Supply Chain - Stormwater Storage -

Campbeltown has experienced flooding over many years, which has affected residential properties, businesses, roads, and community life in general. In June 2022, construction started on a 24-month, £16m scheme, (co-funded 80% by the Scottish Government and 20% by Argyll and Bute Council) under the Flood Risk Management (Scotland) Act 2009 where the council is responsible for developing sustainable flood management solutions. DAWSON-WAM Ltd was appointed as principal contractor, with AECOM employed by the council to provide design and project management support throughout the contract. Flooding sources

The existing sewer network system in the Burnside and Meadows areas could not cope with increasing amounts of surface water. The sewer network combined surface water with sewage water from households. When the system was overwhelmed during periods of heavy rain, fouled water backed up out of the system and flooded streets and premises.

On the outskirts of the town is the Millknowe Burn which, due to several steep catchments, is the most dominant flood source in Campbeltown. Flows from the Balgreggan Burn also caused overtopping during extreme events at the Dalaruan culvert intake and resulted in overland flows and flooding in the town.

[caption id="attachment_18234" align="alignnone" width="1200"] Google Earth image showing Campbeltown FPS site locations[/caption] Scope of works

The scope of the scheme was to reduce the risk of flooding from the Millknowe and Balgreggan Burns and from localised surface water in Campbeltown. The project was rolled out over four working areas:

Millknowe flood storage reservoir. Dalaruan culvert. The Meadown SuDS. Burnside Square. Millknowe Flood Storage Reservoir

Millknowe Field is located beside the watercourse channel, which acts as a catchment area for rainwater. It floods onto nearby roads and streets; particularly the main A83 trunk road which provides the primary access into the town. There are two block of flats beside the field, built essentially on the same floodplain that is prone to flooding during heavy rainfall when the watercourse and culvert cannot cope with the flow of water.

Snipefield Industrial Estate is located on the northern side of the watercourse channel, which is home to many local businesses. This was also prone to flooding from the field during heavy rainfall events.

The scope of works at Millknowe included:

Construction of a flood storage reservoir in Millknowe field to provide temporary flood water storage within the watercourse channel during high rainfall events. Realignment of the watercourse channel along a new sheet piled wall. Installation of over 500m of new back of wall drainage ranging in diameter from 225mm to 600mm and new outfalls with flap valves through the new flood wall. [caption id="attachment_18237" align="alignnone" width="1200"] Installation of AZ sheet piled wall with TM22 piling rig to form reservoir - Courtesy of DAWSON-WAM Ltd[/caption]

Flood storage reservoir

The reservoir was constructed in line with the Reservoir (Scotland) Act 2011 and comprised a 125m long AZ19-700 sheet piled wall encased in a 2.6m (average height) earth embankment dam and concrete spillway. A 325m AZ20-700 sheet piled flood wall formed the enclosed dam structure and provided flood protection to Snipefield Industrial Estate. The sheet piles were driven into cohesive soils and ranged between 6m to 9.5m in length. A new concrete culvert passing through the dam with headwall and orifice plate was constructed to throttle the flows. The exposed piles were capped off with a precast concrete coping.

To facilitate the installation of the 450m of steel sheet pile wall, a 6m wide x 600mm deep stone temporary piling platform was installed to support the 90T ABI TM22 piling rig.

Where space allowed, the piling platform was constructed in Snipefield Industrial Estate to the north of the watercourse. This minimised the length of the watercourse that was required to be culverted and reduced the impact on agricultural soils and the watercourse, and reduced the amount of imported stone required.

DAWSON WAM’s in-house heavy-duty steel mats were utilised which further reduced the amount of imported material for the piling platform. Three temporary bridge culvert crossings were constructed using the mats which allowed full access along the length of the wall during construction.

[caption id="attachment_18243" align="alignnone" width="1200"] Temporary bridge crossing watercourse using DAWSON-WAM Ltd’s heavy duty steel mats - Courtesy of DAWSON-WAM Ltd[/caption]

The sheet piles were purchased from Arcelor Mittal in Luxembourg, and were stored in DAWSON-WAM Ltd’s yard in Bedfordshire. The piles clutches were sealed using Biguma SV and were delivered to site in a just-in-time basis throughout the project.

DAWSON-WAM Ltd used an ABI TM22 piling rig to complete the piling installation works. This rig with its powerful 36VV vibro hammer  is the most powerful leader rig mounted vibro in the UK and Europe. Interchangeable attachments were mobilised, including the preaugering with advanced cased CFA and the HydroPress system, which allowed increased production and mitigated risks where unforeseen ground conditions were encountered.

Service crossings

The new sheet piled wall installation required to perpendicularly cross a 225mm asbestos cement Scottish watermain and a high voltage (HV) Scottish Power cable. These two services were critical to the town infrastructure and any disruption or shutdowns would have caused a extensive disruption to the town’s residents.

Traditionally these services would have to be diverted through the wall in a phased approach, resulting in inefficiencies and several shutdowns on the services. Through DAWSON-WAM Ltd’s in-house expertise and utilising its specialist vibrationless HydroPress piling equipment, installation of the piles adjacent to the utilities was successfully achieved without diverting the services. This involved early liaison and working closely with the designer and utility providers, undertaking trial pits to ascertain the ground conditions, installation of vibration monitors on the services to highlight vibration levels during install and agreeing a short shut down of the services during pile install.

Earth embankment

The earth embankment at Millknowe consisted of an up to 4m high 125m long embankment with a 21m in situ reinforced concrete spillway. The embankment footprint was excavated removing 3000t of topsoil, 6000T of peat and associated soft soils to form the earth embankment foundation on alluvial or raised beach deposits with a Cu value greater than 40Kpa.

To consolidate the formation level prior to construction, trial pits were excavated along the footprint of the structure and with the aid of a shear vein test, the employer-appointed Reservoir Construction Engineer witnessed and approved the formation levels on site.

2,600t of locally sourced Class 6C compacted in accordance with CESWI 7th Edition in a 500mm deep layer formed the foundation of the embankment. The main body of the embankment was constructed using 5,800t of compacted Class 1A granular fill, with the maximum embankment height of 4m with sloped of 1V:3H. Class 6N granular material was used to segregate the Class 6C and Class 1A material around the piles.

The embankment was capped with a 300mm layer of site won screened topsoil and reinforced with coir matting and seeded. Associate embankment toe drainage and a hardstand access road was provided for future maintenance access.

[caption id="attachment_18233" align="alignnone" width="1200"] Realigned burn along sheet piled wall reinforced with coir matting - Courtesy of DAWSON-WAM Ltd[/caption] Dalaruan culvert

270m of the existing Dalaruan storm water culvert required upgrading from the Millknowe flood storage reservoir intake to downstream of Dalaruan Street intake. This online upgrade would convey flows and provide additional inline storage from the Millknowe reservoir and prevent overtopping flows from the Balgreggan Burn during extreme rain events.

The exemplary design proposed the install of concrete culverts however as Dalaruan Street is straddled by terraced houses with listed boundary walls and with a congested mix of utilities of various eras, it was decided from an early stage in the contract a detailed survey be undertaken including trial digs to ascertain existing utility positions. This work proved prudent as a design change was requested to provide a more buildable solution while managing live flows and to avoid extensive service diversions.

The contractor and designer worked collaboratively, and through engaging with the supply chain and by re-modelling validation, the section of culvert was changed to a twin line using AquaSpira pipes.  This solution provided a bespoke steel reinforced twin piped culvert which had both health and safety and constructability benefits and prevented extensive costly service diversions.

In total, 110m of twin 900mm and 45m of twin 675mm pipes were installed including 115m of single 675mm pipe to form the new culvert. AquaSpira’s prefabricated manhole units were used, which proved to be considerably easier to install compared to the traditional concrete manhole rings, and benefited the management of the existing live storm flows, as well as a faster build time.

During the 6-month construction phase of the Dalaruan culverts, as the majority of the works required the closure of the Dalaruan Road, an extensive public relations exercise was undertaken prior to works commencing. Following the extensive liaison, a phased programme of work plan was developed which merged the phasing of the works and the concerns raised from the public events.

This plan was then shared prior to construction commencing and was supplemented with a full-time public liaison officer, this resulted in the successful installation of the replacement culvert in time to take the first storm flows.

[caption id="attachment_18240" align="alignnone" width="1200"] (left) Installation of twin 675mm diameter AquaSpira pipes and (right) installation of prefabricated manholes - Courtesy of DAWSON-WAM Ltd[/caption] The Meadown SuDS

The Meadows catchment area consisted of an existing, out of date, undersized combined sewer network system that during periods of heavy rainfall events caused out of sewer flooding to occur. This, combined with the existing culverts in the Witchburn and Meadows areas that could not convey sufficient volume of stormwater,  resulted in regular localised flooding events.

The solution was to provide 6500m2 of storm separation on the existing road network by installing over 1.2km of new storm pipe ranging in diameter from 150mm to 375mm and connecting over 65 existing storm gullies to the new infrastructure. This new pipe network conveyed storm flows to three new detention basins with a total capacity of 1500m3 to temporarily attenuate surface water runoff before releasing it back into the existing Witchburn Culvert system. The shallow detention basins were constructed in existing green spaces with 1:4 slopes with a 3D cellular confinement system installed on the basin slopes to improve soil stabilisation and control erosion.

Flows from the basins are retained by flow control manholes fitted with Storm Brake valves, which limit storm flows to between 4.5 l/s and 7.7 l/s into the culverts in a more forgiving manner.

Biodiversity within the detentions basins was enhanced by planting a wet meadows mix plant and a combination of birch and willow trees surrounding them. Due to the poor quality of the existing road surfacing, new full width reinstatement was undertaken to all roads that acquired new storm drainage significantly enhancing the whole road network.

[caption id="attachment_18244" align="alignnone" width="1200"] Reinstated detention basis at Meadows - Courtesy of DAWSON-WAM Ltd[/caption] Burnside Square

A new underground geocellular attenuation tank in Burnside Square was constructed to temporarily store and attenuate 540m3 of surface water runoff from 1.6km newly constructed storm separation pipework in the Witchburn and Burnside areas during high rainfall events before releasing it back into the existing culvert network.

As Burnside Square was at the heart of the town, it posed several construction risks none less than excavating near three-storey properties that dated back to the 1700s with stone foundations, while having to maintain safe access to the business and residential premises contained within.

Due to the level of disruption anticipated in this area, the contract had restricted these works to be completed during the winter months and required it to be re-opened for public use by June 2023 in time for the peak tourist season.

Following the completion of a detailed ground investigation, it identified that there was largely granular materials withing the dig area for the new attenuation structure, with a water strike just at dig level. Due to the above mentioned buildability concerns, a battered excavation solution posed many risks, therefore a secant piled box cofferdam solution was opted for which would act as both permanent and temporary works for the tank.

DAWSON-WAM Ltd utilised its in-house cased CFA piling rig to install the 300 (No.) 450mm diameter secant piled cofferdam, with its vibrationless installation method eliminating the risk of vibration on the surrounding properties, with this solution the space around the perimeter of the site was able to be maximized, resulting in sufficient space for pedestrians to access the premises safely during the construction period.

[caption id="attachment_18238" align="alignnone" width="1200"] (left) Secant pile installation using cased CFA at Burnside Square, and (right) Geocell attenuation tank at Burnside Square prior to backfilling - Courtesy of DAWSON-WAM Ltd[/caption]

Upon the excavation of the 2700T of granular material, the area was bedded with single-sized rounded stone which provided the base for Polypipe’s lightweight Polystorm-R crates. The crates were delivered in articulated lorries and were built in situ and wrapped in a welded impermeable liner and geotextile protection fleece.

Inspection chambers and a manifold pipe arrangement was formed to enable the filling, emptying, and maintenance of the tank. To prevent overloading of the tank in the temporary condition, the tank was backfilled with imported aggregate in a sequence using small plant, a 100mm layer of Bitmac then formed the foundation for the 900m2 of new block paving.

As the existing square was predominately used for car parking, the council grasped the opportunity to reinstate the area into a multi-functional space that could be utilised for community events as well as serving as daily parking.As part of the final reinstatement, over 52 new bollards and an array of new street furniture and planters were installed.

Supply chain: key participants Main contractor: DAWSON-WAM Ltd Project management & client design: AECOM Materials testing: MATtest Limited Culvert pipes/chambers & twin pipe culvert (Dalaruan Street): Aquaspira Ltd Sheet piles: Arcelor Mittal Attenuation tank installation: Hydro Water Management Solutions Ltd Precast concrete: FP McCann Civil contractors: McFadyen’s Contracts Campbeltown Landscaping: MW Groundworks Attenuation tank drainage crates: Polypipe Civils & Green Urbanisation Drainage & ducting materials: Huws Gray | Jewson Transformers: Wilson Power Solutions Ltd [caption id="attachment_18241" align="alignnone" width="1200"] Storm separation pipework installation at Witchburn - Courtesy of DAWSON-WAM Ltd[/caption] Community benefits

It was obvious from inception that, engineering challenges aside, one of the key challenges for the project team would be constructing the works within this rural town in Scotland which would not have previously witnessed a construction project on this scale.

Effective consultation with stakeholders, which included road users, local businesses, residents, the general public and affected landowners, would be key to the minimising disruption and to the project’s successful delivery.

Several pre-construction pop-up events were held locally with the residents, businesses and stakeholders. Consultations were carried out through Argyll and Bute Council’s internal communications team and monthly newsletters were issued through the portal giving updates on construction progress and programme of works and queries were managed through the site-specific e-mail and emergency contact number. Public drop-in meetings and letter drops were undertaken in advance of entering a new area of works.

A project of this size and complexity will always have a significant local impact and it was important that the project team endeavoured to engage with and, for the duration of the project, be part of the community.

[caption id="attachment_18235" align="alignnone" width="1200"] Realigned burn along sheet piled wall reinforced with coir matting - Courtesy of DAWSON-WAM Ltd[/caption]

Initiatives to promote community engagement included:

Visits from the construction engineering team to Dalintober and Drumlemble primary school, educating the children on the dangers of construction sites. Donations of PPE waterproofs to the school to help support their learning and exploration group. Donation of a ‘LEGO education spike prime Set’ to help engage STEM learning and problem-solving skills. Donation of Ukuleles to the local kids’ music group. Local charity donations including a donation to the Kintyre Sea Sports charity to purchase new equipment to aid in the training and development of the local disadvantaged children in water activities. Donation of 6T of topsoil to Campbeltown Climate Action for use in their community garden. Voluntary repairs to the Campbeltown youth football team goal posts and changing facilities access paving prior to a kid’s football tournament. Supply and installation of a bicycle rack in Burnside Square.

The most significant community benefit was the employment of a local sub-contractor to undertake much of the construction works. This provided and supported employment to the residents of Campbeltown and supported the local businesses as the money stayed in the community.

[caption id="attachment_18236" align="alignnone" width="1200"] Reinstated Burnside Square - Courtesy of DAWSON-WAM Ltd[/caption] Conclusion

In Autumn 2023 during two periods of heavy rainfall, the Millknowe Reservoir, the Meadows detention basins and the Burnside attenuation tank protected the town from flooding and reduced the load onto the wastewater network.

The successful delivery of the Campbeltown Flood Scheme was a brilliant example of collaboration between the contractor, client, designers, and the local community; ultimately resulting in the success of the project as well as providing an asset that will efficiently protect the town and its residents for years to come.

Section 101a of the Water Industry Act requires that sewerage providers deliver and fund first time sewerage (FTS) schemes where the current drainage for two or more properties is unsatisfactory and where a public solution is viable. Unsatisfactory drainage includes situations where environmental or amenity problems caused by uncontrolled discharges from septic tanks or cess pits result in pollution of ditches or watercourses, odours and public health problems. The FTS process starts when an application is received by the sewerage provider from residents requesting a connection to the public sewer system. On receipt of the application, engineers will assess the viability of providing a new public sewer system in the area - including environmental, technical and financial considerations. The assessment will cover the polluting properties, but may be extended to others on septic tanks in the immediate area if considered beneficial. If this assessment indicates the scheme is viable, then it will proceed to design and onto construction. Reasons for an FTS scheme in Sherston In Sherston, there were a number of properties on septic tanks, including several on a shared septic tank that could only be accessed via a field. This field was only accessible once a year, after crops were harvested, which resulted in the septic tank overflowing raw sewage into an adjacent ditch. This represented a significant risk of groundwater pollution; particularly as the area is within a groundwater source protection zone. [caption id="" align="alignnone" width="1200"] (left) Schematic showing the route of the new gravity sewer and new rising main and location of the new SPS and (right), illustration from the Wessex Water leaflet to property owners showing responsibility split - Courtesy of Wessex Water[/caption] Residents in Sherston applied for an FTS scheme due to this pollution, which led to an assessment by Wessex Water which concluded that a public FTS was viable and could be progressed to design and construction. Wessex Water carried this out using their in-house capabilities at a cost of £2m. Although only a few properties were causing environmental and public health issues, Wessex Water included as many properties as possible within the FTS scheme, taking into account the viability of doing so. As a result, 24 properties (estimated population equivalent of 60) currently on septic tanks will be able to connect to the new public sewerage system by July 2023. Wessex Water’s FTS policy is to construct the new public sewerage system up to the inside the property boundary, where it terminates within a demarcation chamber. This chamber denotes the boundary between the public and private drainage system. Design options Due to the levels of the existing sewers relative to the septic tank outlet pipework, a sewage pumping station (SPS) would be required as part of the solution to connect the new sewerage system to the existing network. [caption id="" align="alignnone" width="1200"] Penstock ready to be installed over the outgoing pipe (on the left) within a manhole chamber upstream of the wet well - Courtesy of Wessex Water[/caption] Several iterations of the design were undertaken to confirm the best route of the pumped rising main, location of the proposed new SPS and access road, as outlined below: Connection to the existing sewerage network in Easton Town (B4040) - Option discounted This solution provided the shortest route for the proposed new rising main and access track to service the new SPS. However, the rising main route would need to pass through a narrow driveway between existing houses. Due to the depth of the excavation, an open cut construction method was discounted as it would be within the zone of influence of the building foundations and consequently had the risk of undermining and causing structural damage to these buildings. Trial holes indicated shallow rock which would have further exacerbated structural damage due to vibration when breaking out the rock. Trenchless techniques were considered, however due to the rock, only micro-tunnelling would be suitable, and would require a reception pit within Easton Town road, which would cause significant disruption to the community. Micro tunnelling would have also resulted in the scheme becoming financially unviable. Connection to the existing sewerage network in Sandpits Lane (access via Sandpits Lane) - Option discounted This solution provided the next shortest route for the access track to service the new SPS, and although the rising main route was extended, it allowed for an additional gravity sewer to be laid within the same trench to connect four additional properties at minimal cost, which increased the cost viability of the scheme. Prior to submitting a planning application for this option, Wessex Water consulted with the community. Significant public objections were raised due to the visual impact of the proposed new access track and SPS, particularly from those who were already on mains drainage and would not benefit from the FTS scheme. In addition, land purchase negotiations for the access track could not be resolved. Connection to the existing sewerage network in Sandpits Lane (access via Tetbury Road) – Selected Option The sewerage network for this option was similar to Option 2, however the access track route and SPS location changed to take into account the points raised from those objecting on the grounds of visual impact and land purchase issues. This option was confirmed to be both technically and financially viable and no public objections were raised, which was valuable in securing planning permissions. Scope of works The selected solution comprised: 1073m of gravity sewer. 257m of pumped rising main. A 280m long access track with post and rail fencing. A sewage pumping station to include a below-ground wet well (2.1m diameter, 5m deep), control kiosk, dosing unit (to prevent odour formation) and concrete turning area for maintenance vehicles. [caption id="" align="alignnone" width="1200"] (left) Excavation of shallow rock for rising main and (right) internal view of the wet well before sump benching - Courtesy of Wessex Water[/caption] Sherston FTS: Supply chain - key participants Designer & principal contractor: Wessex Water Civil sub-contractor: Lynwood Civil Engineering Ltd Electrical sub-contractor: HM Electrical Services 1996 Ltd Pump supplier: Grundfos Electrical kiosk supplier: BW Controls Ltd Dosing unit supplier: WES Ltd Wavin Q-Bic crates: Wavin UK Cell pave: Groundtrax Systems Ltd Wet well cover slab: Marshalls Civils & Drainage PVC pipework: Funke Gruppe DI pipework: Saint Gobain PAM UK Fittings: AVK UK Ltd Construction The SPS was located at the lowest point of the site to allow gravity fed pipework from properties connecting to the FTS scheme. The wet well was 5m deep, and constructed using a ‘bottom-up’ technique due to shallow rock in the area, which was identified during pre-construction geotechnical investigations. This involved excavating to foundation level and building up the shaft in precast 2.1 m diameter concrete rings. The 300mm-thick, 2.85 x 2.6m cover slab was made to order but cast off site. Where possible, valve chambers are constructed as precast sections to save on build time and cost. However, due to the space constraints and the proposed finished ground profile at this site, the valve chamber had to be cast in situ and dowelled into the wet well to reduce its footprint. The valve chamber was constructed relatively deep at 2.7m deep to avoid the need for air valves on the rising main, which would require ongoing future maintenance. [caption id="" align="alignnone" width="1200"] (left) Shuttering around the wet well in preparation for pouring the final layer of concrete surround and installing dowels into the valve chamber and (right) construction of wet well - Courtesy of Wessex Water[/caption] All pipework within the valve chamber was constructed in ductile iron DN80 and the rising main was constructed in MDPE pipe, SDR17, OD 110. The gravity pipework was constructed in PVCu Funke Gruppe pipe (100 mm and 150 mm internal diameter), which was selected to provide due to its light weight and construction durability compared to conventional vitrified clay pipework. Penstocks were required so that incoming sewerage flows can be controlled, to allow isolation of the wet well for cleaning and maintenance. The penstock was designed to be located within the wet well, however concerns were raised by the site team that the proposed opening for the penstock spindle would be difficult to cast in the correct location during the off-site fabrication of the cover slab. To resolve this, the cover slab design was amended to remove the penstock spindle opening, and it was moved to an upstream manhole, where the penstock could be positioned within the clear opening of the manhole cover. An overpumping connection allows for the temporary pumps to be installed and connected to the rising main via a Bauer coupling on the pipework from within the valve chamber. The valve chamber cover was designed to allow for the safe operation of the Bauer coupling by having a split walk-on grid over the coupling. This allowed the operator to place the foot on one of the closed flaps, whilst the other flap is lifted, thereby allowing them to position themselves directly over the Bauer coupling. This avoids the operator having to lean over an opening or handling the tankering hose with one hand. [caption id="" align="alignnone" width="1200"] Valve chamber cover, with split walk-on grid over the Bauer connection - Courtesy of Wessex Water[/caption] Surface water drainage A surface water drainage strategy was required due to the impermeable areas created by the sewage pumping station concrete surfaces and turning area. Pre-construction infiltration tests indicated that the ground had permeability ranging from 2.58 x10-6 to 6.59x10-6 mm/s, which was considered suitable for surface water disposal via infiltration. While it was imperative to keep the turning areas finished in concrete for durability, the straight sections of the access track were able to be constructed of permeable CellPave 50 cellular paving grid system (gravel filled) from Groundtrax Systems Ltd, laid over Type 3 poorly graded stone sub-base to minimise the surface water runoff created by the scheme. Surface water runoff from the concrete turning areas was collected via a french drain and directed into a 150m2 Wavin Q-bic crate system, which is sufficient to store a 1:100 year storm run-off based on the infiltration test results. The crate system was constructed beneath the SPS concrete turning area to optimise land use within the developed area. [caption id="" align="alignnone" width="1200"] (left) Q-bic system construction and (right) concrete slabs being cast over the crates to form the turning area - Courtesy of Wessex Water[/caption] Third party considerations The project is within the Sherston Conservation area as well as the Cotswold Area of Outstanding Natural Beauty, so it was important to provide a solution in the most environmentally sensitive way possible. Wessex Water undertook detailed environmental archaeological surveys in the early design stages to understand third party constraints and ensure these were incorporated within the design and construction process, including: Arboricultural surveys. Ecological surveys, including monitoring of stone debris removal to check for reptiles, and phased vegetation removal to check for hedgehogs. Archaeological surveys, including a geophysical survey ahead of construction and intermittent archaeological monitoring during construction, due to proximity of a Roman villa complex. As a result of the construction of the FTS scheme, permanent removal of the following was required: Three trees. Small area of dense scrub at the SPS location. An area of scrub at the edge of an arable field. [caption id="" align="alignnone" width="1200"] Public drop-in session and (inset) 3D model of the new sewage pumping station - Courtesy of Wessex Water[/caption] All temporarily affected habitats were seeded with a species-rich grassland and wildflower mix, managed to avoid cutting during the late spring and summer to benefit invertebrates and other wildlife. Off-site biodiversity enhancement measures were also undertaken in Sherston Primary School, including planting of additional trees, wildflower meadow, and providing bird nesting pockets to students. Close interaction with residents is a key feature of FTS schemes due to the work within private gardens. All residents were contacted prior to construction via face-to-face meetings and community drop-in session to give them the opportunity to ask any queries direct to the Wessex Water Project Team. A 3D model of the proposed sewage pumping station was created and used during the drop-in session to provide more clarity for the public on the proposed solution. Conclusion The Sherston FTS scheme has been project managed, designed and constructed using Wessex Water’s in-house teams at a cost of £2m. It comprises of a gravity sewerage pipework from properties which drain to the new sewage pumping station, from where it is pumped via a new rising main to the existing sewerage network. This scheme has provided 24 properties on septic tanks with a connection to the public sewerage network, which has been enthusiastically received by residents. Environmental damage from raw sewerage overflowing from septic tanks into adjacent ditches and groundwater has also been prevented as a result of this scheme. Work on site started in October 2022 and completed in July 2023.

This Sustainable Drainage Systems (SuDS) at Bilton School, Hull, aimed to reduce the volume of water entering the combined sewer system by providing storage during rainfall events. The project adhered to the CIRIA SuDS Manual and ensured that the features installed would disconnect flows going directly into the network and slow the return of stormwater back into the network, thereby ensuring controlled discharge into the sewer network while providing essential storage during rainfall events. Mott MacDonald Bentley (MMB), in collaboration with Yorkshire Water, Living With Water, and Stantec, undertook the detailed design and construction of the initiative. Project overview

The scheme involved the installation of various SuDS features due to previous flooding in the local area in previous areas. Installations included underground attenuation storage in the playground area, new perimeter channel drains in the car park area, and a 75-meter-long swale to convey flows to an attenuation pond. Additionally, four planters were installed throughout the school to slow flows from the school roofs. The playground and car park surfaces were redesigned to direct flows into underground storage and to the adjacent pond, with the playground completed with fun and educational surface markings.

[caption id="attachment_22366" align="alignnone" width="1200"] Tree pit cross section - Courtesy of Mott MacDonald Bentley[/caption] Tree pits with permeable paving

Tree pits are an integral component of Sustainable Urban Drainage Systems (SuDS), designed to manage stormwater runoff in urban environments. At Bilton School, tree pits were installed to provide storage for stormwater underneath the playground. Tree pits are engineered structures that integrate trees into urban landscapes while providing a means to manage stormwater. These systems typically consist of a subsurface storage area filled with soil and structural crates that support the tree and store water.

The primary purpose of tree pits is to capture, store, and slowly release stormwater, thereby reducing the volume and rate of runoff entering the sewer system. The operation of tree pits involves several key processes:

Stormwater capture: During rainfall events, stormwater is directed into the tree pits through permeable paving. The water infiltrates the soil and crate system, which acts as a temporary storage reservoir. Storage & filtration: The soil within the tree pit filters pollutants from the stormwater, improving water quality. The structural crates provide additional storage capacity, allowing the system to hold a significant volume of water. Slow release: The stored water is gradually released into the nearby drainage system. This slow release helps to mitigate the impact of flooding by reducing peak flow rates and volumes. Tree health: The stored water also benefits the tree by providing a consistent source of moisture, promoting healthy growth and enhancing the urban environment.

The system was designed to store water in the soil and crate system from Polypipe Civils before releasing it back into the sewer network.

A Controflow flow control chamber was installed to restrict the flow of water to 1 litre per second, as per the instructions of the Internal Drainage Board. The Controflow flow control chamber regulates the discharge rate of stormwater from the tree pits. By restricting the flow to 1 litre per second, the system ensures that the release of water is controlled and gradual, preventing overwhelming the sewer system during heavy rainfall events.

[caption id="attachment_22368" align="alignnone" width="1200"] Tree pit installation - Courtesy of Mott MacDonald Bentley[/caption] Geosynthetic clay liners

Ground investigations at Bilton School revealed high groundwater levels, necessitating the use of a liner for the pond and swale to prevent the pond from being filled with groundwater and not having the required storage capacity during a storm event. The swale and pond system were designed with a geosynthetic clay liner (GCL), which offers several advantages over traditional liners.

GCLs are factory-manufactured hydraulic barriers consisting of a layer of bentonite clay sandwiched between two geotextiles or bonded to a geomembrane. Bentonite clay is known for its high swelling capacity and low permeability, making it an effective barrier against water infiltration. The geotextiles provide structural support and protect the clay layer from damage during installation and operation.

GCLs offer several advantages over traditional liners such as compacted clay liners (CCLs) and geomembranes. These benefits include:

Self-healing properties: One of the most significant advantages of GCLs is their self-healing capability. When bentonite clay comes into contact with water, it swells and fills any voids or cracks that may develop, effectively sealing the liner. This property reduces the need for future repairs and maintenance, ensuring long-term performance. Ease of installation: GCLs are lightweight and easy to install compared to traditional liners. They are supplied in large rolls that can be quickly deployed over the prepared surface. This reduces installation time and labour costs, making GCLs a cost-effective solution. Environmental benefits: The use of GCLs significantly reduces the project’s carbon footprint. Traditional compacted clay liners require extensive excavation, transportation, and compaction, which contribute to higher carbon emissions. Reduced water usage: Compacted clay liners require a significant amount of water for proper compaction and hydration. GCLs, on the other hand, require minimal water for activation, conserving valuable water resources. High performance: GCLs provide superior hydraulic performance compared to traditional liners. The low permeability of bentonite clay ensures effective containment of water and other fluids. Vegetated headwall system

A vegetated headwall system from Geogrow was chosen over a conventional concrete headwall for the outlet from the attenuation pond, where it discharges into the watercourse. This decision was driven by the need to limit carbon emissions from cement production and to enhance the aesthetic and ecological value of the site. Geogrow bags were also used for the headwall at the inlet of the swale.

[caption id="attachment_22371" align="alignnone" width="1200"] (left) Installation of the vegetated wall system and (right) Geogrow vegetated wall system - Courtesy of Mott MacDonald Bentley[/caption]

Vegetated headwall systems are engineered structures that use geotextile bags filled with soil and planted with vegetation. These systems are designed to provide structural support while integrating seamlessly into the natural environment. The vegetated facade not only enhances the visual appeal but also offers several ecological and environmental benefits.

Traditional concrete headwalls require significant amounts of cement, the production of which is a major source of carbon emissions. By contrast, vegetated wall systems use geotextile bags and soil, which have a much lower carbon footprint. They also provide a natural and visually appealing alternative to concrete structures. The vegetation helps the wall blend into the surrounding environment, enhancing the site’s aesthetic value. Additionally, the vegetated walls support local biodiversity by creating habitats for various plant and animal species. This ecological integration promotes sustainability and contributes to the overall health of the ecosystem. The use of vegetated headwall systems supports environmental sustainability by reducing the reliance on non-renewable materials and promoting the use of natural resources.

The installation of vegetated headwall systems eliminates the need for heavy lifting and placement of precast concrete units, which can pose significant health and safety risks. By using geotextile bags and soil, the construction process becomes safer and more efficient, reducing the risk of injury associated with handling and installing large concrete structures.

Bilton School SuDS: Supply chain - key participants Principal designer & contractor: Mott MacDonald Bentley Outline design: Stantec UK Geosynthetic clay liners: Naue Geosynthetics Ltd Planters: Bioscapes® Tree pits: Polypipe Civils & Green Urbanisation Vegetated headwalls: GeoGrow Ltd Channel drain: MEA Water Management Flow control chamber: Controflow from SuDSstore Enhancing biodiversity with Bioscapes planters

A number of Bioscapes planters were installed at the school. These planters are designed to capture and hold water during storm events by connecting on existing rainwater downpipes, reducing the flow rate and input to the drainage network. By managing surface water runoff, the planters mimic natural processes and improve water quality before it enters sewers or rivers. Bioscapes planter was specifically chosen as they also support a wide range of habitats.

[caption id="attachment_22369" align="alignnone" width="1200"] Bioscapes planters and (right) with testing and monitoring equipment - Courtesy of Mott MacDonald Bentley[/caption]

The planters provide essential habitats for wildlife, supporting Biodiversity Net Gain (BNG) by integrating ten wildlife habitats within a compact space, creating microhabitats that cater to various plant and animal species. These planters include features such as houses for hedgehogs, butterflies, bees, amphibians, and birds. This initiative not only enhances the local ecosystem but also contributes significantly to environmental sustainability.

Bioscapes planters also offer significant educational benefits. At Bilton School, Bioscapes provided teaching and planting sessions with the children, engaging them in hands-on learning experiences. These sessions included planting a wide range of plants, such as spring bulbs and winter flowers, ensuring a vibrant display throughout the year.

The educational activities associated with the planters help to raise environmental awareness among students, teaching them about the importance of biodiversity and conservation. By involving children in the planting process, they gain a deeper understanding of ecological principles and the role of sustainable practices in protecting the environment.

Community engagement & educational initiatives

The project included several community-focused initiatives, such as the creation of a woodland path for pond dipping activities, a bug hotel, and various educational activities with the school. These initiatives promote environmental awareness and foster community engagement. Tours of the grounds were conducted with staff and children to explain the project’s benefits, and children were involved in the design process by completing sketches of the playground.

Educational signage was installed to explain the various sustainable features of the project, helping to educate the public about the benefits of SuDS. Collaboration with Hull University involved installing testing and monitoring equipment to assess the project’s effectiveness.

The data collected will be shared with the school, furthering educational opportunities and demonstrating the project’s long-term sustainability and societal impact.

[caption id="attachment_22372" align="alignnone" width="1200"] (left) Educational signage and (right) a bug hotel - Courtesy of Mott MacDonald Bentley[/caption] Summary

The SuDS project at Bilton School exemplifies excellence in sustainable design and construction. Through innovative features, community engagement, and effective project management, the initiative has significantly benefited the school and its surroundings. The project’s success demonstrates the potential for SuDS to enhance environmental sustainability and provides valuable data to support future initiatives in sustainable urban drainage.