Restoring coastal ecosystems reduces wave height 35-71% to protect communities

A new study released by PLOS ONE, Effectiveness of Nature-Based Coastal Defenses, summarizes the substantial body of field evidence demonstrating that coastal habitats can be effective and cost-effective natural defenses.

It extracts and synthesizes data from 121 field studies world-wide, that measured wave reduction, restoration costs, coastal protection benefits, and a combination of these.

From these studies of across five key habitats (coral reefs, mangroves, salt-marshes, seagrass/kelp beds); findings show that coastal habitats reduce wave heights by 35 to 71 per cent and wave energy by 44 to 96 per cent. Effectiveness is determined by the type of habitat: reefs are more effective than marshes, which are more effective than mangroves and seagrass beds. Surprisingly marshes are more effective than mangroves in coastal defense. Effectiveness also depends on environmental parameters. Wider and shallower reefs perform better, and similarly, marshes are most effective when their canopies are close to the water surface.

At present, coral reefs are the most expensive to restore for coastal defenses. Despite their high defense effectiveness, very few of the coral reef restoration projects were designed to provide coastal protection.

Almost all the mangrove projects had coastal protection as their main objective. Mangroves are cheap to restore and thus provide very cost-effective coastal protection. The researchers compare the costs of restoring a habitat, to the costs of building a breakwater, for specific sites in Asia, Europe and the United States.

They find that mangrove restoration projects in Vietnam, cost (on average) $100 less than a breakwater for every meter of coastline. Thus, for a restoration project of just 5 km of the coastline, this translates to a cost-saving of $500,000. The IFRC Vietnam restoration projects – some of the 52 projects analyzed- restored nearly 100 kilometers of dyke lines in the region.

Report Abstract:

There is great interest in the restoration and conservation of coastal habitats for protection from flooding and erosion. This is evidenced by the growing number of analyses and reviews of the effectiveness of habitats as natural defences and increasing funding world-wide for nature-based defences–i.e. restoration projects aimed at coastal protection; yet, there is no synthetic information on what kinds of projects are effective and cost effective for this purpose.

This paper addresses two issues critical for designing restoration projects for coastal protection: (i) a synthesis of the costs and benefits of projects designed for coastal protection (nature-based defences) and (ii) analyses of the effectiveness of coastal habitats (natural defences) in reducing wave heights and the biophysical parameters that influence this effectiveness.

We (i) analyse data from sixty-nine field measurements in coastal habitats globally and examine measures of effectiveness of mangroves, salt-marshes, coral reefs and seagrass/kelp beds for wave height reduction; (ii) synthesise the costs and coastal protection benefits of fifty-two nature-based defence projects and; (iii) estimate the benefits of each restoration project by combining information on restoration costs with data from nearby field measurements.

The analyses of field measurements show that coastal habitats have significant potential for reducing wave heights that varies by habitat and site. In general, coral reefs and salt-marshes have the highest overall potential. Habitat effectiveness is influenced by: a) the ratios of wave height-to-water depth and habitat width-to-wavelength in coral reefs; and b) the ratio of vegetation height-to-water depth in salt-marshes.

The comparison of costs of nature-based defence projects and engineering structures show that salt-marshes and mangroves can be two to five times cheaper than a submerged breakwater for wave heights up to half a metre and, within their limits, become more cost effective at greater depths. Nature-based defence projects also report benefits ranging from reductions in storm damage to reductions in coastal structure costs.

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