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High and dry

Flood protection in the Thames estuary is being improved with the replacement of a 19th century outfall, as Margo Cole reports.

The low-lying areas east of London are not only some of the most popular for new development, they are also the most susceptible to flooding.

Tilbury, on the Essex side of the tidal Thames, is built below high tide level, so the area's flood defences are hugely important.

Surface water from the town drains into a watercourse known as the Tilbury East Dock Sewer, an open channel that runs around the boundary of the Port of Tilbury. The water is discharged into the River Thames at two outfalls on either side of the Port, known as Botney Sluice and Hotel Gardens.

These outfalls form part of the flood defences for the 12,000 -strong population of Tilbury.

While the primary line of defence is a massive sea wall designed to withstand a 1:1000 year flood event, the sewer and outfalls are essential both to prevent build-up of surface water in the town and to prevent sea water getting in.

Although Botney Sluice was upgraded in the 1990s, the Hotel Gardens outfall was not, and is in poor condition. Tidal water seeps in through cracks in the wooden gate at the end of the 19th century outfall, and the brickwork of the structure itself leaks.

Tim Summers, principal engineer at Atkins, says: 'It has limited life, and failure of that infrastructure has some pretty serious consequences.' Working as a framework consultant, Atkins was employed by the Environment Agency to investigate options for improving the entire drainage system.

The result is a £2.1M project to dredge the East Dock Sewer channel, improve bank stability alongside it and replace the Hotel Gardens outfall.

The project is a high priority for the Agency in its drive to provide flood protection to 72,000 homes nationally by the end of this financial year.

Andrew Usborne, the Agency's project team manager, explains: 'We have a national priority programme of 30 projects, of which 11 are in the Anglian region. This scheme is robust because there are 4,000 homes affected here. It is one of the largest projects on the programme. But it was always going to be a high priority. It's a must-do project from the Agency's point of view.' After completing the feasibility stage, Atkins worked with the Agency and its framework contractor Jackson Civil Engineering to put a scheme design together. Jackson has taken this forward under a design and build contract, which started in August and is due to finish next June.

By far the most complex element of the work is the outfall replacement at Hotel Gardens.

The sewer is in a culvert for 300m under the Port of Tilbury, and emerges as a 40m long brick outfall running under the foreshore on the river side of the flood defence wall. At the end is the original - leaking - timber gate. Access from the river side is all but impossible, but constraints on the land side also make construction a far from simple process.

The outfall is located between a floating bridge, from which hundreds of Hyundai cars are loaded and unloaded each day, and a landing stage for the London Cruise Terminal - home to a listed building. Space alongside the line of the outfall is limited to the muddy foreshore.

The only way of replacing the outfall is to work inside a cofferdam, but Jackson considered various options before plumping for the final method.

'Originally the concept was that the cofferdam would be permanent, with the new pipes and head wall suspended off it, ' explains Jackson's project manager Dick Wolton. 'We realised that our piling contractor would be putting in an awful lot of piles that they're not going to remove, which adds a lot of cost. So the solution was to use the same cofferdam but to have circular piles driven within that to suspend the pipes, which dramatically reduces the piles left in the river.' Although the contractor also considered building a temporary platform alongside the outfall to work from, it eventually decided to use the sheet piled walls of the cofferdam as the base for the platform.

As a result, the structure has been built in 10m-long sections, with the crane and piling equipment sitting on a platform supported by steel beams spanning between the sheet piled walls of the previous section. Once the cofferdam is complete - later this month - all the excavation, breaking out of the old outfall and construction of new, will also use machines working from this platform, with the maximum excavation depth being 10m.

The cofferdam itself is made from approximately 150 sections, all 21m long and driven through the river's alluvial deposits into the gravel. The piles extend above normal high tide level, but the cofferdam would flood in an exceptional event. Each pair of piles has been designed to cope with loads of up to 120t - strong enough for both the maximum head of water and the load from the crane and piling rig.

Throughout the excavation and reconstruction phases, the water from the East Dock Sewer will be over-pumped from an existing chamber behind the sea wall.

'At the moment at every tide there is water coming in, ' reports Atkins senior engineer Don Lamont. 'Just by building the cofferdam the area has better protection.' The replacement outfall structure consists of a twinwalled plastic pipe surrounded by reinforced concrete, which acts as a beam spanning between the new, permanent circular piles.

This concrete will then be topped by a concrete fill that will stop load going onto the pipe, and be used as a walkway, enabling the Agency to inspect and maintain the head wall structure.

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