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The Thames Barrier – a Systems Study

Chris Wallace, May 2005

Modified 13 June to improve referencing

Purpose of the case study

This land-mark civil engineering project is one of the success stories of British civil engineering. The web links provide a good background to the project and to the current issues in flood protection for London and the Thames estuary.

This case study may seem a long way from the everyday concerns of an engineer working in the aerospace sector. However, it is an example in the public domain of an engineering project widely considered to be a success, despite large cost and time overruns. Systems engineering issues are present throughout its long incubation and construction, in its day-today operation as a vital defence system, and in the wider flood control and global weather systems of which it is a part.

Notes on the project

The following notes summarise some aspects of the project, mainly drawn from the authoritative history of the barrier and its construction by Gilbert and Horner, 1984. Gilbert and Horner both served in key roles on the project. The interpretation of the case is entirely the responsibility of this author, who was neither involved in the project nor ever a civil engineer.

Alternative designs

Whilst flooding has been a problem in the Thames estuary since Roman times, the floods of 1953 were the impetus for a serious attempt to engineer a solution to protect London from surge tides which threatened to cause untold damage to the infrastructure of the capital. Despite the enormity of the risk, it was 30 years before the barrier was operational. During the twenty years until a solution was settled on, many different forms and locations were considered. The form choice was between raising the height of the banks, a movable barrier and a fixed barrier with locks. Sites from Sheerness all the way up the Thames were proposed. For postwar Britain, trade was a national imperative and its importance ensured that the demands of the Port of London Authority were paramount. Their original demand was for 2 500ft openings. When a site at Crawfordness was proposed, they demanded a single 1400ft opening because of navigation difficulties. Despite the detailed technical analysis of dozens of alternative designs the engineering problems were not able to be resolved and it was only the arrival of containerisation and the move of the main activity of the port of London down river to Tilbury in the 60’s that these constraints could be relaxed. The present Barrier has 4 200ft openings, and it is now thought that these are more generous than needed.

Even now, there are those of the opinion that more consideration should have been given to the barrage option, with its amenity advantages for London.

Prediction

A wide variety of predictive models were developed for the project. Hydrographic models of the Thames river, both scale models and primitive mathematical models were created at great expense to explore the effect of the barrier on flows and siltation. Both finite element models and scale models of the 3000 ton main gates were developed. Wind tunnel tests of the design for the pier cladding (stainless steel on redwood and iroko) were carried out at Bristol University.

A major area of modelling was in the prediction of surge tidal heights. A full model needs to take account of the tilting of the British Isles, the sinking of London as the clay is consolidated, the increase in sea volume and storminess with global warming, and the effects of changes to river and river banks and the reduction in the flood plan. Some of these effects only emerged during the early days of the project. Probability theory to deal with the occurrence of extreme events is still an active research are.

Models of the actuarial cost/benefit of the barrier were also developed and argued over. Famously and controversially Professor Herman Bondi, a mathematician and astronomer who lead a study for the barrier took the view that such models were largely irrelevant in the face of the huge cost of a major flood.

Project Management

The total project can be divided into two main parts to the development: the barrier itself, and the raising of banks and additional floodgates downstream of the barrier. The barrier construction itself involved multiple structures: dredging diversion channels; the civil engineering of the scaffolding (i.e. the caissons), concrete piers ,abutments and gate sills; the gates themselves; the shafts and bearings supporting the gates; the machinery for raising and lowering the gates; power plants; control panel; lifts, navigation lights; ancillary services. In all 21 separate main contracts were let to 18 different contractors. (Gilbert and Horner, 1984, p 171) These components have critical interfaces in space and time: clearance between the ends of a sill and the piers was only 50mm; gate end to the shaft support 1.5mm. Furthermore there are many scheduling constraints which whilst not as critical, caused delay and cost: delay in the civil works meant that the gates and gate ends had to be store for several years.

By contrast, the downriver work could be more simply divided by stretch of bank. Although the Barking and Dartford Creek barriers were significant engineering works, the interfaces between these separate works were much less complex. In all 72 separate contracts were let for this work.

Despite the engineering challenge of the main barrier, the major cause of delay on the project was not engineering but industrial relations. The civil engineering works had started with 2 12 shift working. Whilst common in civil engineering, the local labour force were accustomed to a 40-hour week, and productivity suffered. When a switch to 3 8 hour shifts was finally proposed, this too caused difficulties due to the loss of overtime. This dispute festered for over three years, with several all-out strikes.

Contractual difficulties were another constant problem. Given the length of the project and the uncertainties of the economic climate (the inflation rate in the 1970’s reached 24%), no contractor would agree to a fixed price contract. These circumstances lead to complex negotiations between the customer (the GLC), the contractor and the Government (which funded 75% of the project), leading to what was in effect a cost-plus contract (ibid, p 102).

Other difficulties encountered and countered were problems with the river-bed geology affecting the bed/pier interface; collision of a ship with a coffer dam and of course, bad weather. One unanticipated problem arose in the sheeting of the roofs to the piers. The doubly curved roof had to be laid in narrow strips which were joined by turning the edge of one sheet up and folding the edge of the next sheet over it, in a direction to make the joint water proof. On one side a right handed plumber could do the job normally, but the other side could only be done by a left-handed plumber, or a right-handled plumber working upside down. Fortunately it appears that enough let-handed plumbers were recruited.

Project management changed several time during the project. One quote shows some frustration with ‘modern’ project management:

John Grice brought an entirely new spirit to the site, instead of concentrating on costs… he concentrated on getting h job done. If productivity was low on the night shift during a cold spell in February, he was out on site in the early hours of the morning to see what could be done about it. Such action is worth several tons of computer-aided management monitored data, which after all only tells the man on the site with any feel for the job what he knows already. (ibid, p 102)

Operations

The effectiveness of the barrier relies entirely on the failsafe closure of the gates well in advance of a high surge tide. A whole chain of activities from weather observation and forecasts, tidal and surge predictions, communication to barrier control, navigational warning to shipping in the Thames, manning and timely operation of the barriers must work without fail.

For an apocalyptic prediction of the effect of a multiple-failure situation which includes a ship hitting a gate, read Richard Doyle’s disaster novel ‘Flood’.

Current situation

As sea levels rise this century, the number of defence closures will rise. In 1984, the prediction was for up to 10 closures a year in the 21^st century. In fact the barrier has been activated 80 times in the last 20 years, and half of those incidents occurred in the last five years.

Rising sea levels will slowly increase the chances of a surge tide exceeding the height of the gates to the design limit of 1 in 1000 years (i.e. a 0.1 % chance each year) by 2030. However, Gilbert and Horner claim that the design strength of the gates will allow for a large difference in height between the up-steam and down-stream sides, so that even if the gate is overtopped by 0.8 m, the upriver section would only rise by 0.3m (ibid, p138). Nonetheless the London Assembly is now exploring plans for a replacement beyond the 2030 design life of the Barrier.

Questions

Here are a few questions which are triggered by the case study and the readings around it.

1. According to present Government estimates, a flood would cost around £30 billion, around 2% of the UK’s GDP (Jane Kettle, EDIE 1/4/2005). By comparison, the cost of the barrier is estimated to have been around £1.3 billion pounds in current money.

Should the barrier have ever been built?

2. It is often said that problems aren’t solved, they are simply dissolved. Is waiting sometimes an optimal strategy?

3. Modelling methods and analytic techniques developed along side the use of those techniques on the project itself. Is this a common feature of all complex engineering projects?

4. A key feature of the design is the novel raising sector gate devised by Charles Draper, an engineer with the consulting engineers Rendel, Palmer and Tritton. I believe, although I can find no evidence, that Draper was not a civil engineer. Is it still possible for engineers from outside a discipline to contribute novel solutions?

5. Is the day of hands-on project managers like John Grice gone forever?

6. Would today’s managers have better predicted and handled the labour problems?

7. ‘Our new holistic approach should help to reassure the 5 million people at risk from flooding and coastal erosion that we are working hard across Government to set in place a strategy for the next twenty years that will effectively manage the impact on people, homes and businesses; doing so in a way that benefits wildlife and the landscape'. Elliot Morley Environment Minister (reported on the DEFRA website 24^th March 2005).

One area of concern is the development of housing in the Thames Gateway, on the flood plain. What does Morley mean by ‘holistic’ here? In what way is the Thames Barrier not holistic?

8. Is there any connection between the increased risk of flooding in London and the aerospace industry? Who should be concerned about the relationship if there is one?

9. Is the barrier a system?

References

1. Gilbert, S. and Horner,R. The Thames Barrier, Thomas Telford Ltd 1984

2. Doyle, R Flood Arrow Books, 2003