Hybrid vigour A new form of sea defence is part natural, part artificial

I F THE CORAL reef pictured below looks odd, that’s because it is. It is artificial, but co-opts the local ecology to help out. Fully grown, it will be a defence against erosion and wave damage—and a tourist attraction for divers. Such “hybrid” reefs have been around for a while. But they are now being deployed by commercial developers, and gaining military attention.

Rather than blocking waves, as a sea wall does, a reef slows them, dissipating their energy before they reach land. One estimate, from the University of California, Santa Cruz and the Pacific Coastal and Marine Science Centre, suggests natural reefs prevent $1.8bn a year of flood damage in America alone. Rising sea levels and increased risk of storm surges will require better sea defences in the future. But while natural reefs take centuries to grow, hybrid versions can be conjured up in months.

The idea began with Wolf Hilbertz, an architect with an interest in marine biology. In the 1970s Hilbertz developed a technique that uses submerged electrodes to run electrical currents through seawater. This precipitates calcium carbonate and magnesium hydroxide out of the seawater, forming limestone similar to that of natural reefs. The artificial reef can become the substrate upon which a natural coral ecosystem develops.

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