Innovation: Thames rings changes for London: The ring main links London's networks to central control

THE Thames Water ring main, 50 miles of tunnel 130ft below London, is nearing completion, eight years after the digging began. When the Queen performs the opening in November, she will not only inaugurate the biggest tunnel constructed in Britain, but bring into full operation what is claimed to be the most advanced water control system of any big city.

Construction of the ring main provided the impetus to develop an integrated London- wide water control system: without it the main would be a pounds 250m hole in the ground. The pounds 3.2m control centre on the Thames at Hampton, south- west London, continuously monitors pressures, flows, reservoir levels and the quality of most of the capital's water supply. It replaces 12 centres that could not talk to each other.

Communications around the 50 miles of the main are through an optical fibre cable. This transmits instructions from the centre to operate the 11 pumps that raise water from the main into the local pipe network, and collects data on pressure and water quality from sensors around the tunnel. It also carries closed-circuit television pictures from security cameras. Thames Water found that standard optical fibre degrades when submerged in drinking water, and commissioned specially developed cable from a Norwegian company.

Water is fed into the main from five treatment works. As it flows 140ft into the main, the force of gravity sets the water flowing round the tunnel. For gravity to keep the water moving, the tunnel must be full. This means the control centre needs to know how much water is being tapped. Information on water usage, which may double in some areas during dry weather, comes from flow meters in the local network.

The data is also used to plan pumping operations to take advantage of cheap-rate electricity. Data collected by the optical fibre, and from sensors in the local pipes, gives an instant river-to-tap view of the water supplies of 7 million Londoners. This is shown on a wall- sized 5 x 2m display at the Hampton centre.

Data is also analysed to help predict future demand. The four-day weather forecast is factored into this calculation: demand peaks not only in dry weather but also in cold spells, when pipes burst and water runs to waste. Production requirements are then fed to the treatment works. Bob Turner, control centre manager, says the sensitivity of the system means fewer bursts than before in the capital's older water mains.

This is not only because of the increase in capacity provided by the ring main, but also because water pressure in the mains can be finely controlled. When bursts do occur, householders are less likely to notice. The software in the control system reacts to the fall in pressure caused by a leak by rerouting the supply. There are more than 1,000 routes.