Supply Chain - River Work - Civil Engineering
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Case Studies
Llyn Bran before discontinuance - Courtesy of Welsh Water[/caption]
Llyn Anafon is nationally and internationally protected for specific conservation features. This means prior to any project taking place, a full Habitats Regulation Assessment (HRA) had to be undertaken. The outcome of this assessment was that it was not possible to avoid adverse impacts to protected features and as such, the only route to proceed with the project was under the “Imperative Reasons of Overriding Public Interest (IROPI)” (Article 6.4 of the Habitats Directive) on the grounds of public safety. The Anafon Dam Discontinuance Scheme is the first IROPI case of its kind in the UK and has recently been approved by Welsh Government.
To satisfy the IROPI case, Welsh Water was required to secure ‘compensatory measures’ for the loss of habitat at Anafon. Using a set of criteria agreed with National Resources Wales (NRW), Llyn Bran was identified as the most suitable site to provide the necessary compensation. A key element of the work was to ‘restore and maintain hydrological functioning’ i.e., removal of the dam to create a natural lake and restore connectivity to the downstream watercourse.
Although the primary reason for discontinuing Llyn Bran was for compensation, it also had the added benefit of removing a dam which was no longer needed for operational purposes.
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Llyn Bran dam before discontinuance - Courtesy of Welsh Water[/caption]
Ground conditions - Courtesy of Welsh Water[/caption]
As with all discontinuance schemes the various surveys, permits, consents and negotiations in advance of the physical works were particularly complex, taking more than two years to complete. An overview of the consents and constraints required is below:
(left) Bog mat access track and (right) amphibious excavator used to load barge with coir matting - Courtesy of Welsh Water[/caption]
Peat
The extent of peat encountered on site was far greater than expected once construction began. This resulted in an entire change in the contractors methodology for the coir matting. It was initially planned to use Bog Master excavator and tracked dumper to access the western shore of the reservoir, however it quickly became clear this was not practical. Thanks to the contractor being proactive, the methodology was quickly changed to using an amphibious excavator and barge, to transport the coir matting across the reservoir.
Water voles
Water voles were identified on site during the early site survey stages, meaning a licence was required to temporarily remove the voles during construction.
Six water voles were captured by specialists and were transferred to a facility in Kent, where they were looked after while the work took place. A condition of the licence was that there must be 5-days with no water voles captured before it could be concluded that all voles were removed from site. Rather unluckily, there were two occasions where a vole was captured at the end of the fifth day, meaning the process took far longer than expected and the licence had to be extended. The voles are due to return home to Llyn Bran this summer.
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Water voles were captured and transferred to a facility in Kent for the duration of the works - Courtesy of Welsh Water[/caption]
Compensation flow
A condition of the NRW impoundment licence was for a continuous compensation flow of at least 20 l/s for the duration of the works. This meant pumping to the watercourse, which had to be continually monitored. It was hoped to use a cloud-based telematics tool that makes it easy to monitor the pumps off-site but unfortunately, due to lack of signal, this did not work meaning William Hughes Civil Engineering needed to attend site over the weekends to check on the compensation flows. The summer of 2022 proved to be exceptionally dry, which crated a challenge in meeting compensation flows whist also not allowing the remaining lake to become too low.
Flood risk
The existing reservoir provided significant flood attenuation meaning the discontinuance increased the flood risk to a downstream highway culvert. Initially, there were concerns from the highways department that the culvert would need to be upsized, which would have added significant costs to the project. Following a detailed flood risk and stability assessment, Welsh Water were able to prove that the increased flood risk would not adversely affect the culvert and only minimal slope protection work around the culvert was required.
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Amphibious excavator forming the new channel connecting to the downstream watercourse - Courtesy of Welsh Water[/caption]
Cores being drilled through concrete dam to allow any extreme storm flows to pass through - Courtesy of Welsh Water[/caption]
Dam safety mitigation
During detailed design, the Dam Safety Team suggested coring holes though the concrete dam once the reservoir was drawn down to the required level. This would mitigate the risk of the reservoir refilling during severe weather, before the dam was fully demolished. Fortunately, on this occasion the weather was favourable and the cores were not needed. This approach will be considered for any future concrete dam discontinuances.
Waste removal
It was agreed with NRW that the stone from the haul road and compound could be transferred to a nearby quarry, just down the road from site. This was done under a waste transfer notice, saving costs. The arisings from the dam were removed off site. The dam was demolished far easier and quicker than expected, making the removal of concrete straightforward.
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Dam demolition in progress - Courtesy of Welsh Water[/caption]
Coir matting
Due to the silt being drier than anticipated, there was spare rolls of coir matting left over. Rather than this being wasted, it was moved to nearby Alwen WTW where it is being stored before being used for silt management on the upcoming Cilcain Discontinuance Scheme which is due to start this spring.
Aerial view of the completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]
Modular wall shuttering in place for wall pour - Courtesy of Jackson Civil Engineering Ltd[/caption]
Excavated fish pass awaiting FRC works - Courtesy of Jackson Civil Engineering Ltd[/caption]
Non running fish pass mid-distance - Courtesy of Jackson Civil Engineering Ltd[/caption]
Completed fish pass - Courtesy of Jackson Civil Engineering Ltd[/caption]
(left) Completed fish pass looking towards the intake structure and (right) fish pass in operation showing control radial gate - Courtesy of Jackson Civil Engineering Ltd[/caption]
Existing intake structure (external screens at and below river level behind the debris boom with the access platform above) - Courtesy of South West Water & Fishtek ltd[/caption]
New debris return launder chute - Courtesy of South West Water & Fishtek Ltd[/caption]
(top left) Crane setup within restricted location & approach track, (top right) structural steelwork components being fixed ready for lifting and installation, (bottom left) silt removal prior to installation of steelwork, and (bottom right) accommodation work installing new concrete apron at landowners access gate to provide turning for reversing delivery vehicles - Courtesy of South West Water & Fishtek Ltd[/caption]
(left to right) Hydrolox screen installation, (i) steel support frame, (ii & iii) screen being lowered into position (note proximity to the building), and (iv) final positioning of the Hydrolox screen - Courtesy of South West Water & Fishtek ltd[/caption]
(left) Complete Installation of the S1800 Hydrolox screens and decking system, and (right) new wash water booster set which provides high pressure cleaning of the screens removing debris - Courtesy of South West Water & Fishtek Ltd[/caption]
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.
The existing Woodhouse Mill regulator gate - Courtesy of JBA-Bentley[/caption]
Model of the new Meadowgate regulator - Courtesy of JBA-Bentley[/caption]
(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.
Precast fish pass tiles - Courtesy of Craven Concrete Ltd[/caption]
(left) location of the regulator gates and (right) the new Meadowgate top-down vertical control gate[/caption]
Precast concrete ‘fish tiles’ from Craven Concrete Ltd positioned in the channel - Courtesy of JBA-Bentley[/caption]
(left) The dewatered channel and (right) ‘fish tile’ installation - Courtesy of JBA-Bentley[/caption]
Syphon outlet - Courtesy of JBA-Bentley[/caption]
(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.
Overhead power exclusion zones within model - Courtesy of JBA-Bentley[/caption]
(left) Project overview - Courtesy of Arup and (right) fish passage with adjacent ground lowering works - Courtesy of Walters[/caption]
Tir-y-Dail flood bund following construction - Courtesy of Walters[/caption]
(left) Lightweight shuttering utilised in constrained working corridors and (right) brick cladding works to flood walls - Courtesy of Walters[/caption]
Visualisation of Bonllwyn Green pocket park - Courtesy of Arup[/caption]
Heol Hayden public realm improvements - Courtesy of Walters[/caption]
Llandybie community woodland project - Courtesy of NRW[/caption]
Flood event in September 2023 - Courtesy of Walters[/caption]
(top) Collapsed left bank and masonry weir, (left) left hand cofferdam dewatered, exposing the extent of the undermining to the weir foundations, and (right) sewer pipes diverted through a temporary bridge (PortaDam and cofferdam installed on the left hand side of the river) - Courtesy of Ward & Burke[/caption]
(top left) Left bank foundation RC slab and wall built, excavator grab positioning reclaimed stone wall, (top right) left half of weir stabilised, shutter for right hand cofferdam installed, river bed graded ready for flooding, (bottom left) with the weir stabilisation complete the existing fish pass and part of the weir are demolished to make room for the new fish pass, and (bottom right) sewer pipes reconnected through the fish pass walls - Courtesy of Ward & Burke[/caption]
During construction, large stones from the collapsed downstream weir and left bank were recovered from the river. The design was altered to reuse these stones and incorporate them into the structure, avoiding embodied carbon associated with the original proposed concrete structures and imported aggregate. The stones were used to rebuild the left bank, installed as scour protection on the right bank, and used to landscape canoe landings. The stones helped the structure blend harmoniously with the surrounding environment.
Before flooding the left bank downstream cofferdam, a steel shutter was designed and installed on the new concrete slab to seal the proposed right bank cofferdam to the weir face. This enabled part of the right bank cofferdam to be constructed within the left bank cofferdam, ensuring a better seal and reducing risks associated with piling in the river.
Once the left bank was complete and the left half of the weir stable, the cofferdams were flooded and removed.
A similar process was followed on the right bank to stabilise the weir. Sheet pile cofferdams were installed upstream and downstream of the weir. Temporary pumps kept the cofferdams dewatered.
(top left) Propped cofferdam for upstream section of fish pass, preparing for blinding, (bottom left) fish pass wall rebar, upstream section, and (right) upstream baffles, penstock, and access metalwork installed in fish pass - Courtesy of Ward & Burke[/caption]
Midland Valves supplied a penstock upstream and stop logs downstream to isolate the fish pass for maintenance. The penstock and stop log frames were cast into the fish pass walls, flush with the base of the fish pass. This avoids grit being caught in the frame, preventing a tight seal.
The fish pass was constructed in two halves; upstream and downstream of the weir. The two sewer pipes were cast into the walls of the downstream section. The downstream section was constructed first so that the sewer pipes could be re-energised and the temporary pumping station decommissioned before the end of the whole project.
To construct the downstream section of the fish pass, a section of the weir and sewer pipes were removed. Then the ground was formed and blinded with a 8.53° slope. The base steel was fixed and the base poured. Next the wall steel was tied, shutters erected and the walls poured. The walls were constructed with a tolerance of +-5mm to ensure the baffles could fit and there wasn’t excessive space for debris to get caught. For the construction of the walls, the formwork panels had to be stepped to tie in with the run of the slope. The on-site joiners made relevant angled shutter bases to integrate with the main wall shutters so each wall could be poured in one go.
To construct the upstream section of the fish pass, a new cofferdam was installed. The cofferdam was initially propped with MGF frames. The blinding was designed strong enough so that the MGF frames could be removed once it reached strength. It was important that the MGF props were removed because they would have clashed with the fish pass wall, and limited lifting operations in the cofferdam. The construction process was repeated and the fish pass base and walls were poured.
Surveys and inspections of the fish pass by Yorkshire Water and the Environment Agency confirmed that the overall design and construction of the structure was satisfactory and within the design specification. On confirmation of this, the dewatering pumps were decommissioned and the sheet piles removed from the river to allow flows through the new pass.
A 160t crane was employed to remove the sheet piles upstream of the weir. The excavator that had driven them in could not reach over the new fish pass.
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(left) right bank cofferdam, looking at the rebuilt left bank masonry wall (and right) 160 ton crane and vibro hammer removing the upstream cofferdam - Courtesy of Ward & Burke[/caption]
Dunside Reservoirs prior to breach - Courtesy of George Leslie Ltd[/caption]
Water management plan layout - Courtesy of George Leslie Ltd[/caption]
(left) Dunside Reservoirs during construction July 2023, and (right) after breach - Courtesy of George Leslie Ltd[/caption]
Reinstatement of the watercourse - Courtesy of George Leslie Ltd[/caption]
Mini CS & Company News
- Springfield Surface Water Abstraction
Springfield Surface Water Abstractio...Springfield Surface Water Abstraction (SWA) is a Southern Water pumping station located on the banks of the River Medway in Maidstone, Kent and transfers water to Eccles Lake. The intake is positioned flush with the riverbank and consists of two openings into an underground concrete intake structure, with a busy riverside towpath on top. The picture above is before any works started. There are two coarse bar screens, one in front of each opening and a fixed timber boom in the river in front of the intake to deflect large debris. The flow travels approximately 25m by gravity into the pump house building, then is pumped to Eccles Lake and from there under gravity to Burham Water Supply Works. To comply with The Eel Regulations, Southern Water had to upgrade the intake so that the maximum bar-spacing on the intake was 2 mm and the velocity of the water approaching the screen did not exceed 0.25 m/s. This is to prevent juvenile ‘glass eels’ from being sucked into the intake which would prevent them from moving upstream into the Medway catchment and beginning the long and complex life cycle of the European eel. The 0.25 m/s approach velocity could be achieved by using only half of the existing intake, with the other half blocked up using a concrete stop-log. GoFlo Screens were awarded the contract to supply and assist with the installation of the eel screens at this location, by Trant/BTU Utilities Ltd. The site is particularly complicated because it is in use 24/7, and cannot be turned off for more than a day or two before disruption to Burham Water Supply Works would occur. Coupled with this, the site cannot be de-watered, so would have to be installed by divers in water that is 4.5 metres deep, with very low underwater visibility. The new GoFlo screens are too large to fit inside the existing concrete structure, so had to be accommodated outside of the existing concrete structure, in a prefabricated stainless steel ‘intake box’ that sits on the existing underwater concrete apron. GoFlo designed and manufactured the GoFlo screens, intake box, walkways and debris trough. Everything had to be designed to be simple and modular so that the underwater elements could be installed by divers. The elements supplied by GoFlo are shown below along with the basic specification:- Screen drive: 2.2 kW 3-phase WIMES-spec motor/gearbox
- Maximum bar spacing: 2mm
- (Single) screen mesh area: 4.652m2
- Angle of inclination (from horizontal): 72.5°
- Construction material: 304 stainless steel
GoFlo 3D CAD model of the proposed screens, intake box and walkways[/caption]
Proposed design
The overall design of the system was developed collaboratively between the main contractor (Trant/BTU), the client (Southern Water), Atkins Global who were the M&E designers working for Trant/BTU and GoFlo Screens. GoFlo also worked closely with Commercial and Specialised Diving Ltd to incorporate their ‘diver design friendly’ feedback into the design to make it easier for the divers to install, which was especially important considering it would be installed in the middle of winter. Before the detailed design could begin, the intake and apron area in front had to be cleared of silt and debris. Once in the water the divers found a tangled mess of silt, fishing line, a large quantity of steel railing, two motorbikes, a pushbike and various other debris! Once the debris was cleared, a detailed site survey was undertaken to determine how flat the apron was, the exact profile of the vertical concrete piers which the GoFlo intake box would fix onto (a special 5-metre-long profile gauge was manufactured to achieve this), and check how far the apron extended out into the river. A number of differences were found between the original construction drawings from the 1960s and what was actually built. Following the site survey, a detailed 3D model of the intake was constructed in Inventor software which formed the basis of the GoFlo design. The principle of the GoFlo design was a modular construction, comprising of:- A fixing frame that fixed to the concrete structure and was used as a template for drilling the fixing holes.
- A sacrificial ‘measuring frame’ which bolted to the fixing frame to allow all of the key levels to be measured (hence adjusted) from above-water.
- 2 x intake box sides which bolted to the fixing frame (assembled above-water, after the fixing frame fixing holes had been drilled and the fixing frame removed).
- The standard GoFlo baseplate and pivoting frame (guide rails), 2 off.
- The standard GoFlo screen(s).
- Coarse debris screen/walkway that integrated with the Intake box/debris trough and return channel.
(i) The fixing frame (ii) the temporary measuring frame, fixed to the fixing frame (iii) an intake box side - two are required, and (iv) the baseplate and pivoting frame[/caption]
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(i) Course debris screen (ii) the walkway assemblies (iii) debris trough and return channel with covers (iv) the completed design[/caption]
The whole design was based on simplifying the works on site, particularly in regard to making the works as straightforward as possible for the divers. Key design features include:
- Two reference brackets installed on the above-water concrete upstand, which provided two precise reference points for the fixing frame to locate onto at top end.
(left) Reference brackets fixed to existing concrete structure and (right) fixing frame in position against existing structure[/caption]
- The fixing frame, which once positioned acted as a drilling template for all of the fixings into the concrete structure. At each fixing position there is an 18.5 mm diameter hole, to allow an 18 mm masonry drill bit to pass through which was used as a guide for drilling. These 18 mm diameter holes, once the fixing frame was removed, had Hilti HKD-SR stainless-steel M16 fixings installed.
- For precise positional adjustment, the fixing frame had 2 x M16 height adjusting bolts close to the toe of the intake apron, 2 x M16 stand-off adjusting bolts at the bottom of the vertical and two temporarily-fixed adjustment blocks at the bottom of the vertical frame members to allow the frame to be pushed/pulled both laterally and longitudinally, and once in position locked in that position for the fixings to be drilled. These were removed once the intake box was finally installed.
(left) Close up of the baseplate of the fixing frame, showing the levelling bolts and (right) temporary adjustment blocks used for positioning the fixing frame[/caption]
- The fixing frame was designed to stand-off the concrete by a nominal distance of 20 mm to allow for unevenness in the concrete structure. This gap was taken-up at each fixing point using a stainless-steel shim stack. The height of the required shim stack was measured after the fixings holes had been drilled, but before the fixing frame was removed so that the shim stacks for each fixing point could be pre-prepared. Fine adjustment was still possible by adding/removing shims later as required.
- Once the fixing holes had been drilled, the fixing frame was removed, then the intake box sides bolted on, the GoFlo baseplates and pivoting frames attached. This allowed the whole intake box to be lifted in as an assembled unit and fixed in place using the pre-prepared fixings and shim stacks. Apart for the minor adjustments to levels, this saved a lot of complex underwater assembly work.
- Once the intake box was fixed in position, the coarse debris screen and walkway were fixed onto the intake box, then the GoFlo screens were installed.
Construction
The photos below show the actual parts being installed on site. Bear in mind that the water is 4.5 metres deep, and the GoFlo screen 6.1 metres long. The first photo shows the fixing frame lying on its back, having the measuring frame bolted to it prior to being lifted in. The second photo shows the assembled fixing frame and measuring frame being lifted into position. You can see that the river was very high after several days of heavy rain. [caption id="" align="alignnone" width="1200"]
(left) Fixing frame and measuring frame being assembled and (right) fixing frame and measuring frame being lifted into the intake – note the very high water levels![/caption]
The fixing frame and measuring frame in position, ready for the fixing holes to be drilled. The mild steel measuring frame was sacrificial, but provided above-water horizontal references for a spirt level, which was essential during levelling.
Once the drilling was completed, the fixing frame and measuring frame were removed from the water, then the intake box sides, GoFlo screen baseplates and pivoting frames assembled together.
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(left) Fixing frame and measuring frame in position and (right) the intake box being assembled prior to final installation[/caption]
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(left) The fully assembled intake box, (middle) lifting the assembled intake box into the intake with a 60-tonne crane, and (right) the assembled intake box was attached to the concrete structure by divers which took several days[/caption]
Next, the coarse bar screen was lifted into position, closely followed by the GoFlo Screens. GoFlo screens are lifted in vertically and use the pivoting frames as guide rails to guide the screen precisely onto the baseplate.
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(top left) Lifting the course bar screen into the intake box, (bottom left) lifting the first GoFlo screen into the intake box, and (right) fitting the front part of the walkway structure onto the intake box[/caption]
The screens are designed to discharge the collected debris into a dedicated trough behind the screens. The trough is made from stainless steel (as are the screens, intake box and coarse bar screen). The trough has a high back to catch the spray (2.5 bar water pressure) that is emitted from the screen spray-bars behind the mesh belt.
Each screen has a spray bar with multiple nozzles fed by a remote water booster pump located inside the main building. Debris on the surface of the screen mesh as it rotates falls off as it runs over the top sprockets, while any remaining ‘sticky’ debris is blasted off by water the jets from the inside of the screen and into the trough.
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(top left) Fitting the debris trough and return channel, (bottom left) the spray boom assembly which fits inside the screen mesh, and (right) completed walkway structure attached to the intake box[/caption]
The debris trough includes a dedicated water feed (10 litres/second) to flush collected material back into the river downstream of the screens. The green fence posts are to support the security fence to protect the public from the machinery.
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(left) The debris flushing flow pipework underneath the debris trough and (right) the security fencing being fitted[/caption]
10 cm bar-spaced coarse debris screen protects the GoFlo screens from large object damage, and keeps swimmers out. Due to the proximity of the screens to the footpath and the possibility of ice formation in winter months, a fully enclosed debris trough was required to confine the spray mist. The covers are hinged to enable full unrestricted access to the trough.
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(top left) View from the-screens, looking out towards the river, (top right) the lids covering the debris trough, (bottom left) further lids covering the debris return channel and (bottom right) the completed installation, inside the security fencing[/caption]
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The completed installation, looking upstream[/caption]
For more information: GoFlo Screens Ltd | +44 (0)1453 884400 | www.gofloscreens.co.uk
- Albert Street Eel Screens
- Sandown Surface Water Abstraction
Sandown Surface Water AbstractionSandown Surface Water Abstraction (SWA) is a pumping station abstraction from the River Yar on the Isle of White, which feeds raw water into the Sandown WTW. The pumping station is several hundred metres from the water treatment works, and sits alongside the popular and busy Red Squirrel Trail walking route and cycle path. Along with many other water intakes throughout the UK, Southern Water had to upgrade the water intake screens to be compliant with the Eel Regulations, which meant a bar-spacing that did not exceed 2mm and an approach velocity that did not exceed 0.25 m/s. The existing intake structure was built in the early 1980s and consisted of interlocking steel sheet pile walls, capped with a concrete slab. Manholes on top of the slab provided access to the sump pumps, valves and pipe/cable ducts. The river bed in front of the structure is a flat concrete slab, which could provide a good base for the eel screens to sit on if needed. [caption id="" align="alignnone" width="1200"]
(left) Concrete slab on top of the sheet-pile pump and valve enclosure and (right) sheet-pile walls that make up the pump enclosure[/caption]
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(left) View inside the pump chamber, looking at the sheet piling adjacent to the river. Below the waterline is the 0.2m high x 3m wide slot where the river water is drawn through. (right) close-up of intake drawing showing ‘letterbox’ slot[/caption]
The existing intake did not have any screening; hence fish, eels and other objects could potentially enter the pump chamber.
CMDP was the main contractor for the project, and GoFlo worked closely with them from the early design stages to the final commissioning to ensure an optimal solution was developed.
With a maximum abstraction flow rate of 0.278 m3/ second, the approach velocity through the existing intake slot in the sheet pile wall would exceed the maximum specified in the Eel Regulations. This meant that would it would not be possible to install the GoFlo screens inside the existing pump building and would have to be installed externally to the existing building.
An added complication was that the intake could not be shut down for more than 8 hours without severely disrupting the water supply to the Isle of White, which would not be acceptable, so the solution had to be designed to be installed into a working intake using divers.
GoFlo had a similar issue at another Southern Water site (Springfield), and for that site designed a fabricated ‘intake box’ system that could be installed by divers. This system worked well, so the principle was used for Sandown, even though the water depth at Springfield was 4.5m, whereas at Sandown it is typically only 600mm.
The width of the screens at 3.55m has been determined by the minimum depth of water in the river (0.438m) while still having to meet the 0.25 m/s approach velocity under the Eel Regulations while abstracting the maximum flow rate.
Site survey
The design process started with a detailed survey of the intake area. As is the case with many sheet-pile structures, nothing was exactly plumb or square, so because the fabricated intake box we would have to manufacture had to fit perfectly, first time, we had to enter the river and make plywood templates that fitted the profile of the existing structure perfectly. At the same time a detailed level-survey was conducted on the river bed concrete slab. We took the wooden templates away and turned them into precise profile data points that could be used to build an accurate 3D model of the existing structure in our CAD system. Based on that, we could design the intake box to fit perfectly.Design
Due to how the in-pans and out-pans of the sheet piling were positioned, the bolted connections onto the Intake Box at the upstream end was on an out-pan and at the downstream end on an in-pan, so the left and right-hand intake box sides being different widths (1560mm and 1450mm). [caption id="" align="alignnone" width="1200"]
(left) Intake box side drawing and (right) plan drawing showing new intake box attached to existing sheet-pile structure[/caption]
The intake box is a stainless steel fabricated box made up of the two sides and a baseplate that were assembled into a finished box on-site. The structure is designed to be a rigid structure so it could support the screens in the inclined ‘operating’ and vertical ‘service’ positions. The Intake Box structure also had the walkways fixed to it, which in turn have the security fencing fixed to them, so had to be designed to be very strong.
A course bar screen with 50 mm bar-spacing was incorporated into the intake box, just upstream of the screens. This bar screen will protect the fine GoFlo screen meshes from large debris which could deform or puncture the screen face during flood flow conditions, but also keep any swimmers away from the moving parts!
An advantage of having the screens perpendicular to the river flow will reduce the possibility of the bar screen collecting debris; the river passing flow tending to pull material away from the bars.
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(left) Drawing of intake box with course bar screen attached to upstream face and (right) drawing showing GoFlo screens, debris trough and intake box attached to existing structure[/caption]
The debris captured by the screens is returned to the river via the debris trough and return channel, but about 5m downstream, so the likelihood of material being pulled back into the screens is minimised.
The walkway structure incorporates davit sockets and removable winches. The winches are used for pulling the screens from their inclined ‘operating’ position to the vertical ‘service’ position. From here, the screen’s motor/gearbox and drive system can be accessed safely. Should a motor need to be taken off the screen, a portable davit can be mounted in a dedicated floor socket and the motor lifted up and over the handrail onto a trolley for transport away from the area.
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Complete 3D model of proposed GoFlo system attached to the historic structure[/caption]
Installation
This started with the river bank and floor being cleared of any silt and stones etc. Next, the intake box was assembled, including adding some temporary diagonal braces to keep the sides an baseplate square and to prevent the structure flexing during lifting. Once the intake box was in the river and fixed in position the diagonal braces were removed. The intake box was fixed down to the concrete apron using stainless steel M16 resin-fixed anchors, and to the vertical sheet pile wall by drilling a tapping M12 bolted fixings. Installing such a structure in a flowing river was a hazardous operation, especially with divers in the water to guide it into position, drill and install the fixings into the wall and river bed. Underwater visibility was very poor due to unseasonal heavy rainfall and increased river flow. The divers had to be in constant radio contact with the dive support team on the bank. The divers also had a live video feed to the team in the control room so they could be guided to the fixing locations. [caption id="" align="alignnone" width="1200"]
(left) Lifting the intake box into the river and (right) lifting the first of the GoFlo screens into the intake box[/caption]
Once the intake box was fixed in place the two GoFlo screens were lifted into position. This is a relatively quick operation due to the integrated ‘pivoting frame’, which are guide rails allowing the screen to be lifted in vertically and accurately guide the screen into position, even when the intake is flooded with zero visibility.
At this point the screens were connected to a temporary control panel and a wooden debris trough was constructed to allow the screens to be put into operation. The screens had to be put into service immediately because of the looming deadline to make the Intake Eel Regulations compliant.
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(left) The first screen in position on the intake box and (right) both GoFlo screens in position in the intake box[/caption]
A few months later the walkways, debris trough and security fencing were fitted, which all bolted directly to the intake box. The assembly was all designed in detail using 3D CAD software by GoFlo, then the constituent parts laser-profiled and CNC folded, finally being welded together. The walkways were installed in two days, and the debris trough in a similar time, which is testament to the accuracy of the design and fabrication undertaken by GoFlo.
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Completed GoFlo screen installation[/caption]
For more information: GoFlo Screens Ltd | +44 (0)1453 884400 | www.gofloscreens.co.uk
