Gates Across the Thames
By “Awake!” correspondent in Britain
TONIGHT a million Londoners can sleep more soundly. Until now their lives and homes have been in danger. About 46 square miles (120 sq km) of Greater London are below highest tide levels. The lowest areas are shown in darkest shade on the map above. A serious flood would have caused damage costing at least £3,500 million ($6,000 million, U.S.), paralysing a large part of the city. Today’s answer is a barrier of ten floodgates across the Thames. These became operational in November 1982.
The Thames was not always the well-defined waterway that it is today. There was a time when the area from London to the sea was just a vast marshland, flooded periodically by peak tides. But as the city grew, more and more land was reclaimed and banks were built to keep it dry. Even so, peak tides sometimes breached or surmounted these man-made embankments. An Anglo-Saxon chronicle reported a severe flood in 1099. Later, historian John Stow wrote that in 1236 the Thames overflowed its banks and “a great number of inhabitants there were drowned and in the great Palace of Westminster men did row with wherries in the midst of the hall.”
Flooding became so frequent—every decade or so—that it seems to have been accepted as inevitable. For centuries, flood defence was left to individual landowners who built and maintained the riverbanks to protect their own property. Then in 1879 an Act of Parliament made the local authority responsible. Still the flood danger increased.
Why? As the magazine New Scientist explains: “First, London is gradually sinking. Not only is the bed of clay on which it rests slowly compressing, but over the centuries the whole of Britain is tilting, with . . . the south-east gradually dipping at the rate of 30 cm [1 ft] per century. Secondly, tides in the North Sea are getting higher every year.” This is thought to be due to the melting of the polar ice masses. And thirdly, the tidal range of the Thames—the volume of seawater flowing up and down the river—has increased. Repeated dredging and raised embankments have provided a deeper and clearer channel for it to race along. All these factors have combined to raise the tide levels in the centre of London by some two and a half feet (76 cm) in the past hundred years.
Tidal Surges—The Real Danger
The worst threat, though, comes from surges of water caused by storms in the North Sea. When a trough of low atmospheric pressure moves across the Atlantic and around the top of Scotland, it causes the sea beneath it to rise as a “hump” of water. When this huge volume of extra water is propelled by gales down the funnel-shaped North Sea and superimposed on a high tide, London is threatened. And further complications could arise if the freshwater river itself happened to be in spate because of heavy rain.
The last time that central London itself was flooded was in 1928. Fourteen people drowned, a great quantity of merchandise was ruined and enormous damage was done to buildings and installations. In 1953, farther downstream in the estuary, there was a still more disastrous flood, with 300 lives lost. That same North Sea surge took 2,000 lives in the Netherlands. Central London, however, escaped as the defences there held. Then, on April 8, 1982, the city again came near to tragedy. A surge began coming down the North Sea, coinciding with a high spring tide. With only a few hours to spare, however, the wind changed and the emergency passed.
A Movable Barrier for Protection
Something was needed that would protect London from flooding and yet keep the river open for shipping. Two options presented themselves. One was to raise the walls and embankments a further six and a half feet (2 m). The advantages would be that walls are easy to maintain and are not likely to fail through human error or mechanical breakdown. But to go on raising their height would make them an eyesore and block the view. So that idea was abandoned.
The other option was to erect some sort of barrier across the river and raise the flood defences downstream. The first proposal was for a dam with locks allowing ships to pass. For a century this scheme was stoutly opposed by dock owners who feared that cumbersome locks would drive shipping elsewhere. Later, the newly formed Port of London Authority also objected because a dam would greatly increase siltation in the river and mean immense dredging bills. After extensive discussion, along with a number of feasibility studies and experiments, it was decided to build a movable barrier that could, if necessary, be converted to a dam. In August 1972 an Act of Parliament opened the way for work to begin. The site chosen was at Silvertown on Woolwich Reach, some eight miles (13 km) downstream from Tower Bridge.
How It Works
Put simply, the barrier consists of three main components: gates, sills and piers. The ten gates, side by side between the piers, span the river’s width of 570 yards (520 m). Six of them are “rising sector” gates, as shown in the diagrams. These are designed to lie flat in sills recessed into the bed of the river when not in use. Thus they will not obstruct river traffic, the tides or the flow of the river itself. However, in the raised position they will provide protection from surge tides five and a half feet (1.7 m) higher than the disastrous 1953 tide. Four of these gates are 200 feet (61 m) wide, allowing ample room for ships to pass. In fact, each of them is equal to the opening of Tower Bridge. And these steel gates are 52 feet (16 m) high, which means that when in use they rise higher than a five-storey building above the riverbed.
An interesting feature of the sills, which with ballast weigh 23,000 tonnes, is that they do not depend on the soft riverbed for support. Like the gates themselves, their weight is borne by the piers. And they fit so precisely between the piers that only a negligible amount of water can pass between them.
The nine piers, with the enormous weight of both gates and sills bearing upon them, had to be bedded well down into the chalk, as hard as concrete, 50 feet (15 m) below the riverbed. At the top of the piers is the heavy machinery for manoeuvring the gates. These are protected from the weather by stainless-steel-clad housings that resemble the bows of ships and preserve the nautical motif of the piers themselves.
Coping With an Emergency
All the vital parts of the barrier machinery are duplicated to minimize the possibility of failure during an emergency. Power is available from three sources: the barrier’s own generating plant and the national supply from either north or south of the river. Nothing has been left to chance. In just 30 minutes the gates can all be closed—15 minutes in dire necessity. Twice a month all gates will be closed in succession to ensure their proper performance.
Ships and smaller craft on the Thames are monitored continuously by radar, just like planes above an airport. In the event of an emergency they will be given two hours’ warning. Then the massive lifting beams will raise the gates; they will be locked in position and London will be safe—blocked away from the sea. This was proved for the first time since they became operative, when, on the night of February 1/2, 1983, a combination of high tides and North Sea gales posed a threat to London. Authorities reported that “the system worked perfectly.”
At a cost that has exceeded £500 million ($875 million, U.S.), a question raised is: Is it worth it—especially so since it is expected that the barrier will only be needed two or three times a year during the remainder of this century? But if the tides keep getting higher at the same rate as during the past 200 years, and as London continues sinking, the barrier will be used more and more. It has been built to last a hundred years. With relatively low running cost, it can be looked upon as an insurance premium to protect the capital, paid in full at the start of the policy. London’s population and the colossal investment in property, industry, commerce and essential services will now be secure.
The floodgates across the Thames are indeed a fine example of the benefits that man can enjoy when he puts his remarkable engineering skills to peaceful use.
[Map on page 24]
(For fully formatted text, see publication)
Westminster
Lambeth
City
Southwark
Tower Hamlets
Lewisham
Newham
Site of Barrier
Greenwich
Bexley
Barking
[Pictures on page 25]
A. Gate out of action, recessed in the bed of the river
B. Gate raised for flood protection, blocking water from the sea
A Open position
B Closed position