A global synthesis of field measurements shows that coastal habitats – particularly coral reefs and mangroves, can be physically and economically effective at protecting coastlines.
During my post-grad thesis on coastal risks and adaptation, I came across a port in India situated behind an island covered with mangroves. Despite being within the tropical cyclone belt, the port had no hard structures in place for its protection. In my study, I quantified the contribution the mangroves made (over and above the presence of the island) in protecting the port from cyclone waves.
A few months later, I was able to visit the port and present my findings. During our conversation, the managers recounted the devastation they had seen in a nearby city from a cyclone a few years ago. While they knew the island was an important barrier, they admitted they had not – until then, considered the value of the mangroves on it.
The world’s coastlines are becoming increasingly difficult to protect due to rising risks, and coastal habitats can help reduce this risk. At the same time, these habitats – which sometimes act as defenses, continue to degrade due to direct and indirect human pressures. To help adapt to these collective threats, there is great interest in understanding the contribution, and value of habitats in protecting coastlines. In a study, published today in PLOS ONE, the SNAPP Coastal Defenses Working Group synthesized field measures of the effectiveness of natural coastal habitats and, for the first time, tied these measures to the costs and benefits of restoration projects (also called nature-based defenses).
We know that coastal habitats can protect coastlines, within limits, as long as they are healthy and thriving. They do this, principally, by reducing wave energy that would otherwise hit coastlines. Over the past 3-4 decades, considerable field evidence has been collected, documenting this effect across habitats.
Most of this evidence has been collected under “every day” conditions of low waves and water levels, which is understandable – collecting field data during a storm is not easy! However, in the aftermath of the 2004 tsunami, Hurricane Katrina in 2008, and Hurricane Sandy in 2012, there was great interest in the role of nature-based defenses, and more generally, in finding risk reduction options that are cheaper, more adaptive and less damaging in the long-term than massive gray infrastructure projects.
So far, little attention has been paid to measuring the costs and benefits of nature-based defenses, and relating these to physical effectiveness. To plug this gap, we extracted and synthesized data from 121 field studies world-wide, that measured wave reduction, restoration costs, coastal protection benefits, or a combination of these. Each of these datasets gives us specific information on the role of nature-based defenses.
From the wave reduction studies, we find that coastal habitats reduce wave heights considerably, by 30 to 70 percent. This reduction is determined by the type of habitat: reefs are more effective than marshes, which are more effective than mangroves and seagrass beds. Effectiveness also depends on the environmental parameters: reefs are most effective at certain water depths and for certain widths. Importantly, this physical effectiveness has its limits: reefs and marshes are most effective when they are close to the water surface, and mangroves are usually not exposed to high waves. These results support previous knowledge on the physical effectiveness of coastal habitats.
What is interesting, though, is tying these to benefit and cost information from restoration projects. We compare the costs of restoring a habitat, to the costs of building a breakwater, for specific sites in Asia, Europe and the USA. We find that mangrove projects can potentially be 2 to 5 times cheaper than a breakwater for the same degree of protection. The project syntheses show that coral reefs are the most expensive to restore, though they are usually much smaller than marsh or mangrove restoration projects.
Despite their high physical effectiveness, most coral reef restoration projects focused on biodiversity objectives and very few were designed for coastal protection. Almost all the mangrove projects had coastal protection as their main objective, though they are not usually exposed to high waves.
Mangroves are, however, very cost-effective measures due to low restoration costs. Also, some projects reported that the restored mangroves protected landward villages and structures from storm damages, though this effect was not physically measured. This highlights the less perceived role that coastal habitats often play during a storm, as sacrificial barriers, or as dis-incentives for dangerous coastal development, even in areas that are normally quite sheltered and not exposed to very high waves.
This study summarizes the substantial body of field evidence demonstrating that coastal habitats can be effective and cost-effective defenses, and outlines their relative effectiveness, costs and benefits.
The analyses show that restoration projects do provide coastal protection, and in some cases, this protection can be demonstrably cost-effective. The study also shows that physical and cost effectiveness can vary widely depending on the type of habitat, and its location and environmental conditions. It provides examples of ways in which measurements of effectiveness can be linked to evaluations of economic costs and benefits.
In doing so, it highlights an immediate need for more – and better, inter-linking of physical measurements with economic analyses, and design and management practices for natural and nature-based defenses.
Recognizing and understanding the contribution of habitats to coastal protection is essential, in ensuring that we manage risks appropriately, and in motivating us to preserve these habitats. I am periodically in touch with the officer that invited me to present my findings at the port in India, and the last time I checked with him (and on Google Earth!), the mangroves on the island are healthy and growing, and they, along with the port they protect, have survived two cyclone seasons without damage.