Coral reefs work as nature’s sea walls – it pays to look after them

Stack ‘em high: big, health reefs take the sting out of stormy seas. Dennis M. Sabangan/EPA

Coral reefs: fragile, delicate, and in danger? Actually coral reefs can be the first line in defence against incoming storms, reducing the power of incoming waves by 97%, even during hurricane-force winds. Most (86%) of this wave energy reduction happens at the reef crest, a thin narrow band at the highest point of the reef where waves break first.

Our study published in the journal Nature Communications found that the risk reduction provided by reefs is relevant to some 200m people worldwide, and it is these people that may have to bear the costs if reefs continue to be degraded. These are the people in the villages, towns, and cities in low-lying, risk-prone coastal areas below 10 metres elevation and within 50km of coral reefs, mostly in Indonesia, India, and the Philippines. You may be surprised to learn that the US is ranked seventh on this list of nations, due to the high density of cities living near the reefs around south-east Florida. We conducted a more conservative analysis, looking at only those living below 10 metres elevation and within just 10km of a reef, which still represents 100m people.

The risk these people face is growing because of the rate of coastal development and climate change, compounded by reef loss and degradation in tropical areas. Reef degradation, particularly the kind that we have seen across the Caribbean has particularly devastating effects. Small reductions in the height of reefs, particularly at the reef crest, means substantially greater wave energy passes through the reef to strike the coastline. If you reduce the height of a breakwater that runs the length of your coast by 30cm (12 inches) – and the loss on coral reefs has sometimes measured much more – then you can expect to see a major impact on the coastlines.

But there is reason for optimism. At a time when towns, cities and countries are making major investments in climate and weather-related hazard protection, we found that coral reef protection makes economic, ecological and practical, risk-reduction sense. The average cost of building artificial breakwaters is US$19,791 per metre, compared to $1,290 per metre for projects focused on coral reef restoration. And of course healthy coral also provides a beautiful tourist attraction and has other benefits for fisheries.

But this restoration has to be done properly. Treating coral reef conservation as a joint exercise with storm risk reduction is a new field of science and practical application. It would pay to heed the lessons of past projects that have sought to use other habitats in this way. Tsunamis and storms across the Indian and Pacific oceans in recent years have spurred much interest in restoring mangroves as a storm defence, for example, but some of the mangrove restoration has been poorly conceived – for example, by planting mangroves as “bioshields” in places where they did not naturally occur. These projects are doomed to failure as the mangroves generally die.

On this basis, we don’t suggest creating new reefs in places they did not naturally occur. Reef restoration will almost certainly require adding to the height and complexity of existing reefs in order to enhance their wave-breaking power.

There will inevitably be trade-offs to make between the conservation and risk-reduction goals. Cautious conservation may favour using reef rubble or natural materials to grow the reef, but if these are unavailable, cement or rocks might have to do to. To deliver actual risk reduction means improving those reefs that will protect the most people – not necessarily the most remote and diverse reefs that have often been the focus of conservation.

We know that corals can recover. There has been significant recovery in many places around the world from the severe coral bleaching that occurred in the extremely warm El Niño year of 1998. Recovery was most sustained where other stresses on the reef, such as pollution, were managed well. Done well, restoring reefs can be money well spent with many benefits.

We shouldn’t be investing just in grey infrastructure like seawalls and breakwaters that will further degrade coastal habitats. Instead we have the opportunity to also invest cost effectively in “blue” ecological infrastructure – the sea wall nature has provided us.