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Changing channel The River Thames is at last getting the help it needs to cope with floods. Lisa Russell reports on progress of the Maidenhead, Windsor and Eton flood relief scheme.

For most of the country, heavy rainfall means little more than a soaking on the way to work and the prospect of being able to use the hosepipe in summer. But for people near the Thames in Berkshire, it brings with it the very real threat of another round of flooding.

The river was running bank full earlier this month, and memories are still clear of the floods of 1990 which affected 500 households. Flooding in 1947 was even more widespread, involving 2,000 homes over an area which is now much more developed. But relief is in sight. An £83M project is under way to construct an 11.5km flood relief channel; effectively an extra river to augment the Thames' capacity (NCE Water supplement October 1995).

Under the original plans, work should have been completed a year ago, says Environment Agency's project manager Colin Martin. Delays are due to administration and approval; the public inquiry process made virtually no change to the work.

The main channel is to be complete in 2000, and subsequent work on the west bank - stream improvement and a flood bank - is due to be finished by winter 2001. Tenders are being assessed for the main length of the channel, on which construction starts in earnest later this year.

Balfour Beatty has the £12.5M contract for the northern contract, which includes channel work at the northern end up to the intake, plus a bridge under the A4 and the main water inlet control structure. But although the award was made in October, work will not start until August, and meanwhile preparatory work is under way.

Excavation for the channel will be considerable. Like the main river, the channel will have a standing level of water, with weirs along its length. The channel is designed for 215m3/s, with total flood flow of 515m3/s. Capacity will be exceeded if there is greater than a 1 in 65 year flood, though the extent of flooding would be alleviated. 'It also runs out of steam if there are extremely low flows in the summer,' adds Martin, as there will be twice the retention through the reach that there is now.

Both those situations will be managed using the weirs, he says. 'We can manage the high flow one at the moment better than the low flow one, because we don't know enough about it. That is something we will learn over a period of time.'

The channel will have a trapezoidal cross-section with a 30m base width, a working depth of 5m and 1:1.5 side slopes, giving a top width of 45m, more or less the same as the Thames. In some areas, landscaping will make it a good deal wider, while it will narrow through structures.

'The crucial factor about building all this is that the channel is to be cut through extremely high quality Thames Valley sand and gravel,' says Martin. 'Apart from being a valuable financial resource, it is a valuable mineral resource that can't be allowed to go to waste.' Some 3.5Mt of gravel will be won.

'There is a lot of mineral contractor interest,' says Martin. 'We are aiming that the contractor will pay a royalty for the extraction. The hope is that the royalty will be greater than the cost of extracting it.'

But carrying this material on local roads has to be avoided. 'The whole of our gravel strategy, which really governs the contract strategy, is to avoid using local roads,' he says. The strategy has included the decision to set up a mineral processing plant on site, a dedicated access to the M4, carry out early construction of structures and make use of river barges.

The award of the contract that includes the bulk of the excavation plus some structures is expected in April, and eight consortia are competing. They are also bidding for two other contracts - the eastern length plus a structures contract.

It is on structures that work is concentrating at present. Keeping material movements off

local roads is complicated by obstructions in the form of rail and road routes. 'To get all that material moved, we have first got to get the obstructions out of the way,' says Martin. This includes crossing both road and rail routes; and not just minor ones. Work starts in a fortnight on construction of a temporary six lane diversion of a stretch almost 1km long of the M4 motorway between junctions 7 and 8/9 (see News).

One obstacle already overcome is an area of heavily contaminated land. A waste containment cell was completed in October under a £2.2M contract by CA Blackwell. It is above groundwater level and lined with clay and a polyethylene membrane. The project has led to creation of the first sedtion of what will become the channel giving a first indication of the scale of the scheme.

Several bridge contracts are well under way. Access has to be made under the A355 which runs across the line of the channel. 'We need to be able to get material into the processing plant, and processed material away,' says Martin. The £2.17M contract, being carried out by Alfred McAlpine, started in October 1996 and is due for completion in May.

The north/south running road was moved across, allowing half of the channel structure to be built. In December the road was moved back on to the new bridge freeing the other half for construction work. Once complete, material can be run through without interrupting the traffic flow above.

Two high speed and two local rail tracks on embankments provide further obstacles. One is a £7M thrust bore contract, being administered by Railtrack and carried out by Nuttall. Preparatory work is under way and jacking of the box is due to take four weeks of round the clock working, starting in October. In common with other structures on the project, there will be two openings, 10m wide by 5m high. Overall cross-section of the box is 23m wide by 7m deep and it is 45m long.

Similarly at the east, the channel passes at a skew under the Slough- Windsor Central line. Here track possessions are needed and the embankment will be cut away before the box is installed.

A first possession in September allowed steel piling to be driven; and ground anchors were installed on Boxing Day. The track will be taken up during a 54 hour possession in April, with excavation carried out between the piles, and then the box pushed through in open cut. This £2.5M contract is being carried out by Osborne, administered by Railtrack.

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