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.

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…Later work with Thomas Goreau, a marine biologist, produced both a catchy name—“Biorock”—and the idea of using the stuff as the basis of coral reefs, and, in particular, for repairing damaged reefs.

In 1996 the Global Coral Reef Alliance, a charity, began using Biorock for reef repairs by growing a six-metre structure in the Maldives. Other repairs have followed in Indonesia, Jamaica and Mexico. The Pemuteran Coral Reef Restoration Project, in Bali, is more than 300 metres long and includes dozens of “nurseries” in which Biorock acts as nuclei for the natural extension of the reef….DARPA a research agency run by America’s Department of Defense, also sees hybrid reefs as a means of coastal defence—in this case protecting the country’s seaside military installations. Lori Adornato, head of DARPA’s “Reefense” project, says the goal is a hybrid reef-type system which will be maintenance-free and self-repairing. Reefense therefore involves not only creating reefs and measuring their effectiveness, but also attracting and fostering appropriate organisms to sustain the reefs’ health, ensuring they can survive even when natural reefs are suffering.

Excerpts from Ocean Reefs: Hybrid Vigor, Economist, Sept. 11, 2021