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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Coastal ecosystems provide multiple, valuable services including coastal protection but are globally threatened by human-induced and natural pressures. Traditionally, coastal protection has relied on hardening the shoreline using structures that are difficult to modify or remove once in place and, frequently have negative impacts on adjacent coastlines and coastal ecosystems. Consequently there is growing interest in the design and engineering of «nature-based solutions» (NBS) as elements of coastal protection that are flexible and sustainable and also help maintain or enhance ecosystem health. Despite the increasing interest and body of scientific research around NBS, there remains a gap with regard to guidance and understanding on how these solutions can be managed and integrated within traditional coastal engineering practice. This is partly due to the fact that NBS straddle two distinct disciplines-ecology and engineering. However, the coastal engineering profession has considerable expertise and experience with tools and approaches that can be applied to ecosystem based solutions. Likewise, the vast knowledge on the ecology of coastal ecosystems can help address many of the challenges raised by the consideration NBS for coastal protection. This work reviews the design approach and process within commonly used coastal engineering frameworks in the context of NBS, to find common grounds and overarching principles. Existing guidelines for traditional engineering structures are compared and contrasted with those for NBS to contextualize the position of NBS within engineering practice. A parallelism is thus drawn between NBS and traditional coastal engineering, acknowledging their differences, to provide insights on ways in which NBS can be integrated with standard coastal engineering practice, ultimately, to achieve the dual goals of coastal protection and habitat conservation.@en