Academic Reading — Test 30

Reading passage

London has grown up along the banks of a tidal river, and that location has always carried a hidden danger. Twice a day the North Sea pushes water up the Thames estuary, and under the wrong conditions this incoming tide can rise far higher than usual. When a deep low-pressure weather system tracks down the North Sea, it can drive a surge of seawater southward; if that surge arrives at the same moment as a high spring tide and is funnelled into the narrowing estuary, water levels in central London can climb to dangerous heights. The catastrophic floods of 1953, which killed more than three hundred people along the east coast of England and inundated low-lying districts, finally persuaded the authorities that a permanent defence was needed. The response, after decades of study and debate, was the Thames Barrier, a movable flood barrier built across the river at Woolwich in east London. Construction began in the mid-1970s, and the structure became operational in 1982, although it was not formally opened until 1984. Rather than forming a fixed wall that would block the river permanently, the engineers designed a barrier that normally lies open, allowing ships to pass and the tide to flow freely beneath. The barrier spans roughly half a kilometre and is divided into separate openings by a line of piers founded on the riverbed. Between these piers sit a series of steel gates that can be rotated into position when a flood threatens and then lowered again once the danger has passed. The largest of these gates are remarkable pieces of engineering. The four main navigable openings are each as wide as the central span of Tower Bridge, and the curved steel gates that guard them are tall enough to match the height of a five-storey building. In their resting position the gates lie flat in curved recesses cast into the concrete sills on the riverbed, so that vessels can sail over them without obstruction. When a surge is forecast, hydraulic machinery housed in the piers rotates each gate upward through about ninety degrees until it stands as a solid barrier facing the sea. In this raised position the gate holds back the incoming tide while still allowing the engineers to control how much river water passes through underneath. Closing the barrier is not a decision taken lightly, nor is it taken at the last minute. The Environment Agency, which now operates the structure, monitors weather systems, tides and the flow of fresh water coming down the river from upstream. Predictions of a dangerous combination are made many hours in advance, drawing on data from the North Sea and from gauges along the estuary. A typical closure is ordered while the tide is still falling, so that the gates are sealed across an estuary that holds a relatively low volume of water. The rising surge tide is then kept on the seaward side, while the space upstream of the barrier has room to receive the ordinary flow of the river. Once the high water outside has peaked and begun to retreat, the gates are opened again and the trapped river water is released downstream. When it was first built, the barrier was expected to be used only a handful of times each year. In practice the number of closures has risen markedly over the decades, partly because of rising sea levels and partly because the land in south-east England is slowly sinking. Some closures are made not to hold back a sea surge at all, but to manage high volumes of fresh water flowing down the Thames after heavy rainfall inland; by closing on a falling tide, the operators can create extra storage space upstream for this water. This combination of tidal and fluvial defence has made the barrier a more frequently used asset than its designers originally anticipated. The Thames Barrier was designed with a long working life in mind, and current planning assumes that it can continue to give effective protection until around the year 2070. Beyond that horizon, the picture is less certain. Engineers and planners are already considering what should follow it, since the threats it was built to counter, chiefly higher sea levels and more extreme weather, are expected to intensify. Options under discussion range from raising the existing defences and improving the embankments along the river to constructing an entirely new barrier further downstream. For the present, however, the structure at Woolwich remains one of the largest movable flood barriers in the world and a central element in the protection of millions of people and a vast amount of property in the capital.