Supply Chain - Reservoirs & Storage - Products

Mosswood Water Treatment Works (WTW) is located approximately 14 miles south-west of Newcastle, near Derwent Reservoir. The works has a maximum deployable output of 152 Ml/d and is of strategic importance to Northumbrian Water (NWG), serving approximately 1 million customers across County Durham, South Tyneside, Washington, and parts of Sunderland and North Tyneside  The existing works consists of clarifiers, rapid gravity filters, UV disinfection and disinfection via chlorination, with two final water reservoirs on site. Following a Drinking Water Inspectorate (DWI) notice, NW required the installation of a new run to waste (RTW) system to discharge untreated water in the event of a process failure. This required an attenuation lagoon to be constructed to buffer RTW flows before they were discharged to a local burn, and Mott MacDonald Bentley (MMB) was commissioned under a Concept & Define (C&D) appointment followed by a Design & Construction (D&C) contract to design and implement this solution. Outline design

At the outset of the C&D appointment, an outline solution for the RTW system had been developed by another contractor, transferring non-compliant waste flows from the treatment works, through a new 1.1 km pipeline discharging into the River Derwent. However, the Environment Agency had raised concerns regarding this solution due to the potential impact on river water quality.

MMB held a design sprint with NWG to review the solution and identify alternative proposals. It was highlighted that by incorporating an attenuation lagoon, NWG could construct the works within their existing site boundary and discharge to a local burn within 50m of the lagoon.

Collaborating with the NWG operations team to better understand the requirements of the system resulted in the required volume being reduced by 50%, representing a significant early saving for the project. MMB has significant experience of designing lagoons for water companies across the UK as well as the Coal Authority. The design team sought advice from colleagues involved in these schemes and sourced standard details to reduce design time.

[caption id="attachment_13603" align="alignnone" width="1200"] Field where the lagoon will be sited - Courtesy of Mott MacDonald Bentley[/caption]

MMB changed the proposed RTW point within the treatment process from the clarifier overflow channel to the end of the rapid gravity filter (RGF) channel due to safety concerns over the constructability of the clarifier overflow option.

The outline RGF connection design required a section of the common channels to be isolated behind a temporary dam to enable installation of two 1.2m wide penstocks, which would need two RGFs out of service for weeks, reducing the site deployable output. MMB challenged the design and specified hot tap connections with gate valves located on the external wall, cutting the reduced output period from weeks to a matter of hours. The change kept the valves in the dry, improving maintenance access.

Ground conditions

A geotechnical desk study report was produced by MMB during the optioneering phase of the project. The desk study reviewed two sites and influenced the chosen location of the lagoon. Subsequently a project-specific ground investigation was designed to understand the possible geotechnical and geoenvironmental constraints.

The ground investigation was undertaken in March 2022 by Dunelm Geotechnical & Environmental Ltd and comprised four dynamic sample boreholes with rotary follow-on and 12 machine excavated trial pits. The information collected was summarised in a ground investigation report which identified made ground, variable glacial deposits and sandstone bedrock below the site.

During design, an earthworks specification was produced which allowed for reuse of site-won materials in the lagoon embankment, creating cost and carbon savings. The embankment compaction requirements were also defined in the specification, including in-situ validation testing. Slope stability assessments undertaken for the lagoon earthworks proved that the design of the slopes would be stable during construction and in the long-term scenario.

Discovery of an uncharted deep land drain adjacent to the perimeter of the lagoon within the initial excavation required unplanned diversions and extension of the land drains to mitigate the risk of the land drain carrying water underneath the lagoon liner. MMB installed a french drain system around lagoon to ensure any water would be drained away.

[caption id="attachment_13602" align="alignnone" width="1200"] Early construction of the lagoon - Courtesy of Mott MacDonald Bentley[/caption] Digital delivery

Modelling software enabled the lagoon to be shaped organically to fit the field, increasing the plan area over concept designs which reduced the overall depth of the lagoon whilst still achieving the specified volume. This enabled the lagoon formation level to be raised above the underlaying water table, removing the requirement for lagoon under-drainage but critically also removing the requirement for a discharge pumping station, recognising a significant TOTEX saving and future maintenance. The system was hydraulically modelled using InfoWorks ICM, enabling pipes to be sized efficiently and minimising the number of required manholes.

The lagoon was modelled using Autodesk Civils 3D, which highlighted that the initial cut and fill volumes were imbalanced. The team identified opportunities to minimise over-dig and re-use excavated spoil across the site dynamically to reduce temporary stockpiling. A digital terrain model allowed use of excavators and dozers with in-built GPS and intelligent machine control. The model was combined with Synchro 4D Planning software to create a virtual simulation of the construction works, which, in combination with collaborative planning sessions, meant that MMB was able to identify and realise opportunities within the construction programme.

Mosswood WTW RTW: Supply chain - key participants Client: Northumbrian Water Design & construct: Mott MacDonald Bentley Ground investigation: Dunelm Geotechnical & Environmental Ltd Systems integration: EJ Peak Technology Solutions Ltd Electrical: Armah Switchgear Ltd Electrical installation: Intelect (UK) Ltd Underpressure wall connection: UTS Engineering Ltd Metalwork & pipework: Fluid Sealing & Environmental Ltd Diamond drilling: NML (Diamond Drilling) Ltd Lagoon liner: Celtic Lining Ltd Valves: AFFCO Flow Control UK Ltd Scaffolding: SWL Scaffolding Ltd Landscaping: Stuart Marley Ltd Security systems: Aceda Ltd Security fencing/gates: Middleton Forge Ltd [caption id="attachment_13606" align="alignnone" width="1200"] Lagoon liner installation - Courtesy of Mott MacDonald Bentley[/caption] Sustainability, climate change impact and carbon footprint

NWG own several fields around the WTW which have historically been used for sludge spreading. Some of the fields are now being utilised by the NWG conservation team to improve biodiversity and the old sludge lagoons are now home to great crested newts. MMB considered the ecological impact of the new lagoon from an early stage and was able to identify a field of lesser biological value to the south of the works within which to situate the lagoon. This ensured that the fields containing higher biodiversity value habitats were left undisturbed. MMB worked with Natural England to obtain a District Level License due to the proximity of the pond.

Water treatment typically requires chemical dosing to bring the pH of water into safe levels, which, when discharged to a watercourse can have a negative environmental impact. The RTW system was designed to minimise the impact of chemicals and selected an attenuation lagoon to buffer RTW flows before discharging to the environment. Lime dosing would be reduced before running to waste and temporarily storing RTW waters within the lagoon would allow any ferric sulphate coagulant to settle.

The maximum discharge flow rates to the River Derwent of 1760 l/s were reduced to a trickle flow of 10 l/s into a local burn.

Storing flows in the attenuation lagoon allows for temporary treatment equipment to be brought in, should water in the lagoon require further treatment before discharge. MMB designed out the requirement for a permanent pH correction system downstream of the lagoon by utilising existing dosing systems on site.

Because the negative cut and fill balance identified in the design required spoil to be brought in, an opportunity was identified to utilise site won surplus materials from another MMB site 15 miles away at Horsley WTW. The materials were tested for suitability and moved under a Material Management Plan; significantly reducing the associated carbon with the muck shift on two projects.

MMB utilised the Moata Carbon Portal to develop a baseline carbon footprint and then inform decisions throughout the design. At the end of the concept and define stage, the carbon footprint was calculated to be 270.3 tonnes CO2-eq. During detailed design, the carbon footprint was reduced to 146 tonnes CO2-eq, an overall reduction of 46%. Notable savings in the carbon footprint included:

Changing pipe material from ductile iron to HDPE. Reducing the pipeline depth. Changing lagoon shape to reduce muck shift volumes. Removing the need for lagoon outlet pumps and associated pumping station via lagoon redesign. Combining four chambers into one. Decreasing the manhole chamber size from DN2400 to DN1800. Decreasing the access road length from 250m to 50m. [caption id="attachment_13601" align="alignnone" width="1200"] Security gate installed to allow access to lagoon area - Courtesy of MMB[/caption] Community benefits

A new access gate into the field where the lagoon was situated was constructed, avoiding plant tracking down Wallish Walls Road (public road), removing the risk of damage to the road and providing segregation from public traffic. Consideration of the cut and fill balance allowed wagon movements to be minimised on public highways and reduced temporary stockpiling on site, reducing the visual impact of the construction works.

Conclusion

Despite inclement weather conditions Mosswood RTW has been constructed without any unplanned disruption to the local water supply. The main construction works started in February 2023 and was successfully commissioned prior to the DWI undertaking date of March 2024. Overall completion and handover to NWG was in July 2024.

The use of 3D technology and lean planning techniques helped to minimise wastage on site, maintain excellent safety standards and find opportunity throughout the project programme. Over 26,000 hours were worked, with 1600 Safety Health Environment positive interventions being raised and zero accidents.

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 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.

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 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.

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 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Three possible fill materials were considered:

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

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

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

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

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

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