by Ivor King Piling, following a period of ground pre-auguring to ensure that the circa 10m AZ700-36 sheet piles could be driven with relative ease.

A total of 350 sheet pile pairs with a number of singular piles were driven into the ground to produce the working cofferdam.

Due to the inlet and outlet elements of the fish pass having to be installed within the river flow, it was imperative that the works hit the allowable in-river works period to avoid damage to spawning fish and suitable breeding grounds. This period ran from 15 June 2022 to 2 October 2022; these dates forming an X5 Contract Key Date. Work on the inlet/outlet sections started nine workdays after the period started, and all piling works were completed by the end of August 2022.

There were some significant challenges throughout this period, the depth and complexity of excavating safety behind the sheet wall to install tie piles at a depth of 4-5m requiring a significant portion of the river bank (50m³) to be removed stored and reinstalled and for a number of the tie bar being at a level below the surface of the river, meant a design change was needed to a welded beam solution on the backside of the piles.

[caption id="attachment_12352" align="alignnone" width="1200"] Modular wall shuttering in place for wall pour - Courtesy of Jackson Civil Engineering Ltd[/caption]

Channel design concept

In summary, the channel concept included a full height channel wall at the inlet and outlet, joined on both sides by a 2m high dwarf FRC sheet pile wall. Around the top of the sheet piles, a non-structural concrete pile capping beam (chainage 24m - 150m) was required, at which point the design changed to a structural capping beam.

This design was driven by the ground pressures seen on the sheet pile walls in that the channel floor would act as the only prop needed in the non-structural sections, however, due to the increasing depth of the channel towards the discharge end the top of the channel would also need to be permanently propped as well as using the base slab.

The start of earthworks & FRC

Originally the staging for construction was to temporarily prop the sheets at the top throughout the channel run, excavate down to design invert level, cast the base slab and the install the capping beams. This would require the need for significant soffit support systems and working at height.

Through detailed project discussions with the principal designers and supply chain, the decision was made to excavate only the area needed for the capping beam and use the ground formation to act as the soffit support there for removing a significant health and safety risk. To allow access throughout the construction period a section of capping beam was left out (5m of the 400m required) which would require a conventional support system for install.

Earth removal started in earnest in mid-August 2022 reducing the ground to install 26 (No.) SBS 400-185Hyd struts and 300m linear lengths of SSB wailing beams provided by Mabey Hire Ltd.

[caption id="attachment_12345" align="alignnone" width="1200"] Excavated fish pass awaiting FRC works - Courtesy of Jackson Civil Engineering Ltd[/caption]

Groundwater challenges

From the provided geotechnical report it was noted that groundwater was expected circa 2m below the ground surface and from engaging with a dewater drainage specialist it was possible that installing a dewatering system would require a significant number of well points discharging 25 l/s. The larger issue was that these wells would need to be installed through the capping beam (not aesthetically pleasing) or would be placed in the way of the bulk earthworks removal.

A number of trail pits were excavated to 4m to assess the realistic nature of the groundwater. Little groundwater was evidenced during these tests, therefore the project proceeded without the use of dewatering, with a mitigation measure that a local sump would be created to allow groundwater to be pumped our using a 6” end suction over pump, to aid in the drainage and working surface a 100mm thick layer of gravel (350 tonnes) was installed in the bottom of the channel to allow the water to flow beneath and work plant. The risk of significant groundwater did not materialise.

The installation of the temporary struts and excavation of bulk material were done in sections therefore allowing manoeuvrability of the excavator to reach between the piles. M2 Civils Ltd, the earthworks contractor, removed on average 24 road wagons of earth per day, totalling 1458 loads or 27,935 tonnes (13,760m³).

Holme Sluice Fish Pass: Supply chain - key participants

• Client designers: Team Van Oord
• Principal contractor: Jackson Civil Engineering Ltd
• Temporary works design: Waterman Group
• Mechanical design: KGAL Consulting Engineers Ltd
• FRC: STAM Construction Ltd
• Groundworks: M2 Civils Ltd
• Piling works: Ivor King Piling
• River works: Red7 Inshore Diving
• Ground support: Mabey Hire Ltd
• Electrical design & control equipment: IMAC Ltd
• Radial gate system & stop logs: Centregreat Engineering Ltd
• Eel tiles: Berry & Escott Engineering Services
• Fish monitors: FishTrack
• Precast concrete beams: Banagher Precast Concrete Ltd
• Tarmac & surfacing: CR McDonald Surfacing

[caption id="attachment_12347" align="alignnone" width="1200"] Non running fish pass mid-distance - Courtesy of Jackson Civil Engineering Ltd[/caption]

FRC

The FRC works was undertaken by STAM Construction Ltd and started in October 2023 with the installation of base slab using cut and bent reinforcement steel throughout the construction. This would then allow the high-level shoring props and wailing beams to be removed. Due to the nature of the design being effectively three straight runs of dwarf walls, the wall shuttering system (9m long by 2.5m high) was constructed on an adjustable wheeled cart, thus allowing the shutters to be installed, retracted, moved, and redeployed reducing the use of timber and removing significant lifting and manual handling risks.

To facilitate this the project teams worked on providing a solution to allow the shuttering system to fly past the starter bars required for the wall and floor stanchions. This was achieved by utilising a ‘Startabar’ product whereby the RC bars are encased under a steel cover which is then removed, and the bars bent out to the correct position after the wall concrete has been poured.

To construct the wall and floor stanchions, conventional shuttering systems were employed. In total 329 tons of steel reinforcement was used and 2,250m³ of concrete were pumped.

Bridges

The construction of the fish pass has created an island of the operational sluice gates requiring 24 hour. To service this asset, two permanent 40T rated road bridges were designed and installed. A major project challenge was to sequence the bridge construction start for after the bulk excavation had past the bridge locations and have them finished before the temporary access created across the sheet piles needed to be excavated out. This includes the bridge abutments, deck and on/off ramps. The bridge construction utilised MY3/MY3E precast concrete beams spanning 12.6m utilising seven beams per bridge and used scaffold soffit support system to all for cast in situ sidings and deck. The MY3 beams were planned and delivered in a ‘Just in Time’ strategy allowing a direct lift and install from the wagon to their installation location.

Construction started September 2022 and finished October 2022 including the installation of the vehicle restraint system, notably the bulk earth movement completed by removing the temporary access to the island in December 2022.

MEICA

The water passage through the pass is controlled by an automated radial gate system, using local river levels and area flow gauges. The gate opens when the flow is greater than 49m³ and the level is above 20.34m AOD; should either of these readings become false the gate will close.

The gate spans the full width of the channel with a radius of 3.5m and stands closed at a vertical height of 1.9m. Using a single actuator the gate (10 ton self-weight) is lifted and lowered via a steel cable winch system with a full extent of travel being circa 4m vertically.

The radial gate solution was chosen by the designers as the best suitable control mechanism due to the following benefits:

• Requiring less infrastructure than a vertical gate (no need for structures/lifting frame/gantry above the channel) all reducing the civil reinforcement design in those areas.
• Less energy to operate as loads are transferred through the pivots and the geometry of the gate means upwards water pressure will assist with lifting.
• Longevity of the seals as loads transferred through the pivots and not into guides and seals.
• No need for channels/slots in the walls/base of the channel which could become blocked with debris and would also affect hydraulics.

Even though the size of the fish pass requires significant volume of water to achieve a steady state, this is realised in approximately 90 seconds from the start of the gate opening even though the gate takes 8 minutes to fully clear the pass/river surface.

The isolation of the fish pass for maintenance purposes is undertaken by the installation of five 6.8m long stop logs. These are deployed using an 18m long lifting gantry with dual lifting hoist with a SWL of 2.5T and a hand cranked moveable storage cart.

While the pass is in operation there will be free bidirectional passage for aquatic life, however during non-operational periods eels are still likely to try and pass and with the water during non-operational period only filling the last third of the pass, there is the potential for the eels to become stranded in the pass. Therefore, the pass has been designed with eel chute compliant with the Eels Regulations 2009. The chute and subsequent rebates along the edges of the pass have been fitted with fixed plastic eel tiles therefore providing purchase to aid the eels in their journey. To assist the chute, which is connected to the radial gate and is lifted out of the water during gate operation, while the gate is closed there is a constant flow of water covering the eel tiles guided by the fall of the channel base and surface mounted fixed plastic lumber sections.

[caption id="attachment_12351" align="alignnone" width="1200"] Completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]

Pass/river interfaces

Extensive riverbank and river bend modifications were needed to ensure that the pass joined and exited the river and was engineered to prevent erosion. During the in-river works period of 2022 and detailed as a key date within the contract, specialist dive services were employed to dredge out the riverbank/bed and place 80-100T of scour protection amour using 5-300kg sections of rock.

This was all achieved using bank-side long reach excavators; therefore being more environmentally friendly, with greater efficiency than deployment of float pontoon platforms.

Due to the in-river works, stringent protection measures were placed on the project with regards to fish protection and silt migration. The project team deployed 100m of silt bubble curtains around the river work areas to reduce the any sit mitigation as well as undertaking on-site water quality monitoring for turbidity and dissolved oxygen both upstream and downstream, and within the works area. Data showed that the work area was a factor of 10 higher than the up/downstream areas, which remained within agreed permissible limits.

Further dive works included underwater cutting and removal of the cofferdam end sections to open the fish pass to the river. This solution was chosen as it reduced the risk of delays and damage to permanent structures during a vibration extraction method.

Monitoring fish usage

The fish pass is fitted with state of the art near-field communication hardware (passive induction transmitters) to facilitate the monitoring of fish passage. When the electronics are installed, the system will allow the client to understand how effective the pass is, the direction of fish travel and time spent within the pass.

[caption id="attachment_12350" align="alignnone" width="1200"] (left) Completed fish pass looking towards the intake structure and (right) fish pass in operation showing control radial gate - Courtesy of Jackson Civil Engineering Ltd[/caption]

Conclusion

The project completed on 14 May 2024 having run for 27 months. The following is a statement from Simon Ward, Fisheries Specialist with the Environment Agency.

“After two years in construction, and several years of planning and design, the Colwick (Holme Sluices) Fish Pass on the River Trent is now complete and ready for fish to use enabling them to reach their spawning and feeding grounds. This flagship project opens up the River Trent and its tributaries for migratory fish, including salmon, trout and eels, making more habitat accessible for fish. Whilst a complex task the construction ran smoothly throughout and the partnership of designer, constructor and EA Fisheries technical staff working together is an exemplar of what can be achieved for the environment.

“The fish pass is part of the Environment Agency’s work to improve fish passage across the country and provides a significant step in restoring the River Trent catchment to its former glory for salmon and other coarse and migratory fish. The project also includes an eel pass to help support the critically endangered European eel; and a public viewing platform above the water, with highly visual interpretation boards to inform and advise visitors about the local wildlife in and around the river, including the fish that are expected to use the pass.”

Northbridge is a key drinking water supply location with a large amount of upstream habitat available for eels. The intake is located 11km from the tidal limit on the right-hand bankside of the River Exe, Devon and is the intake for Pynes WTW. Abstraction occurs all year round, with typical abstraction rate between 30,000-40,000m³ per day with a maximum licenced flow is 66,450m³/day. Water is abstracted via two inlet lanes each consisting of a course and fine screen which feed a total of four pumps with the two lanes being able to be operated independently. All four pumps can be operated whilst one lane being closed if flow must be diverted around the closed lane using the actuated penstocks.

Background

The existing screening has coarse (9mm aperture) and fine (5mm aperture) screens within the pump house, which discharge debris and eels back to the river via a chute through the side wall.

The current screening will not meet 2025 statutory requirements and South West Water are addressing the issue now to ensure future compliance.

This project will deliver improved screening at the Pynes WTW intake with Eel Regulations compliance, which is required by the end of AMP7, ensuring continued operational abstraction at the site. There is a need to improve the exclusion of the freshwater stages of the critically endangered European eel from river intakes and abstractions allowing lifecycle completion at sea. Additionally, there is need for protection of downstream migrating salmonids. This includes both Atlantic salmon and brown (sea) trout.

[caption id="attachment_12976" align="alignnone" width="1200"] Existing intake structure (external screens at and below river level behind the debris boom with the access platform above) - Courtesy of South West Water & Fishtek ltd[/caption]

Northbridge design considerations

The proposal was to install two 2.352m wide Hydrolox S1800 rotating band screens within the existing bankside infrastructure to replace the existing screening. This will reduce fish and eel entrapment at the intake and ensure compliance for continued operational transfer to Pynes Water Treatment Works. These screens also offer health and safety benefits, as well as OPEX and TOTEX cost benefits.

To allow the planned work to be undertaken, the inlet lanes had to be closed one at a time for the new screens to be installed and the old screens removed whilst still allowing for the 46,000m³/day abstraction. High water demands during the summer months had to be considered along with other consents and approval conditions, so the earliest date restrictions could be imposed was early September 2023. The work was therefore initially planned for a 4-month duration between September 2023 and January 2024.

During the installation period, South West Water had to allow for ‘recharge periods’ where both lanes could be open to allow for the 60,000m³/day abstraction. This would be over a weekend (Friday-Monday), and this allowed for a total of 50% of the installation period where recharge could be undertaken.

To be able to produce a feasible design, various surveys and reports had to be undertaken. These included dive surveys, ecological surveys, protected species surveys, GPR surveys, existing infrastructure surveys, 3D scans of the pump house building and underwater intake bays, and crane lift surveys.

The results of the surveys found the existing intake structure was structurally sound to be re-used for the new Hydrolox screens, which removed the need for using ‘wet’ concrete to construct the work, which would of entailed using a cofferdam. This removed a hazard from the new work within the design stage, benefitting both programme and cost savings.

[caption id="attachment_12977" align="alignnone" width="1200"] New debris return launder chute - Courtesy of South West Water & Fishtek Ltd[/caption]

Hydrolox screen selection

Hydrolox S1800 rotating band screens were chosen for this scheme as they are considered an industry best practice solution by the Environment Agency. Benefits included:

• Through management and coordination between South West Water Operations and the contractor Fishtek ltd, this option carried less programme risks than alternatives because the ability to abstract river water during construction was retained.
• The solution was considered to be ‘riverside’ and therefore applicable for reduced screen sizes compared to the options available if they were to be set back from the riverbank.
• Being installed above-ground, inspection and maintenance of the screens removed the hazard of requiring confined space access.
• Being an accepted best practice solution, notably less consultation would be required with the National Eel Service Helpdesk, so this solution carried significantly less risk towards compliance.

Restricted site

The work site was approximately 330m from an unclassified single vehicle access road, and having to navigate down this road and track with oversize vehicles, along with having to contend with a section of Japanese knotweed was an early challenge. The rutted access track had to be regraded to accommodate the large vehicles and crane attendances along with protection of the knotweed to prevent cross contamination of the area across the area.

The site was extremely restricted and a 90t crane was required and had to be set up in a small area constrained by the existing building, the Network Rail mainline, 11Kva overhead power cables, outbuildings and mature trees.

Because the location was restricted and unsuitable for larger vehicles, the contractor had to change their methodology and bring the new structural elements in as individual components. These components were transferred onto smaller flatbed vehicles away from the site and reassembled in the small working area.

[caption id="attachment_12988" align="alignnone" width="1200"] (top left) Crane setup within restricted location & approach track, (top right) structural steelwork components being fixed ready for lifting and installation, (bottom left) silt removal prior to installation of steelwork, and (bottom right) accommodation work installing new concrete apron at landowners access gate to provide turning for reversing delivery vehicles - Courtesy of South West Water & Fishtek Ltd[/caption]

Construction phase

One of the first tasks was to remove the existing course trash screens and decking area over the River Exe intake in a safe and controlled manner, utilising a dive team to prop open the existing penstocks which remained in place until the scheme was complete and the equipment removed.

Upon successful removal of the existing equipment, the galvanised steel platforms (as required under DWI Regulation 31) were assembled on site and lowered into position within the existing reinforced concrete inlet structures and fixed with galvanised mechanical fixings. The steel platforms would house the two Hydrolox screens.

Other work streams involved mechanical and electrical interfaces, where decommissioning and removal of existing redundant equipment including the two sets of fine and coarse screens, wash water booster set, pipework and obsolete electrical equipment and cabling, all of which had to be phased in agreement with the Operations team.

Electrical scope included the installation of the instrumentation required to control the new screens and four ultrasonics, including a temperature sensor, and temporary isolation and installation of actuators for the external penstocks so that the new steel decking could be installed. This required good communication and agreement with the South West Water Operations team and the control centre.

[caption id="attachment_12980" align="alignnone" width="1200"] (left to right) Hydrolox screen installation, (i) steel support frame, (ii & iii) screen being lowered into position (note proximity to the building), and (iv) final positioning of the Hydrolox screen - Courtesy of South West Water & Fishtek ltd[/caption]

Northbridge Eel Regulations Compliance Project: Supply chain - key participants

• Client: South West Water
• Principal designer & contractor: Fishtek Ltd
• Diving & civils contractor: Teignmouth Marine Services
• Ecological supervision: Colmer Ecology
• Supplier of eel screens: Intralox Ltd
• Instrumentation & controls contractor: SWW O&T Dept
• M&E contractor: A&T Electrical
• Structural steel supplier: Milestone Communications Ltd
• Wash water booster pumps: Xylem Water Solutions
• Trash screens, launder chute & deflector boom: TWS Ltd
• Lifting contractor: Spence Crane Hire Ltd
• Transportation contractor: ACE Crane & Transport Ltd

Value engineering

The contractor, Fishtek Ltd, worked efficiently, continually keeping the project team informed with progress and any issues that arose to agree a solution to keep the project momentum. During the time that the contractor was on site, value engineering principals were applied. Examples of this include the reuse of existing equipment such as the launder chute where a partial section was manufactured and installed which reduced the material usage by some 80%.

Another example of the value engineering approach was to take most of the removed equipment to Pynes WTW, which would be available to be re-used for spares and parts replacement.

This benefitted South West Water from disposal costs and the costs of new parts.

Another positive outcome of the project was that Fishtek Ltd, engaged South West Water’s O&T team to undertake the instrumentation and controls scope. This worked extremely well to the benefit of all parties, setting a precedent for future collaborative working.

Third party relationships

Good engagement with all stakeholders (EA, Network Rail, National Grid, landowners and South West Water’s Asset & Operational team) was maintained throughout the programme and there were zero complaints.

To get equipment and cranes onto site, the team required access via a landowner’s property so good relationship was developed and maintained throughout the project. By way of thanks, a new reinforced concrete slab was constructed at the field entrance as a good gesture for the landowner’s cooperation. Upon project completion, a final re-grading of the 330m track was undertaken, and the Japanese knotweed protection was kept in place.

[caption id="attachment_12979" align="alignnone" width="1200"] (left) Complete Installation of the S1800 Hydrolox screens and decking system, and (right) new wash water booster set which provides high pressure cleaning of the screens removing debris - Courtesy of South West Water & Fishtek Ltd[/caption]

Conclusion & successes

Though the project was completed in an agreed revised programme due to several issues outside of the contractor’s control, the scheme was still completed almost one year ahead of the regulatory compliance date (March 2025).

The contractor demobilised from site in April 2024, with all that remains to be undertaken being the removal of one fine screen which was left in place at the request of the South West Water Operations team until the new screens had sufficient bedding in time, and after five river spate conditions of the River Exe had been experienced.

While on site, the contractor adopted a ‘see it, sort it’ ethos and several site operational safety improvements were identified and acted upon. Despite this additional work, the project scheme still came in 9% under budget.

Good health and safety was managed throughout the programme and the project was delivered with zero incidents or accidents.

The overall objective of the project was achieved and exceeded, gaining valuable lessons learnt for all involved so future schemes can be optimised throughout the project life cycle from conception through to construction and handover. This will allow for improved programme durations and activities, increased risk identification, improved tender scope and work information, as well as logistical restrictions and operational interface requirements.

The Don Regulators comprise three regulator gates on the River Rother that are owned, operated, and maintained by the Environment Agency (EA). The purpose of the regulators is to reduce the risk of flooding to settlements within the River Don catchment by attenuating flows in the River Rother. JBA-Bentley has completed the design and installation of the first regulator gate. The detailed design of the second gate is currently being undertaken, while the third regulator is being modelled as part of the optioneering phase. In addition to the installation of the first regulator gate, a bespoke full-channel fish pass has been installed directly downstream to improve migratory fish passage.

Meadowgate Regulator

The Meadowgate Regulator is located within the Rother Valley Country Park. The historic regulator consisted of a single fish belly bottom-hinged tilting gate, operated by a single electrically powered hydraulic ram.

[caption id="attachment_14798" align="alignnone" width="1200"] (left) location of the regulator gates and (right) the new Meadowgate top-down vertical control gate[/caption]

In 2023, JBA-Bentley replaced the gate with a top-down vertical control gate, improving the resilience of the structure. Alongside the regulator gate, a new control building and EA Office have been constructed. Both buildings have green roofs and EV charging. The gate was commissioned in February 2024 and is ready to operate during a flood event.

The gate is operated to ensure a maximum of 60m³/s forward flow is always maintained. When flows exceed 60m³/s the new gate is lowered and water levels in the Rother rise and begin to spill into the washlands located within the Rother Valley Country Park, which can retain approximately 1.5 million m³.

Meadowgate full channel fish pass

The removal of the bottom tilting gate provided an opportunity to improve fish passage on the River Rother through the installation of a unique full-channel width fish pass. This fish pass is designed with 4-tonne reinforced concrete ‘fish tiles’ featuring varied-height upstands supplied by Craven Concrete Ltd.

The tiles were designed with the aid of computational fluid dynamics modelling to ensure they influenced the nature of the flow to allow for varied fish species to migrate upstream while also withstanding hydrodynamic forces during extreme events. The fish species focused on are coarse fish, European eels, brown trout, large migratory salmonids, and lamprey.

The 40m long stilling basin has been removed by building up the existing concrete channel bed and grading out the levels to create a uniform slope. The ‘fish tiles’ have been laid onto the regraded channel. This is the first fish pass of its kind in the world.

[caption id="attachment_14802" align="alignnone" width="1200"] Precast concrete ‘fish tiles’ from Craven Concrete Ltd positioned in the channel - Courtesy of JBA-Bentley[/caption]

Meadowgate temporary works solution: Syphon pumps & contingency planning

The fish pass installation required the full channel as a dry working area. To achieve this, an innovative syphon system, designed by Dutch water specialist Vanderkamp UK, was installed. The solution involved complex temporary works and contingency planning.

To form the upstream dam, the new regulator gate was dropped to bed level, allowing upstream water levels to build up. The gate was designed to operate this way; hence no further calculations were required. Four syphon intake pipes were installed upstream of the gate to divert up to 12m³/s of flow out of the channel for 115m, around the working area.

The syphons were primed using electric vacuum pumps which expelled all the air from the system to draw water through the pipes. By utilizing the existing gate and power supply, the syphons had a far lesser environmental impact than a typical cofferdam and over-pumping solution. The total embodied carbon associated with the syphon solution was 464 tons.

Due to the scale of the flows being diverted, the loads imposed on the riverbanks by the pipes when full were considerable. JBA-Bentley and Vanderkamp UK worked together to design a temporary support system involving ground-bearing improvements, steel and timber supports, and retaining structures. Checks were also carried out on the existing channel wall where the forces from the syphons acted upon it.

[caption id="attachment_14797" align="alignnone" width="1200"] (left) The dewatered channel and (right) ‘fish tile’ installation - Courtesy of JBA-Bentley[/caption]

In addition to work to ensure the stability of the syphons, an energy dissipation system was designed and installed at the syphon outlets to combat scour, and a bubble curtain was installed upstream to deter fish from entering the syphons. The installation of the system was done via a temporary crane pad. Due to the nature of the site and its space constraints, this was a challenging design. JBAB worked closely with the landowner, Rother Valley Country Park, to balance the desire to allow continual access into the park with ensuring the crane was safe to carry out the required lifts. To achieve this, the crane had to be placed in close proximity to the riverbank. Slope stability modelling and bearing capacity calculations were undertaken to ensure the crane could be operated safely.

Recent storm events have recorded flows higher than the syphon capacity. To manage the risk of this occurring during the programmed works, a contingency plan was developed.

The basis of the plan was developed from a bespoke JBA flow forecasting system, which forecasted the flow over a 2-day period. Using the data from the forecasting system, flow monitoring data provided by Vanderkamp UK, upstream gauge data, and the storage capacity of the washlands upstream of the gate, JBAB could make informed decisions on whether the gate needed to be raised to allow flow to pass forward.

This decision was made with the aid of hydraulic calculations and decision flow charts within the contingency plan, allowing for quick and effective decision-making, ensuring flood risk was not impacted upstream and the safety and well-being of the site team working in the channel was never put at risk.

[caption id="attachment_14801" align="alignnone" width="1200"] Syphon outlet - Courtesy of JBA-Bentley[/caption]

Don Regulators: Supply chain - key participants

• Client: Environment Agency
• Project delivery: JBA-Bentley JV
  • JBA Consulting
  • JN Bentley Ltd
• Principal designer: Callsafe Services Ltd
• Gate design: Aquatic Control Engineering
• Syphon design: Vanderkamp UK
• Electrical installations: IMAC Ltd
• Building design: JP Chick & Partners
• Building design: Peter Wells Architects
• Supporting ECC Supervisor with new gate & associated equipment: KGAL Consulting Engineers Ltd
• Precast fish pass tiles: Craven Concrete Ltd
• In situ concrete works: Bells Construction Ltd
• Metalwork fabrications: Fabtek
• Green roof: ABG Geosynthetics Ltd

Canklow Regulator

The Canklow Regulator is the furthest downstream of the three Regulators, approximately 3.5km upstream from the confluence of the River Rother and the River Don at Rotherham.

The existing structure consists of a single electrically powered vertical lifting sluice gate operated by an electric wire rope hoist, comprising a drive motor gearbox assembly, cross shaft with integral rope drums, and support bearings.

To improve resilience and the operation and maintenance of the asset, JBA-Bentley is completing the detailed design and build of a new vertical regulator gate and associated MEICA and civils infrastructure.

The MEICA structure consists of a 20.3-ton vertical gate, mounted on a new steel bridge and working platform, which shall also house 13.5 tons of drive equipment for operating the gate.

Two 4kW motors, working in a duty/standby configuration, will provide drive to the gate via bevel and gearboxes, which turn a single driveshaft spanning the width of the gate. On this driveshaft, one at either end of the gate, are rack and pinion gear assemblies, which convert the rotation of the driveshaft to the vertical motion of the gate. The total gear ratio from motor to driveshaft is an approximate reduction of 4410:1, allowing relatively small motors to be specified.

[caption id="attachment_14807" align="alignnone" width="1200"] (top left) the existing Canklow regulator gate (bottom left) 3D model of the new Canklow Regulator and (right) bathymetry of Woodhouse Mill Washland - Courtesy of JBA-Bentley[/caption] The power supply to the gate will be a three-phase supply routed via a new MCC, which will contain the control modules, centralised drive units, and the telemetry system for remote monitoring and operation of the gate. The operation of the gate will be from on-site or via telemetry, with some semi-automated functions. The facility for future full automation of the site has been incorporated into the design. The MCC and electrical components will all be located within a kiosk mounted on a raised steelwork platform to lift the electrical components above the 0.1% AEP flood level. Also on this raised platform will be the DNO kiosk bringing power onto the site from the national grid, and a standby generator to provide resilience in case of power failure.

To support the new gate structure over the next 50 years, a new anchor restraint system has been designed. A buried sheet-piled ‘dead-man’ wall will be installed with ten 36mm anchor rods, evenly spaced, connecting to a waling beam on the wet side of the existing channel wall. This will be mirrored on both riverbanks.

The new infrastructure replicates the existing system, which has been determined to be past its design life and insufficient in supporting the new gate.

The existing superstructure of the control houses will be demolished, the foundations will be retained and extended where required to support the new gate structure using cast-in bolt sets.

A new permanent crane pad will be constructed on the south bank for the construction, maintenance, and decommissioning of the gate. This will double as parking spaces for EA operatives when not being used by a crane.

The JBA-Bentley team worked collaborative to optimise the delivery. This meant that site constraints were identified, and the design was completed with buildability and safety as the focus. A key constraint on the scheme is the proximity of the Northern Power sub-station and overhead power lines. The design of the dead-man walls, crane pad, and steelwork platform was completed to ensure that the plant and temporary works required for construction and installation were outside of the powerline exclusion zones.

[caption id="attachment_14796" align="alignnone" width="1200"] Overhead power exclusion zones within model - Courtesy of JBA-Bentley[/caption]

Woodhouse Mill

The Woodhouse Mill Regulator is situated between Canklow and Meadowgate. It uses a single electrically powered vertical lifting gate to retain water. This gate is operated by an electric wire rope hoist, which includes a central drive motor and gearbox assembly, half shafts, spur and pinion driven rope drums, and support bearings, all counterweight assisted.

JBA-Bentley is currently carrying out hydraulic modelling on behalf of the EA as part of the optioneering phase. The options include passive solutions, such as a baffle or orifice plate spanning the width of the channel to continue to utilize the upstream storage. Due to the size of the Don catchment, the modelling required three individual models to be run together:

1. The Rother.
2. Don/Dearne.
3. Lower Don.

Multiple return periods are being considered, resulting in hundreds of models runs. The modelling also uses a continuous simulation approach, which adds to the complexity of the model runs.

Summary

The upgrading of infrastructure of the Don Regulators will ensure continued flood risk benefits into the future for the Don Catchment and surrounding area with improved resilience and reliability of the new assets. Alongside the reduction in flood risk, the environment and biodiversity will improve in the River Rother as a result of the Meadowgate Fish Pass.

The town of Ammanford, Carmarthenshire has a history of flooding. Several significant flood events have been recorded since 1979 and most recently in 2009, with homes, businesses, roads, the Heart of Wales railway line and a local college inundated. Natural Resources Wales (NRW) commissioned Arup and Walters to design and construct a solution to reduce the community’s flood risk and improve fish passage. This project innovatively aligns solutions to reduce risk to the community from flooding with two other previously separate schemes; safeguarding a critical sewer crossing found to be in poor condition and addressing a significant barrier to migratory fish.

Project scope

With limited public funding and the increasing need for multifunctional infrastructure enhancements, the scheme offers lessons in the delivery of multiple benefits. The scheme comprises:

• Seven concrete flood walls.
• Two earth flood bunds.
• Four blockstone pre-barrages step water levels to allow fish passage, with adjacent ground lowering to increase flood capacity.
• Landscape planting and public realm enhancements, including nearby Bonllwyn Green and the Llandybie Community Woodland Project.
• Ecological enhancements for otters, bats, birds and invertebrates, focussed on improving connectivity, diversity and ecosystem resilience.

Reducing flood risk to homes and businesses

Ammanford is located at the confluence of the rivers Loughor, Lash and Marlas. The rivers respond rapidly to rainfall and their channel capacity is limited. Much of the town is built on low-lying land adjacent to the river and overtopping is predicted at multiple locations. If nothing was done to reduce flood risk, 198 homes and 25 business would remain at risk of flooding from the 1 in 100 (1%) annual chance event.

NRW studies demonstrated a strong case for change. The study area was one of the top communities at risk of flooding from main rivers within Western Wales, and facing one of the largest increases in flood risk as a result of climate change. Flooding in the area was predicted to cost the UK some £7.9m in today’s prices over the next century.

With options to slow flows upstream infeasible, optioneering focused on better containment of flood waters in the river corridor. No single flood defence would be able to meet the project objectives, so a combination of raised defences throughout was adopted as the preferred solution.

The scheme was designed to reduce flood risk up to the 1% AEP event, allowing for the impact of climate change over the scheme’s 100-year lifetime; benefiting 349 homes and 37 businesses.

[caption id="attachment_14722" align="alignnone" width="1200"] (left) Project overview - Courtesy of Arup and (right) fish passage with adjacent ground lowering works - Courtesy of Walters[/caption]

Improving fish passage

Separately, NRW had identified the case for improvements to Tir-y-Dail weir at the downstream extent of the scheme. The weir was a partial barrier for upstream migration of adult Atlantic salmon and trout, and a complete barrier to adult lamprey and juvenile European eel. Fish passage improvements would unlock 20km of river to migratory fish. The weir was in poor condition but encased a critical main sewer and enabled upstream river level gauging.

Scheme design and innovation

The design phase of the scheme involved comprehensive investigations and modelling to understand flood dynamics and identify effective solutions. Key design considerations and innovations are presented below.

Positive solutions to mitigate detriment

At the outset, the team identified a risk of conflict between the initially separate fish passage and flood scheme objectives. A further challenge was mitigating potential flood detriment to other areas. Flood modelling indicated that raising flood defences and making fish passage improvements to the Tir-y-Dail weir would cause unacceptable detriment elsewhere. Typical measures to mitigate the detriment were investigated but were found to be either ineffective or excessively costly (such as additional raised defences upstream) jeopardising the business case justification for the scheme.

The team identified and developed from first principles an innovative solution to locally address issues in proximity to the weir, including an area of ground lowering within the field adjacent to the existing weir and fish passage pre-barrages. This provided additional conveyance around the existing weir and proposed pre-barrages in more extreme flood events, mitigating potential detrimental increases in flood risk associated with both the flood defence and fish passage improvements.

To maximise the benefits of the ground lowering, soils generated from this area were used within the construction of the Tir-y-Dail flood bund, improving the scheme’s earthworks balance and reducing import volumes. The area was also landscaped to provide additional area of riparian habitat.

Balancing earthworks

Two factors made creating an earthworks balance on the project challenging. Firstly, the nature of the scheme and site constraints resulted in limited areas of fill. Secondly, the engineering and chemical parameters of some of the soils excavated were unfavourable.

[caption id="attachment_14724" align="alignnone" width="1200"] Tir-y-Dail flood bund following construction - Courtesy of Walters[/caption]

To minimise the volume of excavated soils that would need to be disposed of off site, the team first addressed the chemical properties of the made ground. Site-specific risk assessments were undertaken to derive appropriate re-use criteria for human health and controlled waters. These were developed into a material re-use strategy, which considered appropriate legislative frameworks and was able to demonstrate that the majority of made ground was chemically suitable for re-use in appropriate scenarios.

To address the imbalance in volumes, the design of the Tir-y-Dail flood bund was reviewed as the primary fill area within the scheme. Flood bunds are most commonly constructed from a homogenous clayey material. This would need to be imported due to a lack of suitable on-site sources. The flood bund was therefore designed to have a ‘cap’ of imported clay to ensure performance against flooding, while the main core was formed from variable site-won material. Slope stability assessment undertaken under various scenarios demonstrated that, as long as the clay cap material was appropriately controlled, the geotechnical parameters for the core material could be significantly reduced.

To allow the contractor flexibility of the site-won materials placed in the core of the bund depending on the sequencing of their works, a project-specific soil material class was created. This was based on a combination of observational and basic material classification controls. This approach resulted in a reduction of 270 wagon loads of soil movements, reducing carbon and impact on local residents.

Sensitive detailing of flood defences

Measures were adopted to ensure the flood defences were sensitively integrated into the local landscape.

Flood walls in publicly viewable areas were finished with brick cladding, form liner finish and imitation stone copings to match local architecture. Coping stones were designed to be structurally integral to the flood wall to minimise their overall height, as well as reduce maintenance and public safety risk. Pillar features were incorporated into the facing of brick cladding at regular spacings to provide visual interest along the expanse of the wall.

[caption id="attachment_14725" align="alignnone" width="1200"] (left) Lightweight shuttering utilised in constrained working corridors and (right) brick cladding works to flood walls - Courtesy of Walters[/caption]

Design of the Tir-y-Dail flood bund was tailored to mitigate its impact on the open views from a nearby public right of way. In plan, a curved alignment was specified to conserve an irregular field pattern. In section, slopes were slackened and rounded at the top and bottom to create a naturalistic landform.

A fundamental element of the scheme design was determination of the alignment and form of the flood defences to minimise tree loss along the riparian corridor. An arboriculturist, who was integral to all major design decisions, was embedded within the design team. Every tree was considered individually so that as many trees as possible could be maintained. The resulting designs/design adaptations included micro-siting of flood walls and localised modifications to flood wall foundations in the vicinity of root protection zones.

Ammanford Flood Risk Management: Supply chain - key participants

• Designer & ECC project management: ARUP
• Design sub-consultant: FiskTek Consulting
• Main contractor: Walters UK Ltd
• Environmental consultants: Yellow Sub Geo
• Ecology: Sylvan Ecology
• Arboriculture: Mackley Davies Associates
• Coping stone design: Quad Consult
• Floodgates: MM Engineering
• FRC works: Trueform Civil Engineering Ltd
• Waterproofing: Tywi Damp Proofing Treatments
• Metalwork: Cambrian Foundry
• Reinforcement: Express Reinforcement
• Asbestos disposal: Gavin Griffiths
• Brick & stonework: JW Masonry
• Surfacing: Laymak Ltd
• Rail contractors: Protech Infrastructure
• Rock armour: Tarmac Ltd
• Landscapes: Afan Group

[caption id="attachment_14728" align="alignnone" width="1200"] Visualisation of Bonllwyn Green pocket park - Courtesy of Arup[/caption]

Nature and people thriving together

The project is in the heart of Ammanford, with interventions required in a combination of residential, commercial and public spaces. Multiple rounds of public engagement supported the scheme development, including:

1. Public drop in events with questionnaires on specific issues and feedback forms.
2. Online consultation during Coronavirus period, as well as letterbox information drops to ensure all parts of the community were engaged.
3. Regular newsletters, press releases, social media updates and a project website were provided throughout the design and construction phase of the scheme.
4. For public realm enhancements to Bonllwyn Green, visualisations and schematics were prepared to effectively communicate the scheme proposals and help local residents have their say on the design.

In line with NRWs strategic objectives, enhancements to public space and ecological habitats were fundamental to the scheme design, in addition to reducing flood risk. To provide biodiversity net benefit through increasing connectivity, diversity and ecosystem resilience of the high value riparian corridor, enhancements delivered as part of the scheme included:

1. Improved fish passage for multiple species through the existing weir on site.
2. Otter ledges beneath two existing road bridges.
3. Log piles for invertebrates, hedgehog nesting boxes and bat and bird boxes.
4. Where tree loss was unavoidable, they were replaced at a ratio of 3:1. Appropriate native species of local provenance and were designed to maintain and enhance habitat connectivity, while maintaining an appropriate amenity setting for the local community.

[caption id="attachment_14721" align="alignnone" width="1200"] Heol Hayden public realm improvements - Courtesy of Walters[/caption]

Improvements to public spaces in the vicinity of the flood defence works, but also within the wider community, were also delivered as part of the scheme as summarised below:

1. Where flood defences were constructed in public spaces and within the local college, the surrounding areas were enhanced with additional trees, swathes of bulb planting and new seating areas. Timber from trees felled as part of the scheme was reused to construct new benches.
2. At the Heol Hayden Flood Wall, the need for improved delineation of space between an existing riverside footpath and the adjacent gardens which fronted directly on to it was identified through community engagement and the Landscape and Visual Appraisal. A new hedgerow with small clusters of trees better defined the space and provided additional privacy to the local residents.
3. The existing Bonllwyn Green Park in the heart of the town was enhanced offering an amenity for local residents. The parks existing features were kept, but the space enhanced through avenue planting of specimen trees; drifts of bulb planting; interpretation boards; seating areas and informal play features formed from trees felled as part of the scheme.
4. As part of the scheme, NRW worked in partnership with Carmarthenshire County Council to deliver the nearby Llandybie Community Woodland Project, which comprised1.58ha of community broadleaved woodland with public access.

Protecting the environment

Environmental impacts were assessed through the Environmental Constraints and Opportunities Record (ECOR), with mitigation and enhancements integrated at an early stage into scheme development, heavily influencing the design process. Potential impacts upon trees, protected species, local community, archaeology, and local landscape, are some of the many that were assessed and mitigated.

The ECOR informed the Environmental Action Plan which formed part of the works information for the scheme and enabled key environmental mitigation measures to be communicated and tracked during the construction phase. The mitigation and protective measures implemented were comprehensive, covering the local community, protected species and environmental quality.

[caption id="attachment_14723" align="alignnone" width="1200"] Llandybie community woodland project - Courtesy of NRW[/caption]

Considerate construction

The works were spread across nine locations in Ammanford comprising residential, commercial and public areas. Each location presented unique challenges and constraints. To ensure that the project was executed safely and efficiently, significant effort was made to tailor methods of construction in each location.

The team’s focus on careful planning and attention to detail ultimately resulted in the successful completion of the project.

Early contractor investigation works captured potential clashes with existing utilities and structures. A collaborative working approach from all parties ensured potential issues were rectified before the main construction works commenced.

Working methodology, plant and equipment choices were tailored to the environment at each location to minimise disturbance. Due to the nature of restricted access in some locations, alternative construction methods were implemented. This included the use of a lightweight shuttering system that could be safely manually handled into position.

Collaborative working between the client, contractor and designers allowed early access for in-river works. Note only did this result in the fish passage works being completed with minimal impact on the ecosystem, but also reduced the overall construction programme by many months.

Summary

The Ammanford Flood Risk Management and Fish Passage Scheme is a testament to the importance of comprehensive planning, innovative design, and community involvement in flood risk management. By addressing the unique challenges of Ammanford, the scheme significantly reduces the flood risk for hundreds of properties and unlocks 20km of river to passage, as well as providing numerous other environmental enhancements. This case study highlights the critical role of such projects in building resilient communities and enhancing the environment.

The scheme was successfully tested whilst still under construction in September 2023.

[caption id="attachment_14726" align="alignnone" width="1200"] Flood event in September 2023 - Courtesy of Walters[/caption]

The Don Regulators comprise three strategically important regulator gates on the River Rother that are owned, operated and maintained by the Environment Agency (EA). The purpose of the regulators is to reduce the risk of flooding to settlements within the Don catchment, by attenuating flows in the River Rother. JBA-Bentley, in a joint venture relationship, are working on the replacement of the three regulators, each being at a different stage of design or construction. Figure 1 illustrates the locations of the three sites.

Canklow

The Canklow Regulator is the furthest downstream of the three Regulators, approximately 3.5km upstream from the confluence of the River Rother and the River Don at Rotherham. It is located approximately 50m downstream of the A630 Rotherway near junction 33 of the M1. The existing structure, consists of a single electrically powered vertical lifting sluice gate operated by an electric wire rope hoist, comprising drive motor gearbox assembly and cross shaft with integral rope drums and support bearings. To improve resilience and the operation and maintenance of the asset, JBA-Bentley were required to provide a new guillotine gate and associated MEICA and civils infrastructure. The Canklow Regulator is usually the first of the three gates to operate and is activated when trigger levels are reached elsewhere within the catchment. Operating the regulators on the Rother during a significant flood event allows the flood peak on the river to be delayed until the peak on the River Don has passed Rotherham and prevents flooding from the Don. Lowering the gate at Canklow can hold back over 1.5 Mm³ of flood water across seven washlands. [caption id="" align="alignnone" width="1200"] The existing Canklow regulator gate - Courtesy of JBA-Bentley[/caption] Canklow is currently in the detailed design phase of works. Alongside the gate design some additional civil engineering works are required. This includes some remedial works to the existing sheet piled channel walls including new anchors, a new crane pad to be utilised during construction and to facilitate future gate maintenance as well as the substructure for the new motor control centre (MCC). One of the key constraints of the site are high voltage overheads; therefore, the crane pad location and the siting of the EA’s compound with MCC had to carefully considered. To assist in undertaking the design and developing the business case for the project, 3D survey was undertaken to create a 3D Matterport tour of the site. It allows the user to quickly navigate around the site and view all the infrastructure remotely. This proved particularly useful during the pandemic when restrictions were in place and site visits were limited.

Woodhouse Mill

The Woodhouse Mill Regulator is located on the Rother between Canklow and Meadowgate, upstream of the B6200 Retford Road. It retains water using a single electrically powered vertical lifting gate. It is operated by an electric wire rope hoist, comprising a central drive motor and gearbox assembly and half shafts, with spur and pinion driven, counterweight assisted, rope drums and support bearings. The proposal at this site is to decommission the current regulator and replace this with a passive solution, such as a baffle or orifice plate which spans the width of the channel, to further utilise some of the upstream storage in the washlands. JBA-Bentley are currently undertaking modelling of options for Woodhouse Mill regulator to inform the next phase of the project and the viability of the proposed solution. [caption id="" align="alignnone" width="1200"] The existing Woodhouse Mill regulator gate - Courtesy of JBA-Bentley[/caption]

Meadowgate

The Meadowgate Regulator is located within the Rother Valley Country Park. The existing regulator consisted of a single fish belly bottom hinged tilting gate, operated by a single electrically powered hydraulic ram. As per Canklow, the new gate will be a top-down vertical control gate improving resilience of the structure; as per the model above. When the new gate is lowered, water levels in the Rother rise and begin to spill into the washlands that are located within the Rother Valley Country Park, which can retain approximately 1.5 Mm³.

Temporary set up

The civils elements of the scheme have included demolishing the existing buildings and installing new control building. To ensure the gate remained operational during this time a temporary control centre was established during the early part of the scheme; once the gate has been commissioned this will be removed from site.

River Don Regulators: Supply chain - key participants

• Project delivery: JBA-Bentley JV
  • JBA Consulting
  • JN Bentley
• Principal designer: Callsafe Services Ltd
• Gate design: Aquatic Control Engineering
• Electrical Installations: IMAC Ltd
• Building design: JP Chick & Partners
• Building design: Peter Wells Architects
• Supporting ECC Supervisor with new gate & associated equipment: KGAL Consulting Engineers Ltd
• Precast fish pass tiles: Craven Concrete Ltd
• In situ concrete works : Bells Construction Ltd
• Metalwork fabrications: Fabtek
• Green roof: ABG Geosynthetics Ltd

[caption id="" align="alignnone" width="1200"] Model of the new Meadowgate regulator - Courtesy of JBA-Bentley[/caption]

HAZOP

Working closely with the Environment Agency’s Operational and Maintenance teams the gate design and associated infrastructure has been developed to ensure it allows safe access and operation; a Hazard & Operational Study (HAZOP) was undertaken during the early design stages to ensure all requirements were included.

Full channel fish pass

By removing the bottom hinged tilting gate, there was an opportunity to improve fish passage on the River Rother. Currently, there is a stilling basin immediately downstream of the gate approximately 40m in length, 12m wide and up to 3m deep at the upstream end. Through design development a unique full channel fish pass has been developed which removes the stilling basin by infilling to grade out the bed levels before installing reinforced concrete fish tiles with upstands of varying heights.

Computational fluid dynamics (CFD) modelling of fish pass

Due to the engineered nature of the channel in this location and the operation of the gate, the fish pass is required to withstand high velocity flows up to 8m/s during extreme events. This proved challenging to the more traditional fish pass construction leading to a heavily engineered solution. [caption id="" align="alignnone" width="1200"] (left) Screenshot of fish pass model - Courtesy of JBA-Bentley, (top right) fish tile mould - Courtesy of Craven Concrete Ltd, and (bottom right) the Meadowgate site - Courtesy of JBA-Bentley[/caption] To refine the design and prove passability for a range of species, the proposed arrangement was modelled using CFD. This allowed the upstand heights to be adjusted to provide the optimum depth and velocities through the range of design flows.

Fish pass tiles

Once the height and arrangement of the upstands was agreed with the Environment Agency’s National Fish Pass Panel, the structural design of the fish pass tiles began. Each tile measures 2.5m x 2.0m and weigh 4 tonnes; therefore, the design not only had to withstand hydrodynamic forces but also the lifting requirements for construction. In total, 80 fish pass tiles will be manufactured and placed into the channel, within a dry working area. [caption id="" align="alignnone" width="1200"] Precast fish pass tiles - Courtesy of Craven Concrete Ltd[/caption]

Temporary works consideration

To allow the fish pass to be installed within a dry area of channel, the full river channel needs to be dewatered. This will be achieved by installing an upstream dam and fluming the flow through the working area. The decision to install precast units reduces the risks of in-channel pours and to reduce programme duration for the in-channel works.

Summary

The replacement Don Regulators will protect Rotherham and the surrounding area from flooding, improve resilience, and ensure the gates can be operated as required both now and as we see the effects of climate change.