Letter: Cost of flooding

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Sir: The Yangtse floods again and a flash flood occurs in the Pyrenees ("Flash flood turns camping site into 'Dante's vision of hell' ", 9 August) causing loss of life and devastation.

Demand for extension and reinforcement of embankments is the immediate reaction to flooding demanded by the public. It is the inevitable cause of higher river levels, swifter currents and transfer of sediment downstream further and faster than would occur in the natural river. Once constructed, embankments have to be extended and progressively raised; the river bed level rises in the embankment section, so that eventually, when failure occurs, a flood that would have been a minor disaster becomes a major calamity. In the meantime, the problem is transferred downstream where sediment is deposited nearer to the sea-face.

Dam construction can give temporary relief, but reservoirs silt up and thus have a limited life, while the effect upstream and downstream involves a complete change in ecology.

One approach is to do as little as possible and to adapt to the inevitable floods. The cost of dams and embankments could be spent instead on monitoring the evolution of the river basin and providing reasonably flood-proof housing, adapted from time to time to changes in the river basin. Minimum guidance of the river would be needed, but straight-jacketing must be avoided. The policy is attractive in many cases, but each river basin has its own problems, for example due to deforestation, population growth, mineral exploitation, agriculture and urban development.

Engineers have to try to educate planners and politicians, who are responsible to the public, about the nature of natural hazards and realistic ways of living with them.

Emeritus Professor D M McDOWELL

Brighton, East Sussex

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