Defending the island
A drainage project on Canvey Island in Essex formed part of the Environment Agency's rolling programme of improving flood protection to housing. The scheme was completed in November last year and is estimated to cost £5.24M.
Canvey Island is a low-lying, heavily urbanised island on the north side of the Thames Estuary in Essex covering about 16km2. The land is below high-tide level and is protected from inundation by the Thames Tidal Defences sea wall.
In addition to the tidal protection, an efficient internal storm water drainage system is necessary to prevent fluvial flooding.
Surface water is drained by a series of interconnecting sewers, culverts, natural and artificial dykes and lakes, which are discharged over and through the sea defences by a series of seven high-flow pumping stations and low-tide gravity sluices. Five low-flow pumping stations maintain a low level within the drainage system giving essential storage capacity in the event of a storm event.
The overall estimated cost of the Canvey Island Drainage Scheme project is £5.24M. The scheme involves works to all 12 pumping stations that had exceeded their design life and had deficiencies in structural integrity, health and safety features, and performance capabilities.
This has included the construction of four new high-flow stations and discharge structures along with reconstruction and refurbishment of the other stations.
The four existing pumping stations constructed in the 1960s were based on tidal defence levels established after the tragic flooding of 1953. Subsequent enhancement of the flood defences, in the 1970s substantially increased the potential discharge head for the pumps to overcome and this, in addition to their natural deterioration over time, resulted in significantly reduced performance in higher tide conditions.
The rising silt levels in the estuaries also made the low-level discharges increasingly difficult to maintain.
The project is part of the Environment Agency’s (EA) rolling programme of improving flood defences under the National Priority Programme (NPP) to improve flood protection to housing.
This scheme, which was completed in autumn 2006, will provide improved flood protection to 3,199 residential properties and numerous commercial properties, including 1,762 properties that were protected by the end of March 2006.
To enable this and to satisfy the consent conditions applicable to a number of the sites, the project has been carried out using a staged design and construction programme. This allows the lessons learnt during construction to be fed into the designs of the later stations.
The integrated project team lead by the EA comprises Atkins (consultant) and J Breheny Contractors (principal contractor) with specialist subcontractors Dabbrooks (electrical works) and Spaans Babcocks (screw pumping stations).
Jacobs Babtie (ECC project manager), EC Harris (cost consultant) and Michael Murphy Associates (land agent) have also played key roles within the project team.
The project delivery has needed close liaison with the local community. The 12 individual pumping station sites, located across the island together with works on both sides of the sea wall, has required multiple consent applications, consultations and resolution of a number of land issues.
The construction sites are scattered around the island, each with their own constraints. The two new pumping stations on the southern side of the island, adjacent to the Thames, are located within the seasonal tourist area of the island and work on these stations during the summer months would have caused considerable disruption and discontent from the local traders.
These two stations have, therefore, been constructed through the winter and spring period.
The two new stations on the northern side of the island have greater environmental implications in terms of over-wintering and nesting birds.
This resulted in a short construction window being available, particularly for the outfall works on the foreshore area.
The initial site at Croppenburg, preferred due to the existing installation’s poor reliability, had potentially the most risks associated with it in terms of the number of consents required, environmental issues and being the largest construction undertaking.
It was therefore decided by the project team to progress with the detailed design and make full consent application for the other summer pumping station at Knightswick as a potential alternative construction site, should Croppenburg be delayed.
The new pumping stations have been designed to enable all the new ABS submersible pumps to be interchangeable between the sites. The poor ground conditions, high water table and close proximity of roads, structures and housing had potential for damage if dewatering of the surrounding area was not carefully controlled during construction of the new sumps.
To minimise the risk, a wet caisson design with a sealing plug of concrete placed underwater was the chosen method of construction. This allowed the existing ground water level to be unaffected until a seal was achieved and the sump could be drained safely without any dewatering of the surrounding area.
The cascade outfall structures for the new pumping stations allow discharge at any tide level, while dissipating the flow energy to prevent undesired scouring of the estuaries at low tide, particularly in the environmentally sensitive Tewkes Creek area (Benfleet and Southend Marshes SSSI, SPA and Ramsar site).
The construction of the outfalls on the foreshore was carried out within temporary cofferdams to isolate the construction areas and permit uninterrupted working. The integrity of the tidal defences during the installation of the new discharge pipelines was also a priority.
Many of the sites are in open, publicly accessible areas and this has required secure unobtrusive designs to be incorporated, while ensuring safe maintenance access. Security and safety of both the public and the operational staff were a priority throughout the design process.
Confined spaces entries have been avoided, where possible, and access openings to deep wells incorporate secondary safety grills.
The location of Canvey Island along the Thames estuary designated it a medium- to high-risk area for unexploded WWII ordnance. The soft silty ground meant that any likely remaining bombs would
be located well below ground level, but potentially within the reach of the ground
investigation boreholes and construction excavations. Investigations were carried out to identify any potentially hazardous foreign objects within the areas.
The low-flow pumping stations were originally constructed with sheet pile walls and reinforced concrete floor and cover slabs. Sheet pile corrosion had become an issue at a number of the low-flow stations and with limited space available to build new stations the solution developed was for new reinforced concrete walls to be created within the existing chambers.
To achieve this the existing cover slabs were partially removed to allow works to carried out within the chambers and new cover slabs cast in-situ complete with openings for the new style access covers and equipment/cabling. The two screw pump stations at Canvey Island – Hilton and Dutch Village – have had a complete mechanical refurbishment comprising three new screws and one refurbished screw, new drives, gearboxes and bearings. Associated works included new covers to ensure security of the sites incorporating openings for easier inspection by maintenance personnel.
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