Supply Chain - Reservoirs - Raw Water - Dam Maintenance & Repair

Northumbrian Water are constructing a new 43 ML service reservoir at Springwell Village near Washington, Tyne and Wear, along with associated pipework and connections to the Derwent and Carr Hill mains. The project will supply water to the village and support the wider Wearside and South Tyneside areas, improving water storage and benefitting around 50,000 properties in South Tyneside, and supporting resilience for a further 200,000 properties in South Tyneside and Wearside. Mott MacDonald Bentley was appointed as principal designer and contractor for the £26m project which started on site in June 2023 and is due for completion by March 2025. Project details

The twin cell reservoir is 100m by 75m x 9m tall with a storage capacity of 43 ML. The construction of the reservoir is semi-precast, utilising precast wall panels with an in situ concrete base, in situ infills between the walls, precast columns and roof planks and an in situ screed placed over the top of them. The site is comprised of a sloping grassland field with a fall of 1 in 10 from north to south.

[caption id="attachment_15453" align="alignnone" width="1200"] Springwell Service Reservoir 3D model - Courtesy of MMB[/caption] Planning & environment

The new service reservoir and pipework was subject to planning and full statutory Environmental Impact Assessment. Environmental assessment and planning submission was undertaken by others during the definition design stage. Environmental constraints and any required mitigation were passed to MMB for consenting and implementation. This included the discharge of numerous planning conditions from local Councils, applications for: protected species licences (including great crested newt); prior consent under Section 61 of the Control of Pollution Act, and Scheduled Monument Consent from Historic England for tunnelling work under the Bowes Railway.

Landscaping upon completion of works will implement a non-statutory biodiversity net gain on the reservoir site through planting other neutral grassland using a locally sourced seed mix on the reservoir banks and surrounding land, the reinstatement/gapping up and planting of new species rich hedgerow, and the planting of new native trees within the site. A retention basin will be constructed within the site boundary to manage runoff.

Design strategy

Once appointed, MMB began to consult with the pre-construction designers at WSP and the client to further develop and detail design the structure and pipe network. MMB worked with FLI Precast at an early stage of the development to engage with them to design a semi-precast solution for the main structure, utilising lessons learned, and knowledge gained by both organisations from constructing multiple service reservoirs in this manner.

[caption id="attachment_15452" align="alignnone" width="1200"] Springwell Reservoir base construction - Courtesy of MMB[/caption]

From the beginning of their appointment, MMB has engaged with all stakeholders to develop solutions through collaborative working and to ensure that water quality standards are met throughout the construction process. MMB has particularly engaged with WSP and NWG, who have had an on-site presence throughout the works and have been able to identify opportunities within the design for cost and carbon effective design amendments, with shorter approval windows due to the collaborative nature of the design.

One opportunity highlighted by the design team was the proposal to remove the reservoir underdrainage from the scope. By studying available guidance, in particular the UK Water Industry Research Limited, Treated Water Storage Assets: Good Practice for Operation and Management, Version 2 report.

Underdrainage is more prevalent on structures that are not considered monolithic. Due to the construction sequencing and the type of jointing techniques considered, with no movement joints along with the required fall and depth of the drainage system due to project constraints, reservoir fall direction, the lack of presence of groundwater and subsequent uplift concerns, the underdrainage system would not have been beneficial to the structure. This was presented at an early stage to the Qualified Civil Engineer appointed to oversee the structural design and was deemed surplus to requirement and omitted.

The use of BIM360 as a shared working environment allowed the client, their consultants, MMB and their subcontractors to collaboratively work on drawings, showing real time changes to the design in 3D and allowing quicker design discussions, with the aim to shorten the design program and keep the site program on target. This worked particularly well when MMB re-designed the valve house layout and sizing using virtual reality.

[caption id="attachment_15456" align="alignnone" width="1200"] Springwell valve house - Courtesy of MMB[/caption]

It was noted on receipt of the outline design that the valve house was not sufficiently designed for operation and maintenance of the assets housed within it. Therefore, MMB had to increase the size of the building itself, change pipework arrangements to ensure adequate access to assets. To make this process as efficient as possible the 3D model was used in a VR headset to aid meetings and acceptances with the client. This was better than using 2D drawings as the team could all see in real space what the proposals were, and pick up any issues to be addressed.

Due to constraints regarding planning consents, works on the project began in earnest in May 2023, whilst the detail design of the reservoir was still ongoing, leading to a truly ‘design and build’ approach on the scheme. This approach sharpened attention on the project’s critical path and detail design works proceeded with construction programme in mind.

Significant design works were conducted in relation to the geotechnical operation, which required approximately 55,000m3 of sandstone and superficial materials to be excavated. The excavated materials will be reprocessed at a local quarry to become the embankment backfills, reducing waste.

Geotechnical & earthworks

One of the initial challenges with the construction of the scheme was the variance in geotechnical properties of the founding material. Covering approximately 80% of the reservoir footprint is an area of sandstone, and the remaining 20% an area of superficial materials, comprising weathered sandstone and granular glacial till to the south side of the reservoir.

Due to this variance, there was a significant risk of differential settlement of the base, which was highlighted during the definition phase. Initial settlement calculations led to large differential settlements which would have created intolerable stresses within the structure, so a ground improvement solution was sought. The solution adopted was to use a large temporary berm of material, comprising of the excavated material for the reservoir, to surcharge the softer material to the south. This berm of material (measuring approximately 8m tall, generating greater bearing pressures than the final structure would when filled to maximum capacity) was constructed, along with monitoring points using settlement gauges within this material, to drive out the anticipated settlement.

[caption id="attachment_15459" align="alignnone" width="1200"] Springwell settlement bund - Courtesy of MMB[/caption]

The monitoring results were analysed showing that the settlement was achieved quickly, with a significant tail off from any additional settlement recorded. The trendline of the settlement indicated granular properties over more cohesive, which confirmed that rebound of the material was unlikely. The berm was removed and then an assessment of rebound of the material was made. Initial programmed time for this berm was significantly longer than eventually needed and due to the close monitoring of this settlement it resulted in some programmed time savings to the works.

Precast construction & value added

The use of the precast walls reduced the scale of wall reinforcement fixing reducing both a risk of the reinforcement cages moving during the temporary construction phases and the time duration of works at height for setting the reinforcement and pouring operations, making construction safer. The reduction in concrete pours reduced the number of deliveries to site and the number of concreting operatives at any one point.

Off-site production of the walls aided in finish control of the concrete and allowed for higher accuracy of the placement of pipe wall couplers and setting in submarine door frames with less likelihood of cracking around these openings.

Precast wall panels installation is quicker than traditional wall pour construction, which has aided the construction programme, however this has led to a significant number (231) of wall infill pours required. As these locations are a potential area for leakage, good curing and placing procedures were required. The infill pours are tall and thin (approximately 8.5m tall and nominally 500mm to 1m wide) and significant care was given for the placement using tremie pipes and rate of concrete pour, with considerable care on concrete delivery times taken. Bar spacing within the infill was designed with a view of construction procedures, ensuring adequate bar spacing for effective concrete compaction via vibratory pokers. [caption id="attachment_15455" align="alignnone" width="1200"] (left) Wall panels and (right) wall infills - Courtesy of MMB[/caption]

SMR Projects Ltd supplied a Xypex additive for the concrete mix, which reacts with water and forms crystalline structures closing small voids/cracks that may occur from various thermal effects. Controlled permeability formwork was used to densify the surface of the concrete by limiting evaporation of water from the concrete surface. This increases resistance to soft water attack which is prevalent in this area of England. An impermeable sheet was also used over the infill following striking of the formwork for the curing process to prevent the concrete from drying out too quickly, again to reduce early cracking of the concrete surface creating a higher quality product.

From an operational perspective, using submarine doors to access the reservoir cells through the valve house allowed MMB to design out a requirement for access directly through the reservoir roof, thus reducing the number of openings in the roof helping to prevent contamination ingress to the reservoir via additional roof hatches and reducing fall from height risks associated with such access.

A further saving was made during design by ensuring that the valve house was capable of housing the electrical and signal board components needed for monitoring of flows and operation of valves. This removed the need for an external electrical control building that was considered in the planning application.

Early planning & stakeholder engagement

The Springwell Project, located near a new housing estate close to the village, has led the team to focus on early engagement with local stakeholders to minimise disruption, including road closures, bus and footpath diversions, construction traffic, and the impact on local businesses, as well as noise and dust concerns. To ensure the community feels informed and involved, MMB have participated in meetings and workshops with residents to provide project updates, get local input and answer questions.

The approach to community engagement has also included regular drop-in sessions, quarterly information events, newsletters, and site tours to keep residents up to date throughout the project. These initiatives have helped build a strong, positive relationship with the community, reflecting MMB’s commitment to minimising disruption and improving the local environment.

There has been significant emphasis on supporting education, particularly in STEM. By partnering with Newcastle College, we’ve organised site visits and provided two students with work experience, while local primary schools have been engaged through activities such as site safety aiming to spark interest in engineering and construction. Beyond education, MMB have collaborated with local organisations and the project’s subcontractors have also contributed to the community by installing a reading area at Springwell Village Primary School, demonstrating how MMB have harnessed resources and partnerships to deliver meaningful improvements.

[caption id="attachment_15451" align="alignnone" width="1200"] Springwell Reservoir under construction - Courtesy of MMB[/caption] Springwell Reservoir: Supply chain - key participants Client: Northumbrian Water Main designer & contractor: MMB Client’s designer: WSP Earthworks: Thompsons of Prudhoe Excavations: Cheetham Hill Construction Ltd Soils material testing: Dunelm Geotechnical & Environment FRC: Cidon Group Precast design & install: FLI Precast Solutions Ltd Precast roof panels design & supply: Stresslite Precast Xypex concrete waterproofing chemical additives: SMR Projects Ltd (UK Xypex Distributor) Pipeline cleaning & dechlorination: Pipeline Testing Consultants Ltd Tunnelling works: Active Tunnelling Ltd Directional drilling: Ken Rodney Construction Underpressure connections: Power Technologies Services Ltd Connections to NWL network: EJ Peak Technology Pipe welding: Serimax Ltd Pipework & fittings: UTS Engineering Ltd Pipework & fittings: Electrosteel Castings (UK) Ltd Pipework & fittings: George Green (Keighley) Ltd Flow controls: AFFCO Flow Control (UK) Ltd Pipe brackets: Deritend Group Ltd Metalwork: JHT Fabrications Fencing: Middleton Forge Ltd Conclusions

The reservoir is part of a broader package of works to improve the resilience of Northumbrian Water’s clean water network and the reservoir aspect of the scheme is approximately 70% complete at the time of writing (August 2024).

[caption id="attachment_15454" align="alignnone" width="1200"] Springwell Reservoir columns and roof - Courtesy of MMB[/caption]

Through MMB’s commitment to protecting the environment and social engagement the project has limited disruption where possible and aimed to build a stronger connection with the local community.

Direct engagement with the client during the enabling works and beyond has expedited the programme of works, allowing longer lead items to be procured earlier and aid in achieving this challenging programme.

The use of a large berm of excavated material during the enabling works for the scheme has induced settlement within the ground prior to the structure installation, thus reducing stresses on the structure and allowing a more efficient design to be made.

Engaging with the precast concrete panel designer at early stages of the construction programme allowed construction efficiencies to be achieved with respect to structural arrangements and thickness, reducing embedded carbon for the structure. Through the team’s collective experiences of building modern service reservoirs, MMB have endeavoured to create a structure which should be less prone to leakage. Further works and testing are ongoing to realise this so far successful project.

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%.

West Hallington Reservoir is a Northumbrian Water Group (NWG) asset situated 1km to the northeast of the village of Colwell in the Tynedale district of Northumberland. The reservoir was constructed between 1884 and 1890 for Newcastle & Gateshead Water Company for the purpose of municipal water supply. The reservoir was built adjacent to the earlier East Hallington Reservoir and operates as a non-impounding structure. It has a maximum depth of 12.1m, a capacity of 3.3Mm3 and a surface area of 50 hectares at the Full Supply Level (FSL) of 155.1m AOD. Scope of works An inspection carried out under Section 10 of the Reservoirs Act 1975, identified that the reservoir had insufficient drawdown capability, and thus needed to be increased such that the reservoir level can be drawn down by 4.8m in 8 days. Mott MacDonald Bentley (MMB) developed a design solution which utilises two 700mm diameter siphons on the west embankment to operate at a combined flow rate of 3.2m3/s. The new siphon pipeline, alongside a temporary pump arrangement and the existing scour pipes, will meet the required drawdown capacity. The siphon flows will be conveyed across a stretch of adjacent field which NWG has purchased. The flows will then either percolate to ground or gravitate to the Coal Burn and eventually to the River North Tyne. Detailed design and engineering challenges Hydraulic analysis MMB has designed and constructed over 25 siphons for various water companies in the UK. The siphon arrangement was analysed using an in-house design spreadsheet, and demonstrated that the siphon would break when the water level dropped 3m below top water level which was deemed acceptable. The siphon location was moved to the west embankment due to concerns about stability and leakage through the south embankment (the original proposed location of the siphon) and because it was easier to access this area of the site. [caption id="" align="alignnone" width="1200"] Summary chart from the reservoir drawdown hydraulic analysis - Courtesy of MMB[/caption] Pipes and thrust restraint Hoops stresses within the DN700 mild steel siphon pipes were checked and a DWI Regulation 31 coating specified. The stainless steel pipe supports were analysed for bending and shear and concrete thrust blocks were designed for the system test pressures. The pipe wall thickness was reduced from 10mm to 8mm as an efficiency. The pipework arrangement drawing was uploaded into the federated model in BIM360 to check for clashes and when installed on site was found to fit perfectly. Ground investigation and geotechnical analysis Following the original construction of West Hallington Reservoir (between 1884 and 1890) the Resident Engineer Charles Henzell wrote a report to the Institution of Civil Engineers describing the works and detailing a number of challenges which included: Areas of soft book leaf marl overlaying harder boulder clays on the route of the embankment needed to be excavated. Embankments sinking or slipping and the toe of the slope reinforced with concrete walls, sheet piles and additional stepped embankments. Springs in the vicinity of the works. [caption id="" align="alignnone" width="1200"] (left) Cable percussive borehole rig on the crest of the embankment and (right) scaffold decked area required for rotary bore works on the slope - Courtesy of MMB[/caption] A desk study and intrusive in situ ground investigation (cable percussive and rotary bored boreholes, standard penetration tests (SPTs), piezo installation, lab testing) were carried out to classify and determine the thickness of the superficial and solid geology along with groundwater levels, identify any contaminated or aggressive ground and determine the engineering properties of the native soils. Slope stability and ground bearing pressure checks were carried out to ensure large plant would not compromise the stability of the embankments. The clay core and embankment material of the Pennine-type dam was found to be very silty and exhibit poor engineering qualities and so was disposed of, and a source of good quality puddle clay and Series 600-2C material was found locally at a brick factory in Birtley, Gateshead. This was installed in compliance with the test and inspection plan (T&IP) to a good standard. Thankfully the three springs in the west embankment mentioned in the Henzell report were not encountered. [caption id="" align="alignnone" width="1200"] Extreme weather conditions during the construction stages: (left) winter 2022 and (right) flooding from Storm Babet (October 2023) - Courtesy of MMB[/caption] Temporary works A borehole at the toe of the embankment on the dry side indicated the presence of water-bearing sands and gravels and so interlocking sheet piles were specified that punched through the sands and gravels into the underlying clays and so cut off groundwater flows into the excavation. A Portadam supplied and installed by OnSite Central was employed to provide a dry area to construct the intake bay. The Portadam was erected in the wet by operatives wearing buoyancy suits, and then pumped out and an excellent seal was achieved. The Portadam also allowed plant access around the pipework whilst the stone pitching was reinstated. Flood risk assessment A flood risk assessment (FRA) was carried out to support the planning permission and, as due diligence, to ensure MMB would not flood a property by accident. The assessment looked at the extent of flooding during operation of the siphon along with fluvial, surface water, groundwater and finally reservoir flood risk due to an uncontrolled release (which of course the siphon is designed to help prevent). The FRA concluded that:

The maximum modelled flood extent from a drawdown of Hallington Reservoir is in agreement with the flow path identified by the Environment Agency in the long-term flood risk map and no communities are shown to be at risk due to the proposed drawdown. As periodic testing of the reservoir drawdown will occur during dry periods, it is not considered that a drawdown of West Hallington Reservoir at this location would increase flood risk elsewhere.

Synchro 4D MMB chose SYNCHRO 4D from Bentley Systems during the latter stages of the design process. SYNCHRO 4D is software that enables MSP/P6Programme to be linked with a Revit model and to run a construction simulation. [caption id="" align="alignnone" width="1200"] A screen dump from the SYNCHRO 4D simulation - Courtesy of MMB[/caption] The simulation highlighted a number of pinch points when materials must be delivered or when water levels must be lowered or raised and helped drive efficiencies. SYNCHRO 4D was also useful for explaining the construction process to the client, to site operatives and to visitors. Solar power The mechanical equipment associated with the siphon is housed within a high security kiosk. Due to the remote site location, the cost of a permanent power supply for the kiosk building services was prohibitive. A solar powered solution has been installed utilising roof mounted PV cells, DC/AC converter and battery storage. This sustainable solution provides sufficient energy for lighting and heating within the kiosk with negligible running costs. West Hallington Reservoir: Supply chain - key participants Construction delivery partners: Mott MacDonald Bentley Feasibility study & ARPE: Stantec UK Ecological surveys: OS Ecology Ltd Temporary water barrier (Portadam): OnSite Central Ltd Large diameter pipes: Powerrun Pipe-Mech Ltd Electrical/instrumentation installation: Intelect (UK) Ltd Large diameter valves: AVK UK Ltd Large diameter valves: Glenfield Invicta Valve actuators: Varley Hydraulics Ltd Stonemasons: Stuart Marley Ltd Steelwork fabrication: AJ Engineering & Construction Services Ltd Steel fixing: D&N Reinforcements Ltd Kiosks: Technocover Ltd Bespoke steel access cover: EJ Solar panels: Green Leaf Engineering [caption id="attachment_9915" align="alignnone" width="1200"] Solar panel display - Courtesy of MMB[/caption] Construction: Problems, successes and lessons learned? Intake bay The intake bay was located along the Small Burn which originally ran across the middle of the reservoir and was diverted into the adjacent Coal Burn in the 1880s via a DN375 clay gravity drain. The ground around the intake bay was found to be poor and was excavated down to competent sub-grade and reinstated with 340 tonnes of gabion stone and 60 tonnes of Type 1 capping material. As this area is normally under a 5m depth of water it was inaccessible during the ground investigation stage. Pressure testing As the DN700 pipes are fabricated from coated mild steel, a Type 1 pressure test (water loss method) was required. All bolts had been tightened using Hytorc© equipment and the torque readings recorded in the T&IP, but achieving a pass proved difficult due to sunny weather. A temperature rise on the above -ground pipework led to a pressure loss of more than 0.2 bar in the water filled pipe voiding the test on a number of occasions. Eventually cloud cover enabled a valid test; one of the only times a site team has prayed for the sun to go in! [caption id="" align="alignnone" width="1200"] Siphon intake bay constructed within a Portadam barrier supplied and installed by OnSite Central - Courtesy of MMB[/caption] Stone pitching Reinstatement of the original stone pitching (to resist wave action) took seven weeks longer than planned; mainly down to availability of stone masons. Productivity increased as the works progressed and the three man gang went from replacing 3 to 5m2 per day. The stone pitching was reinstated to a good standard, but it was felt that the detailing around one of the pipe supports would be built differently next time. The 2,500m perimeter of the reservoir had been faced with stone pitching down to original ground level; installation of which in the 1880s without an excavator and selector grab must have been a job and a half! Ecology Prior to the construction works, NWG employed OS Ecology to undertake trapping for white clawed crayfish, a native species that is in decline due to the introduced North American signal crayfish. No white clawed crayfish were found within West Hallington Reservoir but three were trapped within the adjacent East Hallington Reservoir. A pied wagtail nest was spotted by eagle-eyed site workers next to the engine of an excavator. The excavator was withdrawn from service until the eggs had hatched and the fledglings left the nest. [caption id="" align="alignnone" width="1200"] Reinstatement of the stone pitching - Courtesy of MMB[/caption] Testing, commissioning & conclusions The siphon is designed to operate above a water level of 152.5m AOD and to self prime above a level of 154.1m AOD (TWL = 155.1m AOD) and so the All Reservoirs Panel Engineer (ARPE) requested that during the testing and commissioning stage, MMB should demonstrate the following: That the siphon can be primed between the water levels 152.5m AOD and 154.1m AOD. That the siphon can self-prime above a water level of 154.1m AOD. That the Full Supply Level of 155.1m AOD can be achieved. The water level rate was restricted to 300mm/week and any seepage through the clay core will be monitored in future via a flow measurement chamber. During operation of the siphon, receiving watercourses and culverts were monitored at a number of locations to ensure scour or carriageway flooding were recorded and the test halted if required. The works were completed within budget (final NEC3 contract value £3.3m, £60k gain pot) and on time. The original compliance date of 31 September 2023 was extended to incorporate obtaining the Section 166 agreement and a new date of 30 June 2024 was agreed with the ARPE. [caption id="" align="alignnone" width="1200"] DN700 siphon pipework during installation - Courtesy of MMB[/caption] Leaving a legacy Northumbrian Water is very keen on creating community projects and “leaving a legacy”. MMB widened the access from the main road and provided three passing places as the access route is used by both residents and members of the fishing club. Looking ahead Northumbrian Water are investigating another 10 reservoir sites that potentially require siphons, and so Mott MacDonald Bentley hope to use the good practice and experience learned at West Hallington Reservoir on other schemes in the future. A Northumbrian Water Engineering Briefing Note for siphon design and construction is also underway, which captures all the learning and good practise and will sit alongside the newly published CIRIA 813 Siphons in Dams: Design, Installation, Operation, Management and Testing.

King George V Reservoir (KGV) is an operational raw water storage reservoir located in the Lea Valley at Walthamstow and is a Site of Special Scientific Interest (SSSI) with restricted access. It is controlled by Thames Water’s Lea Valley/North London Abstraction Group based at Coppermills AWTW. Seepage through the clay core at the north-east of the reservoir was identified by a Willowstick survey in 2020. MWH Treatment was awarded a contract to address the seepage issue, with the works taking place 1.4km north of the main entrance from Lea Valley Road to the north of the tidal break on the east side of the reservoir. History of the site The reservoir was conceived as part of an overall plan for the Lea Valley and was laid before the Royal Commission of Water supply in 1893. At this time, the responsible authority was the East London Waterworks Company. Construction took place between 1908 and 1912, and the reservoir was opened by Their Majesties, King George V and Queen Mary in March 1913. At the official opening there was a ceremonial guard of honour review, and The King signed a Grant of Arms to the Metropolitan Water Board (owners and operators at the time). [caption id="" align="alignnone" width="1200"] Google Maps - Location of King George V Reservoir[/caption] Description of the current facility The reservoir was formed by the construction of a continuous embankment on the floodplain of the River Lea. An earth embankment divides the reservoir into two compartments that are connected by three large diameter culverts. The external grassed embankment supports a central puddle clay core. The reservoir has a maximum capacity of 12,492 ML at its top water level (TWL) of 20.18m AOD. It is normally operated on a ‘put and take’ basis which at typical through‐flows of 60 to 100 MLD gives a theoretical retention period between 100‐150 days. The crest level is 22.50m AOD with a length of approximately 7km. The reservoir is operated in conjunction with other local reservoirs to provide a continuous supply of raw water to Coppermills AWTW and Hornsey WTW. The embankment, wave wall and clay core were raised between 1938 and 1941 to restore the 1.5m design freeboard that had been lost due to gradual consolidation settlement. The reservoir was, however, kept 1.5m below original top water level, as a wartime precaution, until February 1945. At the higher TWL leakage occurred at several points, being attributed to dryness of the very high plasticity alluvial clays within the core; consequently, the current TWL was established. A 4.5m deep grout screen was installed along an 800m length of the southwestern embankment in 1983 in an attempt to address the leakage. Access to the reservoir is located at its southeast corner from the Lea Valley Road. This access is shared with a sailing club and activities club. Sheep graze the embankments as an environmentally sustainable way of managing grass growth. Scope of works The key scope of works for the project was the provision of a sheet-pile cut‐off wall. The wall was designed as 15m deep; based on the pile toe being driven into the London Clay a minimum of 0.5m, and 165m long; being installed between crest chainages 1,715m and 1,880m. A low vibration technique of pile driving was promoted to minimise potential for stability issues within the embankment during construction. The scope also included provision of access to the crest and provision of temporary access road, strengthening of existing site roads and protection to existing pipework and services. [caption id="" align="alignnone" width="1200"] Google Maps - location of site compound and works area[/caption] Technical description A survey was carried out late 2019 to early 2020 in the area around chainage 1,750m as this had been identified as an area of interest in a previous survey. Willowstick surveying is a non-intrusive geophysical method used to map and model the subsurface water flow paths. The procedure involves energising the study area with a signature electric current between paired electrodes. The surveying equipment consists of GPS system and an instrument known as a Willowstick which measures the magnetic field response of an electric current. As the electric current flows through the survey area the preferential flow paths are identified, mapped, and modelled. The information gathered can be used in making informed, guided, and cost-effective decisions concerning how to further, monitor, intercept and mitigate seepage. The main findings from the survey were: A weakness zone of preferential flow identified between chainage 1,723m and chainage 1875m. The flow path shows the characteristics of uniform sheet flow and did not indicate a concentrated leakage path, rather a 150m zone where water was passing evenly through the core. The preferential flow was identified at elevation 12m AOD through the foundation underneath the embankment. The preferential flow path strongly correlated to the location of the original stream channel. Its characteristics indicated that seepage was concentrating in flood plain sediments deposited by the original river channel meandering through and beneath the survey area before the dam was built. The survey report recommended remediation works in this area within a short timeframe to reduce any movement or damage to the reservoir embankment. King George V Reservoir: Supply chain - key participants Principal contractor/designer: MWH Treatment Designer: Atkins Temporary works design: Noble Works Ltd UXO survey: 6 Alpha Associates Ground investigation, soil testing: Structural Soils GPR surveys: Technics Group Environmental surveys: Middlemarch Environmental Willowstick survey: Willowstick UK Noise & vibration monitors: RSK Acoustics Silent piling contractor: Sheet Piling (UK) Ltd Piling verticality survey: CMCS Ltd Civil works, labour/plant: Derryard Construction limited Electrical (site setup, gate works): SRS Electrical Ground maintenance: UK Landscapes Scaffolding/access: Clocktower Scaffolding General plant hire: GAP Group Ltd Welfare supply: Wernick Group Supply of Trakway Matting: Sunbelt Rentals Ductile iron pipework: Electrosteel Castings (UK) Ltd Valves: Glenfield Invicta CCTV, timelapse cameras: Wireless CCTV Ltd [caption id="" align="alignnone" width="1200"] Finished temporary road before track matting - Courtesy of MWH Treatment[/caption] Toe road and crane pad Atkins were instructed to advise on embankment loading constraints, construction methodology and the sheet pile solution. They developed the initially suggested remedial works to achieve a detailed solution which was then agreed with Thames Water. The existing track at the toe of the embankment was the only access route to the works area. The track had a loading capacity of 12t and the crest road a 5t weight limit; both limiting machinery and plant selection. A previous project adopted an 8t excavator and 6t tracked dumpers and Atkins used this information within their assessment to complete a slope stability analysis of the embankment. This confirmed that the use of an 8t excavator was acceptable for the works on the crest road. An existing 30” pre-stressed concrete (PSC) pipe main under the road at the toe of the embankment further constrained the size of vehicles to be used during construction and required enabling works. A disused section of pipe was recovered and tested to confirm dimensions and the material characteristics to be used in the loading analysis. Atkins confirmed that the pipe had sufficient capacity to support a maximum axle load of 12t. A temporary road was installed to minimise rutting and dynamic loading on the existing track. Trackway matting was installed from the sailing club ramp to the crane pad to disperse vehicular loading and mitigate impact on the pipe and vehicle speed was restricted to 5mph for all vehicle movements. An emergency plan was put in place to ensure the Thames Water Operations team were made aware of any incident and to cover the eventuality of a pipe burst. The temporary road was designed by Noble Works Ltd and installed by Derryard Construction Ltd over a period of 8 weeks. Over 1km of track matting was hired from Sunbelt Rentals and installed over a period of 2 weeks. Along with the temporary road, a crane pad was installed to facilitate the 200t crane used to lift the piling equipment and sheet piles from the toe to the crest of the embankment. The crane pad was designed by Noble Works Ltd and constructed in MOT Type 1 granular fill by Derryard Construction Ltd. A section of the adjacent aquifer recharge pipeline was dismantled to facilitate construction of the crane pad which also included a concrete protection slab above existing underground HV and telecommunications cables. [caption id="" align="alignnone" width="1200"] Finished installation of the track matting - Courtesy of MWH Treatment[/caption] Piling decision Following previous successful remediation works carried out at Sir William Girling, Staines South, and Island Barn Reservoirs, MWH Treatment selected Giken silent piling. This is a vibration free press-in method that allowed the pile installation to be virtually silent. This technique enables the piling plant to ‘walk’ over the tops of the driven piles. The initial press-in to install the dummy piles was carried out utilising a reaction platform. The Giken rig then used the previously installed piles to generate reaction to enable on-going pile installation. The sheet piling works (undertaken by Sheet Piling (UK) Ltd) commenced at chainage 1,880m which is the junction of the perimeter and the reservoir dividing embankments. A wider toe area had been identified as a turning and laydown area for the storage of equipment. It was also used for the reaction platform for the installation of the dummy piles. The first 100 piles were fed to the rig by the crane at the base of the embankment. Once the radius of this crane was exceeded, the ‘non-staging’ method was adopted utilising a clamp crane and pile runner to move the piles along the crest and to feed the piles to the Giken rig. A 15m long PU32 section pile was adopted being dependant on the chosen method of installation, length of pile and expected ground conditions. The pile clutches were lined with sealant to ensure low permeability through the wall joints. Press-in driving of the piles with the Giken rig was assessed as feasible with hard/difficult driving being a low risk when reaching the London Clay layer. Subject to the approval of the reservoir Qualified Civil Engineer (QCE) water jetting was an option to achieve required toe penetration. [caption id="" align="alignnone" width="1200"] Reaction stand setup excavation - Courtesy of MWH Treatment[/caption] Crest road works The Giken piling method minimised the access width requirement to work on the crest, however some excavation was still required. Using an 8T excavator the crest road was excavated to expose the clay core to allow the alignment of the sheet pile wall with the centre of the core. The base width of the trench was specified at 2m to allow the Giken rig to operate. The top of the core was covered with plastic sheeting to prevent desiccation and protect the clay core from being damaged. Following the completion of the sheet piling works, the trench and crest was reinstated. Site installation of the piles Sheet Piling (UK) Ltd commenced work on site at the end of October 2022, with a planned 10-12 week site duration. The 200T attendant crane was stripped of all non-essential weight in order to maintain the 12T per axle load weight restriction. Once in position on the crane pad a smaller 45T service crane off loaded the initial crane ballast and commenced rigging; once part rigged the remaining ballast was self-rigged. The reaction stand was lifted into position by the 200T crane to the 6m x 6m hardstanding area finished 600mm below the top pile design level. The Giken rig was then positioned onto the reaction stand followed by counterweights. The first pile was then driven utilising the weight of the rig and counterweight as reaction. After the first pile were installed, it became a reaction pile for subsequent piles to be installed. When the rig fully sat on the dummy piles, the reaction stand and counterweights were removed. Once a pile was sufficiently driven and stable the rig releases its clamps from the reaction piles and travels forwards. This ‘self-moving’ system eliminates the need for support by a crane during the piling operation. Following installation of the first 100 piles a clamp crane was utilised as the piling works had exceeded the working radius of the 200T crane. After the clamp crane had travelled along the piles as far as possible inside the 200T crane working radius a track, formed of a temporary pile welded along the completed wall, allowed a handling train to deliver the piles to the clamp crane. Pile verticality was critical as the piles must not exit the puddle clay before penetrating the underlying London clay. During piling verticality was monitored via spirit level. Once completed Sheet Piling (UK) Ltd employed CMCS Ltd to undertake a verticality survey. This included the first five 5 piles and then 10% thereafter, resulting in a total of 32 tests. CMCS Ltd utilised the lancing tubes on the piles that were welded prior to installation. The data reported the tilt of the pile and to what extent, whether leaning towards or away from the reservoir. [caption id="" align="alignnone" width="1200"] Train moving a pile to the clamp crane - Courtesy of MWH Treatment[/caption] Success of piling solution After the first week of initial set up and pressing of the 3 dummy piles, Sheet Piling (UK) Ltd installed 275 piles over the next 6 weeks. The site team completed the work in advance of programme despite difficult weather conditions. The crest of the embankment was very exposed; strong cross winds resulted in several days when piling was prevented, but the team were able to boost productivity to recover lost time. The Sheet Piling (UK) Ltd team de-mobilised mid-December; this was during the weekend of a severe snowstorm with over 6 inches falling over the Sunday night. The MWH Treatment team and subcontractors mobilised additional resources to clear the site of snow, including the 1km toe road, enabling several lorries and the 200T crane to safely leave site. Site demobilisation took place earlier than planned realising 3-weeks’ efficiency on the track matting hire period. After the Christmas shut down, the trench on the crest road was reinstated and an embankment survey was carried out by Derryard Construction to confirm the final ground levels were within tolerance. A post-installation Willowstick survey was conducted which reported that the previous flow path had been cut-of. The sheet pile wall appears to be working as designed by cutting-off the preferential flow of electric current through and beneath the northern half of the sheet pile wall; interpreted as cutting off the seepage flow. The post completion survey confirmed that the remediation works had been successful. Biodiversity The King George V Reservoir is a major wintering ground for wildfowl and wetland birds, including nationally important species. A total of 85 wetland species have been recorded and the reservoir also forms a moult refuge for a large population of wildfowl during the late summer months. Over the course of the project environmental consultants Middlemarch visited site monthly to monitor bird activity and to ensure site works were not affecting wildlife. The site compound took up the space of the sailing club/water sports club car park so MWH Treatment constructed a temporary car park providing sailing club patrons with continued safe parking. On project completion it was agreed with Thames Water that the car park should become a permanent overflow car park. A wildflower meadow is being planted to compensate for the biodiversity loss of the car park. Conclusion MWH Treatment have delivered a very successful project overcoming access, logistic, weather and environmental challenges. The sheet piling cut-off wall achieved the desired project outcome: mitigation of water seepage. The project was delivered early with an excellent safety and environmental performance. Thames Water additionally gained an upgraded toe road and improved protection for critical services. The project also fostered and maintained excellent relationships with neighbours at the sailing and activity clubs.

Gouthwaite Dam impounds the River Nidd on its route from the head of Nidderdale, North Yorkshire, and is the last in a cascade of three reservoirs within the valley; those upstream being Angram and Scar House. The Category “A” Dam was constructed between 1893 and 1901. The project undertaken by Mott MacDonald Bentley (MMB) on behalf of Yorkshire Water was for the construction of a new reservoir drawdown siphon pipe with a hydraulic capacity of 13,500 l/s to allow the water level in Yorkshire Water’s second largest reservoir to be lowered by 5m in 5 days in the event of an emergency, following best practice and guidance from the appointed Qualified Civil Engineer (QCE). Background to the project

In 2017 (AMP 6), MMB was contracted to produce an acceptable design and construction of the downstream leg of the siphon. In 2021 (AMP 7), MMB resumed works, assessing opportunities in a period of detailed design, with construction of the upstream and crest elements of the siphon commencing in November 2022.

Design

The additional outflow capacity required to achieve a 5m drawdown after five days with a Q10 inflow was confirmed during the 2017 concept design phase to be achievable by installing a siphon. The hydraulic design of the siphon was acceptable to the QCE. The design of the siphon inlet was required to prevent the development of air-core vortices forming at the inlet end of the siphon at levels approaching 5m below Top Water Level (TWL). This was achieved by ensuring the inlet of the siphon was suitably submerged, with a 3m x 3m bar cage proposed, with a solid roof situated on suitable rock armour.

The design constraints within the reservoir related to the ground profile, which is normally below water. To map the profile, a bathymetric survey was undertaken by APEM Ltd to ascertain the surface profile, with a sub-bottom profiling survey (silt survey) also undertaken by APEM to understand the potential levels of soft, silty material on the reservoir bed.

[caption id="attachment_12577" align="alignnone" width="1200"] Pontoon excavator - Courtesy of Mott MacDonald Bentley[/caption] The design of the upstream leg of the siphon included a 70m length of 2000mm Ø flanged steel pipework leading from the inlet to the crest, with excavation of the reservoir bed to install at the designed depth necessary to attain the required submergence depth of the inlet end of the siphon. Excavated material could then be utilised as backfill and provide sufficient resistance to thrust forces about bends near the crest. Prior to passing through the crest, the siphon tapers from 2000mm Ø to 1600mm Ø.

The constraints through the crest of the dam were with respect to minimising the impact to the earth embankment dam and the overflow channel downstream of the gravity dam. The level of the siphon through the crest were dictated by the hydraulics of the siphon, with the horizontal alignment of the pipe such that it was sat on the concrete core, some of which required removal, of the earth embankment dam. The vacuum pump priming system, air release pipework and 1600mm Ø isolation valve was positioned within the crest, with the siphon proceeding downstream towards an actuated gate valve installed just prior to the discharge location into the spilling basin, with the route taking consideration into its proximity to, and level of, the existing right-hand overflow collection channel.

Enabling works

With the dam sited along the River Nidd, one of the major challenges of the scheme was the extent to which the reservoir could be drawn down, given the compensation flow requirements required to provide a constant feed to a downstream fish farm throughout the year. There is little to no control of inflow into the reservoir, with the compensation pipework being the only means of draining it down.

[caption id="attachment_12582" align="alignnone" width="1200"] Offloading pipes - Courtesy of Mott MacDonald Bentley[/caption]

Any drawdown required a variety of factors to be assessed; the catchment is very ‘flashy’ with levels directly associated to rainfall in the large catchment. Any drawdown needed to be low enough to provide protection to the crest works within the cofferdam and beach area for pipe construction, but not too low that pontoons may ground out. With works programmed over a 16-month construction period, designs had to consider periods of very dry and very wet weather, whereby a drop in reservoir levels would see the crane unable to service the pontoon, and an increase in them leading to the in-reservoir works area being flooded out. A 2.7m drawdown was calculated and agreed with the client and other stakeholders, prior to being implemented in March 2023.

The drawdown provides 1-in-5-year storm protection without the reservoir overflowing, whilst retaining 50% of the reservoir’s water stock to be used at compensation flows, which feed the downstream fish farm.

A temporary pumping arrangement was also installed, the primary use of which being to draw off clean water from the top of the reservoir to maintain compensation flows should siltation be an issue when installing the siphon within the reservoir basin. The pumps and bagging were situated on a purpose-build pontoon cited 200m behind the dam, with flows discharged into the existing spillway channel. The pumps could also be utilised to assist in the drawdown of the reservoir as required.

Given the water level fluctuation, a means of isolation from the reservoir to the works within the crest was required to facilitate construction and protect the embankment during site works. A cofferdam was designed to protect the crest from up to a 1-in-1000-year storm event, allowing the site team to carry out works even when the reservoir is at top water level. The design had to be flexible, given that the concrete base and chamber were to be installed within it, alongside the large 1600mm Ø pipework and gate valve.

[caption id="attachment_12578" align="alignnone" width="1200"] Cofferdam installation around the concrete core - Courtesy of Mott MacDonald Bentley[/caption] Working closely with Groundforce, MMB assessed different phases of the work and how the cofferdam could be optimised to suit construction requirements. The existing concrete core was utilised in the design to resist frame loadings throughout, alongside whilst the bottom frame was designed to be removed following the casting of the slab to facilitate further construction of the siphon.

Alongside frequent reviews with Groundforce, MMB undertook internal reviews for the cofferdam, highlighting possible failure mode, such as frame rotation, slope stability and the strength of the concrete core, implementing additional design checks and site to ensure a safe and effective design that could be constructed and maintained on site.

The project has aimed to minimise the impact of the enabling works by incorporating them into the permanent works where possible. It was proposed to install an area on the beach of the reservoir once it had drawn down to facilitate the forming of the upstream pipework and to position a crane, which could be utilised to lift pipes and equipment onto the pontoon. A ramp from the crest to the beach was installed using reclaimed stone from another scheme, which shall be retained for future maintenance purposes, reducing 1500T of muckaway from the site and reducing the number of vehicle movements.

The placement of a crane within the reservoir as opposed to on the crest reduced loadings on the embankment, as well as mitigating potential access issues to the remainder of the site. Embodied carbon within the scheme has also been reduced by utilising surplus stone from the site compound to construct the permanent vehicular access to the downstream valve, further reducing imported materials, alongside ensuring careful removal of wave wall masonry and stone pitchings, to ensure that they can be reinstated, maintaining the heritage of the structure.

[caption id="attachment_12574" align="alignnone" width="1200"] Installation of the large diameter siphon pipes - Courtesy of Mott MacDonald Bentley[/caption]

Surveys and previous workings within the reservoir had encountered peaty soils, which would require excavation prior to the installation of the upstream leg of the siphon. A silt mitigation plan was compiled by MMB with input from Yorkshire Water, the Environment Agency and Jifmar Group; the specialist marine subcontractor undertaking the in-reservoir works. Physical measures taken to contain silt included the installation dual silt curtains anchored to the bed of the reservoir, with an additional curtain installed around this.

Monitoring points installed in three locations by Meteor Communications Ltd provided real-time readings of pH, turbidity and dissolved oxygen, enabling out of hours remote monitoring/alarms and reducing the need for people and water interface. Text alerts were sent to the project team when readings exceeded those denoted in the silt mitigation plan, with MMB and Yorkshire Water operatives undertaking agreed actions and tests dependent upon the works being undertaken at the time (enabling works, low-risk and high-risk).

[caption id="attachment_12572" align="alignnone" width="1200"] Installation of the large diameter siphon pipes - Courtesy of Mott MacDonald Bentley[/caption] Gouthwaite Reservoir: Supply chain - key participants Design & build: Mott MacDonald Bentley Bathymetric survey & silt survey: APEM Ltd Silt monitoring: Meteor Communications Ltd Upstream pipework installation methodology: Jifmar Group Cofferdam: Groundforce Piling: Sheet Piling (UK) Ltd Siphon pipework: Powerrun Pipe-Mech Ltd Siphon pipework: Freeflow Pipesystems Divers/pipework installation: Jifmar Group Valves: AVK (Supplied by Glenfield Invicta) Actuators: AUMA Actuators Ltd Air release pipework: Dax Air Ltd Site electrics & power: Total Automation & Power Kiosk: Morgan Marine Construction

MMB mobilised to site in November 2022, with the crest and downstream siphon leg constructed by JN Bentley, and the in-reservoir works subcontracted to Jifmar Group. At this stage, collaborative planning sessions and Synchro software were utilised to enable teams to visualise the scale of the project and highlight potential programme clashes, driving efficiencies within the scheme from the offset.

[caption id="attachment_12571" align="alignnone" width="1200"] Screenshot of Synchro software showing workings around the crest - Courtesy of Mott MacDonald Bentley[/caption]

Having drawn down the reservoir, the silt curtains were installed, and the beach area utilised to accommodate the lifting pad for the crane and pipework storage. A pontoon was lifted in, with a 35T long-reach excavator positioned on it to commence excavations within the reservoir. As this was undertaken, the 2000mm Ø pipe assembly was joined together on the beach, complete with buoyancy aids to ensure the pipe would not sink should reservoir levels rise. Once both elements were completed, the pipe was floated out into position and floats cut to ease the pipework into the excavated trench.

Meanwhile, MMB commenced the construction of the downstream leg of the siphon, connecting onto the length previously installed in 2017 as well as installing the cofferdam. To facilitate the length of siphon through the crest, a section of the concrete core required removal. The use of wire cutting reduced HAV’s to zero and provided a perfect surface for reinstatement; with 82 tonnes of concrete removed.

Once the siphon pipework was installed within the aperture, the core was sealed by dowelling into the existing core and pouring a new fibre-reinforced concrete surround, saving time constructing reinforcement and providing carbon savings.

Hydrotite was applied around the pipe-concrete interface, with imported puddle clay compacted on top to Peak Flood Level.

[caption id="attachment_12576" align="alignnone" width="1200"] Concrete core: (top left) before and (bottom left) after wire cutting with 82t of concrete removed, and (right) Reinstated concrete core following installation of 1600mm pipe prior to connection with the upstream leg - Courtesy of Mott MacDonald Bentley[/caption] Project status

As of May 2024, the scheme had in excess of 300 ‘Perfect Days’, with zero service strikes or lost time incidents. Multiple work experience students have visited and spent time on site, to witness the scheme and sample construction work life. Significant elements including siphon pipework and priming apparatus have been installed, with final installations and site reinstatement ongoing prior to full commissioning taking place in June 2024. The Gouthwaite Reservoir Project has prioritised being sympathetic to the local environment, bringing a 19th century structure up to 21st century safety standards and ultimately providing safeguards to the downstream community.

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.

United Utilities and Managed Service Provider (MSP) Costain have delivered improvements to the existing spillway at Rivington Upper Impounding Reservoir in Lancashire. The team worked collaboratively, undertaking a complex construction programme in challenging conditions, while managing the operational needs of the reservoir site and popular recreational area. The main objective was to upgrade the existing spillway. This was driven by a requirement to meet regulatory reservoir protection enhancement requirements. The remedial works required comprised: Works to the spillway to allow the design flood Probable Maximum Flood (PMF) plus air bulking to be safely passed down the spillway and prevent establishment of vegetation Protection to the toe of the Yarrow embankment and provision of a reinforced concrete (RC) base to reduce the probability of it being damaged in a PMF. A compensation flow of 1.223Ml/d was to be maintained during the works and levels in feeding reservoirs were to be held down by 1.5m to enable the works to progress. A scour flow not exceeding 100M/ld was diverted around the working areas in the lower spillway. Early involvement in the design process A physical model was produced to illustrate the flow levels down and across the spillway, which determined the new wall heights and gradients down the spillway. Early engagement between MSP design and construction teams meant that options for construction methods and flow diversions were discussed and assessed for viability. The preferred solution was presented to the Independent Qualified Civil Engineer who was able to comment and challenge the solution. The nature of the scope and working environment offered significant challenges which through collaborative planning and design meetings, the team overcome to meet the regulatory date. A key priority on such a project is to ensure operational requirements are maintained appropriately throughout construction. Costain worked on various phasing plans to manage flows whilst achieving the regulatory output. The project team engaged regularly with United Utilities catchment controllers to manage the flows safely during construction, ensuring necessary temporary diversions were managed and in place. A flood contingency plan was developed and reviewed regularly to establish a triggered response to rising water levels and ensure the safety of all workers and minimise potential damage to the works. The job was completed successfully with an exemplary health and safety record with no reportable incidents. [caption id="" align="alignnone" width="1200"] The existing spillway prior to the work - Courtesy of Costain[/caption] Rivington Upper IR Improvements: Supply chain - key participants Client: United Utilities Designer: Costain RC works: Westshield Ltd Masonry wall construction: JVT Construction Handrailing & gates installation: Pure Utility Solutions Valves: Glenfield Invicta Fencing & landscaping: Lowther Forestry Group Ltd Construction methods Due to damage caused to the spillway from previous storm events, and the need to tie in with the existing spillway levels, much of the existing sandstone rock base needed to be broken out prior to construction of the new reinforced concrete spillway. To reduce loadings and vibration on the dam, the preferred solution was to use hydro-demolition to reduce the level safely without jeopardizing the embankment and other surrounding structures. This method allowed the team to prepare the spillway for the new construction safely and cause less disruption than traditional rock breaking methods. Another key temporary works solution deployed was to divert scour flows up to 100Ml/day around the working area. This required temporary dam structures and sequence planning to allow the project to construct the spillway walls and base on to land that was previously a riverbed. The lower spillway (approximately 90m in length) was split in half, on one side we created a ditch along with a line of 100 Visqueen wrapped sandbags in order to carry scour flows around the working area and out to river whilst the other side was excavated and the RC base constructed. Upon completion of the works on one side, the temporary dams were moved over to divert flows onto the newly constructed base. This then allowed the opposing side to be completed, working safely away from the diverted water whilst ensuring the embankment was protected. [caption id="" align="alignnone" width="1200"] Looking  down the new spillway - Courtesy of Costain[/caption] Sustainability Rivington Upper Reservoir was one of the first batch of projects to form part of Costain’s baseline initiative; part of the wider WiiSE Strategy launched in 2022 focussing on the company’s approach to climate change, Wellbeing and Inclusion. The initiative looked at materials used and operation consumables including the use of petrol diesel, with a view of achieving a reduction in carbon during the construction programme.  The project targeted a 10% reduction vs the baseline and with reductions in the thickness of slabs and walls and the introduction of HVO instead of diesel the project is predicted to achieve that reduction target. Collaboration Costain were granted early contractor involvement, enabling the team to work alongside United Utilities at the early stages. This allowed constructability issues to be highlighted and challenged early in the process. Alternate alignments to the outline design were suggested to reduce significant temporary works requirements, whilst saving time and cost in the construction period. This included reducing the spillway width by proposing double sided wall shutters in certain areas instead of the outline scope to use single sided shutters against the existing wall which was in poor condition and had restricted access. Another collaborative solution which involved both client and contractors was to provide a way of capturing and measuring flows that historically leaked through the rock formed spillway. The design team was able to design and construct a series of under slab drains capable of capturing water penetrating through the rock and providing an outfall where they can be safely measured and inspected to check for potential issues with the embankment. Costain and United Utilities worked collaboratively, from ECI works, to regular collaborative planning and progress meetings to ensure all information and suggestions were openly shared and discussed to ensure the best solution was put forward. Completion Effective temporary works and phasing were key to delivering the project on time, whilst working in challenging conditions and managing the operational needs of the reservoir site. The formal spillway base and walls in the lower spillway were completed in the winter months, dealing with scour flows successfully. Rivington is popular with walkers, cyclists and other users. The team were able to successfully manage the PRoW, providing a safe alternate diversion route for the public and ensuring the road was open and suitable each night, minimising disruption to the local community. A robust flood contingency plan was key to delivering the project successfully. From the outset a collaborative plan was formed and constantly reviewed to ensure its effectiveness. Extensive monitoring and measuring of surrounding reservoirs allowed the team to plan for flood events and ensure sufficient time was available to clear the works area in preparation of flood events.

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.

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.

Since the mid-1800s, Whittle Dene has served Newcastle-upon-Tyne and surrounding areas with potable water, and continues do so, albeit with more modern processes and technology. When constructed in 1992, the current water treatment works replaced slow sand filters with sedimentation tanks, rapid gravity filters (RGFs) and chemical dosing, alongside provision of further on-site single cell service reservoir storage to compliment the two pre-existing reservoirs associated with the original works. A scheme followed soon after in the mid to late 1990s to add additional treatment processes by way of granular activated carbon filtration, contact tanks, and abandonment of the two original service reservoirs (likely due to their construction being no longer acceptable for modern water treatment and storage standards), leaving the new reservoir as the sole storage facility on the site. Project drivers

The main objective of the project was to allow an existing asset, the remaining operational service reservoir, to be taken out of service and inspected, whilst maintaining the treatment works output to the network.

Backwashing of the rapid gravity filters, approximately every 48 hours, requires a significant volume and source of stored water, hence the use of the sole remaining on-site 17 ML in-service reservoir. However, a single source of stored water represented a operational and maintenance bottleneck for Northumbrian Water as no other structure on site was available to store treated water for supply or backwash during maintenance or repairs without disruption.

[caption id="attachment_23904" align="alignnone" width="1200"] 3D Revit model: (left) Looking west and (right) looking east - Courtesy of MMB[/caption]

The site faced the following issues:

Existing backwash pumps, associated drives and MCC were aged. The last remaining reservoir in service was the only significant body of stored potable water. Limited availability of spare land or unused structures that could be re-purposed on the site to construct new assets. Finite spare capacity for power available for new additional assets. Lack of spare compartments on existing MCCs from which to take a power supply and install new drives. Integration with an on-going, site-wide, fibre optic upgrade project undertaken by others. Project summary

Northumbrian Water awarded Mott MacDonald Bentley (MMB) a Concept & Define (C&D) contract to review potential solutions, followed by a Design & Construction (D&C) contract, latter valued at £3.1m. The scope of works included:

Installation of 1,050m3 workable volume of water storage by way of twin glass fused to steel tanks, 525m3 each. Installation of two new duty/assist 11 kW storage tank feed pumps each delivering 70 l/s. Installation of two new duty/standby 110 kW backwash pumps to deliver 600 l/s. Associated buried and above-ground pipework including provision of block and bleed valving for double isolation, flow monitoring and pressure regulation. Modification of the existing MCC to add additional tier for new drives. MCC, PLC and system integration into existing site-wide SCADA. [caption id="attachment_23903" align="alignnone" width="1200"] 3D Revit model: Section through tanks - Courtesy of MMB[/caption]

Throughout the project, Mott MacDonald Bentley worked with Northumbrian Water to find solutions that would benefit the project, realising CAPEX, OPEX and carbon savings by replacing the pre-existing backwash system which was fed from the existing reservoir.

Consideration for upgrading the existing fixed flow backwash to a temperature compensated flow rate (to ensure consistent levels of filter bed expansion) was given, however would have required upsizing the existing distribution header pipework. These works would have resulted in significant disruption to the treatment site and was unfortunately ruled out.

Seeking an alternative source of water was a key theme to the success of the project.

Whittle Dene WTW - Backwash System: Supply chain: key participants Client: Northumbrian Water Principal designer & contractor: Mott Macdonald Bentley Flow surveys: Hydraulic Analysis Ltd Topographical/GRP surveys: Castle Keep Surveys Ltd Contaminated land & asbestos survey: Lucion Environmental LV & ICA cabling: Intelect (UK) Ltd Backwash switchboard: Blackburn Starling Ltd Additional tier to main switchboard: Technical Control Systems Systems integration: March Engineering (formerly EJ Peak) Cleaning, swabbing & disinfection: Excel Pipelines Ltd Coring: NML (Diamond Drilling) Ltd Coating works: RAM Services Above-ground pipework: Fluid Sealing & Engineering (FSE) Under pressure tappings: UTS Engineering Ltd Feed pumps: Xylem Water Solutions Backwash pumps: SPP Pumps Ltd Permastore Trifusion tanks: Goodwin Tanks Limited Valves: AVK UK Ltd Valves: Affco Flow Control (UK) Ltd Pressure sustaining valve: Cla-Val UK Limited Pipework: Electrosteel Castings (UK) Ltd Pipework: North East Pipeline Solutions Ltd Pipework: Saint Gobain PAM UK Pipework: EGEPLAST (UK) Kiosks: NPS Engineering Group Instrumentation: SWAN Analytical UK Ltd Instrumentation: Siemens Instrumentation: Endress+Hauser Group Metalwork & covers: Steelway Fensecure Ltd Security covers: Technocover Ltd Security fencing: Middleton Forge Ltd Scaffolding: SWL Scaffolding Ltd [caption id="attachment_23911" align="alignnone" width="1200"] Above-ground backwash pumps, stainless steel pipework and valves - Courtesy of MMB[/caption] Alternative source of water

The original brief was to source potable water by alternative means. This led to a review of options for intercepting fully treated water before it left the site, or if available, from the local distribution network utilising storage capacity at an existing service reservoir approximately 1 mile away. The latter was soon ruled out due to cost and programme considering the length of new mains pipework required to be laid, in third party land.

Focusing on intercepting potable water from within the site, MMB worked collaboratively with our client to review locations to construct new storage and investigated how this could be filled and returned via new backwash pumps to the RGFs for backwashing.

Whilst tees and tappings to divert potable water from existing transfer pipelines within the site were achievable, the ability to create new storage proved more challenging.

The disused original two service reservoirs were early candidates in which to construct storage using modern materials to current potable water quality standards, but the structures lacked:

As-built records accurately representing their construction. Details of foundation to determine re-use to sustain a new loading regime. The structures were buried and risked accumulation of surface water and hence required additional pumping CAPEX and OPEX. Combined with demolition to remove the existing roofs, the access issues to construct and then maintain new plant represented significant cost.

An area of land immediately adjacent to the existing in-service reservoir and backwash system looked available upon initial walkover, but further investigation revealed extensive made ground with low bearing strength which contained a complex network of services.

The extent of disruptive ground modification necessary to avoid extensive surcharge loading on adjacent structures and service infrastructure rendered this location unfavourable.

[caption id="attachment_23907" align="alignnone" width="1200"] (left) Tank internal view on draw off pipe and (right) tank internal view on overflow pipe - Courtesy of MMB[/caption]

Due to the need to locate the storage vessels further from the planned source, Mott MacDonald Bentley challenged Northumbrian Water’s requirement that washwater from the rapid gravity filters needed to be potable, and whether partially treated water (pre-final chlorination) i.e. filtered water from the RGFs, could be sourced and re-circulated as RGF backwash water. Northumbrian Water confirmed that this would be an acceptable arrangement not affecting downstream processes.

Thus a solution based upon this philosophy was developed and proposed, requiring buried pipework of the order 100m to connect RGF outlet, new feed pumps, new storage tanks, new backwash pumps and back to the RGF backwash inlet manifold. The tanks and backwash pumps were designed at ground level resulting in low static head and hence low pumping OPEX.

Civils consideration

The material forming the stored water structure was greatly debated for suitability for purpose, ease of construction, programme duration, Regulation 31 compliance, and embedded carbon. Historically the material of choice would have been concrete, whether cast in situ or semi-precast, but other materials were considered, including HDPE, stainless steel, and glass fused to steel (GFS), along with exploration of opportunities for DfMA for off-site fabrication and assembly on site.

Ultimately, despite no precedence within Northumbrian Water’s portfolio of assets whereby GFS has been used to store potable or partially treated water, GFS nonetheless succeeded as the preferred solution, scoring highly in nearly every category considered.

Electrical consideration

Through careful and detailed planning, full works shutdowns were limited to one half day loss of production, to confirm the condition of the main works distribution switchboard busbars for continued use, but equally importantly, for checking the ability to connect the additional tier to supply power to support the new backwash pumps MCC.

[caption id="attachment_23905" align="alignnone" width="1200"] New backwash MCC in kiosk to power and control new plant and equipment - Courtesy of MMB[/caption]

A harmonic survey was also undertaken to provide a representative profile of the background harmonic distortion in order to demonstrate that the equipment incorporated within the new MCC would conform with the requirements of the Energy Networks Association document Engineering Recommendation G5/5.

Whilst a shutdown was granted to expediently check the main switchboard busbars, a full board condition survey would have required much longer to have informed an outline design and enquiry with suppliers for the detailed design, manufacture and installation subcontract package for the additional tier. To mitigate risk, MMB engaged with the original panel supplier for provision and installation of the new tier to benefit from their knowledge of the original panel design, which proved highly successful upon site acceptance testing and commissioning.

Mechanical consideration

To ensure no loss in RGF backwash performance, Hydraulic Analysis Ltd was engaged to survey the flow and pressure seen at each of the various filters. The pumps were then sized to be able to provide a constant flow of 600 l/s to each of the filters independently, regardless of which filter in the 130m long gallery was to be washed.

The varying discharge head requirements was compounded by sizing the pumps for maximum to minimum suction head variation of 8m in the storage tanks. This required the use of a pressure sustaining valve and variable speed pumps to provide the required flow and pressure irrespective of the discharge and suction conditions.

[caption id="attachment_23908" align="alignnone" width="1200"] Hydrostatic pressure transmitters - Courtesy of MMB[/caption] Digital delivery

Following site surveys and collation of information physically held on site and in digital format e.g. as-builts, a 3D Revit model was developed showing all known assets above and below ground including services. This was fully utilised throughout design development for siting the new structures avoiding construction over known assets, and for routing new pipelines and cables through the myriad of pre-existing services.

All stakeholders were invited to an access, lifting and maintenance (ALM) review whereby the model was viewed on screen, but also through a VR headset, where a walk-through simulation identified several ALM issues including access to valve handwheels to operate them, resulting in chains being provided, and hop-over stairs and platforms where above-ground pipework prevented access to all areas.

The model was linked to the construction programme through Syncro, creating a 4D planning and project management resource for all stakeholders to visually understand sequencing and interfaces throughout the construction phase.

Collaboration and proactive management

The project was a complex multidisciplinary replacement of assets requiring integration to a strategically important treatment works.

This required proactive management of work-faces to achieve its implementation combined with adaptable planning to meet changing constraints evolving through daily management of the treatment works. To achieve this, MMB employed regular collaborative planning sessions to look ahead and plan out forthcoming work activities and management of interfaces.

The collaborative planning sessions brought all parties together including client capital delivery team, treatment works operators, and key subcontractors.

The sessions aimed to highlight opportunities, identify blockers and agree a collaborative plan for not only upcoming tasks, but to map the route to ensure efficient and seamless delivery of the scheme to completion. Held every month, and coupled with buildability reviews on the practicality of the developing methodology, MMB enabled a clear plan to be developed which was bought into by all stakeholders.

[caption id="attachment_23909" align="alignnone" width="1200"] Installation of new tanks, backwash pumps, stainless steel pipework and valves - Courtesy of[/caption] Summary

Whittle Dene WTW’s status as a key asset within Northumbrian Water’s portfolio means that the community benefit it currently provides, in terms of safeguarding water supply for major parts of the north east, cannot be understated.

After over 100 years of service, the scheme implemented as a part of this project will mean that the treatment works can continue to operate for decades to come, with less demand on multiple electrical and mechanical upgrades of aging assets, and by having a more serviceable and maintainable reservoir.

Overall use of collaborative planning enabled the site team to ensure our client and their operations team were fully briefed of the forthcoming works and interfaces, and as a tool to engage with subcontractors and suppliers for sufficient notice of the optimal time to attend site to undertake their works. In total, the project had over 50 personnel on site and involved over 25 different sub-contractor organisations.

Located in the heart of the picturesque Scottish Borders, the towns of Galashiels and Selkirk are central to a significant upgrade of the region’s water infrastructure. Galashiels, situated approximately 38 miles south-east of Edinburgh, serves as a vital hub for the surrounding rural communities. In line with Scottish Water’s vision, the £30m Borders Central Strategy focuses on improving water quality and reliability in these communities by transforming Galashiels’ Manse Street Water Treatment Works into a Distribution Service Reservoir site only and enhancing Selkirk’s Howden WTW’s borehole supply and treatment output to serve over 20,000 houses and business premises. The construction includes a new 12 ML clear water tank, 7 miles of 400 mm HPPE main, and significant upgrades to Howden WTW; promising to support the region’s growth and resilience for years to come. The need for improvement

Manse Street WTW was built in 1964 and has a capacity of 5 ML/D; equivalent to two Olympic sized swimming pools. The process consisted of an inlet clarifier, four RGFs and associated backwash equipment (backwash pumps, blowers and dirty wash water system), and poly, caustic and sodium hypochlorite dosing. Since the 1960s there has been no significant capital maintenance and plant operation is reliant upon significant manual and knowledgeable operator input. Manse Street WTW has eight outputs with risk concerns of contaminant breakthrough due to process issues with the filter backwashing, flocculation formation, and clarification.

[caption id="attachment_14780" align="alignnone" width="1200"] Completed access road to new CWT - Courtesy of ESD[/caption]

Howden WTW was built in 2000 and in 2009 a membrane stage was added. The setup is comprised of four abstraction boreholes (duty/assist/assist/standby), a membrane package plant including six membranes, CIP cleaning plant and equipment, sodium hypochlorite and orthosphoric acid dosing, contact tank and four high lift pumps. Howden’s design capacity was 16 ML/D although it only ever managed a peak flow of 12.5 ML/D due to issues with raw water quality and limitations of membranes performance.

The average demand is 8-9 ML/D and the clear water tank (CWT) was undersized at only 5 ML and required significant repairs.

The Central Borders Strategy Project aims to address these issues by upgrading Howden WTW to meet a demand of 16.2 ML/D that will enable a main out of Manse Street WTW in addition to providing additional capacity in the Central Borders Supply Network for the 25-year growth design horizon that is anticipated.

Scottish Water’s Non-Infrastructure Alliance Partner, ESD (a joint venture between MWH Treatment, Binnies, and Galliford Try), started the design of the £30m project started in June 2018 and work commenced on site in February 2020.

Process selection

Studies to assess and identify potential solutions to tackle the issues and optimise delivery demand were undertaken, with each scenario also considering the optimum solution for neighbouring areas. The following were considered for Howden and Manse Street:

Maintain and enhance Manse Street and Howden. Main out Manse Street from upgraded Howden. Main out of all WTWs from new Peebles works. St Mary’s Loch. Main out Howden from new enhanced Manse Street.

Options 1 and 5 were deemed too costly and had significantly more risks for a successful outcome, while Option 3 was discounted due to negative impact on Edinburgh and Lothian yield. Option 4 had a lower yield of only 13 ML/d versus demand of 16 ML/d for Howden and Manse Street.

Option 2 was selected as it best fit the demands of Scottish Water’s SR15 framework drivers, being able to deliver the capacity needed with a lower anticipated cost.

[caption id="attachment_14779" align="alignnone" width="1200"] (left) Geographic extent overview and (right) Option 2: the chosen solution - Courtesy of Scottish Water[/caption] Central Borders Strategy: Supply chain - key participants Client: Scottish Water Principal designer & contractor: ESD Binnies Galliford Try MWH Treatment Pipeline & tunnel design: Caledonia Water Alliance MGroup AECOM Excavation works: RJT Excavations Construction: Careys Pipe construction: AG Wilson Borehole pump changeover & pipework: Dustacco Engineering Ltd Pipework: Alpha Plus Ltd Borehole pumps: Wilo Actuators: Rotork Controls Air compressors: Kerr Compressors Membranes: DuPont Water Solutions Membrane feed pumps: Grundfos Systems integrator: IDS Systems integrator: Lintott Control Systems Systems integrator: Saftronics Electrical: ICL Electrical: Main Electrical Ltd Pipework & metalwork (Howden CWT): AJ Engineering & Construction Services Ltd Cryptosporidium unit: Watercare Systems Ltd GRP kiosks: Kingsley Plastics MCC: Lintott Control Systems Ltd PE pipes: FPP Ltd Gate valves & pipe fittings: AVK UK Ltd Metalwork (Howden WTW): Archibald McAulay & Son Ltd Metalwork (Manse Street): GT Fabrications Metalwork (Manse Street): Homer Burgess Ltd Security & fire systems: Pointer Ltd Overview of process and scope

The existing treatment works at Manse Street, Galashiels is to be closed and decommissioned to create a Distribution Service Reservoir (DSR) site only. Meanwhile an increased borehole and treatment output being installed at Howden WTW, means larger sections of the customer network will be fed from the Howden WTW into a new clear water tank to replace the 5 ML Thornbush Quarry tank. This project will be supported by upgraded infrastructure, including new main laying from Howden-to-Howden new tank to Manse Street.

[caption id="attachment_14778" align="alignnone" width="1200"] New water tank at Howden WTW - Courtesy of ESD[/caption]

The Borders strategy was split into 4 key projects outlined below:

Galashiels WTW

At Galashiels, the existing works, including the raw water source, will be decommissioned and abandoned along with the nearby Thornbush CWT once the new upgrades at Howden and Manse Street are operational and fully integrated. The following tasks to accomplish this at Galashiels, undertaken by ESD, are outlined below:

New MCC and standby power generator to replace ageing asset. Construction of a new twin compartment 12 ML CWT nearby Galashiels and Howden replacing Thornbush CWT, known as the new Howden CWT. With the CWT compliant with all security requirements. Installation of associated valves and overflows, scour pipework, flow meters, and connections into three transfer/distribution mains. Installation of a new kiosk housing quality instruments, level instruments, flowmeter display, telemetry Install a new power supply to the instrument/telemetry kiosk. Decommission the existing Thornbush Quarry CWT, TWS power, instrumentation and existing security Conversion of Manse Street WTW to a DSR within the existing treatment site a new standby, decommissioning of the supply transformer, generator and telemetry.

Improving resilience of supplies

The pipework installation to tie all the works together and deliver the Central Borders Strategy was led by Caledonia Water Alliance; Scottish Water Alliance’s Partner. The works included:

Pipelaying 11.55km of 400mm HPPE main from Howden to Manse Street Clear Water Tank/Distribution Service Reservoir. Upsize 400m of the Central Borders Trunk Main from upstream of Melrose DSR to 350mm HPPE. Lay new gravity fed 400mm HPPE main to cater for a 5.56 ML/D (3.24 M/LD from current Manse Street site plus an additional 2.12 ML/D for the re-zoned). Abandon the existing 300mm supply pipe from Manse Street. [caption id="attachment_14783" align="alignnone" width="1200"] (left) Borehole set up and (right) CWT valve chamber - Courtesy of ESD[/caption]

Howden WTW upgrades

At Howden WTW, the following improvements to upgrade the existing WTW capacity from 12.5 ML to 16.2 ML were undertaken by ESD:

Replacing a pump in Borehole No. 2 with a 10 ML/D pump. Replacement of the existing scour and supply valves on each borehole wellhead and the installation of electronic actuators to enable automated scouring of the boreholes. Construction of new brick built walk-in enclosures over each wellhead to replace the existing chambers. Installation of a 300m3 raw water tank to buffer flows from the borehole pumps to the membrane plant. The borehole pumps are to operate on level control. Installation of three membrane feed pumps in a duty/assist/standby arrangement to feed the membrane plant from the raw water tank. Construction of a new membrane treatment building to house additional membrane units. Installation of three additional membrane units (3 x 48 (No.) L20N modules) to increase the current number of units to nine. Alterations to the membrane plants ancillary components to include increasing the capacity of the air system by installing an additional compressor, installing sodium hydroxide and sodium bisulphate for chemical neutralisation removal of the existing GAC neutralisation system, and increasing the heating capacity of the existing CIP tank. Installation of flow meters, relocation of the site boundary security fencing, alterations to power and control cabling PLC and SCADA modifications. [caption id="attachment_14781" align="alignnone" width="1200"] (left) Membrane feed pumps and (right) expansion to membrane plant - Courtesy of ESD[/caption]

Howden WTW chloramination

Another project to ensure quality at Howden WTW was installation of an ammonia sulphate dosing skid. This system will be housed in a walk-in kiosk. The chemical will be dosed downstream of the chlorine contact tank (CCT) and upstream of the high lift pumps. The overall components of the chloramination dosing plant include:

Storage tanks. Dosing pump skid. Fill point. Carrier water header tank. Carrier water pumps. Motor control center (MCC). Dual-contained dosing lines (duty/standby). Dosing static mixer. Sampling equipment. Chemical spill tank. Associated pipework and instrumentation. Project challenges

Howden’s construction phase began just before the onset of the COVID-19 pandemic, which brought unprecedented challenges.

With the implementation of social distancing measures, lockdowns, and restrictions on workforce movement, the project faced significant delays and disruptions. The team had to quickly adapt to new health and safety protocols to ensure the safety of all workers, which included staggered shifts, remote coordination, and strict sanitation practices.

Implementing the SCADA system posed substantial difficulties as the existing chip needed to be replaced due to incompatibility issues with the new installations due to age. Ensuring seamless communication between various components and achieving real-time data acquisition was challenging. The technical intricacies and need for precise calibration and testing led to additional time and resource investments. Following this, there was a 12-month delay for the Siemens’ chip needed for the new system, due to the Ukraine-Russia war. This delay significantly impacted the project’s timeline, necessitating adjustments in the scheduling and prioritisation of other project activities.

[caption id="attachment_14782" align="alignnone" width="1200"] New South Common CWT and valve chamber - Courtesy of ESD[/caption] Successes The new clear water tank stores water and monitors the quality before it goes into the supply. It also provides additional resilience to the network by providing additional storage to supply the area. The Howden Membrane upgrades increased membrane plant surface area allowing for increased treatment and production capacity from 12 ML/D to 16 ML/D. Leveraging off-site construction, when possible, allowed for precise, high-quality prefabrication of components in a controlled environment. This minimised on-site disruptions, reduced construction time and enhanced safety. Prefabricated elements were seamlessly integrated into the site, improving efficiency and sustainability by lowering emissions and waste. Weekly coordination meetings between Scottish Water (the client), ESD (for Galashiels and Howden WTWs), Caledonian Water Alliance (for infrastructure and pipe implementation) and other stakeholders were instrumental in ensuring the project’s smooth progression. These meetings facilitated effective communication, timely resolution of issues, and alignment of project goals, contributing to the overall success of the water infrastructure upgrade. The groundworks at the CWT site demonstrated exemplary resource utilisation. Materials excavated during construction were crushed, regraded into different sizes, and reused effectively. This not only reduced waste but also minimised the environmental impact and lowered material costs. At the Galashiels Manse Street site, the installation of a Hush Pod for the generator significantly reduced noise pollution. This implementation was particularly important for maintaining community relations and adhering to noise regulations. The Hush Pod ensured that the generator operated efficiently without disturbing the surrounding residential areas. Adherence to Scottish Water’s DOMS and TOMS Protocols, and working closely with Scottish Water’s Public Health Team ensured water quality compliance and avoidance of any issues while interfacing with a live water treatment works (Howden WTW) and distribution network (Howden CWT including inlet and outlet pipework). [caption id="attachment_14785" align="alignnone" width="1200"] The completed Howden CWT - Courtesy of ESD[/caption] Status

Howden’s chloramination project began in 2016 and was completed in 2019. The additional enhancements at Howden WTW, which commenced in 2020 alongside the Galashiels project, are both expected to be completed and operational by winter 2024.

The pipeline is currently undergoing the final stages of commissioning with the aim to commence the final flushing and sterilisation in November 2024 to allow the main out of manse street water treatment works.

Client: Scottish Water Project delivery (WTWs, CWTs, pumping stations etc): ESD Project delivery (pipelines and tie-ins): Caledonia Water Alliance Project cost: £30m Design start date: June 2018 Construction start: February 2020 Completion: October 2024

Usk is a large raised reservoir formed by an earth embankment dam, which completed constructed in 1955 with an approximate capacity of 12,268,000m3. The dam is 480m in length, with a maximum height of 31m, and supplies raw water to Bryngwyn Water Treatment Works and as well as providing compensation water to the River Usk, which is classified as a Special Area of Conservation (SAC) and a Site of Special Scientific Interest (SSSI). The project focuses on the replacement of the reservoir draw-off pipework within the dam’s outlet tunnel. Project drivers Welsh Water appointed Mott MacDonald Bentley (MMB), who form part of the Capital Delivery Alliance, to design and construct the replacement of both historic 18-inch scour mains in the tunnel to improve emergency drawdown capacity and to provide facility for enhanced releases to the River Usk. The works will improve drawdown capacity and as such form part of the works required to address a statutory measure in the interest of safety (MITIOS) under the ambit of the Reservoirs Act 1975. Works to provide ‘enhanced releases’ allow a range of discharges from the reservoir, with the aim of providing benefits to the River Usk and its habitats, which is in line with the European Habitats Directive. Furthermore, the works provide enhanced releases to contribute to the United Nation’s Sustainable Development Goal (SDG) 6. In particular, it contributes to achieve the target of 6.6: ‘By 2020, the aim is to protect and restore water-related ecosystems, including mountains, forests, wetlands, rivers, aquifers and lakes’. Optioneering The historic pipework and tunnel characteristics caused a variety of constraints on the new system that needed to be considered when finalising the desired pipework arrangement. The project aimed to safely maximise the potential drawdown capacity whilst working within these constraints, some of which are described below. The historic arrangement did not allow for easy inspection or maintenance. At the upstream end of the tunnel, the historic 18-inch pipes pass through a concrete plug, which cannot be replaced without a full drawdown of the reservoir. Emptying the reservoir was not feasible due to Usk Reservoir supplying large volumes of raw water for supply and compensation purposes. As a result, the historic 18-inch pipe section formed a constraint on the design and construction of the permanent works. [caption id="" align="alignnone" width="1200"] Pipework and valve arrangement within the valve chamber – Courtesy of MMB[/caption] The tunnel cross-section is a 2.4m diameter horseshoe shape. Access to the upstream end of the dry side of the tunnel is necessary to operate, inspect and maintain the new pipework and valve arrangement. The pipework arrangement and diameter needed to consider the ability to remove sections of pipework if required at a later date. A long list of options were developed, however the chosen solution was to replace the twin 18-inch pipework with a single larger diameter pipe, offset to one side of the tunnel. This option maximised the outflow from the reservoir and maximised space for access, inspection and maintenance. To merge the two 18-inch pipe sections from the tunnel plug, an asymmetric manifold was designed to combine the flow, with guard and duty valves upstream of this. Enhanced releases are provided by two flow control valves installed offline to the new drawdown pipework, that could be remotely operated using a telemetry system located in a new control kiosk. Other options considered during the optioneering included: Increase diameter of one 18-inch diameter pipe downstream of plug, with the other 18-inch diameter pipe to discharge into tunnel downstream of plug. Larger single pipe solutions, up to 1200mm diameter. Twin 600mm diameter pipes. Moving valve house upstream 50m to immediately upstream of a bend in the tunnel. Design considerations Another consideration in the pipework design, was to limit the flow velocity through the twin 18-inch sections of pipework and valves. If the system was operated for a prolonged duration with excessive velocities, there would be a risk of causing damage to the system through cavitation and excessive wear. [caption id="" align="alignnone" width="1200"] (left) Gate valve transportation and (right) bulkhead valves with concrete encapsulation - Courtesy of MMB[/caption] Following the feasibility stage, the pipework system was further optimised to improve hydraulic efficiency whilst maintaining velocities to a suitable level. The manifold was optimised to achieve balanced flows between both legs, to prevent significantly higher velocities within a single leg. The results of the optimisations enabled the diameter of the larger pipework to be reduced from the initial proposal at the feasibility stage, leading to a reduction in material costs and embodied carbon by approximate 10%. The diameter of the larger pipework was 800mm, and the flow rate measured in the commissioning phase was approximately 5m3/s. Concrete tunnel investigation As some cracking had been observed to the tunnel soffit, a condition assessment of the concrete tunnel was completed. Intrusive investigations were undertaken to obtain cores to determine the cause and depth of cracking, mechanical properties, and petrographical assessment of the concrete and the section thickness. Several crack monitoring locations were monitored over a period of 21 months to determine ongoing and potential future movement. The results suggested that limited future movement will take place. Some of the intrusive investigations also showed evidence of alkali silica reaction (ASR). Following a suite of expansion testing, the results showed that future effects of ASR would be limited and unlikely to cause significant deterioration. To help mitigate the risk of deterioration due to water ingress into the cracks, a proactive repair to the cracks was undertaken, and long-term monitoring of the cracks was established. Usk Reservoir: Supply chain - key participants Principal Designer: MMB Principal Contractor: MMB FRC works: Doohan Civil Engineering Electrical & control: Bridges Electrical Engineers Ltd Diving works: Edwards Diving Services Metalwork: D Hughes Welding & Fabrication HPPE siphon pipe: Aliaxis UK - GPS PE Pipesystems Pipework supplier: UTS Engineering Ltd Submersible pumps: Selwood Valves: Glenfield Invicta Flow controls: AFFCO Flow Control (UK) Ltd Flow meters: Siemens Concrete repairs: RAM Services Concrete cutting & drilling: Gnat UK Ltd Concrete supply: Hanson UK [caption id="" align="alignnone" width="1200"] Siphon and over pumping intakes - Courtesy of MMB[/caption] Key temporary works To maintain mandatory water compensation from the reservoir to the River Usk once the historic pipework removal had commenced, a twin siphon system was installed from the reservoir basin to the spillway’s stilling basin. Each siphon consisted of 280m long, 400mm OD HDPE welded pipework, with ductile iron fittings. The siphon was designed to operate down to -4m below top water level, which was typical for Usk Reservoir during the summer as the water level fluctuates. It used a pump to prime the siphon, and the pump could also be used in the event the siphon loses prime to discharge the minimum compensation requirement. The siphon was found to be reliable, and throughout the construction phase, saved approximately 260,000 litres of diesel, and 700 tonnes of CO2, when compared to over-pumping. Construction phase and commissioning Scope of work: Temporary siphon for compensation releases. De-commission of existing Portis supply main (including diving works). Demolition/re-construction of the valve house. Construction of new control kiosk and compound. Removal of three existing pipes within the tunnel, each at 180m in length. Installation of new electrically actuated guard and duty isolation valves, whilst working under single isolation. De-commissioning of the existing manual guard valves. Installation of the new 450mm>800mm pipework inclusive of manifold. Installation of valve house control valves/access metalwork. Security provisions. [caption id="" align="alignnone" width="1200"] Valve chamber and tailbay - Courtesy of MMB[/caption] The historic valve house was demolished and replaced with a new buried valve chamber, and the tailbay was relined with a new reinforced concrete structure. This was due to the visual condition of the historic structures, to create a larger area for the new valve and access arrangements, and to facilitate the installation of the new pipework and valves. The first task within the tunnel was to remove the three existing pipes. During the feasibility surveys it was found that the existing pipework and fittings contained asbestos, MMB commissioned a specialist asbestos contractor to remove the pipework adopting a pipe wrapping containment and use of a wire saw cutting technique. Once the pipework was removed all existing concrete pipe supports were also removed and the existing tunnel invert made good. Once all of the existing pipework had been removed up to the existing guard valves (at the tunnel bulk head), works to remove the existing duty valves and replace with new continued under client Gold Command control due to the single isolation nature of the works. The works then progressed onto installing the new twin 450mm to 800mm diameter carbon steel pipework in tandem with installing a new level concrete tunnel floor. Due to the limited space within the tunnel and lack of lifting facility MMB used a small pick and carry crane which was able to transport the new pipework into the tunnel for installation. The tunnel was 180m in length with two 45° bends, this presented some construction challenges due to the limited flexibility in the flanged system. The new valve house arrangement was then installed and incorporated the compensation, enhanced releases, and scour valves complete with extension spindle and pedestal arrangements for operation via electrical actuation on the new raised flooring arrangement. [caption id="" align="alignnone" width="1200"] (left) Scour pipework arrangement at the bulkhead and (right) scour pipework & thrust block - Courtesy of MMB[/caption] Programme The scheme was mobilised on site in July 2020 which presented additional unprecedented safety challenges due to the global COVID-19 pandemic. The site adopted stringent social distancing working on site and use of respiratory PPE when working in close proximity within the confines of the tunnel. The works were undertaken under the supervision of the QCE, and the pipework and valve system were successfully wet commissioned in April 2022.

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.