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Food thought

INSPECTION & TESTING; Some testhouses are not afraid to try unusual techniques. Andrew Mylius reports from deepest Oxfordshire.

Lunchtime conversation at a hydraulics research firm, HR Wallingford turns to oranges and cucumbers. Someone has told coastal research facility manager James Sutherland that, in the field, cucumbers are the tool for visually measuring the flow of tides and currents.

Cucumbers have the right relative density - a cucumber will float at the surface rather than on the surface, which ensures it responds fully to the movement of water rather than air. Cucumbers are also appealingly torpedo shaped and, because they are bio-degradable, there is no need to retrieve them once the experiment is over. But, queries Sutherland, what about visibility? Cucumbers are green and water often murky. Oranges perform just as well and they are, well, orange.

Warming to the theme of ad-hoc science, senior executive Michael Thorn says you can tell a lot about the properties and behaviour of mud by throwing stones into glistening banks and flats at low tide.

HR Wallingford's status as a pioneering centre for research has been evolved by enthusiasts and lateral thinkers. However, its work is underpinned by rigorous engineering and an impressive track record in physical modelling.

Physical modelling will continue to distinguish the company even as computers become more sophisticated: 'There are still very significant things you can't do on a computer,' comments Thorn. He cites wave studies for coasts and harbours or complex flooding studies.

'On a computer you have to understand flows. You have to be able to read the physical equations. As computers become faster and bigger more things become possible. But in order to make them possible we have to understand physical processes,' he says. The role of physical modelling is changing.

However, the best way to test a scenario or observe an effect is to create it for real.

Right now HR Wallingford is carrying out scour and erosion tests on rock barrier coastal defences. The 30m-40m long mounds were first introduced to UK coastlines around 10 years ago and, as yet, there is no definitive guidance for their design or distribution along the shore.

Sutherland's coastal research facility is carrying out the 12 week, £50,000 study for the European Union's Engineering and Physical Sciences Research Council. He says findings will help fill gaps in data produced during the 1980s by theoretical, numerical modelling and, in so doing, should dramatically improve understanding of the ways wave action can undermine the barriers. It will also illustrate the movement of sediments around the structures.

The research will give a much needed boost to the confidence of marine engineers, Sutherland says. In due course, information from actual rock mound coastal defences will be cross-referenced with the numerical and physical models to improve the knowledge base further still.

In a 36m by 22m tank Sutherland's team has built a 1:20 scale marine environment. At its deepest, water depth is 0.5m. The sea bed rises 0.2m giving a minimum water depth of 0.3m. A rock barrier has been built above sand towards the upper end of the slope, and an energy-absorbing gravel shore constructed to minimise wave reflection.

Waves of up to 200mm are generated by 72 paddles linked to a central computer and powered by independent hydraulic rams. On this project the average interval between waves is 2.2 seconds and changes in the sea bed are being measured in series of 3,000, 6,000, 10,000, 15,000 and 30,000 waves. The structure is washed by up to 10,000 waves in a day.

Sutherland has tried to produce conditions that will provide most long- term scour.

The first of four tests delivered waves of irregular height but regular incidence straight on to the structure. In the second test waves were oblique, with a 20 degree angle of incidence. Test three introduced a long shore current into the equation. And in the fourth test, without long shore current, scour protection was placed around the structure.

The tests are being concluded with simulated storm conditions.

Before each test the tank was returned to start conditions so that, under each set of wave conditions, an equilibrium would be established. Measurement of bed depths was carried out to monitor distribution of sediments and changes to the structure itself over the course of each phase.

Data is still to be interpreted. However, Sutherland notes that storm conditions have been less damaging than predicted - erosion under the earlier test conditions was far more severe. The tests have also pointed up the vulnerability to undermining of a barrier's landward side.

Publicity manager Richard Wooldridge says research on this kind of coastal defence is symptomatic of a shift in hydraulic engineering from hard to soft solutions - designs that attempt to understand and work in harmony with the environment in which they are placed. With hard structures 'you are always in danger of creating concentrations of energy that are contrary to what you want. The marine environment is a very dynamic system. Putting in a hard solution often creates other problems further along the coast, perhaps in another county'.

Working in sympathy with an environment to achieve a desired end will increasingly characterise research, Wooldridge thinks. And increasingly projects will be evaluated in light of what is practicable and cost-effective environmentally. Particularly on projects undertaken for clients in Europe, Australia and the USA, environmental awareness will have an impact on all areas of research, he predicts.

HR Wallingford has grown in scope and capability in its fifty year history. It now offers services in river hydraulics and morphology, flood risk assessment and hydraulic structures.

Its offshore capabilities include estuary and coastline studies and research of marine sediments and the company has a major ports and harbours division. Thorn believes growth in future years will be in the water resources management, environmental research and offshore power sectors.

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