More often than not, a bilateral deal unblocks one trade route only to silt up its tributaries.

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But by the time it was completed in 1598, the Guadalquivir River had begun to silt up, causing all New World trade to move to Cadiz and the building became a rather opulent filing cabinet instead.

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Ian Barclay, a project manager on the silt scheme, said it would be good to add the extra barrier to stop the silt building up again so quickly.

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The theory is relatively simple: Seawater and freshwater, filtered to remove silt, are fed into pipes which lead to a membrane system, made up of spiral coils to maximize surface area.

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Some recalled a warning given by one of China's most famous critics, Huang Wanli, before his death ten years ago that the dam would silt up the reservoir basin and sooner or later have to be blown up.

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