The Thames Barrier

Friday l6th November, 1984 at the Central Library, Worthing.
Speaker: Mr. R.W. Horner, O.B.B., T.D., Project Manager.

An audience of some eighty persons comprising members, their ladies, friends and visitors assembled in the lecture theatre. The President introduced the lecturer, whose wife was also present, as having been the G.L.C. Project Manager first for the preliminary studies beginning in 1968 and then for subsequent stages of the construction of the Barrier from the passing of the Act of Parliament in 1972. The first full closure of the completed Barrier took place in November 1982.

Mr. Horner started by mentioning that in the audience was Mr. Peter Block who had appointed him in 1968. He first gave a brief account of tidal flow effects in the Thames and referred to the sluices at Richmond comprising 20m spans which were used to control water upstream by holding back outflow at low tides. Locks provided for continuous river traffic movement. The Barrier, in contrast, provided for control of the flow of tidal water in the upstream direction. It was continuously held in the fully open position, except for test and maintenance requirements, only being closed to control the inflow of water in the rare event of exceptional very high tides, known as surge tides, so as to prevent flooding upstream of the Barrier. The use of a barrier, rather than a barrage, removed the need for navigational locks, interfered less with shipping and minimised siltation and ecological problems.

Records showed that, measured at London Bridge, a surge tide height above Ordnance Datum of about 4.3. m. occurred in 1791 and over subsequent years the peaks tended to increase progressively, with 5.5 m. in 1953. This tendency to increase seemed inexorable and is partly due to settlement of the land mass relative to mean sea level, about 4 mm per year at London, and partly due to increasing range of surge tides.

The speaker explained how movement of atmospheric depressions around the north of the British Isles caused a hump of water in the sea to funnel down the East Coast of England. A modest depression might cause the tide to have a surge of 3m. above the normal high tide level but occasional surges could be of 5.5 m. to 7.5 m. Such surges if occurring at high spring tides posed a serious potential threat of flooding from the Thames. After the 1953 surge tide the defences in the Thames Estuary were raised and when a similar surge occurred in 1965 the Thames water lapped the tops of the river side walls in Central London. Studies and projections showed that if no action were taken there could be serious danger of flooding over 5.25 x 10/10sq. metres of land along the Thames up to Richmond, including about 700,000 residents.

Studies were carried out of barriers at various places in the Thames Estuary and in the lower reaches and finally the site at Woolwich Reach chosen. This site provides a firm chalk bed for the foundations of the works and allows good navigational features. An Act of Parliament approved the G.L.C. plans and now the worlds largest moveable flood barrier is erected on this site. Rendel, Palmer and Tritton, Consulting Engineers, were responsible for the detail design, and supervision of the work.

The barrier spans about 550 m. in all and consists of ten separate moveable steel gates built side by side across the river. There are four main navigational openings each of 6l.5 m, two secondary navigational openings of 31.5 m. one at each end of the barrier and four 31.5 m. openings for tidal flow and without navigation. Nine concrete piers spaced across the river provide support for the moveable gates and house their operating machinery.

Gates open and close by rotation about a horizontal axis at right angles to the river direction. Each gate comprises two circular end members between which, the gate leaf is fixed. The leaf is segmental in cross section and when the gate is open the leaf is horizontal with its outer cylindrical surface fitting into the concave concrete sill on the river bed, the gap being only 50 mm. Each circular gate end member has a journal at its centre, on the opposite side from the leaf, to allow the gate to be carried in two spherical bearings of aluminium bronze using solid lubrication. The gates each weigh about 1200 tonnes and operate by hydraulic power. By rotating the gate through 90 degrees the leaf becomes vertical and prevents water flow.

Foundations for the piers required the driving of 30 m. piles down to chalk, to form a coffer dam, and the placing in it of 12,000 tonnes of concrete under water in one continuous pour over a period of 4 days to form a 5 m. thick raft. The piers were built after pumping out the water.

The hollow concrete sills were cast on shore, floated out and sunk into position between the piers. The south half of the barrier was constructed first whilst shipping was diverted to the north side. Mr. Horner quoted many parameters showing the enormous sizes and masses of items being handled and assembled. The accuracy with which they were to be placed was almost incredible.

The Barrier complex includes a shipping control tower on the south Bank and a power generating station having diesel driven 11 KV generators. Connections are made to the grid supply. Flood prevention walls and other defence features were built or augmented downstream of the Barrier.

The whole project cost about £780 million of which about 56% was for the barrier system itself and the remainder for downstream works. The lecture lasted about an hour and included slides of many excellent illustrations, diagrams and views.

In the discussion Mr. Horner explained, in answer to questions, that incoming ships would be given a thirty minute warning of intended raising of the Barrier, that closing normally took about thirty minutes but that if a ship were to collide with a gate it was expected that the gate with its 50 m.m. steel plate construction would withstand the impact. He said the gates were raised sequentially and the speed of closing was kept low because of the desire to limit the electrical power loading. Asked about the 50 m.m. gap between the gate and the sill Mr. Horner said no problem of jamming was expected from small intruding objects as most would be crushed by the enormous forces. A questioner asked about frequency of operation and was told that one or two per year was expected in the late 1980’s. Act allows its use only for flood protection and this would only be necessary in the event of high surge tides. A maintenance staff of about 60 is employed whose duties include painting of the gates which are then rotated completely clear of the water. However, cathodic protection is provided so no serious corrosion is expected.

The vote of thanks was proposed by Dr. E.A. J. Hall. He expressed the appreciation of the audience for the extremely competent way this thrilling engineering project had been explained and illustrated. Mr. Horners sense of humour had added to the enjoyment. Approval was shown by all present.