Supply Chain - Designers - Water Treatment Design

Essex and Suffolk Water, part of Northumbrian Water Ltd, supplies drinking water to 794,000 properties. Maintaining a supply to those customers requires the distribution of up to 670 Ml/d of high-quality drinking water. In Essex, the two primary treatment works are Hanningfield WTW, a physio-chemical works near Chelmsford and Layer WTW, a biological works, near Colchester. These are supplied by raw water stored in two large reservoirs at Abberton and Hanningfield. In 2016, and more recently in 2022 low levels of rainfall and hot weather compromised the raw water quality in Abberton Reservoir which led to significant challenges to the biological process at Layer WTW. This meant unprecedented demand was placed upon the chemical works at Hanningfield WTW, which in turn drew down the water level in Hanningfield Reservoir to levels not seen since the summer of 1996.

The challenge

Whilst this low water level at Hanningfield Reservoir was observed, the level in Abberton was healthy due to the process constraints of the biological works at Layer. To mitigate the scenario of having two reservoirs, one relatively full, and another extremely low, occurring in the future, Northumbrian Water applied for enhancement funding in AMP7 to construct a 20km pipeline between Layer WTW and Langford WTW, also a physio-chemical works, where water from Abberton could be reliably treated. This in turn would allow water from the River Chelmer and River Blackwater that would normally be treated at Langford, to augment the water level in Hanningfield Reservoir by utilising existing transfer pumping facilities between Langford and Hanningfield Reservoir. Providing this connectivity, albeit by substitution, will allow Northumbrian Water to balance the water levels in both Abberton Reservoir and Hanningfield Reservoir, and mitigate a potential scenario where no water is available to supply Hanningfield WTW.

The project

In 2019 Essex & Suffolk Water teams began to plan a pipeline route between Layer WTW and Langford WTW for a pipe that could support flow rates up to 50 Ml/d. This figure was determined as the optimum flow rate to allow balancing of the reservoirs to take place and ensure security of supply when faced with long term drought conditions. The pipeline will take advantage of the capacity and capability of the existing pumping station at Abberton which can deliver up to 255 Ml/d to Layer WTW. The route would begin with a strategic connection into the existing raw water mains that deliver water from Abberton Reservoir into Layer WTW. From this point a route was sought that would allow water to run under gravity conditions to Langford WTW where it would be discharged into two existing bankside storage reservoirs each with a capacity of 35 Ml. The pipeline from Layer to Langford would operate as a gravity main, with the upstream head being set by the pressure in the rising main at the take-off point. With approximately 5 bar of available driving head, the flow out of the pipeline will be controlled by a downstream flow control valve. The pipe sizes have been selected to ensure a positive pressure is available throughout the pipeline and to provide sufficient differential pressure at the outlet at maximum flow.

Hydraulic profile

The hydraulic profile shown below (pipeline pressure is shown in red) extends from Abberton Pumping Station to the outlet at Langford WTW. Chainages are measured back from the Langford outlet (at chainage 0m) and the start of the new pipeline construction is at approximately chainage 20,000m. Note the falling available head due to friction losses along the pipeline and its change in gradient at approximately chainage 12,000m. This change in gradient is due to a change in pipe size which is the result of the pipeline optimisation. [caption id="" align="alignnone" width="1200"] Hydraulic profile - Courtesy of Stantec UK[/caption] The direct connection from the pumped rising main to Layer introduced a number of hydraulic complexities. A key issue was the control of transient pressures, particularly associated with a sudden loss of output at Abberton Pumping Station. Existing surge vessels provide robust surge control, however, the addition of the 20km extension to Langford would require careful consideration. The time to close the outlet valve at Langford, particularly following a sudden loss of pumping at Abberton, would need to be sufficiently long to avoid exacerbating transient pressures while a closing time too long would risk depressurising the pipeline and the risk of drawing air into the pipeline. The occurrence of transient air pockets at high points could create further transient pressures if encountered by the initial pressure waves. While these challenges could be largely avoided by separating both pipelines entirely with a break tank, the tank would need to be sufficiently large and elevated to provide the same driving head. The design team are, instead, progressing an optimised solution to avoid these additional capital works through thorough detailed understanding of the problem, careful selection of the control valves and the implementation of a control system that remains simple yet capable of maintaining pressures within acceptable limits.

Ground investigations

To support the design of the pipeline Essex & Suffolk Water has now completed ground investigation work along the route to determine ground and groundwater conditions and help inform excavated material management decisions. The ground investigation work comprised:

• 63 dynamic sampler boreholes up to 6m in depth.
• 11 cable percussion boreholes up to 10m in depth.
• 27 soil resistivity tests.
• Post-installation groundwater monitoring at approximately 12 locations.

This work was completed by Essex & Suffolk Water’s framework partner Allied Exploration & Geotechnics (AEG) in October 2022 based on a specification written by Stantec UK. This ground investigation work will help Stantec to determine the appropriate pipeline material. [caption id="" align="alignnone" width="1200"] (left) Ground investigation work and (right) pipe installation - Courtesy of Essex & Suffolk Water[/caption]

Pipe material selection

The use of steel, ductile iron (DI) and polyethylene (PE) were all considered. Due to aggressive ground conditions, appropriate corrosion protection would be needed for ferrous materials. PE would remove the corrosion risk but the large diameters needed and the associated pipe thickness would require careful control of welding during construction. At the time of writing (June 2023), final selection is ongoing but, following initial assessment of cost, constructability and maintenance issues, either an all DI pipeline or a mix of DI and PE are considered most favourable.

Project planning

The project is currently moving through the detailed design phase and one activity that has been conducted concurrently is the liaison with the council districts that the pipeline crosses, both Colchester City Council and Maldon District Council. In the experience of Essex & Suffolk Water, large diameter cross country pipelines have typically required Environmental Impact Assessments to support the planning process. For this project, an alternative approach was taken where the team designed the pipeline and its construction methods, including associated compound areas, to have the least possible impact from a number of perspectives including ecology, visual impact and traffic and transport in advance of submitting our EIA Screening Request. This design, including examples of how measures were introduced to provide a less impactful route than one that may have been presented prior to completing these mitigations, helped support the EIA Screening request. In response to that request both Councils viewed the screening request favourably and allowed the team to complete the work using permitted development rights recognising the efforts of Essex & Suffolk Water to plan and design the least impactful route possible.

Archaeology

Essex & Suffolk Water now plans to complete the archaeological work associated with the project over the summer period of 2023. Throughout the early phase of the project Essex & Suffolk Water has worked closely with AECOM to complete a desk based assessment and then develop a Written Scheme of Inspection. This has now been approved by the council archaeologist and comprises 552 trenches measuring 2m x 30m across a 40m corridor along the 20km pipeline route. This work will be completed over a 16 week period, 8 weeks for the field work and a further 8 weeks for recording and report preparation. Due to the archaeological history in the area it is expected that some archaeological artefacts of interest will be uncovered. This may in turn necessitate further mitigation trenching along the route.

Layer to Langford Pipeline - Supply chain (as of June 2023)

• Principal designer: Stantec UK
• Ground investigation: Allied Exploration & Geotechnics
• Land owner engagement: Savills
• Technical input: Saint Gobain PAM UK
• Archaeology: AECOM
• Pipe installation: Northumbrian Water
• Pipe supplier: Electrosteel Castings (UK) Ltd

Schedule

The installation of the majority of the 20km pipeline will begin in February 2024 in order to complete the work in March 2025, allowing Essex & Suffolk Water’s in house team to complete the installation in one pipelaying season. Work has already commenced on a number of minor road crossings to coincide with the work of other developers in the area and minimise impact on our customers. Work is also scheduled to commence on the directional drill crossing that will be required beneath two main rivers as part of the project, the River Blackwater and the Langford Cut in September 2023.

Derrycrin Wastewater Treatment Works (WwTW) is situated in a small village in County Tyrone, on the western shores of Lough Neagh, approximately 8 miles east of Cookstown and serves the Ballinderry area. The Ballinderry River is a short distance north of the village and marks the boundary between Counties Tyrone and Derry/Londonderry. The existing works at Derrycrin had been in service for almost 50 years with NI Water having to undertake continued investment in maintenance to achieve discharge compliance levels. Increasing population demands, resulting in greater pressure being exerted on the wastewater treatment works, meant that the existing assets were approaching the end of their design lives and part of the site was prone to flooding.

Introduction

As part of their six-year business plan (Price Control Period 2021-2027) and infrastructure programme, NI Water invested £3.2m to replace Derrycrin WwTW with modern new infrastructure and technology on the existing site footprint that included the installation of state-of-the-art treatment tanks, along with advanced electrical and mechanical systems to provide a robust wastewater treatment solution.

Existing treatment process

Site investigations discovered that the existing Derrycrin treatment process, originally constructed in the 1970s with a design population equivalent of 285, was under capacity to treat the flow and loads from the current population (PE 403).

[caption id="attachment_15997" align="alignnone" width="1200"] The existing Derrycrin WwTW located near the Ballinderry River - Courtesy of NI Water[/caption]

The works comprised of an inlet receiving chamber and rectangular primary settlement tank with a cover, a percolating filter and final settlement (a steel hopper bottomed humus tank). The original tertiary treatment reed bed was decommissioned due to structural failure and removed to create space for a temporary Submerged Aerated Filter (SAF) plant.

While the SAF plant had assisted achieving the final effluent compliance requirement, the plant was ageing overall, and at times of high river levels, the plant was prone to increased risk of flooding. To address these issues and maintain discharge compliance to the Ballinderry River, a new works was required.

The replacement WwTW for Derrycrin has been designed to serve a projected population equivalent of 556, constructed within the confines of the existing NI Water-owned land.

Project drivers

During the planning stage, the main driver for the Derrycrin WwTW project was to provide a new WwTW to cater for development of the surrounding area and ensure compliance with the stricter Northern Ireland Environmental Agency (NIEA) Water Order Consent that will be imposed in future.

The existing works was biologically overloaded and operating beyond its full design hydraulic capacity. Age, condition of the site, increased PE and regulatory requirements were all significant drivers for the project as follows:

1. Future growth: To meet the future growth of the local population in the Derrycrin area to 2042 (25-year population growth projection of 556 PE).
2. Design flexibility: To provide flexibility in the new design to cater for future extensions or modifications in design flows, loads or consents with minimum disruption.
3. Water quality discharge compliance: To deliver environmental improvements through enhanced water quality discharge to the Ballinderry River in accordance with the Water Order Consent standard and Urban Wastewater Treatment Regulations (NI) 2007.
4. Environmental impact: A secondary driver was to maximise operational efficiencies, and promote sustainability through reduced energy costs, minimised visual impact, waste, disruption, noise, air quality (odour) and socioeconomic effects.
5. Health & safety/structures: Removal of the Health and Safety risks of the failing structures and to ensure all new structures and equipment were above the Q100 flood level to avoid recurrence of flooding at the Derrycrin site.
6. Construction phase discharge consent: To phase the construction of the new treatment works on a restricted site whilst keeping the existing process operational to meet all construction phase discharge consent standards for the works duration.

[caption id="attachment_16000" align="alignnone" width="1200"] Derrycrin during construction (August 2023) - Courtesy of NI Water[/caption]

Early contractor involvement

DLJ Water Ltd, a vastly experienced joint venture between Deane Public Works Ltd, Lowry Building & Civil Engineering Ltd and Jacopa Ltd, was appointed by NI Water as the main contractor in February 2021 under an NEC 4 Early Contractor Involvement (ECI) contract.

DLJ Water, specialists in civil, process, and MEICA design and engineering services of water and wastewater treatment plants and infrastructure assets, provided multi-disciplinary design, project and supply chain management for the contract and delivered a full turnkey design and build solution for replacement of Derrycrin WwTW from start to completion.

Process/MEICA designs were provided by treatment process specialist and joint venture partner Jacopa Ltd. AECOM was appointed as civil/structural designers for the scheme on a Professional Services Contract to DLJ Water and worked with joint venture partner Lowry Building and Civil Engineering to finalise the civil design requirements.

The ECI phase allowed DLJ Water to align resources and integrate objectives with NI Water and their project management team from RPS Consulting Engineers at an early stage to benefit delivery. This proactive approach led to enhanced collaboration, innovation, value, efficiency and reduced whole life costs for the project.

Derrycrin WwTW: Supply chain - key participants

• Principal contractor & designer: DLJ Water Ltd
  • Deane Public Works Ltd
  • Lowry Building & Civil Engineering Ltd
  • Jacopa Ltd
• Client project management: RPS Consulting
• Civil/structural designers: AECOM
• MEICA design: Jacopa
• Temporary works: Cassidy Geotechnical
• Piling contractor: Hamilton Bogie
• Formwork & concrete: McMenamin Construction
• Electrical installation & ICA: Profitec
• Mechanical installation: Bann Mechanical
• Rotating biological contactors: KEE Process Ltd
• Sludge pumps: Xylem Water Solutions
• Water booster set: Dutypoint Ltd
• Concrete: Creagh Concrete
• Precast concrete: FP McCann
• Pipework: Associated Pipeline Products
• MCC: Motrol
• Inlet screen: M&N Electrical & Mechanical (Hydro International Ltd's UK Wastewater Services)
• Fencing: O’Neill Fencing

[caption id="attachment_15995" align="alignnone" width="1200"] Temporary treatment plant used at Derrycrin - Courtesy of DLJ Water Ltd[/caption]

Civil design

AECOM provided civil, structural and geotechnical design services, including the initial enabling and investigatory works carried out at the ECI stage, as well as the detailed design for construction. They were also employed throughout the construction phase to address any issues.

Civil design included the temporary works to facilitate delivery of the new WwTW, which comprised a primary tank, SAF plant, final settlement tank, sludge holding tank, a pumping station, drainage, interconnecting pipework, ductwork and valve chambers as well as site drainage to meet NI Water’s EMSD requirements.

Other civil design requirements included decommissioning, demolition and removal of existing redundant assets, temporary pipework layout and phasing plan, upgraded vehicular access and entrance from Ballinderry Bridge Road, fencing, signage and lighting.

Key civil design considerations were ensuring that the new structures were elevated relative to the existing site/works to reduce the risk of flooding while maintaining gravity inflows to the WwTW and that formation levels maintained the correct hydraulic profile throughout the site.

[caption id="attachment_15996" align="alignnone" width="1200"] Derrycrin WwTW inlet works - Courtesy of DLJ Water Ltd[/caption]

Regular design and HAZOP meetings were important to develop sequencing of construction works for the major structures and coordinating the positioning of new pipework whilst the existing pipework and services were kept operational. These design workshops gave the integrated project team and NI Water operations staff the opportunity to review the proposed completed works to aid clash detection and advise on design changes or pinch points.

The replacement WwTW was designed on the existing confined site footprint in a phased manner whilst the existing works remained operational. This meant that NI Water didn’t need to make any land purchase to facilitate the plant upgrade.

Process design

Jacopa designed a full turnkey replacement process/MEICA plant for Derrycrin considering the best available technology/technique to provide low OPEX costs to serve a projected population of PE 556 within the confines of the existing site boundary.

Following design reviews of the existing WwTW, it was apparent that the available site footprint for a replacement works was limited and maintaining treatment capabilities (in line with current discharge consent standards), while constructing the new works, would prove difficult.

[caption id="attachment_16002" align="alignnone" width="1200"] The new rotating biological contactors - Courtesy of NI Water[/caption]

To overcome the challenge of the restrictive site, Jacopa identified redundant assets under the client’s portfolio that could be refurbished to form part of a temporary treatment solution. Jacopa engaged their supply chain to fully develop a compact, low maintenance temporary treatment solution that would provide effective treatment during the construction phase. The incoming flows at the existing inlet works were diverted to the temporary treatment plant.

Incoming flows gravitated through a 6mm bar screen which then passed flows to a treatment plant, comprising of an inlet lift pumping station, primary tank, SAF unit with an aeration blower kiosk for biological treatment, a lamella tank downstream for final solids removal and finally, a sludge holding tank.

Construction phasing was kept to a minimum, with a small number of assets from the existing site kept online, specifically the existing final pumping station to lift the flows from the temporary plant to an existing SAF and modified final settlement tanks. The temporary plant was operated and maintained throughout the construction phase by Jacopa/Lowrys.

The new Derrycrin WwTW comprises of a new inlet works (6mm screening) and a 10mm bar screen with improved flow control consisting of a greater than Formula A and FFT overflow discharging to a copasac chamber. Downstream of the inlet, the flow goes into a flow split chamber that is designed to split the flow between two rotating biological contactor (RBC) package plants with provision for a third to be installed in the future.

[caption id="attachment_15994" align="alignnone" width="1200"] Flow split chamber - Courtesy of DLJ Water Ltd[/caption]

The new RBCs provide biological treatment for the removal of organic pollutants, such as ammonia. Each unit comprises loss of rotation sensors, GRP covers, clamps, local control stations, easy-lift lids and end covers, internal auto greaser that had been moved to a hatch (this is for easy replacement by one operator), inlet flow balancing and desludge/scum pump.

Desludging of the sludge tanks within the RBCs is an operation that should be completed every 45 days. The treated effluent from the RBCs flows from the final tank in the RBCs to a joint MCerts chamber. The return liquors pumping station has been designed to protect the watercourse and receive flows from various sources such as the automatic decanted liquors from surface water run-off from potentially contaminated areas; for example, tanker loading areas and inlet screening/skip areas, drainage from process sumps and any general containment.

The new works has been supplied with a new MCC housed within a GRP kiosk including HMI and upgraded telemetry. A new 1000 litre potable wash water booster set and two hose reels in kiosks have been provided and the MCerts chamber measures the flows from the site with temperature and turbidity being measured in the tank also. A flow meter has been installed in the wash water booster set to measure the true usage of water on a wastewater site. All this key information - to assist future decision making - can be tracked and accessed by NI Water through the SCADA network.

[caption id="attachment_15992" align="alignnone" width="1200"] MCert chamber - Courtesy of DLJ Water Ltd[/caption]

Challenges/constraints

DLJ Integrated Project Team overcame some major constraints/challenges through a collaborative team approach to deliver an operationally efficient works including:

• Construction of a new treatment works on a very confined site with difficult terrain.
• Maintaining operation of the existing process for the duration of the works.
• Boundary issue during construction - adjacent land was acquired for the duration of the contract on which the temporary treatment plant was located.
• Phased demolition of existing structures meant the site was even more confined.
• Seeding of RBCs proved difficult as temperatures were low due to the winter period.
• Period of hyper-inflation for material and labour costs during design period.
• Supply chain delays during very uncertain time due to Brexit and volatile markets etc.

The Covid-19 pandemic also interrupted the construction programme, however, DLJ managed all issues with strict site-specific Coronavirus Risk Management Plans and the introduction of an innovative paperless work system to further reduce risks.

In advance of work commencing, joint venture partner Lowry Building and Civil Engineering (LBCE) set up the work compound and Jacopa installed a temporary treatment plant in lands adjacent to the existing treatment works, located off the Drumenny Road.

Once this had been successfully installed, DLJ were able to begin decommissioning and demolishing the old works in a phased approach. This included mechanical and electrical stripping of some assets which were refurbished by Jacopa and delivered to NI Water before preparation of the site for the new treatment units commenced.

[caption id="attachment_15993" align="alignnone" width="1200"] Desludge valves - Courtesy of DLJ Water Ltd[/caption]

Although space-saving RBCs were part of the design, the additional treatment capacity required a large footprint within the existing NI Water site and there was limited room available for stockpiling spoil or construction materials. As a result, ‘just-in-time’ deliveries were employed.

DLJ Water worked closely with RPS and AECOM at design stage to agree construction and sequencing of all structures to ensure temporary and permanent loadings. Thermal restraint considerations were also considered in the reinforcement design and detailing.

The new infrastructure was elevated to ensure all new structures and equipment were above the Q100 flood level to protect key assets during flooding of the Derrycrin site.

The construction phase at Derrycrin WwTW commenced in November 2022 with a target date for completion of March 2024. Despite all he challenges highlighted, DLJ Water successfully delivered the project on time and to budget.

Public/stakeholder engagement

Collaboration was instilled throughout every aspect of the planning, design and construction phases for the new Derrycrin WwTW. NI Water along with the DLJ Water Team and RPS developed a proactive communications and public relations strategy for the project to ensure that residents and the wider public were aware of the planned installation and to keep the various stakeholders informed of developments with the project.

These included: erection of a detailed information board providing details on the project to passers-by; house calls to nearest residents; circulation of letters; progress briefings for elected representatives; circulation of press releases in the local media and social media announcements.

In addition, continuous stakeholder liaison ensured no stakeholder-driven delays.

[caption id="attachment_15991" align="alignnone" width="1200"] NI Water Capital Delivery and Operations personnel pictured with the contract team from DLJ Water and RPS - Courtesy of NI Water[/caption]

Key benefits/project success

The collaborative working relationship developed between DLJ Water, NI Water, RPS, AECOM and the wider supply chain through the ECI process and subsequent construction phase allowed for greater innovation and successful delivery of a sustainable, state-of-the-art wastewater treatment solution, capable of meeting its stringent consent standard and serving the community for many years.

The solution - designed and constructed by DLJ - was chosen based on its capability, flexibility and reliability to serve the future catchment needs over a 25-year projection horizon ensuring that NI Water targets were met. This impressive scheme has the added benefit of reduced risk to pollution with inclusion of EMS zones and flood levels considered during design and the new treatment process can operate during flood conditions up to 300mm over the Q100 level. Water turbidity can also be monitored remotely to ensure the process is meeting compliance.

Other benefits include providing a safer site: a new vehicular access was provided resulting in reduced risk to NI Water Operatives. This required extensive involvement from the DLJ Water team during the design phase to improve vehicular access to the site. The site is now also designed with space for an additional RBC tank in the future if required.

Sean Milligan, NI Water’s Senior Project Manager commented:

“The investment to replace Derrycrin WwTW with modern new infrastructure will support local development, deliver environmental improvements and ensure NI Water meets EU standards for many years to come.”

The new works was successfully tested, commissioned and handed over to NI Water in June 2024. It is producing excellent results and is comfortably meeting the NIEA final effluent quality standard of 40 mg/l BOD, 60 mg/l suspended solids and 15 mg/l ammonia.

This innovative wastewater treatment solution will accommodate development in the Derrycrin area and is providing increased environmental protection through enhanced water quality for the adjacent Ballinderry River and thus Lough Neagh.

Forming part of Southern Water’s (SW) Hampshire supply region, the Isle of Wight (IoW) is located just off the south coast of England and is divided into two water distribution zones (WDZ), namely IoW North and IoW South. The IoW South comprises major towns of Ryde, Sandown, Shanklin and Ventnor with multiple villages and more remote rural areas forming the wider distribution network. The water sources and distribution systems have been brought together, with their origins being from former smaller localised systems developed in the late 19th century, which were added to in the 20th century as the IoW population grew, to form an integrated system. Today, there are five existing small and medium sized water supply works (WSW) with a combined daily output of 18 ML supplying customers via several small water service reservoirs (WSR). The water distribution network totals approximately 547 km of mains, serving a population of approximately 45,000 people.

Project driver

The Cooks Castle WSR has been identified for replacement based on asset condition and the associated security of supply risks to customers. This project aims to provide a long term solution for storage requirements and enhance the network associated with the removal/replacement of this existing WSR. To validate the need and determine the optimum size, top water level and location of any new WSR, a solution needed to be developed considering a wider, integrated view of water sources, network storage and water distribution now and into the future. The business planning objectives were broadly defined as:

• Create a more resilient water supply network for future generations (Southern Water Network 2030 rationalisation).
• Maintain levels of service to customers and mitigate water quality risks.
• Ensure future growth and proposed new sources can be integrated (Water Resource Management Plan - WRMP) and reduce dependency on the bulk transfer from the mainland via the Cross Solent Mains.
• Increase operational flexibility through optimisation of existing/new assets.
• Improve security of supply to customers (zonal storage provided in line with Southern Water Asset Class Policy).
• Decommission surplus assets to reduce cost of maintenance.

[caption id="" align="alignnone" width="1200"] Figure 2: The planning process developed - Courtesy of Southern Water[/caption]

Optimisation

The Optimatics’ Optimizer^TM tool was used, allowing the team to analyse up to 35,0000 plans (the group of assets the Optimizer chose to test) in a single optimisation run (see Figure 1 - top of page). Optimizer^TM was allowed to generate plans incorporating both new asset options and removing or retaining existing assets. New options tested included a total of approximately 80km of strategic mains in varying diameters, over 20 WSR configurations in five different locations, decommissioning of existing WSRs and associated rezoning, rationalisation of WSWs and provision of new pumping stations. The Optimizer^TM evaluated plans using input ‘design criteria’ which were formulated and aligned with the project planning objectives and Southern Water standards. More complex ‘rule-based adjusters’ were formulated to allow automation of controls and prevent Optimizer from selecting options which conflicted with one another, e.g. to prevent Optimizer’s ability to select multiple sizes of WSR at the same location. The evaluation of these design criteria by the Optimizer^TM produced a cost and performance evaluation for each plan. The best performing plans were then compared with each other on a Pareto curve - see Figure 3. [caption id="" align="alignnone" width="1200"] Figure 3: Typical Pareto curve showing best performing plans - Courtesy of Southern Water[/caption] Incorporating environmental performance into the optimisation was a particular challenge. The team approached this by taking the results from a high level environmental feasibility assessment and factoring in WSR and pipe costs proportionally to reflect better and worse performing options. This then carried through into the overall cost/performance results for that plan. In the example, the Pareto curve shows about 216 Optimizer^TM plans (from 30,000) from which 10 plans have been identified as feasible options. The Optimizer^TM tool is very powerful at testing several configurations (plans) based on the given inputs, design criteria and formulations. Review and validation are critical parts of the process, and this involves the team checking outputs (individual plan performance), tweaking inputs (adjusting costs/penalties/formulations) and refining the optimisation until several tangible plans are derived. Figure 4 shows typical age of water results for several WSRs with shortlisted plan 7639, 8142 & 8041 compared against the existing system performance. This demonstrates how the optimised plans can significantly reduce the age of water meeting one of the projects key objectives. [caption id="" align="alignnone" width="1200"] Figure 4: Age of water optimisation - Courtesy of Southern Water[/caption]

Defining outputs: the optimal integrated solution

Taking a short list of tangible plans from the optimisation is a starting point which allowed the team to define the pathway to the final integrated solution. Not all factors can be simply incorporated into Optimizer^TM therefore these external factors need consideration alongside the optimisation outputs to form the final integrated solution. One important factor was implementation of solutions over several AMP cycles and ensuring that staged development did not detrimentally impact on customer levels of service. Similarly, consideration of outage scenarios, e.g. WSW shutdowns and mains bursts were considered outside of the optimisation, then retrospectively tested against the shortlisted network configurations.

The solution

The final integrated solution comprised:

• Network connections to a single rationalised WSW using three input water sources (currently two separate WSW with two sources). This aligns with WRMP and improves IoW drought resilience.
• Removal of seven existing WSRs reducing asset risk through deterioration, maintenance costs and age of water issues.
• Provision of one new strategic WSR providing wider security of supply to several district metered areas (DMAs) compared to current configuration.
• Trunk mains reinforcements and new installation/upgrading of network pumping stations to support strategic transfers from proposed WSWs to WSRs and customers.
• Increased operational efficiency and lower OPEX by considering rezoning, triad periods and exploiting opportunities to modify the current network via simple valving operations.

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

Meadowgate Regulator

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

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

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

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

Meadowgate full channel fish pass

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

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

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

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

Meadowgate temporary works solution: Syphon pumps & contingency planning

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

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

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

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

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

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

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

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

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

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

Don Regulators: Supply chain - key participants

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

Canklow Regulator

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

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

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

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

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

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

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

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

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

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

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

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

Woodhouse Mill

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

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

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

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

Summary

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

Guildford Borough Council (GBC) are progressing with the development of approximately 41 hectares of brownfield land located along the west bank of the River Wey as it runs through central Guildford and north through Slyfield Industrial Estate. Guildford Sewage Treatment Works (STW) lies within this land and requires relocation in order to enable the development to proceed. This relocation forms a critical part of the Slyfield Area Regeneration Project, and will allow the delivery of up to 1,550 new homes, partially on land occupied by the existing STW, to enable local housing needs to be met.

The new sewage treatment works will be built to the north of Slyfield Industrial Estate on land that has been used for quarrying, and in turn used as a landfill site, and is currently largely unused. In addition to the construction of a new works, relocation of the works to this location will require the construction of a new transfer tunnel to divert existing incoming flows to the new site, and the construction of a new outfall to the River Wey. The existing STW must also be decommissioned to enable use by the Weyside Urban Village (WUV) Development.

Design flows

The new Guildford STW is designed for a population equivalent (PE) of 125,300, with room to expand the site in the future.

Design criteria	Flow rate
Dry weather flow (DWF)	322 l/s
Average flow	368 l/s
Flow to full treatment (FtFT)	746 l/s
IPS peak capacity	2,250 l/s

Location of new works

The site was formerly quarried for sand and gravel during the 1960s and 1970s, with the resultant voids filled with waste materials progressively moving from west to east (i.e., towards the River Wey), which continued until the late 1980s. There is no record relating to the exact nature of the fill materials. The site was understood to have taken household and commercial wastes, but not special or difficult wastes. Following this, the site has remained vacant and undeveloped. [caption id="" align="alignnone" width="1200"] (left) Location of new sewage treatment works and tunnel route and (right) Slyfield Regeneration Project area - Courtesy of Adams Hendry[/caption]

Transfer tunnel

The STW transfer tunnel will consist of a 1.4km long, 1500mm internal diameter, 1780mm outer diameter tunnel, made of precast concrete pipe, which will be installed by pipejacking (microtunnelling) method. Six shafts are required to facilitate its construction and will serve as temporary launch and reception shafts. Network modelling by Arcadis verified the designed flow velocities remain self-cleansing and provide for the flow rates required. A geotechnical interpretive report has been prepared in review of the ground conditions for the scheme as whole, and to ascertain suitability for shaft sinking and tunnelling. The principal geological stratification for the site consists of Made Ground overlain principally with River Terrace Deposits, with sections of overlapping Alluvium, overlaying the solid geology of firm to very stiff clay of the London Clay Formation (LCF). The selected method of microtunnelling was chosen as it allows long pipeline sections to built, and its suitability to the ground conditions within LCF at Guildford, while limiting the exposure to personnel to underground works, against traditional methods. The microtunnelling is operated from a central control cabin at ground level, and only limited personnel access into the tunnel is required to service the microtunnel boring machine during construction. A ground movement assessment has been prepared and modelled using XDisp software by Oasys. The analysis constitutive model is based on the O’Riley & New method. The method predicts ground movements resulting from ground loss which produces strain of the soil within the influence zone, and projects the settlement in a gaussian distribution curve.

Ground improvement & remediation

The project team has consolidated decades of site investigation data into a unified format and 3D model. The process involves first combining data within OpenGround software and applying scripted translation to a common description - for example having all London Clay called ‘London Clay’, rather than ‘LC’ or ‘Clay’. [caption id="" align="alignnone" width="1200"] RIC 1 and 2 pass areas - Courtesy of BAM Enpure Ltd JV[/caption] This combined dataset is then used in Leapfrog 3D geological modelling software, which applies intelligent interpolations, and highlights anomalies that can be quickly assessed and removed. The resultant 3D model has then been used to enhance communication in the engineering AutoDesk BIM 360 federated models, and in the proprietary Arcadis SmartAtlas GIS viewer. Rapid Impact Compaction (RIC) is a method of imparting energy to the ground using a repeated but relatively short drop-weight, where a base plate (or “puck”) is in constant contact with the ground. RIC is preferred over a larger drop-weight technique due to its more progressive and less disturbing impact on the ground it is being used on. The method is also faster and uses smaller scale setups. A challenge for the RIC method was whether the energy it imparts is sufficient to densify the ground, versus other higher energy drop-weight methods. A trial was deemed necessary to demonstrate the suitability of this method. RIC is proposed to be applied over the majority of the site with an expected densification of landfill down to approximately 4.5m. Areas where assets will not be supported on driven full depth piles will undertake a second pass of the RIC method, reducing the long-term settlement of the landfill material. The predicted reduced surface will be levelled as required by the infill of depressions with imported stone or other suitable material. Improvement of the existing ground by reinforcement through installation of Vertical Rigid Inclusions (VRI) method across the site. Supporting of level sensitive and heavier elements will be on precast driven piles founded below the landfill material. The combination of methods will be designed to achieve Thames Water’s settlement performance values over 40 years, and remain within the allowable values of 50mm and 100mm. This will reduce the need for full depth driven piling which would otherwise be required. Reduction of infiltration to the landfill materials by ensuring impermeable surfaces drain to the treatment works and are released at the outfall away from the site. Gas and leachate well monitoring is provided along the perimeter of the site to monitor any impact and provide leachate removal points if required. [caption id="" align="alignnone" width="1200"] (left) Rapid impact compaction rig and (right) pile under test - Courtesy of BAM Enpure Ltd JV[/caption]

Guildford STW Relocation Project: Supply chain - key participants

• Client: Thames Water
• Principal contractor & designer: BAM Enpure Ltd JV
• Client designer: Jacobs
• Civil designer: Arcadis
• Planning consultant: Adams Hendry Consulting Ltd
• Process & M&E: Enpure Ltd
• CFD analysis: The Fluid Group
• Physical modelling: Hydrotec Consultants Ltd
• Tunnel design: Joseph Gallagher Ltd
• Temporary works/shoring: MGF Ltd
• Precast structures: A-Consult Ltd
• Precast structures: FLI Precast Solutions
• Rapid impact compaction: Cofra Ltd
• Piling: Aarsleff Ground Engineering Ltd
• XDisp software: Oasys
• Controlled modulus columns: Vibro Menard Ltd
• Inlet works and PST odour covers: Power Plastics Ltd
• Inlet pumps: Grundfos
• Imported sludge Strainpress: Huber Technology
• Escalator screens: Longwood Engineering Company Ltd
• Grit removal plant: Jacopa
• PST scrapers: Colloide Engineering
• FST scrapers: Purac AB
• Odour control plant: Odour Services International Ltd
• FBDA grids and blowers: Xylem Water Solutions
• Sludge thickening & dewatering: Kent Stainless Ltd
• Cake Silo: Saxlund International
• Cloth pile filters: Eliquo Hydrok Ltd
• Polymer batching & dosing: Richard Alan Engineering
• Ferric plant: Colloide Engineering
• Tank logging system (WASP): JR Pridham Services Ltd
• Glass lined steel tanks: Balmoral Tanks Ltd
• Sludge tank mixing: Landia UK Ltd
• Pumps, mixers & flocculators: NOV
• Welfare building: M-AR Limited

Guildford STW - the new sewage treatment works

Inlet pumping station The new STW is being designed to maintain inflow from the external catchment ranging from dry weather to 1 in 100-year storm (plus climate change allowance) with future-proofing beyond the 2041 design horizon. [caption id="" align="alignnone" width="1200"] (left) inlet pumping station physical model and (right) inlet pump testing physical model - Courtesy of Hydrotec Consultants Ltd[/caption] The pump design has been developed to fulfil the project brief load requirements (future peak IPS capacity 2585l/s) and some flexibility either side of the requirements is available due to the standard capacity equipment design offered from the supply chain. The proposed arrangement now consists of a wet well 12.5m diameter, approximately 20m deep. A common inlet will be provided to dissipate any entrained air, and two selectable pumped sections with double isolation will be provided by wall mounted penstocks on full height walls. The pumping well will be constructed in two halves, hydraulically connected in normal operational mode however each half will be able to be isolated (using motorised penstocks) in accordance with specification requirements surrounding double isolation, to allow person entry into half of the wet well at a time. Each half of the wet well will have three pumps installed giving a capability to provide full flow to treatment from two pumps with a standby permanently installed. The pumping station was first modelled by computational fluid dynamics (CFD) to demonstrate the assumed flow patterns that will occur. The pumping station was then physically modelled to comply with the specification and process design requirements particularly with respect to distribution, mixing, settlement, turbulence, dispersion, and freeboard for the full range of flows that each structure is expected to receive. [caption id="" align="alignnone" width="1200"] Storm overflow physical models - Courtesy of Hydrotec Consultants Ltd[/caption] Inlet works The inlet works is being designed to receive all pumped flows from the inlet pumping station (including future peak IPS capacity 2585 l/s), inlet works drainage and screening liquor return flows. Flows will enter the inlet works and then separate across the four escalator screens inclined at an angle of 45°. Each screen will be provided with a local proprietary control panel. Each panel will incorporate a differential ultrasonic level control. An emergency bypass will be provided upstream of the screens to accommodate all four screens blinding at the same time. Three 1200mm wide duty screens will be required to screen flows from low flow up to 2585 l/s, screening 6mm in two directions. The inlet works and grit treatment plant will be covered with odour covers which will be connected to the inlet works and PST odour plant. The inlet works has been classified as a Zone 2 Area under the odour covers and all equipment beneath the odour covers will be ATEX rated. Storm tanks Storm flows from the inlet work storm separation chamber will flow into two storm tanks via a 1400mm above-ground pipe. Within the 1400mm pipe between the storm separation chamber and the storm tanks there will be a partial pipe MCERTs certified flow meter to measure the total flow into the storm tanks. During normal operation the blind tank will fill first, then the spill tank will fill. In the event that both tanks are full to capacity (and still in storm flows) the spill tank will overflow via a weir and the storm tank will discharge to river. [caption id="" align="alignnone" width="1200"] Elevated inlet works - Courtesy of BAM Enpure Ltd JV[/caption] The storm tanks contents will automatically return to the inlet pumping station via 700mm gravity pipe when the flow being pumped to the inlet works drops below a predetermined set point. The return flows will be controlled to approximately 0.5 DWF via an automated valve controlled by an electromagnetic flow meter located in the gravity main, returning to the common inlet well of the inlet pumping station. Storm tank cleaning will be a pumped system with the two pumps located external to the storm tanks. Storm tank cleaning will be initiated during the latter part of the storm tanks draining back to the inlet pumping station and will not operate when the tanks are filling. The pump discharge pipe work will include connections for flushing with washwater. Ferric storage & dosing Two 45m³ ferric storage tanks will be located in a concrete bund with the dosing skid packages and control system located within an adjacent kiosk. The bund will be capable of containing 110% of the one of the storage tanks plus 10% allowance for rainwater. A drive-in bunded area for chemical deliveries will be provided and an underground chemical interceptor will contain any local spillages, complete with a Castell interlocked 3-way valve to allow the bunded drainage area either to drain into the interceptor (during a chemical delivery) or to drain into the site drainage (normal operation). Chemical dosing points will be provided upstream of the PSTs, on the common outlet channel of the ASP, and upstream of the tertiary treatment plant. The dosing points will utilise sparge systems to ensure adequate distribution of chemicals into the channels. Odour control plant The odour plant is a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 8432 m³/hr (of which 4813 m³/hr is from the inlet works and 3619 m³/hr from the primary settlement tanks). Service water A potable water main supplies a service water break tank via a ballcock (float valve). The service water break tank has a working volume of approximately 30m³ which gives a nominal retention of approximately three hours at average throughput. Service water booster pumps draw water from the tank and transfer it to the service water main network where it feeds the polymer make-up system for all thickeners and dewatering belt presses. Primary settlement distribution chamber The distribution chamber has been CFD modelled to calculate the flow split to each downstream primary settlement tank from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. This required the inlet bellmouth to be turned down to improve flow splitting. [caption id="" align="alignnone" width="1200"] PST distribution chamber CFD models: (left) with four PSTs in service and (right) with six PSTs in service - Courtesy of The Fluid Group[/caption] Primary settlement Primary settlement consists of four 47.15m long by 10m wide tanks, with a depth ranging between 6.24m at the hopper end and 5.3m at the outlet length (equating to 1 in 50 floor slope). Each PST tank has three sludge hoppers at the inlet end. The outlet of the PST discharges into a common 1000mm wide outlet channel. From this outlet channel the 1000mm outlet pipe from the drop box combines with the 700mm RAS lines into an above-ground 1200mm pipe. Retractable odour covers will be provided across the PST tanks to allow for future inspection, cleaning and maintenance. The odour covers will be connected via ducting to the inlet works/primary settlement tank odour plant. All mechanical equipment installed below the covers will be constructed from 316 stainless steel or high performance plastics. The primary settlement tanks have been classified as a Zone 2 Area, and all electrical equipment beneath the odour covers will be ATEX rated. The design of the PST Tanks is based upon a maximum inflow of 3022m³/hr to all four in-service tanks, and a peak flow to each tank of 1007m³/hr with one PST out of service. In normal operation all four PST tanks will be in service. Within each PST tank a chain and flight scraper system will be provided which will utilise high-tech non-metallic materials as the main materials of construction. Scum from the primary tanks will gravitate via a rotating surface skimmer pipe to the above-ground primary settlement scum tank. The primary scum return pumps are progressive cavity pumps, operating in a duty standby configuration. The pumps are fixed speed pumps operating between a high and low level in the scum tank. The PST sludge hoppers will desludge based upon a SCADA-adjustable interval and duration. The primary desludge pumps are progressive cavity pumps, operating in a duty/standby configuration. The pumps are variable speed pumps operating to maintain a SCADA-adjustable flow set point. Once the effluent has flowed through each PST it will enter into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the ASP distribution chamber via a drop box and 1000mm above-ground pipe. [caption id="" align="alignnone" width="1200"] ASP overview - Courtesy of BAM Enpure Ltd JV[/caption] Activated sludge plant The ASP plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The activated sludge treatment tank consists of four ASP lanes. Each ASP lane has a selector tank followed by an anoxic zone and then the three aerobic zones. The main aeration lane will be fitted with aeration pipework and fine bubble diffusers. The aeration vessel is 80m long and 6.1m wide with a water depth of 7.05m. The total instantaneous maximum air demand of the ASP is 10,438 m³/h. The first third of the aerated fraction of each lane will contain 50% of the aeration capacity, the middle third 30%, and the final third 20%. Each aeration lane will have three passes, arranged as channels of 6.15m width. The air for the diffuser grids will be supplied from three ASP air blowers operating on a duty/assist/standby basis. The aeration system will be controlled by an ammonia based real time control (RTC) system utilising a feed forward ammonia load input and a feedback ammonia concentration. Once the effluent has flowed through each ASP lane it enters into a common 1000mm wide outlet channel. From this outlet channel flows transfer to the FST distribution chamber via a drop box and 1200mm above-ground pipe. [caption id="" align="alignnone" width="1200"] Anoxic and aeration zones flows - Courtesy of BAM Enpure Ltd JV[/caption] Final settlement tanks The FST plant will have a distribution chamber using the same general arrangement as the primary settlement distribution chamber. The four FSTs will consist of 32m diameter flat bottomed circular tanks with a side wall depth of 6.15m. The FST will separate solids from the flow via settlement, producing a clarified effluent which overflows a peripheral weir into a launder channel, before gravitating to the tertiary treatment distribution chamber via a 1000mm below-ground pipe. The combined clarified outlet flow from the four tanks will be monitored by a common flow meter, as well as suspended solids and phosphorus monitoring instrumentation, all provided on the common line to tertiary treatment. The design of the FSTs is based upon a maximum inflow to each tank of 1741m³/hr with one FST tank out of service. In normal operation all four FST tanks will be in service. Each tank will have a 700mm inlet pipe, which will flow into a central diffuser drum before entering the FST tank. Each tank features a 600mm outlet pipe fed via a peripheral launder channel, as well as a 400mm RAS gravity outlet pipe. Each of the four tanks features a full bridge rotating scraper system which will run continuously in normal operation. [caption id="" align="alignnone" width="1200"] 32m final settlement tanks & sludge return (RAS pumping station) - Courtesy of BAM Enpure Ltd JV[/caption] Sludge return (RAS/SAS pumping station) Sludge from each pair of FSTs will enter one of two RAS/SAS desludge chambers. The two chambers will be located side by side and will share a set of three duty/duty/standby RAS transfer pumps, which will recycle the settled sludge from both RAS/SAS desludge chambers to the activated sludge plant distribution chamber feed line. The variable speed centrifugal RAS transfer pumps will be capable of pumping 2200m³/hr at a maximum head of 8.9mhd. The pumps will operate between high and low level in the duty RAS/SAS desludging chamber, with both chambers provisioned. A pair of duty/standby SAS pumps is provided, which will facilitate wastage of surplus sludge, with each pump drawing from a single RAS/SAS desludge chamber. The fixed speed centrifugal pumps will transfer sludge to the SAS buffer tank. Control of the SAS pumps will be carried out either on a timer or based on the level in the SAS buffer tank, with the common discharge line featuring flow monitoring. Tertiary treatment Clarified effluent flow from the FSTs will pass to the tertiary ferric mixing and flocculation (post-precipitation) stage, where the option will exist to dose ferric sulphate solution into the flow in order to encourage flocculation of remaining solids. This stage will be split into two parallel streams, with flow from the FSTs initially entering via a single bellmouth inlet into a common inlet channel, before passing via twin overflow weirs into two rapid mix chambers, and subsequently overflowing into two flocculation chambers. The design of the rapid mixer chambers will ensure complete mixing (>95% CoV) of chemical in less than 4 seconds. Flow from the flocculation chambers will weir into a common outlet channel leading to a flow distribution chamber feeding the tertiary treatment stage. The distribution chamber has been CFD modelled to calculate the flow split to each downstream cloth pile filter from first principles, and assess whether the proposed design would deliver an equal flow split (+/-5%) for seven future flow scenarios. The tertiary treatment stage will comprise six cloth pile filters, which will operate with all filters in service - however the peak flow will be able to be treated with only five filters in service. The filters will be sized to accommodate a peak flow of 840 l/s, producing a final effluent of 20 mg/l TSS (95%ile) and 0.2mg/l TP (average). Bypass pipework is to be provided adjacent to the cloth pile filter structure, by which clarified effluent from the FSTs may bypass the tertiary ferric dosing/flocculation and cloth pile filter stage. Each of the six cloth pile filters will feature a central rotor drum onto which filter discs are attached. The rotor and discs will be fully submerged within the tank, with the discs supporting the physical cloth filter media. Flow entering the filters will flow from outside the discs through the cloth into the discs’ interior, with solids collecting on the external surface of the cloth pile, creating a headloss. As this headloss increases, the water level in the tank will rise and at a pre-set water level (as detected by an ultrasonic level sensor) a cleaning cycle will be initiated. [caption id="" align="alignnone" width="1200"] Tertiary treatment flocculation chambers: Eliquo Hydrok Mecana cloth pile filters - Courtesy of BAM Enpure Ltd JV[/caption] Filtered final effluent (FFE) from the inside of the filter disc and rotor drum assembly will pass forward through the unit via a low-level aperture to an outlet chamber, from which flow will weir into the common final effluent outfall chamber. An emergency overflow weir is to be provided, allowing unfiltered effluent from the filter tank (external to the discs) to weir over into the outlet chamber. Backwash liquors from the backwash pumps will exit the filters via a common manifold, draining by gravity to the grit plant liquor PS. Eight backwash pumps will be provided per machine, giving a total of 48 pumps across all six machines. The pumps will be submersible units placed within the tank, with each pump being dedicated to three filter discs. The backwash flow rate is rated for 10 l/s (subject to the installed pipework design/headloss). Each of the six filter tanks will see some solids settlement occurring (and potential accumulation of other debris), which will create a build-up of sludge in the bottom of the tank. This sludge will be periodically removed by a set of two duty/standby sludge pumps per machine (i.e. 12 sludge pumps installed in total). The discharge from each sludge pump will be connected to the common backwash manifold, leading to the grit plant liquor pumping station. Final effluent discharge Flows leaving the cloth pile filters will pass out from the main filter chamber via a low level aperture into a small outlet chamber, from which flows will weir over into the common outfall channel. The final effluent (FE) will flow under gravity through an FE flow measurement flume, with an ultrasonic instrument for measurement of final effluent flows discharged to outfall. The FE outfall channel will feature a pipe leading to the FE washwater system, with a further line branching off this pipe servings as the suction line for FE Meteorburst Analyser. On exiting the FE outfall channel, flows will enter a dedicated FE outfall main, which will drain by gravity, discharging to the river north of the STW site. [caption id="" align="alignnone" width="1200"] New Guildford STW: 3D model night-time overview - Courtesy of BAM Enpure Ltd JV[/caption]

Guildford STW - Sludge Treatment Plant

Sludge thickening equipment Primary sludge from the PSTs will be pumped via the primary sludge pumps to the primary sludge buffer tank, which will have a volume of 120m³ and will be sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby primary sludge thickener feed pumps will be provided to transfer sludge (at up to 7.0%DS) from the primary sludge buffer tank to the primary sludge gravity belt thickeners. Two primary sludge gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation, although the streams will be able to run as duty/assist if required. Each machine will be able to handle up to 51m³/h (771 kgDS/h) of primary sludge, at up to 7%DS achieving a solids capture rate of 95%. Surplus activated sludge (SAS) from the FSTs will be pumped via the SAS pumps to the surplus activated sludge (SAS) buffer tank, which will have a volume of 67m³ and has been sized to provide 4-hours of intermediate storage prior to thickening. A set of two duty/standby SAS thickener feed pumps will be provided, which will transfer sludge (at up to 2.0% DS) from the SAS buffer tank to the SAS gravity belt thickeners. Two SAS gravity belt thickeners (GBTs) will operate in a duty/standby configuration in normal operation but will be able to run as duty/assist if required. Each machine is sized to handle up to 55m³/h (332kgDS/h) of SAS, at up to 2%DS, achieving a solids capture rate of 95%. Sludge balancing/blending tanks Thickened primary sludge, surplus activated sludge, and screened sludge imports will be pumped to two thickened sludge blending tanks. The tanks will have a combined volume of 2150m³ (1075m³ each), and will be sized to provide a combined total of 4-days retention time prior to dewatering of the blended sludge. The tanks will operate in a duty/standby configuration, with either tank being available for selection as the duty unit for either filling or emptying. [caption id="" align="alignnone" width="1200"] Thickened sludge blending tanks - Courtesy of BAM Enpure Ltd JV[/caption] Each tank will feature separate inlet connections for the three types of sludge being received, with actuated valves and crossover lines allowing all sludges to be directed to either of the two tanks. The tanks will also feature outlet actuated valves, which will allow either tank to be isolated from the sludge belt press feed pumps downstream, thereby ensuring the pumps only draw from a single tank at any given time. A set of three duty/duty/standby sludge belt press feed pumps will be provided, which will transfer blended thickened and imported sludge from the thickened sludge blending tanks to the sludge belt presses. The pumps will be variable speed progressive cavity units, with a peak capacity of 30m³/h (i.e. 15m³/h per pump) at a maximum head of 48.5m, based on sludge of up to 7.0%DS. Dewatered cake leaving each belt press will enter a discharge cake hopper leading to the suction end of a sludge cake pump. Three pumps will be provided, with a single pump dedicated to a single machine; each pump will operate in conjunction with the associated belt press upstream. The pumps will be variable speed progressive cavity units, with a peak capacity of 7.4m³/h of sludge (up to 30%DS). The pumps will transfer the cake via individual discharge lines to the sludge cake silos, with manual discharge crossover facilities allowing sludge from the pumps to be directed to either of the two silos as required. Cake silo storage & discharge Dewatered sludge cake will be pumped to two truck-loading cake silos, with a combined storage capacity of 600m³, sized to provide 7-days of storage at average cake production. Each silo will feature a discharge screw conveyor (with a sliding frame ‘anti-bridging’ device) which will discharge the cake through a hydraulically operated slide gate valve. Odour control plant The odour plant will be a 2-stage system. Stage 2 filters will discharge into an 18-metre-high odour stack. The plant has been designed to achieve a minimum removal efficiency of 98% of H2S and 99% removal of odour, based upon a total system duty of 6629m³/hr, extracted from the various tanks and thickening/dewatering machines in the sludge plant area.

New STW outfall

Outfall route The outfall route from the new works site location was assessed thoroughly in the ITN 2 stage of the project’s life and from exiting towards the middle of the works site, proceeds north towards the River Wey. The route deviates initially to minimise any impact on significant trees that exist within the floodplain, and any impact on the band of vegetation around the perimeter of the site. Final effluent & stormwater pipeline The final effluent pipeline will be a 750mm diameter pipe. This will be situated above ground from the final effluent channel to the edge of the site, and will be supported by piles when within the treatment works site. The pipeline then proceeds below ground through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. A jetting/access point will be provided just before the pipeline dives below ground, giving the facility to clean this arrangement and flush from a more direct point than the pipes exit from the final effluent channel. Air relief may also be provided at this point, subject to detailed design. The storm overflow pipeline will be a 1050mm diameter pipe. This will be situated below ground from the storm tanks to a sampling manhole at the edge of the site; supported by piles when within the treatment works site. The pipeline will then proceed from the manhole through the embankment (to be reinstated using clay stanks) and under the adjacent ditch via an inverted siphon arrangement. The sampling manhole will provide an ability to clean this arrangement. [caption id="" align="alignnone" width="1200"] (left) Outfall route plan - Courtesy of Bam Enpure Ltd JV, and (right) outfall low flow scenario CFD model - Courtesy of Arcadis[/caption] Outfall structure The outfall structure will consist of a reinforced concrete headwall, set back from the river channel, to be founded in the river terrace deposits and flared to direct flows downstream and minimise the impact of upstream debris from collecting. The headwall will have duckbill valves on the discharging pipes, and erosion control products utilised in the surrounding area to tie the headwall into the natural riverbank and bed. The erosion mattress employed following the headwall structure will provide a secondary point at which turbulent flows can dissipate energy without eroding the natural bed of the river. Where the headwall ties into the natural riverbank, the hard/soft interaction between headwall and natural soil will be reduced by the installation of Rock Rolls erosion control and willow stakes at the base, and the installation of a natural hydro-seeded CocoBN blanket or equivalent (to be seeded with local seed varieties) which will retain the slope shape until vegetation establishes and provides an effective natural erosion control. This will also provide a gradual slope to access down to the river.

Programme

The project obtained planning permission in October 2022, with construction works starting on site in July 2023. Detailed design of the civil, mechanical and electrical works are due for completion by December 2023. Construction of the works will be complete by August 2025, with commissioning due to commence in from April 2025 through to December 2025. Flows from the existing works will be diverted to the new works from December 2025 (10% flow) in a 7-stage commissioning process with 100% flows diverted to the new STW by March 2026. Following 100% flow diversion the existing STW will be de-commissioned ready for handover to Guildford Borough Council.

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

Canklow

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

Woodhouse Mill

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

Meadowgate

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

Temporary set up

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

River Don Regulators: Supply chain - key participants

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

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

HAZOP

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

Full channel fish pass

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

Computational fluid dynamics (CFD) modelling of fish pass

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

Fish pass tiles

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

Temporary works consideration

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

Summary

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

St Augustines Road is a busy thoroughfare in Chesterfield linking the town to Matlock and the south-eastern edge of the Peak District. Lying within the Yorkshire Water (YW) waste catchment area, the road is steep in its nature with gradients often exceeding 7%. Due to the local hydraulic catchment and topography, during heavy rainfall events flooding occurred at the road’s intersection with Derby Road (A6). The flooding lifted manhole covers from their frames which caused significant damage to the highway. It also caused internal and external damage to the surrounding properties. To combat this, mitigation was carried out by Yorkshire Water and Chesterfield Borough Council (CBC), including the implementation of mitigation measures such as non-return air bricks and flood gates, however, this had limited success. CBC also commissioned the construction of a detention basin to the rear of the flooding properties; but again with limited success.

Hydraulic study

As a result of the mitigation’s limited success, a hydraulic study was commissioned by Yorkshire Water. This confirmed the root cause of the flooding was hydraulic overloading of both the foul and surface water sewers. A notional solution was developed by Yorkshire Water which included the upsizing of these sewers, with some form of temporary storage for the foul system to retain flows in a rainfall event. Later iterations of the hydraulic model revealed that improvements to the surface water network would in fact also reduce the frequency of flooding on the foul water network due to interactions between the two systems during storm conditions. In light of this information, in May 2021 Yorkshire Water secured funding to implement a series of significant improvements on St Augustines Road.

Preferred solution

The preferred solution included the construction of a new outfall into the River Rother and the transfer of all flows from the surface water sewer in Derby Road. This would provide a much greater flooding resilience in the existing system. The existing 225mm surface water sewer was to be upsized to Ø750mm along its 320m length. It would then be diverted via a tunnel underneath Derby Road into the wetland area immediately adjacent into the River Rother. Here, a new outfall would be constructed to discharge the surface water sewer into the river network. [caption id="" align="alignnone" width="1200"] (left) The preferred option and (right) flooding on St Augustines Road - Courtesy of Galliford Try[/caption]

Constraints and engineering challenges

The preferred solution progressed to detailed design with several technical challenges to be overcome. Several of these challenges were exacerbated by the need to construct the outfall into the River Rother outside of the salmonoid spawning season. An Environment Agency (EA) bespoke Flood Risk Assessment Permit, (FRAP) was required to construct the outfall and all in-channel activities were required to be undertaken before 1 October 2021 in order to mitigate the impact on any spawning salmon. A significant flow rate of up to 0.89m³/s would be discharged into the river during storm events and river scour was a concern. In order to combat the scour on the riverbed and riverbanks, there was a requirement to reduce the velocity of the flow before it interacted with the River Rother, causing excessive turbulence. To achieve this, box culverts were selected due their unique cross section and significant channel width. This slowed velocities down to near 1m/s, which is widely regarded to have negligible effects on turbulence and thus reducing scour. The headwall was also designed to allow flows to enter the River Rother at a 45° angle, as per EA Standard Rules SR2015 guidelines, further reducing the risk of scour on the opposite riverbank and riverbed.

Outfall design

This design however did not come without complications, such as the need to carry out the structural design, build and installation of a bespoke outfall within a few months. BSP Consulting was commissioned to carry out the structural design of the outfall which was to be precast, reducing the need for in situ concrete handling adjacent to the sensitive receptor. Off-site build (OSB) also reduced the construction programme and allowed the headwall to be manufactured concurrently within the determination period of the FRAP. [caption id="" align="alignnone" width="1200"] Outfall under construction - Courtesy of Galliford Try[/caption] Poor ground and underfoot conditions adjacent to the River Rother proved too risky and problematic for traditional mobile crane lifts, so all lifts of the precast sections were to be undertaken with a tracked excavator. This further constrained the design and restricted the maximum mass of each section to 13 tonnes. Consequently, in its final iteration the precast headwall was designed in two sections and was combined with an in situ toe and infill slab/wall which completed the structural design.

Design

During the design stages, extensive ground investigation was carried out. This identified water bearing sand and gravel strata adjacent to the river which transitioned to mudstones and clays towards St Augustines Road. It was this change in strata which posed a significant challenge for the no-dig section of the work beneath Derby Road. This section of work was critical as it linked the outfall to the upsizing work on St Augustines Road. Discussions were held with Active Tunnelling Ltd who proposed the use of the guided auger bore technique to install the sewer in this transitional strata. Although initially designed as a 750mm diameter sewer, this was reduced to 600mm which was the maximum diameter which could be accommodated by the tunnelling contractors. Hydraulic modelling was undertaken by RES Environmental to confirm the viability of this proposal and in order to compensate for this localised throttling, the pipeline had to be upsized locally to 900mm immediately upstream of the tunnel. The alignment of the tunnel was severely limited due to proximity of existing buildings on Derby Road. BSP Consulting was again called into action as pre-condition surveys and settlement calculations were commissioned in order to understand the impact of the tunnelling works on these at-risk properties. [caption id="" align="alignnone" width="1200"] (left) Installation of drive pit on St Augustines Road, (middle) guided auger bore rig, and (right) the transition between 600mm and culverts - Courtesy of Galliford Try[/caption] Settlement calculations were undertaken using the “Guide to best practice for the installation of pipe jacks and microtunnels technique” and a predicted settlement of <3mm was deemed to be acceptable. The pre-condition surveys highlighted only minor cosmetic issues and were used as reference to the state of repair of the properties prior to construction work commencing. The majority of the civil design was undertaken in house. This element included a design coordination role combining and processing the information received from the consultants and stakeholders. An example of the numerous stakeholders which were consulted is that of the Coal Authority. Initial searches indicated the presence of historical coal workings. More detail was sought and records indicated the presence of an adit within 20m of the proposed pipeline. A coal permit was sought from the Coal Authority and the design progressed whilst this was processed.

St Augustines Road (Chesterfield) Flood Alleviation: Supply chain: key participants

• Principal contractor & civils design: Galliford Try
• Temporary works & headwall design: BSP Consulting
• Hydraulic modelling: RES Environmental Ltd
• Ecological consultants: Middlemarch Environmental
• Traffic management: Amberon Traffic Management
• GRP & utility mapping: Cloud Conversion
• Ground investigation: Grange Geo Consulting Ltd
• Pipelaying & outfall construction: Arthur Civil Engineering
• Trenchless installation: Active Tunnelling Ltd
• Cofferdam design & supply: National Trench Safety UK
• ‘EndSafe’ safety barrier: Groundforce Shorco
• Cofferdam installation: Ivor King Piling
• Dewatering: Alba De-Watering Services Ltd
• Precast concrete: Bonna Sabla
• CCTV, CIPP & cleansing: CPS Drainage Solutions
• Handrailing & grille design: Galliford Try (Fabrications)
• Silt curtain: Murlac Land and Marine Environmental Solutions
• Pumping: Selwood Ltd
• Ridgistorm pipes: Polypipe Civils
• Pipe & flap valve supplier: Keyline Civils Specialists
• Landscaping & site clearance: Landscape Maintenance Services Ltd
• Reinstatement of traffic signals & street furniture: Swarco
• Aggregates & muck away: JC Balls & Sons
• Brickwork: Derwent Joinery & Construction
• Gates & fencing: SFC (Midlands) Limited
• Surfacing: BDS (Yorkshire) Ltd

[caption id="" align="alignnone" width="1200"] Service detection - Courtesy of Galliford Try[/caption]

Installation

The Environment Agency granted the FRAP on 13 August 2021, and the detailed design was finalised and submitted to Yorkshire Water’s technical approval committee on 16 August 2021. Work began on site a few days later, leaving just six weeks to construct the headwall and cease all in-channel activities as per the conditions of the FRAP. Work commenced immediately on the enabling works, with particular attention paid to the temporary works. Haul road To provide drainage to the waterlogged land adjacent to the River Rother, a land drainage scheme was commissioned, designed and installed by Lincolnshire Drainage. This provided a more suitable foundation on which to install the haul road and working laydown areas. The haul road was designed by BSP Consulting and was of substantial construction given the underlying strata, at 700mm thickness with geotextile layers to improve longevity. Ground dewatering After the installation of the haul road, wellpoint dewatering was installed around the outfall area and along the proposed culvert alignment. The dewatering was installed by Alba De-Watering Services Ltd at 1.5m centres. However, due to the underlying mudstone, each well had to be pre-augered and was then installed using the more traditional spout method to ensure that a reasonable well depth was achieved to adequately dewater the working area. A silt curtain was supplied and installed by Murlac Land and Marine Environmental Solutions prior to the channel works to minimise silt and sediment damage downstream from the works. It proved to be very effective during the cofferdam installation. [caption id="" align="alignnone" width="1200"] Cofferdam and silt curtain - Courtesy of Galliford Try[/caption] Cofferdam A cofferdam was required to retain the River Rother and this was designed and supplied by National Trench Safety UK and installed by the primary subcontractor Arthur Civil Engineering. The cofferdam was unique in design in that it utilised Larsson piles in a trapezium shape (plan) to minimise over-dig yet still accommodate the headwall and first culvert. The cofferdam was installed by Ivor King Piling using a 35t excavator with a Movax side grip pile driver attachment. The cofferdam was effective but 4” end suction pumps supplied by Selwood Ltd were still required to maintain the cofferdam and keep water levels low. Headwall installation Once the cofferdam was watertight, the headwall toe was cast in situ, after which the precast headwall arrived in two sections as planned. It was lifted into position by Arthur Civil Engineering using a 50t tracked excavator. An administrative issue led to the need to negotiate a 2-week FRAP extension with the EA and the headwall was completed and the cofferdam removed by 14 October in accordance with the conditions of the revised FRAP. Pipeline works Construction works on the remainder of the project continued in March 2022. A temporary one-way system was implemented to provide a safe working area to install the remainder of the pipeline with open cut works commencing on 14 March. The open cut works progressed steadily with over 100 services safely crossed. The later spring and summer months provided excellent conditions for online sewer relay with drought-like conditions, culminating in hosepipe bans for the majority of the UK. [caption id="" align="alignnone" width="1200"] Open cut pipelaying - Courtesy of Galliford Try[/caption] Prior to the tunnelling works commencing, concern was raised over the proximity of the tunnel as it crossed under a 225mm foul water sewer in Derby Road. With a clearance of just 300mm, the existing foul sewer was inspected by CPS Drainage Solutions using CCTV and a precautionary 5m cured-in-place patch liner was installed to strengthen the sewer prior to the guided auger bore operation. No-dig works commenced in early June and progressed well without incident and were completed towards the end of the summer. No settlement was detected during the installation and no visual deformations were reported from the nearby structures. Open cut works continued late into the summer with flooding benefits delivered by 5 October. Significant rainfall events have since occurred and the feedback from residents previously affected by the flooding has been positive.

Off-site build

Off-site build was utilised heavily without which it would not have possible to abide by the conditions set in the FRAP and achieve the tight programme when constructing the outfall into the River Rother. The headwall and culvert manhole cover slabs were precast by Galliford Try off site and the hand railing and grille were fabricated by Galliford Try (Fabrications). The flap-valve was fabricated off-site by Keyline Civils Specialists. [caption id="" align="alignnone" width="1200"] 900mm Ridgistorm pipe from Polypipe Civils ready for installation - Courtesy of Galliford Try[/caption]

Public engagement & community impact

Throughout the project customer engagement was a priority for Galliford Try and Yorkshire Water. Although COVID-19 curtailed some of the engagement meetings which were planned at the start of the project, regular communication with residents of St Augustines Road and surrounding area took place. A significant quantity of the communication was made to the local residents Facebook group, which provided timely and accurate updates on progress. Traditional letters were also posted as back up and for those who didn’t have access to Yorkshire Water’s online updates. Towards the end of the project volunteers from Galliford Try and Yorkshire Water spent a day helping out at the local food bank on St Augustines Road and made a donation of £500 to assist with their ongoing charitable cause.

Project success

The project is considered huge a success for Galliford Try and is the largest single project completed on the Infrastructure Framework for Yorkshire Water within the AMP7 period.

The Southampton Shellfish Scheme comprises the upgrade to four wastewater treatment works and one combined storm overflow (CSO) discharging effluents to the Solent. The Water Industry National Environment Programme (WINEP) No Deterioration Shellfish Drivers (ND_SW) have dual targets: (i) to reduce the number of intermittent storm discharges to 10 significant spills or less per year (as a 10-year average) for the agglomeration of assets impacting Southampton Shellfish Water, and (ii) for the final effluent discharges, achieve a 6.6 log reduction in E. coli between crude sewage and the boundary of Southampton Shellfish Water. Additionally, WINEP U_IMP5 drivers resulting in an increase in flow to full treatment (FFT) were applicable at some of the sites, therefore design and construction for all drivers were combined under the same project.

Site	Pre-U_IMP5 initial calculated volumes (m3)	Post Post U_IMP5 impact to agglomeration (June 2021) (m3)	Final Volumes (m3)
Millbrook WwTW	Yes	Yes	Yes
Slowhill Copse WwTW	Yes	Yes	Yes
Woolston WwTW	Yes	No	No *
Ashlett Creek WwTW	Yes	No	Yes
Ensign Park CSO	Yes	No	No
Table 1: Southampton Shellfish Scheme drivers		* N/A, but FFT is being increased as a mitigation for ND_SW driver

Southampton Shellfish Scheme sites

The technical solutions considered during design included increasing the storm tank capacity and the hydraulic capacity of the wastewater treatment works (WwTW) to reduce the intermittent storm discharges, with UV irradiation or membranes being considered to achieve the disinfection requirements.

[caption id="attachment_14597" align="alignnone" width="1200"] UV irradiation system at Slowhill Copse WwTW (December 2023) - Courtesy of Southern Water[/caption]

Details of the five sites are as follows:

1. Millbrook WwTW treats for a population equivalent (PE) of 147,000 with biological treatment provided using a 4-stage Bardenpho Activated Sludge Process (ASP).
2. Slowhill Copse WwTW treats for a PE of 77,000 PE and provides biological treatment via a Modified Ludzack-Ettinger (MLE) configured ASP.
3. Woolston WwTW treats for a PE of 70,000 and has an MLE configured membrane bio-reactor (MBR).
4. Ashlett Creek WwTW treats for a PE of 15,000 and provides biological treatment via a sequential batch reactor (SBR).
5. Ensign Park is a combined sewer overflow (CSO).

Modelling & optimisation of storm volumes

The project drivers required a reduction of spill frequency at storm overflows for all the catchments that discharge into Southampton Shellfish Water. This impacted five separate wastewater treatment works catchments.

Early in the design, the Southern Water (SW) project team identified an efficiency opportunity of combining the FFT U_IMP5 (increase in flow to full treatment) Driver with the Shellfish Driver, which had a significant impact on reducing the volume of the storm storage tanks. SW held regular meetings and agreed the approach with the Environment Agency (EA) including alignment of Regulatory output dates to the end of the AMP period.

The assessment of storm storage is based on the agglomeration approach outlined in the EA PR19 shellfish guidance and methodology. The definition of a significant spill was agreed with the EA at the conceptual stage of the scheme. This assessment treated all the discharges as a single agglomeration.

[caption id="attachment_14601" align="alignnone" width="1200"] UV irradiation system at Millbrook (May 2023) - Courtesy of Southern Water[/caption]

In summary, the integrated combined driver approach provided the following benefits:

• Avoided abortive investment:
  • Storm tanks for ND_SW being oversized (increased FFT from U_IMP5 reducing storm flows).
  • UV irradiation system and associated feed pumping for ND_SW being undersized (increased FFT from U_IMP5).
• Reduced carbon impact:
  • By avoiding assets no longer required.
  • By avoiding an inefficient 2-stage construction approach.
• Avoided potential abortive third party land acquisition and development.
• Reduced construction, traffic movements and the impact to the local community, particularly within the urban areas.
• Avoided unnecessary impact to the environment and ecology.
• Avoided potential abortive works for 3rd party infrastructure (for example, power upgrades).

To develop the solutions required at each wastewater treatment works, the sewer network performance for the five catchments was designed using a 10-year rainfall dataset. The observed spill frequencies across the catchment had to be agglomerated to ensure that 10 significant spills or less per year was achieved over a long-term (10 year) average.

This meant the modelling was more complex and took a lot longer than a usual model build and verification exercise because of the need to assess all five catchments together.

Following subject matter guidance from across the industry, the project team used an innovative approach of using an extended rainfall series duration and a single rain gauge. In addition, the team targeted the storage volumes at sites where there was available capacity to arrive at the most efficient storage volumes that were significantly lower than the original estimates, all while complying with good industry practice. In summary, this included:

1. Extending the rainfall series for a longer period than 10 years to be more representative of long-term weather patterns, addressing extremely ‘wet’ periods within the original 10-year series.
2. Using a single rainfall profile as this coastal rain gauge is more representative of the rainfall falling on the main urbanised sections of the catchments.
3. An extended rainfall series to help address the annual frequency, extremeness and year-on-year variability of rainfall patterns and particularly wet or dry periods.

[caption id="attachment_14602" align="alignnone" width="1200"] Storm Shaft in Ashlett Creek (June 2024) - Courtesy of Southern Water[/caption]

Optimising storm storage volumes

The project team considered the best location for increasing the capacity of storm storage considering the cost/benefit for reducing significant storm spills. For example, at Millbrook WwTW redundant storm tanks were utilised that provided a volume of 3,140m³. The increase in volume at Millbrook enabled the storage volumes to be optimised across the agglomeration of sites with volumes being offset against other more constrained sites.

Southampton Shellfish Scheme: Supply chain - key participants

• Client: Southern Water
• Delivery partners & outline/detailed design: GTB
  • Galliford Try
  • Binnies
• UV design & coastal & network modelling: Stantec UK
• Coastal & network modelling: Southern Water
• Civils & pipework: Knights Brown
• Civils & pipework: Impact Formwork
• Storm shafts & tunnelling: HB Tunnelling
• Piling works (Woolston WwTW): Rock & Alluvium
• Piling works (Woolston WwTW): Keller Foundations
• Storm tank remedials: Concrete Repairs Ltd
• Mechanical installation: Bridges Electrical Engineers Ltd
• Mechanical installation: Steelway
• Electrical installation: Field Systems Design
• Electrical installation: Loxton Electrical
• ICA: Total Automation & Power (TAP)
• MCCs: MCS Control Systems
• GRP kiosks: Industrial GRP Ltd
• Penstocks: Ham Baker
• UV: Trojan Technologies
• Membranes: Koch (KMS)

[caption id="attachment_14599" align="alignnone" width="1200"] Refurbishment of redundant storm tanks at Millbrook to provide 3,400m3 capacity - Courtesy of Southern Water[/caption]

Infiltration - optioneering

1. To optimise the significant storage volumes at the works, sustainable catchment opportunities were considered for the large, urbanised Millbrook and Woolston catchments by the removal of specific areas of groundwater infiltration and surface water from foul sewers.
2. The optioneering was undertaken using an optimisation software package to target impermeable areas (roads and roofs) connected to the network and working through simulations (up to 20,000) to assess parts of the catchment where connected roads and roofs could be removed from the foul sewer.
3. The benefit was not as expected when considering the agglomerations due to spills from the other assets. In addition, the timescale to achieve long-term reduction from impermeable area removed or groundwater infiltration would require a multiple AMP period.
4. Future opportunities using the optimisation techniques will be considered for similar CSO and storm spill frequency schemes to achieve sustainable catchment opportunities, by the removal of specific areas of groundwater infiltration and surface water from foul sewers.

Woolston WwTW - increasing FFT

The initial significant storm volumes at Woolston were extremely challenging given the location of the site within an urban environment and lack of available space. The project team identified an opportunity of increasing the FFT at Woolston, which provided a significant reduction in storage. The scope involved utilising hydraulic headroom within the works, and through the installation of additional MBR capacity and associated pump upgrades, enabled a 20% increase in FFT at Woolston WwTW (427 l/s to 534 l/s).

[caption id="attachment_14603" align="alignnone" width="1200"] Construction of the shaft at Woolston - Courtesy of Southern Water[/caption]

The programme to deliver the increase in FFT at Woolston was short and provided an early reduction in E. coli concentration discharging to the Solent. This early delivery reduced 20% of the total E. coli concentration across the agglomeration of sites and was fundamental in agreeing new regulatory dates for the SW_ND (storm storage) drivers. The combination of the integrated driver approach to reducing spills, the innovative modelling and optimisation of assets reduced storage volumes by circa 46,000m³.

The final volumes are as shown in Table 2:

Site	Initial calculated volumes (m3) (pre-U_IMP5)	Post Post U_IMP5 impact to agglomeration (June 2021) (m3)	Final Volumes (m3)
Millbrook WwTW	3,684	200	3,400
Slowhill Copse WwTW	13,678	2,000	2,000
Ashlett Creek WwTW	4,176	1,650	1,650
Woolston WwTW	27,422	13,120	5,000
Ensign Park CSO	3,806	280	280
Blechynden Terrace CSO	5,000	0 *	0 *
Newtown CSO	562	0 *	0 *
Table 2: Southampton Shellfish Scheme storm storage volumes		* Reduction driven by reduced spills in the agglomeration

New final effluent disinfection at Slowhill & Millbrook WwTWs (AMP7 & AMP8):

In response to a number of drivers, the EA has recently revised its approach to design and permitting of UV irradiation systems for the disinfection of final effluent. The aim is to move away from the use of generic assumptions, to follow international best practise with an approach which employs the use of a site-specific data in conjunction with performance validated UV irradiation equipment. This revised approach enables development of a significantly better understanding of performance and limitations on a site-specific basis.

Understanding the variation and impact of key wastewater characteristics on parameters such as UV transmittance and the UV dose-response relationship enables selection of a representative design envelope and minimum target UV dose. This enables the capital solution to be optimised, minimises power usage, operational costs and carbon impact while securing environmental performance.

[caption id="attachment_14609" align="alignnone" width="1200"] UV irradiation system at Millbrook (June 2024) post commissioning/FST under construction and (inset) UV irradiation system at Slowhill Copse WwTW - Courtesy of Southern Water[/caption]

The steps undertaken for the development of the design parameters and permit conditions for the UV irradiation systems at Slowhill and Millbrook WwTWs can be summarised as follows:

1. Coastal/estuary modelling was used to determine the dilution/dispersion factor available from the effluent discharge location to the Shellfish Water boundary. The boundary is due to extend closer to the discharge points in AMP8 (i.e. reducing the available dilution of the final effluent discharges), therefore disinfection targets were evaluated for both AMP7 and AMP8 during design. This enabled the overall system (channels, MCC and power requirements) to be designed with the ability to install the equipment required in AMP7 and accommodate further banks to meet the lower E. coli targets required to avoid abortive and/or extensive additional construction in AMP8.
2. A programme of sampling and analysis was carried out (from April 2020 to May 2021) to collect data on the variation of microbial and sanitary characteristics of the final effluent. The results were compared to the parameters available from historical site data in order to ensure that the sampling was representative of the expected range. Variation in UV transmittance was recorded using online instruments for assessment with flow data to understand their relationship. The UV dose-response relationship for E. coli for the final effluent was developed using collimated beam testwork to determine the target UV dose required to achieve the disinfection targets for permitting for both AMP7 and AMP8.
3. The data was used to develop a range of design parameters that were then used in conjunction with UV suppliers’ performance validation information to determine a range of UV irradiation system configurations. The options were evaluated considering the different levels of resilience, redundancy and risk of non-compliance with the EA’s proposed new permit conditions.

The new UV irradiation systems have been in operation at both sites since Autumn 2023 and are consistently achieving concentrations of E. coli in the final effluent discharges that will maintain the concentration at the Shellfish Water Boundary below the post PR19 target value of 5 cfu/100ml as a geomean.

[caption id="attachment_14600" align="alignnone" width="1200"] (left) Millbrook UV pumping station and (right) MBR cassette capacity being increased in Woolston WwTW - Courtesy of Southern Water[/caption]

Woolston WwTW - working with care

Due to the limited availability of operational land in Woolston, the new storm shaft is being constructed beneath a car park and green area own by Southampton City Council (SCC), the area also had a Multi-Use Games Area (MUGA). The project team and the representatives of the Council engaged in extensive discussions prior to submission of the planning application which ensured a positive outcome for both parties.

From those conversations the project team learnt that the Council had worked to create a low nutrient coastal grass environment in the zone affected by the project. The topsoil had been amended and sown with the current species in previous seasons. Therefore, the agreed approach was to retain the topsoil stored on site and reinstate it at the end of the project, enabling the green area to grow and cover the old disused carpark.

The first activity after site mobilisation was relocating the reptiles that naturally inhabit coastal grass biomes. All areas with long grass that were part of the construction compound had the grass cut in two passes to encourage reptiles to move to nearby unaffected zones. Additionally, the top 100mm of soil was hand excavated with an ecologist carefully moving those reptiles still on site.

SW, working closely with the local Council, has ensured that the pre-existing facilities not only are kept despite the new structures being built, but that they are also improved. The number of parking bays and EV charging points will be maintained with improved access arrangements to avoid unwanted camping vans. The MUGA will be improved so the excessive slope that the old court had is removed and the pedestrian access to an adjacent park is protected with bollards, as previously vans used to park using part of the footpath.

[caption id="attachment_14596" align="alignnone" width="1200"] Landscaping over Woolston storm shaft - Courtesy of Southern Water[/caption]

Progress to date, successes and future phases

The storm tank at Millbrook WwTW and the increase in flow to treatment at Woolston WwTW were both commissioned in July 2022 while the projects to install UV irradiation at Slowhill and Millbrook WwTWs were commissioned in Autumn 2023. In combination, these improvements represent a reduction of 72,500 cfu/s of E. coli being discharged to the Southampton Shellfish Waters, which is more than 60% of the total E. coli concentration reduction when all the projects are completed.

Results from sampling carried out after commissioning have demonstrated that both UV irradiation systems at Slowhill and Millbrook have reliably achieved the target efficacy, with concentrations of E. coli in the effluent post disinfection regularly below 200 cfu/100ml. Additionally, concentrations of E. coli in the final effluent discharge from Woolston have been consistently below 35 cfu/100ml since July 2019, following commissioning of the MBR.

The next key milestones are 31 December 2024, when the new storm tank at Ashlett Creek WwTW will be commissioned, and 31 March 2025, when all the remaining storage and increase in flow to treatment drivers will be completed.

Located near Maerdy in South Wales, Lluest Wen Reservoir is a Category A Large Raised Reservoir as defined by the Reservoirs Act 1975. The reservoir is impounded by a 200m long, 25m high earth embankment dam across the Rhondda Fach River and currently impounds 1.07Mm³ of water. Construction of the reservoir was originally completed in 1898. Lluest Wen Reservoir is owned and operated by Welsh Water and has historically been used with a smaller, downstream reservoir, Castell Nos for water supply.

Investigation and scope

Following an inspection of the reservoir in February 2018 under Section 10 (3) of the Reservoirs Act 1975, the Inspecting Engineer made a series of recommendations for measures to be taken in the interests of safety (MITIOS). These recommendations included that flows through the spillway chute be modelled under the 10,000 year and probable maximum flood (PMF) conditions, that the performance of the chute arrangement be evaluated in safely conveying the associated flows to aid scoping remediation works and to action any works as necessary to preserve dam safety. The improvement works were statutory and enforceable by the regulating authority Natural Resources Wales. Mott MacDonald Bentley (MMB) was commissioned to undertake the design and construction of the works under Welsh Water’s Capital Delivery Alliance framework. [caption id="" align="alignnone" width="1200"] (left) Base concrete pour and (right) the upper spillway - Courtesy of MMB[/caption]

Lluest Wen Reservoir Spillway Remedial Works: Supply chain - key participants

• Principal designer & principal contractor: MMB
• Flow modelling: Mott MacDonald
• Ground investigation: Strata Geotechnics
• Geophysical surveys: Terradat
• FRC works: Concrete Structures & Floors Ltd
• Metalwork: Industrial Pipework Services Ltd
• Diving works: Edwards Diving Services Ltd
• Manholes: Marshalls CPM
• Drainage pipework: Polypipe Civils and Green Urbanisation
• Concrete cutting & drilling: Holemasters
• Concrete supply: Hanson UK
• Scaffolding: Mii Engineering
• Site buildings: Wernick Hire

Flow modelling and spillway design

Prior to the scheme commencing, the existing flood study was reviewed and updated to reflect current industry guidance. A computational fluid dynamic (CFD) model was developed to simulate conveyance of the PMF and 10,000 year conditions. The PMF scenario anticipated flows approaching 100m³/s, with peak velocities approaching 20m/s discharging through the spillway. Outputs from the model indicated that the existing spillway was unable to contain flows arising during each flood scenario therefore representing a risk to dam safety through erosion of the earth fill embankment. An Options Report was prepared to identify possible solutions to remedy the shortfall in spillway capacity. The report considered the following options for remediation:

• Construct a new spillway off-line along a straight alignment.
• Rebuilding the spillway along the existing alignment.

[caption id="" align="alignnone" width="1200"] Armourstone and lower spillway - Courtesy of MMB[/caption]

Preferred option

The preferred option was to re-build the spillway along its existing alignment. This would involve re-lining the upper section of the spillway which had previously been re-built in concrete during the 1960s. Lining would take the form of a 500mm thick reinforced concrete base connected to the underlying structure via a system of dowels. A similar approach would be taken for the downstream section of the spillway following demolition and removal of the historic masonry structure. Through this section, the new spillway base was doweled to a layer of mass concrete provided for global stability. New spillway walls were constructed on the hydraulic left and right hand sides capable of containing the PMF flows having utilised the CFD model to rationalise the wall heights as far as practicable. Additionally, a central training wall was provided through upper and lower sections of the spillway to evenly distribute flows across the width of the spillway and reduce water superelevation at the two changes in direction. The CFD model was also used to extract hydrodynamic pressures which allowed optimisation of the reinforcement within the walls and bases. Global stability checks for bearing, sliding, overturning and seismic were also undertaken to verify the suitability of the design. The new spillway construction would be approximately 200m in length and 15m at its widest point. Additionally, new drainage networks would be provided to the back of walls to control groundwater levels and underneath the spillway base to protect the formation and prevent undermining of the spillway base. At the downstream end, armour stone was provided for scour protection.

Temporary works

A 90-tonne crawler crane was used to provide full coverage of the spillway footprint. In order to access the embankment side of the spillway, it was necessary to construct a number of culverts, embankment and roadway at the downstream end of the spillway to cross the receiving watercourse. The upstream face of the bridge was concreted to prevent erosion and migration of fill into the downstream watercourse. [caption id="" align="alignnone" width="1200"] Finished spillway - Courtesy of MMB[/caption]

Design considerations

The gradient at the lower section of spillway approaches a fall of 1 in 2 which presented significant challenges both in terms of pouring and finishing the concrete bases as well as the flow velocities generated within the spillway. Base pours lengths were reduced to 3m and the bays were installed in a hit and miss sequence through this section in order to prevent slumping of the concrete prior to curing. High flow velocities necessitated that the joints be protected with steel plates to prevent scouring of the joint sealant and significant hydrodynamic pressures developing on the underside of the downstream base. Particular care had to be taken to ensure the absence of abrupt deviations across the joints which could invoke cavitation and ultimately lead to concrete degradation. The water level in the reservoir is known for rising quickly during periods of extended or heavy rainfall due to its location. A temporary drawdown of 3m below top water level was initially agreed with Welsh Water and a robust management plan was implemented to prevent the spillway from operating during the construction phase.

Completion

The scheme was mobilised to site in October 2020 beginning with improvements to the access track to the reservoir. Concrete works were scheduled to commence in late March 2021. Following a dry summer, concrete works were completed by January 2022 with landscaping and other finishing works completed by June 2022 whereupon the Qualified Civil Engineer issued the 10(6) certificate ahead of the required October 2022 statutory deadline.