5 th November 2003 - Geoff Miller-Richards
"The Development of the Thames Barrier"
Geoff explained that London was often subjected to flooding in the past and there are reports of this happening in the Middle Ages and during Tudor times. However, it was not until the 1953 floods, during which over 300 people were drowned and 160,000 acres of farmland were flooded with salt water, that the Government appointed a committee to develop anti-flooding measures.
Following this flood in 1953 the Government appointed the Lord Waverly to examine the problem. They recommended the formation of a Technical Panel, to carry out a detailed examination, comprising the chief engineers from all the Local Authorities concerned together with River Authorities. They in turn recommended the appointment of Consulting Engineers to examine the feasibility of a Barrier. This resulted in the formation of a Technical Feasibility Studies team leading to the appoint of Rendel Palmer and Tritton and Sir Bruce White, Wolfe Barry and Partners, Various types of Barrier were studied at many different sites between Blackfriars and Crayfordness on an intermittent basis between 1958 and 1965.
In 1968 the Consultants were re-appointed to make a final recommendation to the newly formed GLC who were responsible for making a final selection of a Barrier site. Many schemes were examined by the Consultants to suit the various sites until the PLA (Port of London Authority) accepted that the London docks were redundant and a site at Woolwich Reach was finally selected and the PLA accepted that four main navigation channels each of 61 metres or 200 ft clear opening would be acceptable.
Several schemes were proposed and rejected and it was only in 1965 that government consent was given to build a barrier. The design influenced by Charles Draper was chosen from 41 proposals because it was compact, attractive, practical, and environmentally sensitive. Geoff gave his personal view that Charles although not technically qualified as a professional engineer was in fact a very creative and lateral thinking designer who gave the inspiration to the global concept
. Charlie Draper was given recognition of his contribution by being made a Freeman of the City of London. Sadly he contracted cancer and died before the Barrier was completed.Inaugurated by Queen Elizabeth II in 1984, the Thames Barrier is the city’s most important insurance against surging tides and crippling floods. It took eight years to build the barrier, the cost of the Barrier (at the Final Account stage) was £454million, it is projected to function until 2030. So far, the floodgates have been raised many times, when tide forecasts have reached critical levels.
The Thames Barrier was designed to protect the capital from flooding until at least the year 2030. Based on the estimates of rising sea levels at the time it was designed, there is no current reason to doubt that the Barrier will serve its full-intended term. The high water level at London Bridge has risen by about 750 millimetres each century, these changes explained Geoff are due to a combination of four factors.
1 Global warming, now an established fact, but in the early 50's not well known, resulting in the melting Polar ice caps and hence a larger volume of free water.
2 The tilting of the south of England sinking at a rate of 300millimetres per century and Scotland raising, Geoff had the meeting in fits of laughter when he suggested the solution was for all the Scotsmen who had come to the south to return home and this would produce a solution.
3 The activities of man by building out to both sea and into the Thames
4 Extraction of bore hole water hence lowering the water table which in turn allowing the London clay to sink to fill the void. Over pumping of boreholes in London reduced the groundwater table by about 76 metres or 250 ft. Water is no longer abstracted by that method and the water table is now rising
The Thames Barrier is the world's largest movable flood barrier. It spans 520 metres (a third of a mile) across the Thames at Woolwich, where it protects London from tidal flooding.
Early proposals for a flood control system were defeated by the Port of London Authority insisting for the need for a large opening in the barrier to allow for vessels from London Docks to pass through. When containerisation came in and a new port was opened at Tilbury a smaller barrier became feasible, this also started the decline of London as a port, so that today the barrier is a transit for tourist vessels, Geoff indicated that future designs would doubtless be a fixed barrier with a sufficiently large lock.
The Barrier was designed to prevent surge tides, a surge tide is caused by a sequence of meteorological events:
A trough of low pressure travels across the Atlantic Ocean towards the British Isles, creating a great surge of seawater.
The depression passes around the North of Scotland, down the East Coast, carrying the surge to the mouth of the Thames estuary.
Geoff clarified further, as the mass of water from the deep seas hits shallower water, this surge that travels down the North Sea is a "Hump" of water of great mass. It is misleading to describe it as a wave like the Severn bore since the surge is of much greater mass and is not visible as a wave. If the arrival of this hump of water at the Thames Estuary happens to coincide with high tide in the Thames then London would have been under the threat of flooding. In the surge of 1953 the tide was at approx. Mean Tide level which meant there was sufficient room in the river above Canvey Island to absorb the mass of the surge.
Strong northerly winds drive the mass of water down the east coast.
The "hump" coincides with a high ‘spring’ tide at the mouth.
The combined mass of water rushes up the River Thames to London – a surge tide.
The barrier was designed by the Consulting organisation Rendel, Palmer and Tritton for the GLC, our speaker was part of the initial evaluation and design team, the concept of the rotating gates was devised by Charles Draper's innovative design which uses radial gates on sills developed from the gas tap principle. The site at Woolwich was chosen because of the relative straightness of the banks, and the underlying river rock was strong enough to support the barrier. Work began at the barrier site in 1974 and construction was largely complete by 1982. In addition to the barrier itself the flood defences for 11 miles down river were raised and strengthened. Since 1982 the barrier has been raised over 70 times; further, it is always raised every month for testing. The National Rivers Authority operated the barrier operation until April 1996 when it passed to the Environment Agency.
More than 50 staff operate and maintain the Barrier. Tides from Northern Scotland to the Netherlands are monitored by sophisticated computers in the Barrier Control Room and at the East Coast Storm Tide Warning Service at Bracknell, Berkshire.
How it works - see diagram below
Half a million tonnes of concrete were used in the coffer dams inside which the piers were built, when the gates are in the open position they are Not supported on the concrete sills. There is a 25millimetres gap between the gate and the sill and any silt that gathers there is flushed out when the gate is rotated. The pre-cast sills are constructed from heavily reinforced concrete and span between the piers. When raised, each of the 4 main gates is as high as a 5-storey building and as wide as the opening of Tower Bridge. The pivoted rocking beams which are linked to the gate arms at each end of the gate, in order to turn the gate, weigh 420 tons each. The hydraulic power packs (sheltered by the stainless steel shells) are electrically driven; using 3 alternative supplies, routed via each of the riverbanks, and, should these options become unavailable, from 3 on-site power generators.
The Thames Barrier comprises 10 separate movable floodgates, positioned end-to-end across the 520-metre span of the Thames at Woolwich Reach, east of London.
The gates are mounted on pivots and supported between concrete piers. Under normal tide conditions, six of the gates are out of sight, resting on concrete sills in the riverbed. The four biggest gates weigh about 3,700 tonnes each and leave a navigable span of 61 metres.
When needed, powerful electro-hydraulic machinery raises the floodgates to stop the encroaching river from reaching downtown London. Each of the nine concrete piers is partly covered by stainless steel roofs that look like billowing sails, giving them a faint resemblance to Australia’s well-known Sydney Opera House.
Foundation Construction
Chalk was first removed from the riverbed to make room for the foundation. Piles were then driven into the riverbed and interlocking steel beams were used to form cofferdams. Horizontal steel joists were located within the foundation to withstand pressure. The pile driving was followed by the placement of 250,000 tons of rock on the riverbed to counter the tidal flow effect. The foundation was completed by pumping concrete into the riverbed.
Barrier Construction
Elements of barrier were constructed simultaneously – either via on-site construction or pre-fabrication. A total of half a million tons of concrete was used to build the piers and sills.
The piers were constructed in the river first. These support the gates and house the machinery. All that is visible of the piers above the water level are the stainless steel domes. These domes house the electric supplies required to drive the gate arms. The sills are assembled next. These are located on the riverbed to support the gates when the gates are not in operation. The sills are prefabricated and are concrete with steel reinforcing bars for added strength. Cross-sectional hollow steel tubes are located in the sills, providing access, service and power to the piers. After the piers and sills have been constructed, the sills are positioned accurate to a few millimetres in between the piers. The sills are flooded, then lowered into the river.
Each main sector (moveable) gate has a semi-cylindrical shape and is constructed out of 4000 tons of steel. Computer controlled cranes manoeuvre each gate into place during construction. Each main sector gate is as high as a five-storey building and as wide as the opening of Tower Bridge (61 metres). The gates are raised by hydraulic machinery, also known as hydraulic power packs. The reciprocating gate arms used to raise and lower the gates weigh 420 tons. Power is supplied from 3 alternative sites, and 3 on-site power generators are on hand in case of emergency.
Geoff Miller-Richards gave in summary an amusing interpretation of the stages of the design of a major Project. As a project engineer he needed to find a solution to keep the team on track, that is apart from the final point, those professional engineers in the membership can fully sympathise we all these sentiments especially the last!
Enthusiasm
Disenchantment
Panic (when a mistake is found)
Search for the Guilty
Punishment of the Innocent,
The Decoration of all those who took no part.
This was a splendid lecture much appreciated by the members; our only regret was that such a complex project was covered in such a short space of time, well done Geoff. The members showed their appreciation after a moving vote of thanks in our normal manner.
Please note the report has been supplemented by more data kindly supplied in part by Geoff, with thanks.
