Supply Chain - Reservoirs & Storage - Spillways

Located near Maerdy in South Wales, Lluest Wen Reservoir is a Category A Large Raised Reservoir as defined by the Reservoirs Act 1975. The reservoir is impounded by a 200m long, 25m high earth embankment dam across the Rhondda Fach River and currently impounds 1.07Mm3 of water. Construction of the reservoir was originally completed in 1898. Lluest Wen Reservoir is owned and operated by Welsh Water and has historically been used with a smaller, downstream reservoir, Castell Nos for water supply. Investigation and scope Following an inspection of the reservoir in February 2018 under Section 10 (3) of the Reservoirs Act 1975, the Inspecting Engineer made a series of recommendations for measures to be taken in the interests of safety (MITIOS). These recommendations included that flows through the spillway chute be modelled under the 10,000 year and probable maximum flood (PMF) conditions, that the performance of the chute arrangement be evaluated in safely conveying the associated flows to aid scoping remediation works and to action any works as necessary to preserve dam safety. The improvement works were statutory and enforceable by the regulating authority Natural Resources Wales. Mott MacDonald Bentley (MMB) was commissioned to undertake the design and construction of the works under Welsh Water’s Capital Delivery Alliance framework. [caption id="" align="alignnone" width="1200"] (left) Base concrete pour and (right) the upper spillway - Courtesy of MMB[/caption] Lluest Wen Reservoir Spillway Remedial Works: Supply chain - key participants Principal designer & principal contractor: MMB Flow modelling: Mott MacDonald Ground investigation: Strata Geotechnics Geophysical surveys: Terradat FRC works: Concrete Structures & Floors Ltd Metalwork: Industrial Pipework Services Ltd Diving works: Edwards Diving Services Ltd Manholes: Marshalls CPM Drainage pipework: Polypipe Civils and Green Urbanisation Concrete cutting & drilling: Holemasters Concrete supply: Hanson UK Scaffolding: Mii Engineering Site buildings: Wernick Hire Flow modelling and spillway design Prior to the scheme commencing, the existing flood study was reviewed and updated to reflect current industry guidance. A computational fluid dynamic (CFD) model was developed to simulate conveyance of the PMF and 10,000 year conditions. The PMF scenario anticipated flows approaching 100m3/s, with peak velocities approaching 20m/s discharging through the spillway. Outputs from the model indicated that the existing spillway was unable to contain flows arising during each flood scenario therefore representing a risk to dam safety through erosion of the earth fill embankment. An Options Report was prepared to identify possible solutions to remedy the shortfall in spillway capacity. The report considered the following options for remediation: Construct a new spillway off-line along a straight alignment. Rebuilding the spillway along the existing alignment. [caption id="" align="alignnone" width="1200"] Armourstone and lower spillway - Courtesy of MMB[/caption] Preferred option The preferred option was to re-build the spillway along its existing alignment. This would involve re-lining the upper section of the spillway which had previously been re-built in concrete during the 1960s. Lining would take the form of a 500mm thick reinforced concrete base connected to the underlying structure via a system of dowels. A similar approach would be taken for the downstream section of the spillway following demolition and removal of the historic masonry structure. Through this section, the new spillway base was doweled to a layer of mass concrete provided for global stability. New spillway walls were constructed on the hydraulic left and right hand sides capable of containing the PMF flows having utilised the CFD model to rationalise the wall heights as far as practicable. Additionally, a central training wall was provided through upper and lower sections of the spillway to evenly distribute flows across the width of the spillway and reduce water superelevation at the two changes in direction. The CFD model was also used to extract hydrodynamic pressures which allowed optimisation of the reinforcement within the walls and bases. Global stability checks for bearing, sliding, overturning and seismic were also undertaken to verify the suitability of the design. The new spillway construction would be approximately 200m in length and 15m at its widest point. Additionally, new drainage networks would be provided to the back of walls to control groundwater levels and underneath the spillway base to protect the formation and prevent undermining of the spillway base. At the downstream end, armour stone was provided for scour protection. Temporary works A 90-tonne crawler crane was used to provide full coverage of the spillway footprint. In order to access the embankment side of the spillway, it was necessary to construct a number of culverts, embankment and roadway at the downstream end of the spillway to cross the receiving watercourse. The upstream face of the bridge was concreted to prevent erosion and migration of fill into the downstream watercourse. [caption id="" align="alignnone" width="1200"] Finished spillway - Courtesy of MMB[/caption] Design considerations The gradient at the lower section of spillway approaches a fall of 1 in 2 which presented significant challenges both in terms of pouring and finishing the concrete bases as well as the flow velocities generated within the spillway. Base pours lengths were reduced to 3m and the bays were installed in a hit and miss sequence through this section in order to prevent slumping of the concrete prior to curing. High flow velocities necessitated that the joints be protected with steel plates to prevent scouring of the joint sealant and significant hydrodynamic pressures developing on the underside of the downstream base. Particular care had to be taken to ensure the absence of abrupt deviations across the joints which could invoke cavitation and ultimately lead to concrete degradation. The water level in the reservoir is known for rising quickly during periods of extended or heavy rainfall due to its location. A temporary drawdown of 3m below top water level was initially agreed with Welsh Water and a robust management plan was implemented to prevent the spillway from operating during the construction phase. Completion The scheme was mobilised to site in October 2020 beginning with improvements to the access track to the reservoir. Concrete works were scheduled to commence in late March 2021. Following a dry summer, concrete works were completed by January 2022 with landscaping and other finishing works completed by June 2022 whereupon the Qualified Civil Engineer issued the 10(6) certificate ahead of the required October 2022 statutory deadline.

Lower Barden Reservoir is the lower of two reservoirs located near Bolton Abbey within the Yorkshire Dales National Park. The reservoir is an impounding reservoir with a crest length of approximately 640m and a maximum height of 28m with a capacity of 2.23 Mm3. The overflow is located on the right-hand side of the embankment and consists of a curved masonry weir, a tumble bay, then twin masonry culverts before discharging into a straight stepped spillway. The spillway chute consists of a series of curved steps with vertical upstands forming pools. Lower Barden Reservoir is owned and operated by Yorkshire Water Services (YWS). Investigation works Due to its capacity, the reservoir is subject to The Reservoirs Act 1975. Within Section 10 of the Act, safety inspections are required to be carried out by an Inspecting Engineer at least every 10 years. Under the most recent report, there were several matters in the interest of safety (MITIOS) including: Improve the drawdown capacity so that it may be lowered by 925mm per day. Install under-drainage to intercept leakage and carry it to a remote measuring point from the spillway culverts. Improve the hydraulic performance of the stepped chute to eliminate out of channel flow to a distance of 150m downstream. Modify the auxiliary spillway channel (above the culverts) to prevent floodwater escaping out through gaps in the crest access track. Following investigations into the drawdown by Stantec UK and the spillway condition and capacity (investigated by MMB in the previous AMP and continued through to definition by Stantec), the following recommendations were given: Increase the spillway wall height and remove the pooled element of the spillway structure to contain the probable maximum flood flows and reduce hydraulic jump issues due to the pooled nature. Replace the top layer of masonry in the tumble bay with concrete and introduce under-drainage. Install a new 800mm diameter siphon pipe to be able to lower the reservoir a minimum of 925mm per day down to 5.0m from top water level. [caption id="" align="alignnone" width="1200"] The original spillway channel - Courtesy of MMB[/caption] Planning application The scheme is situated within a national park and due to the size and scale of the modifications, it was subject to planning application. The development must be aligned with the National Planning Policy Framework (NPPF), including conserving and enhancing the historic and natural environment, achieving sustainable development and achieving sustainable development. To aid in ensuring that these items within the planning frameworks were addressed, specialist reports were prepared, utilising specialists within the Mott MacDonald group to develop them, including Heritage Statements to analyse the historic significance of the existing structure and the impact the new development may have, landscaping and visual assessments and photomontages to show visual impact of the new development from public view points and preliminary ecological appraisal (PEA) to assess the ecological impacts of the development. These reports along with a Planning, Design and Access statement and relevant design drawings were provided to Yorkshire Dales National Park and a constructive dialogue between the designer, the planning officer and Natural England for issues with respect to site access began, with planning permission received in April 2022. Lower Barden Reservoir Safety Improvement Works: Supply chain – key participants Client: Yorkshire Water Planning application: Mott MacDonald Outline design (siphon & spillway): Stantec UK Outline design (spillway) and detailed design & principal contractor: Mott MacDonald Bentley Concrete works: Offafix Formwork Ltd Masonry cladding: Colin Richards Various steelworks: George Green (Keighley) Ltd Pipework supplier: Powerrun Pipe-Mech Ltd Valve Supplier: Glenfield Invicta Hydraulic actuation: Varley Hydraulics Ltd [caption id="" align="alignnone" width="1200"] Completed masonry cladding on spillway, and (inset) wall raising showing new and existing masonry - Courtesy of MMB[/caption] Spillway The spillway channel was designed as a combination of wall raising and new reinforced concrete construction. Wall raising was used as much as practicable to utilise the existing structure. Some minor amendments were conducted in the detailed design stages for the spillway structure, such as maintaining the sloped nature of the walls to the top, which maintained the heritage of the structure, along with maintaining the curved nature of the steps within the spillway and cladding the internal faces with masonry. The masonry selected came from a nearby quarry so that the stone was in keeping to the surrounding area, however it was requested that the colour did not match in order to show the history of the development from a planning and heritage perspective. The original design included steel reinforced concrete throughout the construction. This was challenged and where applicable, for bases which were not carrying large moments and infill sections of clad walls, a change to fibre reinforced concrete was used. This mix was developed with aid from the fibre supplier and fibre designer to make sure that the fibre dosing could take the maximum loadings anticipated. Using fibre reinforced concrete instead of steel reinforced concrete led to a reduction in construction programme due to no steel fixing and lead to an approximate reduction in embedded carbon of 9000kgCO2. Siphon The initial design of the siphon was carried out by Stantec UK and passed across to MMB for detailed design. The siphon was designed to be self-priming at top water level, requiring manual priming at levels lower than its pipe crest level. The siphon is capable of drawing down the reservoir to approximately 5.0m from top water level, achieving the requirements stipulated in the S10 report. During the initial design stages, MMB aided in the development with Stantec UK due to the interaction with the spillway. A solution was reached where the new siphon would discharge through the floor of the newly developed spillway, along with some minor amendments to the hydraulic design of the siphon. A chamber was proposed to house the downstream valve adjacent to the stilling basin. [caption id="" align="alignnone" width="1200"] Upstream leg of siphon - Courtesy of MMB[/caption] Due to the levels of communication between Stantec and MMB, this additional chamber was designed out and the chamber was moved into the stilling basin, prior to it being backfilled to contain the spillway flows. This reduced additional excavation works and aided in the construction programme, promoting some carbon efficiencies. Good communication between both parties ensured that both projects were capable of being delivered within programme and reduced abortive works. Construction phase Enabling works began in May 2022. Due to the restricted access, crane pads could only be placed on one side of the spillway and the programme of works needed to be carefully created to ensure that the two schemes worked in tandem, without blocking off access to each other during the spillway and siphon construction. Offafix Formwork Ltd came on board to deliver the reinforced concrete structure and collaborated with the design team to ensure that the reinforced structure was buildable and if they had any comments to increase the effectiveness of the structure in terms of buildability and increase speed of construction this was welcomed and included in the design. [caption id="" align="alignnone" width="1200"] Fibre reinforced concrete tumble bay slab - Courtesy of MMB[/caption] The site team and stone masons chosen for the scheme comprised of the majority of those who had worked on the previously delivered Swinsty Spillway structure and had an in-depth working knowledge of constructing these structures. Their knowledge and experience greatly aided with the construction of the scheme and achieving the challenging programme. The siphon works required a significant draw down of the reservoir level to be able to work on the inlet structure safely. As this was quite a long duration for the works, it would impact the programme significantly and equally impact the spillway structure. It was decided to work from both ends of the pipe where practicable and to use a make-up piece at a bend on the downstream side of the embankment. This would aid in any minor tolerance issues in the pipework and ensure that the pipe closed correctly. To aid the construction of the pipework, the ICON system was used for setting out. This involved directly using the 3D modelling from the design of the pipe into a handheld tablet device and enabled accurate setting out, without the need to update drawings or continually check that the drawings were the most relevant. This also aided in delivering the as built positions of the pipework for the updates to the completion drawings. [caption id="" align="alignnone" width="1200"] 3D model showing the interaction between the spillway and siphon schemes - Courtesy of MMB[/caption] Conclusion At the time of writing (April 2023), the scheme is in the final stages of construction with short sections of coping stone installation remaining and commissioning of the siphon when water levels in the reservoir are high enough for this to take place. The scheme had a very challenging programme to reach and was able to beat the programme set at Clause 31. The main successes of the scheme were the reduction in embedded carbon using fibre reinforced concrete where applicable, the high level of communication across the scheme between third parties, planners, consultants, client, the design team and the site team and the quality of the finishes of the structure.

Clough Bottom Impounding Reservoir (IR) is an earth-fill embankment reservoir located on the eastern edge of the West Pennine Moors, around 4km north east of Rawtenstall, Lancashire. The reservoir is currently used for public water supply and compensation flows. The reservoir was formed by the construction of an earth embankment across the Whitewell Brook and Clough Bottom.  The dam is an earth embankment with puddle clay core approximately 23m high and 381m long. The embankment has a clay core shown to extend into the foundation over the central section of the dam through the original stream channel. Current facilities and reason for work The main objective of the work was to future-proof the existing dam structure and Victorian spillway, increasing resilience and helping to mitigate against the potential impacts of climate change, while meeting United Utilities’ requirements and regulatory commitments. The scope of work included: Construction of a new wave wall structure. Lining of the existing spillway including along with construction of a portal opening across the scour entrance to the spillway. Removal of the existing gauging weir and replacing with newly constructed flow monitoring. Redesign and rebuild and the lower access bridge to a more accessible location. There was an obligation for United Utilities to achieve the scope of improvements to meet regulatory commitments, while ensuring the integrity of the reservoir structure and maintaining supply and compensation flows to the water course throughout the works. The project set out to achieve the objectives and project requirements by applying a culture of efficiency and collaboration through collaborative design and construction. Collaborative design process Due to the complex nature of the work and site constraints, there was a clear need to innovate in both solution and design. From the outset, the team established a culture of collaboration, engaging at the earliest opportunity to identify constraints and opportunities. The initial design solution was undertaken by United Utilities in collaboration with the reservoir panel engineer. Upon instruction, Costain’s in-house engineering team developed the detailed designs working collaboratively with the United Utilities engineering team to ensure design efficiencies, constructability and timely construction. The team undertook extensive method and temporary works/enabling works planning to ensure the work could be undertaken safely, both in terms of conventional construction safety, and also considering dam safety constraints, through appropriate dialogue with the resident panel engineer. Clough Bottom Reservoir: Supply chain - key participants Client & outline designer: United Utilities Designer: Costain Wave wall design: GHD Civil sub-contractor: JVT Electrical connections: Water Engineering Services Ltd Bridge & electrical diversions: Pure Utilities [caption id="" align="alignnone" width="1200"] (left) The new wave wall upon completion, (top right) construction of the wavewall and, (bottom right) construction work on the spillway - Courtesy of Costain[/caption] Construction of the wave wall In terms of legacy, the most visual aspect of the project is the new wave wall, designed with a high-quality stone finish, in keeping with in keeping with the local context and area which is overlooked by a small residential community and used regularly for recreation purposes. Initial investigations were undertaken to explore the possibility of retaining the existing dry-stone wall while meeting safety requirements. However, as the original wall had little or no structural integrity this was not deemed possible. Different options were evaluated and assessed again the criteria set by the client. A concrete wall solution was agreed which would be built into the clay core and then upwards to create a new wave wall, with a stone facing tied into both sides and using the original 1895 topping stones to cap the wall. Dam safety was at the forefront of solution development, for example how could crest loading be limited during construction, and how could the length of time excavations being left open be minimised. The solution was to prefabricate the reinforcement cages in 5m sections, so that the removed area could be closed off quickly in the event of a storm in accordance with flood protection requirements. The new wave wall structure was built into the existing clay core through excavation, the prefabricated rebar was placed in the excavation, formwork installed and checked prior to pouring. The linear nature of the construction allowed learning and efficiencies to be implemented progressively. This approach reduced construction time along the 370m wave wall from an initial 7-day turnaround of each 5m section, to 4 days (-40% efficiency). Lean construction techniques such as these, where adjustments were made to the construction process in response to previous results, were testament to the success of the project as a whole. Considering the environment A project carbon baseline was established at an early stage which looked at the proposed materials and operational carbon. This formed a strategy to target and achieve reduction during the programme. The project targeted a 10% reduction vs the baseline. This included the use of use fibre concrete in the lower section reducing the amount of rebar required. The works were also connected to the mains power supply, which prevented the use of a diesel generator for the site welfare. Upon completion of the works, the haul road to the far side of the spillway was retained to give better permanent access to the client in the future. Another key priority was consideration of the environment used socially by walkers in such a scenic location. Investment in the design phase to consider visual impacts and legacy led to the design of a stone-clad wave wall utilising a combination of re-used and sourced stonework selected to be in keeping with the existing natural environment. The overall product was built to a high operational and aesthetic quality, providing a real tangible benefit for the surrounding community and those visiting the area for recreation. Completion Works were completed in a live reservoir environment, so it was important to ensure that operational access was maintained during the works. This required a collaborative approach to allow access for monitoring and regular statutory inspection of the dam. The works were delivered to a high standard of health and safety throughout, in accordance with the plan and ensuring regular communication with all stakeholders. An excellent Health & Safety record was maintained with no reported incidents throughout the project. The finished product provides not only the function but also in terms of the wave wall, a visually enhancing product in keeping with the environment. This asset was delivered with a real focus on efficiency through a ‘production thinking’ approach. From the outset, the team adopted collaborative behaviours during an ECI phase prior to the main D&B contract award, which enabled a ‘production thinking’ approach to maximise efficiency and dam safety considerations. Planning and production control techniques were developed throughout construction to ensure efficiency and enable the team to learn from one section to the next. Through this effort the team were able to reduce the construction duration by over 15%.

Harthill Reservoir near Sheffield, was built in 1796 to supply water to the Chesterfield Canal and is operated by the Canal & River Trust, the UK’s largest canal charity. It has a capacity of 295,045m3. The reservoir is a popular commercial fishery owned by the Canal & River Trust and the Rotherham Sailing Club is based at the reservoir, with a car park at the east edge of the dam. The Yorkshire Outdoor Swimmers Club has a licence for swimming, kayaking and use of a rescue boat in the reservoir.  In January 2020, in accordance with the Reservoirs Act 1975, a Section 10 inspection identified the need to upgrade the reservoir’s spillways to increase its resilience for extreme flood events. A new spillway was to be constructed at the western end of the reservoir dam, replacing the function of the previous spillways. Reservoir’s history

Harthill Reservoir was originally a cascade of three reservoirs constructed between 1771 and 1796 to provide water supply to the Chesterfield Canal. Two of the three reservoirs were later joined together to form Harthill Reservoir, fed by the smaller Harthill Old Reservoir and the upstream Pebley Reservoir.

Harthill Reservoir has a single earth-fill embankment dam at its northern end, 440m long and 9.2m high. The dam crest is 3m wide and carries a stone pathed public footpath. There is a curved concrete wave wall running along the upstream face of the embankment crest. The underlying geology is Middle and Lower Coal Measures, comprising sequences of coal, mudstone, seatearths and sandstones.

[caption id="attachment_14856" align="alignnone" width="1200"] (left) Original primary spillway prior to decommissioning. New heritage-approved brickwork will be used to match existing red and blue imperial brickwork of bridge abutments and central support pier and (right) the original downstream masonry apron of the primary spillway. The apron is being retained to allow for seepage through the new rear wall, embankment and garden drainage. Improved access for future maintenance will be added via a new galvanised steel footbridge and stairs to the rear of the apron - Courtesy of Kier[/caption] Existing spillways

The reservoir had two spillways; the original masonry spillway at the east mitre of the dam at a level of 101.38mAOD; and a larger 45m auxiliary concrete spillway in the middle of the reservoir dam at 102.13mAOD, built in 1961. It also had two draw-off pipes that went through the dam and into valve tunnels, which were used to control the feed to the Chesterfield Canal. The upper level outlet was near the eastern end of the dam with an invert level of approximately 99.34mAOD, and the lower level outlet was to the west of the auxiliary spillway, with an invert level of approximately 95.30mAOD.

Both spillways emptied into the Broadbridge Dyke Feeder channel that runs into a culvert under Woodall Lane, and on to the Chesterfield Canal, two miles away at Kiveton Park.

The 2020 S10 report determined the spillways did not meet modern reservoir standards, identifying the potential for hidden erosion of the auxiliary spillway, as well as a lack of capacity in the culvert and potential for deterioration of the masonry on the main spillway. The upstream face of the spillway had evidence of erosion.

Planning

Local people and stakeholders were consulted and plans were submitted to Rotherham Metropolitan Borough Council, including a construction environmental management plan and a construction traffic management plan.

[caption id="attachment_14855" align="alignnone" width="1200"] Archaeological investigation undertaken prior to start of main works. Full footprint of planned works plus 10m easement either side was investigated for historical materials - Courtesy of Kier[/caption] Construction constraints

A range of geotechnical investigations and topographic surveys were carried out, as well as heritage, archaeological and ecological assessments.

Harthill Reservoir is mainly surrounded by agricultural land and woodland, but the village of Harthill is close to the dam, so disturbance during the works needed to be minimised. There are public footpaths around the perimeter of the reservoir, through its car park and across the dam crest.

The reservoir is designated a non-statutory Country Wildlife Site. Arcadis carried out an ecological appraisal of the site, with surveys for protected and notable species, including bats, birds, reptiles, amphibians and badgers.

The project aims to deliver in excess of 10% Biodiversity Net Gain (BNG). To compensate for the removal of short stretches of hedgerow and a section of semi-natural woodland, new trees, hedgerow, broad-leaved woodland and grassland will be planted, and new bird and bat boxes and a hibernaculum will be installed.

Other measures to compensate for the disruption to wildlife included:

Pre-construction nesting bird checks. The removal of refugia under supervision by an Ecological Clerk of Works. Lighting design to minimise light spill. Providing ramps to enable trapped animals to escape from incomplete excavations.

There are no statutory heritage designations, but the Canal & River Trust considers all historic structures to have heritage value, in particular the draw-off tunnels and original spillway.

[caption id="attachment_14844" align="alignnone" width="1200"] Footbridge over newly built spillway and riprap erosion protection upstream of concrete apron and labyrinth weir - Courtesy of Kier[/caption] Harthill Reservoir: Supply chain - key participants CRT framework partner/principal contractor: Kier Group Designers: Arcadis Consulting (UK) Ltd Construction delivery partner: AE Yates Group Flood studies & modelling scenarios & ecological appraisals: Arcadis Consulting (UK) Ltd Fish rescue & welfare: MEM Fisheries Consultants & Services Ltd HDPE pipe: AE Yates Directional Drilling Sewer cleaning, CCTV & drainage: OnSite Central Ltd RC concrete works: FAB Formwork & Reinforcement Precast concrete: Moore Concrete Products Ltd Flow controls: Aquiko Penstocks: Aquatic Control Engineering Ltd Steelwork/new spillway footbridge: Ekspan Handrailing & access steps: PA Welding Ltd ArmorLoc blocks: RPC Contracts Ltd Drainage & manhole chambers: Burdens Steel reinforcement: Express Reinforcements Ltd Concrete: Right Mix Concrete Ltd Imported aggregates: Rotherham Sand & Gravels Imported aggregates: JA Jacksons Plant hire: Buckhurst Draw-downs

Ahead of the works, both Harthill and Pebley Reservoirs were drawn-down and Arcadis prepared a detailed flood study, modelling scenarios for water levels and rainfall events during the works.

Harthill Reservoir was completely drained ahead of construction works. A specialist contractor rescued the fish ahead of the full draw-down and they were re-homed elsewhere on the Canal & River Trust’s network. Pebley Reservoir continued to be held at 1.5m below the top water level to provide attenuation and some water supply to the canal. Harthill Old Reservoir was slightly lowered.

[caption id="attachment_14849" align="alignnone" width="1200"] The original upper draw-off inlet has been discontinued by grouting up the inlet and full pipe running through the embankment and (right) lower draw-off pipe and guard valve in the access tunnel within the reservoir embankment - Courtesy of Kier[/caption] New spillway design

Following a review of the potential options, the team concluded building a new spillway was the best way forward. The auxiliary spillway was located at the tallest part of the dam where failure could release the entire volume of the reservoir. The channel downstream of the original spillway was insufficient to convey ordinary floods and posed a risk to the houses close to the right-hand of the bank.

The new spillway has been built on the left-hand abutment of the dam where the embankment has limited height and where it could be built on natural ground, reducing the risk of failure due to erosion under the concrete slabs. It takes over the function of the two previous spillways and the design aims to avoid the dam overtopping, taking into account the potential effects of climate change.

The new spillway was constructed from reinforced concrete grade C32/40, with an exposure class of XD2 providing a design life of 100 years. All reinforcement is 60mm (including +10mm tolerance) and is high yield strength, fyk = 500N/mm2. The concrete has been designed in line with water retaining structures standards.

The design does not alter the flood risk downstream, keeping outflows in line with the previous configuration.

The new spillway has been designed to comprise a two stage labyrinth weir with lower slots, where the slots are equivalent to the original primary spillway, and the remaining weir length is equivalent to the auxiliary spillway.

A labyrinth weir with three sections was chosen in place of a straight-axis weir to minimise the width of the breach through the dam, reducing the amount of concrete and excavation required for construction. Minimising the width also limited the span of the new bridge crossing the new spillway and the channel width downstream of the weir.

The weir has three lower sections acting as a weir at 101.38 mAOD, two penstocks and two low flow pipes for canal supply. The top of the weir crest is at 102.13 mAOD.

The feeder flows are contained within two low-flow channels positioned along the sides of the spillway, so it can be inspected whilst the feeder flow is operating. They carry the low flows from the penstocks, providing improvements to emergency drawdown capacity.

[caption id="attachment_14852" align="alignnone" width="1200"] (top left) Underslab drainage with clean aggregate layer being installed prior to new spillway slab construction, (top right) : fixing of steel reinforcement within the spillway base beneath the position of the new labyrinth weir. Steelwork and concrete of this section of base slab is tied into a 1.4m deep concrete cut off/shear key tying into the bedrock below, (bottom left) breaching of the reservoir embankment commenced once downstream sections of the new spillway were in place and all necessary approvals were gained, and (bottom right) New spillway construction during embankment breach - Courtesy of Kier and Canal & River Trust[/caption] Movement and contraction joints

There are several movement and contraction joints across the spillway, with a central longitudinal joint. The joints were designed to meet new Environment Agency (2021) guidance for spillways, with drainage pipes beneath.

The majority of the movement and contraction joints were positioned to align with joints in the structure. But there is a single movement joint within a section of curved wall that does not align with a joint, allowing for the large section of curved wall. The contraction joints in the weir do not pass into the slab. This allowed the weir to be constructed in stages, so sections could be operational during any flood events experienced during construction.

Spillway freeboard

The wall heights of the spillway section were built to best suit the required freeboard. Downstream of the labyrinth weir, where the flow transforms from super-critical to sub-critical, the walls have their maximum elevation (the same as the dam crest) and therefore a very large freeboard (>2m). The USBR formula was used to assess a suitable freeboard. Calculations using the preliminary results from CFD analysis provided values between 600 and 700mm, which was easily accommodated in the design. This method double-counted waves and splash, but compensated for the lack of knowledge about spray, and provided additional protection against uncertainties (from modelling, hydrology, climate uncertainty).

The spillway approach walls extend out past the embankment. Reinforcement continues between the wingwalls and the base slab, so the approach walls act integrally with the other structural elements.

Spillway discharge channel

The spillway discharge channel conveys flows along the existing ground contours until the channel bed is higher than natural ground. It has a mild slope and is routed away from the embankment along the natural contours, to further improve the embankment resilience and reduce the amount of excavation required.

[caption id="attachment_14853" align="alignnone" width="1200"] Spillway discharge channel linking the new spillway to the existing culvert via a drop shaft - Courtesy of Canal & River Trust[/caption]

The natural hillside slopes down from left to right. At the end of the bend the channel shallows with decreasing embankment heights before passing down a shallow channel. This section of the spillway is away from the dam and has less effect on its safety, so the discharge channel was designed to have a softer, greener engineering approach than the spillway.

The discharge channel terminates at a new drop shaft positioned over the existing culvert. This connects the lower draw-off tunnel to the Chesterfield Canal feeder.

Drainage

A new drainage system of perforated pipes runs beneath the new spillway, to monitor any potential leakage through the construction joints and drain groundwater. They exit the spillway through a drainage junction with an inspection chamber and manhole. A land drain carries flows from the connection manhole to a new outfall on the southern side of the spillway channel.

A new perforated drainage pipe runs along the toe of the dam from the right-hand side of the new spillway to the right-hand side of the auxiliary spillway. The primary spillway channel drains the rest of the dam. A new channel was built to take water from the new spillway to the existing culvert under Woodall Lane.

The ditch running along the toe of the main embankment was formalised using a French drain to prevent water from ponding at the toe of the dam, protecting it from erosion.

A separate drainage pipe runs along the access track on the western and northern side of the spillway, collecting water from the field and road.

[caption id="attachment_14854" align="alignnone" width="1200"] Decommissioning the original primary spillway with double concrete walled structure with fascia to match original spillway - Courtesy of Kier[/caption] Decommissioning the old spillways

The 1960s concrete spillway in the middle of the dam was removed, and an earth-filled section of embankment was reinstated. The upstream face of the dam was covered in Armorloc erosion protection blocks. The section of wave wall removed was replaced with a new handrail.

The original masonry spillway was blocked off with a U-shaped reinforced concrete wall, clad with heritage-approved bricks matching those from the original heritage structure. The base slab includes a 700mm cut-off keying into the formation layer of the original spillway. New handrails and footpath surfacing will provide safe access across the new structure. The timber footbridge crossing the original spillway is being replaced with a new 1m wide galvanised steel footbridge, with gated steps down to the channel on each side for access/maintenance. The bridge is designed for a live load of 5kN/m2.

The original upper draw-off has been discontinued by infilling the pipework and inlet with non-shrink grout. The original lower draw-off has been investigated by remote CCTV equipment, cleaned using ultra high pressure jetting and relined with a new HDPE pipe. The existing downstream control valve has been reused and a new upstream control valve has been fitted at the inlet within the reservoir. This will have an actuator fitted to be controlled hydraulically from a GRP cabinet on the reservoir crest.

Access & water control

The footpath above the dam was resurfaced and a new 30.6m long and 3.5m wide steel footbridge was installed across the new spillway. It has a deck level of 103.9m AOD and 1.4m high handrails. The bridge deck has 12mm thick anti-slip panels and is designed to support 5kN/m2.

The two new drawdown penstocks are controlled from an enclosed lockable section on the new bridge. New access steps and footpaths will be created for reservoir maintenance work.

[caption id="attachment_14847" align="alignnone" width="1200"] (left) Access and water control for the reservoir team to operate the penstocks within the new labyrinth weir. The wheel is lockable to prevent unauthorised use and (right) One of two control penstocks located within the labyrinth weir, viewed from upstream. Control headstock located above with access from the spillway footbridge - Courtesy of Kier[/caption] Conclusion

Steps were taken to minimise the amount of excavation and concrete required for the construction, reducing disturbance to local residents and the landscape, while maximising flood resilience. Unfortunately, the winter of 2023/24 was the wettest on record and caused significant delays to the project.

At the time of writing (July 2024), the spillway is nearing completion and the reservoir will start to be re-filled in late August/early September. Harthill Reservoir is now better placed to withstand the weather events brought about by our changing climate, minimising the risk of local flooding while continuing to supply the Chesterfield Canal with water for navigation, and provide a valuable public amenity for local people and wildlife.

Birds Park Reservoir is no longer used for water supply and is enjoyed by the community for recreation. Constructed around 1850, the dam is an earth embankment, 45m long and 13.8m high, perched above a residential area near Kendal, Cumbria. There are no historical drawings, and limited site investigation data was available but it was assumed to have a central water retaining puddle clay core. The aim was to reduce the storage capacity to below 10,000m³, and reduce the risk of overtopping to a probability of less than 1 in 10,000, with no detriment to downstream flood risk. Located in a constrained, environmentally sensitive area meant that only limited initial ground investigation could be undertaken without major temporary works. Design of the new spillway and weir structure therefore had to use engineering judgement and an observational approach to validate assumptions. The project team worked collaboratively to modify and redesign the structure as more information became available. Delivery approach United Utilities procured detailed design and construction of the reinforced concrete intake and weir structure via Maintenance Service Provider, Costain. Eric Wright Civil Engineering was awarded a design and construct sub-contract, engaging GHD in February 2021 to undertake temporary and permanent works design for the 72m long, 4m deep channel and transition channel into existing spillway. The structure had to be constructed through a 170-year-old embankment using assumed ground parameters for design due to site access constraints. It was tied into the ground to prevent leakage in the permanent scenario, whilst maintaining a functioning reservoir. Covid-impacted delays to project start drove the main works into the winter period requiring substantial temporary works and increased costs to manage the health and safety challenges of rising water levels in this flashy catchment. The emergency flood plan required deployment of 6 high flow-rate emergency pumps and a temporary spillway to manage several storm events. [caption id="" align="alignnone" width="1200"] Site compound across undulating glacial topography - Courtesy of Eric Wright Civil Engineering[/caption] Construction methodology Site access posed significant challenges, requiring creation of a substantial haul road down the steep valley side, ramp cut though weathered sandstone, and widening of the embankment crest to facilitate access for construction plant. Slope stability and vibration assessments of the dam were completed with limited GI data due to access restrictions prior to the main works. A pragmatic approach was agreed involving reverse analysis of the embankment to determine geotechnical parameters assuming full utilisation. Slope stability analysis was undertaken using Rocscience Slide in accordance with EC7. A comprehensive site monitoring system was agreed using cost-effective scaffold tubes embedded 1m into the dam. These not only provided level monitoring but also acted as simple but effective inclinometers. The crest was reduced in height and widened from approximately 1.5m to 4.0m utilising structural fill on the upstream face. Track matting was also placed on the haul road to further reduce the loading enabling use of a 21T excavator, M36 concrete pump and 8m3 concrete wagon to construct the works. [caption id="" align="alignnone" width="1200"] Bulk bag bund to inlet structure and excavation of notch - Courtesy of Eric Wright Civil Engineering[/caption] During excavation for the spillway, rockhead was encountered at or above formation which meant that the planned sheet piled excavation through the crest was not feasible. After collaborative discussions to review risks to the embankment, an agreement was made to excavate a wide battered notch through the crest. The excavation was blinded with concrete and this innovative semi-permanent structure acted as a channel to manage flood risk prior to completion of the upstream and downstream works. The main overflow structure and weir, to be constructed within the reservoir basin, proved extremely problematic. The concept construction method for the structure was to build a granular stone working platform into the basin and install a cofferdam to reduce environmental impact. The presence of a 2m thick layer of silt necessitating a redesign. Following discussions with the Environment Agency, an aquatic survey was carried out and consent obtained to fully drawdown the reservoir. An impounding bund was formed using bulk bags which enabled safe isolation of the working area, allowing 200m3 of silt to be removed and replaced with dry lean compacted concrete to bring levels up to the structure formation to facilitate construction of the outlet structure. Birds Park Reservoir: Supply chain - key participants Client: United Utilities Principal designer & contractor: Costain Delivery contractor: Eric Wright Civil Engineering Ltd Designer: GHD Environmental advice: PBA Applied Ecology Masonry: Eden Stonework Fencing: Lowther Forestry Group Ltd Underwater surveys: Underwater Diving Services Landscaping: Ashlea Ltd Crane supply: Baldwins Crane Hire Ltd [caption id="" align="alignnone" width="1200"] Spillway construction in progress - Courtesy of Eric Wright Civil Engineering[/caption] Collaborative approaches to constructability GHD completed both temporary and permanent works design, working as part of the collaborative team effort needed to effectively investigate, verify the nature of the ground, and undertake redesign as required based on site conditions. This required trust amongst parties, a communication plan to ensure open dialogue and fast turnaround of approvals to manage change and reduce delay. Bespoke temporary works designs included: Demountable timber stank wall isolating the spillway. 3m high bulk bag temporary bund. Composite timber chute supported by spillway starter bars providing safe flood routing over Christmas. The new spillway had to be tied into the core with an in situ reinforced concrete key to prevent seepage and internal erosion, but due to shallow rock outcrops it was not clear if this could be achieved. Core depth was investigated using large diameter rebar attached to the arm of the excavator, negating the need for specialist equipment. This confirmed the presence of core around the spillway enabling construction to be completed.  Placement was undertaken in 100mm thick layers using trench-rammers and timber boards to stop punch through, whilst away from the structure a remote-controlled sheep-foot roller was adopted. Summary The works on this £2 million project commenced in June 2021 following a 9-month covid delay with the impoundment licence. The team worked collaboratively, adapting planned phasing to reduce impact on programme and cost, to complete works April 2022, 3 months ahead of programme. [caption id="" align="alignnone" width="1200"] Aerial view of completed spillway with basin refilled and footpath reopened - Courtesy of Eric Wright Civil Engineering[/caption] The project adopted a pragmatic approach to the work by client, contractor, designer, and the Reservoir Qualified Civil Engineer which, given the limited ground information, challenging timescales, and the need to adapt to ongoing investigation as works progressed, was vital to the success of the project. The highly professional team effort delivered a technically demanding project in exceptional circumstances. Efficient delivery of quality data using innovative and bespoke solutions meant that it was still possible to complete a major capital project whilst adapting to a changing environment. A high level of site support and a collaborative approach to problem solving, using pragmatism and engineering judgement, delivered simple, effective solutions to problems that might otherwise have been overcomplicated.

Tunstall Reservoir is located approximately 5km to the north of Wolsingham, in County Durham. It is an impounding reservoir retained by an earth filled embankment with a central clay core. The dam is approximately 25m high and 300m long. The reservoir was previously used to feed a water treatment works located at the toe of the dam; which is now decommissioned. The reservoir is now used for amenity purposes and to provide environmental compensation flows to the downstream valley. Following a Statutory Inspection under Section 10 of the Reservoirs Act, a number of measures in the interest of safety were recommended by the Inspecting Engineer in order to ensure the safety of the dam. Section 10 of the Reservoirs Act

Following a Statutory Inspection under Section 10 of the Reservoirs Act, a number of measures in the interest of safety were recommended by the Inspecting Engineer in order to ensure the safety of the dam. These recommendations involved the following:

Recommissioning the existing siphon that used to feed the works and was no longer in use had to be intercepted and provided with a new route in order to provide additional drawdown capacity to the reservoir. Repairs to the lower part of the spillway where the existing blocks were damaged. Filling an existing tunnel within the dam structure. [caption id="attachment_18531" align="alignnone" width="1200"] (left) Condition of the spillway blocks prior to the works and (right) new concrete slab with stainless steel protection to the joint - Courtesy of Esh-Stantec[/caption] Siphon re-commissioning

Various options were considered at the concept stage in order to restore the use of an existing siphon that was abandoned in the early 2000s when the treatment works were decommissioned. The preferred option involved the following:

A siphon discharge chamber to break the siphonic effect. This was decided given the uncertainty about the existing system and the significant difference in levels in the downstream leg. Thus, the water would flow by gravity between the chamber and the outfall. This discharge chamber also functions as a thrust block to restrain the thrust acting on the new gate valve that has been installed immediately upstream of the chamber. The use of the existing settlement tanks to retain the water and aid to the settlement of suspended solids. A penstock chamber has been provided between the tank and the new discharge point to retain the water for a day before it is discharged into Waskerley Beck once the majority of the sediment has settled. Routing the new pipework away from the areas of the embankment that present the most valuable Waxcap habitats. Waxcap mushrooms are rare as they are restricted to grassed areas that are not grazed, and are locally protected in Durham. [caption id="attachment_18533" align="alignnone" width="1200"] Tunstall Reservoir dam - Courtesy of Esh-Stantec[/caption] Spillway repairs

The original Victorian spillway structure is formed by sandstone blocks bedded in concrete. However, the Inspecting Engineer identified that the majority of the blocks in the lower part of the spillway presented signs of wear and tear and needed replacement.

The proposed solution consists of four new reinforced concrete slabs that have been placed over the existing spillway, except the most upstream slab where blocks had to be removed to tie-in with the existing structure and avoid a step. An expansion joint was necessary in the centre of the new slabs. The flexible sealant in this joint has been covered by a stainless steel plate that has been designed to protect the sealant from the predicted high velocities expected in the flows and reduce the maintenance requirements.

A number of groundwater relief drains have also been incorporated into the design in order to capture the emerging groundwater at the lower part of the spillway and both reduce the hydraulic load on the new slabs and improve the long-term reliability of both the new concrete and the remaining sandstone blocks. These discharge to a new headwall in the bank downstream of the end of the spillway.

[caption id="attachment_18535" align="alignnone" width="1200"] Tunstall Reservoir spillway - Courtesy of Esh-Stantec[/caption] Tunnel infill works

When the dam embankment at Tunstall was completed and the filling of the reservoir began, significant leakage was observed through the eastern abutment. A concrete cut-off wall extending into the hillside was constructed in 1879 which significantly reduced the leakage rate. Nevertheless, as some leakage continued, a tunnel and shaft were constructed downstream of the cut-off wall, and an 18-inch cast iron diameter pipe was installed inside the tunnel to capture the leakage and convey it to the treatment works.

Following the Section 10 inspection of the dam in February 2021, a number of Measures in the Interest of Safety (MITIOS) were raised recommending works to address the leakage and the stability of the tunnel. One of these IOS measures recommended to fill the tunnel whilst providing some drainage, as a result of the concerns about its structural integrity and the potential risk to the integrity of the dam embankment and the spillway in the event of a collapse of the tunnel.

The project team scoped and contracted a ground investigation (GI) in Autumn 2022 to investigate the leakage paths into these tunnels.

Based on the findings of the GI and the review of the reservoir levels, it was concluded that the cut off wall is in good condition and that the main flow path goes around the concrete cut off wall. There is also a strong correlation between reservoir levels and the recorded flows, which decrease significantly when the levels are 6 m below the top water level (TWL). This was observed again during the construction works, as the water level was kept 6 m below the TWL for health and safety reasons while there was work ongoing in the tunnel and the spillway.

[caption id="attachment_18544" align="alignnone" width="1200"] (left) View in the unlined section of the tunnel (notice the beams across the soffit) and (right) view of pipe inlet and partial headwall (notice standing water on the invert) - Courtesy of Esh-Stantec and CC Informatics[/caption] Various options were considered during the Concept stage. The option of not filling the tunnel was disregarded as it would not eliminate the potential risk to the integrity of the dam embankment and spillway channel. Filling the entire extent of the tunnel was considered, however it was not preferred due to the significant constraints associated with it. These constraints include the difficult access and requirement to work in a confined space, potential land ownership issues, the presence of a SSSI immediately to the east of the abutment, and the increasing depth to the tunnel as it continues beyond the spillway. For all of these reasons, it was agreed with that filling the extent of tunnel beneath the dam and spillway would be sufficient as that would eliminate the risk to these structures.

Three possible fill materials were considered:

Cement grout was discounted due to concerns of potential pollution as a low viscosity mix would be likely to mix with groundwater and filter through the rock, causing a pollution incident. Foamed concrete was discounted as a result of the potential constraints to place it at the furthest point inside the tunnel, and the remote location of the site and access within the site would further complicate the timely delivery of concrete. Expanding geopolymer foam was selected as the preferred material as it reduced the risks associated with grout and foamed concrete; and it had successfully been used by Esh-Stantec in the delivery of other projects.

The expanding geopolymer foam was deemed to be more cost-effective than the alternatives as it offered significant savings to the installation process, because:

It avoided the risk of pollution and therefore any mitigation works to stop grout leakage; Simple installation from a mobile compound, avoiding the need for complex plant and equipment; Geopolymer mix materials delivered to site in barrels, allowing for easy storage and transport within the site. Installation from the surface via boreholes every 10m along the line of the tunnel. Lower carbon footprint that cement based alternatives. [caption id="attachment_18545" align="alignnone" width="1200"] (left) View of the geopolymer in the tunnel, and (right) view of the pea gravel stop end - Courtesy of Esh-Stantec and CC Informatics[/caption]

The filling was undertaken via a number of injection holes that were drilled from the surface at 10 metre intervals, and expanding geopolymer was injected via these injection points. The progress on site was as follows:

A pre-drilling survey took place to confirm the condition of the pipe in the tunnel ahead of the works. Eleven injection holes were drilled along the dam embankment and across the spillway. Prior to the injection, after the drilling had been completed, high resolution cameras were used in each injection hole and a drone survey was carried out to confirm that there was no damage caused to the pipe and that the expanding geopolymer would not enter the pipe. Pea gravel was introduced through the final injection point in order to create a mound that would stop the advance of the geopolymer and prevent it from blocking the pipe. The use of the gravel mound avoided the need to carry out works inside the tunnel with the associated risks of working in the confined space. Once the project team was satisfied that the drilling had not cause any damage to the pipe, the injection began, advancing at 10 metres per day on average. [caption id="attachment_18534" align="alignnone" width="1200"] Overview of the siphon diversion route in preparation for the works - Courtesy of Esh-Stantec[/caption] Tunstall Reservoir MITIOS: Supply chain - key participants Principal designer & contractor: Esh-Stantec Esh Group Stantec UK Geopolymer injection subcontractor: Geobear Ltd High resolution cameras & drone survey: CC Informatics Confined space entry specialist: MRS Training & Rescue Ground investigation: Ian Farmer Associates Topographical survey & laser scan of tunnel: Academy Geomatics Ltd Pipework modifications: UTS Engineering Steel fabrications: JHT Fabrications Ltd Conclusion

The contract has been delivered by Esh-Stantec and the recommendations in the interest of safety made by the Inspecting Engineer have now been addressed. The siphon will provide additional drawdown capacity to the reservoir in the event of an emergency, the damaged part of the spillway has been repaired to a satisfactory standard, and the potential risk of collapse of the tunnel has been removed.