Supply Chain - River Work - River Restoration

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.