During my master's 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 by the mangroves (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.
Coastal
habitats like mangroves and reefs can be valuable in protecting coastal areas, especially
as the world’s coastlines become increasingly risky. At the same time, these habitats –
which sometimes act as defenses, continue to degrade due to direct and indirect
human pressures. To adapt to these collective threats, we need to understand
the contribution, and value of habitats in protecting coastlines. In a study published 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
(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 has been 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 – 70%. 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. We find that that coral
reefs are the most expensive to restore, though they are usually much smaller
than marsh or mangrove restoration projects. Indirectly, however, this provides
a sense of urgency to conservation efforts to protect and preserve the reefs we
have. Despite their high physical effectiveness, most coral reef restoration
projects focus on biodiversity objectives and very few are intended for coastal
protection. Almost all the mangrove projects have 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 report 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 periodically find myself on Google Earth, investigating the port behind the mangrove island in India; the last time I checked, the mangroves on the island are healthy and growing, and they,
along with the port they protect, have survived two cyclone seasons without
damage.
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