Coral reefs and mangroves save billions of dollars in flood damage

Published by Flemmich Webb on

By Michael W. Beck

The risk of flooding in coastal cities around the world is growing so there is great interest in identifying effective solutions that can reduce it. It is increasingly clear that natural defences such as wetlands and reefs can play significant roles in doing so, and it is now possible to quantify the value of this to all nations with coral reefs and mangroves.

In 2016 the World Bank teamed up with The Nature Conservancy and scientists from the public, private and academic sectors to calculate the value of the flood protection offered by coastal habitats (1) using tools from the engineering, risk and insurance sectors.

Coral reefs act as natural, submerged breakwaters that reduce flooding by breaking waves and reducing wave energy. In a newly published study on The Global Flood Protection Savings Provided by Coral Reefs, The Nature Conservancy and IHCantabria (2) compared the flooding that occurs currently on coastlines protected by coral reefs with the flooding that would occur if just the top-most one metre of living reef was lost (as is happening in the world today).

We applied this method to all coral reefs globally, covering more than 60 countries, to estimate the flood protection savings offered by natural defences.

The findings are staggering.

Overall the annual expected losses from storms would double with the loss of just one metre of coral reefs. The costs from frequent storms would triple. If coupled with sea level rise, flooding costs could quadruple. For the big one-in-100-year storms, flood damage costs could increase by 91 per cent to US$272bn.

The countries with the most to gain in terms of risk reduction from reef conservation and restoration of their coral reefs are Indonesia, the Philippines, Malaysia, Mexico and Cuba, where annual expected flood savings exceed $US$400m for each nation. The US also benefits from coral reefs (ranking eighth globally), accruing almost US$100m annually in direct flood reduction benefits.

We have also applied this analysis to mangroves in more than 115 nations across more than 700,000km of coastline. The study found that mangroves protect millions of people around the world, and reduce property damage costs caused by flooding by several billion dollars annually.

The study (3) estimated that if today’s mangroves were lost, 18 million more people would be affected by flooding every year, an increase of more than 39%. The annual damage to property would increase by 16% and would cost US$82bn globally. In Vietnam, India and China alone, mangroves protect more than 12 million people annually from flooding. Together with the US and Mexico, mangroves in these countries currently save US$57bn in damages to residential and industrial property every year.

This, then, is a clear incentive to restore and protect these coastal ecosystems.

An added benefit is that restoring natural coast defences is particularly cost-effective. The University of California Santa Cruz and The Nature Conservancy recently compared the cost effectiveness of nature-based (4) (e.g. wetland and oyster reef restoration) and artificial (e.g. dikes) solutions for flood reduction across the Gulf of Mexico. Overall, wetland and reef restoration can yield benefit-to-cost ratios greater than seven to one, equating to more than US$7 in direct flood-reduction benefits for every US$1 spent on restoration. Many artificial solutions (such as levees and elevating homes) have benefit-to-cost ratios of near to or below one-to-one; in other words, their benefits can be high, but they are expensive to implement at scale.

Taken together, this evidence shows natural defences can help reduce flood risk in many coastal cities and that they can be cost-effective to implement. But the fact is we need important changes in policy, practice and perception to realise these benefits.

What should we do? There are many solutions for supporting natural defenses. I would suggest two things to start — one public and one private.

First, disaster recovery funding must support the recovery of natural defences. In past storms, such as Hurricane Sandy, only about 1% of recovery funding from the federal government went towards rebuilding resilience using natural solutions – despite their proven benefits. That must change. This is critical as decisions are being made now by US federal agencies (e.g. FEMA and HUD) about how best to invest more than US$100bn in congressionally approved recovery funding across the southeast US and northern Caribbean after the devastating hurricanes of 2017.

Second, the insurance industry should redouble its efforts towards offering incentives to policyholders that carry out habitat conservation and restoration. It is starting to do this by ensuring habitats are included in industry risk models, and by setting up the first-ever trust to fund an insurance policy for coral reefs in Mexico.

The insurance industry has also played an important role in creating the evidence base for when, where and how these natural solutions can help. For example, Lloyd’s Tercentenary Research Foundation funded a two year study (5) into the role of coastal habitats in managing natural hazards and risk reduction.

The evidence is clear: natural defences work, and furthermore, it makes good economic sense to conserve and restore them. One of the biggest remaining obstacles to carrying out this work is the perception that artificial solutions are better than natural solutions. With a mix of on-the-ground actions and incentives (both public and private), we can show how a combination of natural and artificial solutions can strengthen resilience in cities around the world. Investing in our first line of coastal defence is good for nature and good for ourselves.

Mike Beck is lead marine scientist for The Nature Conservancy and a research professor at the University of California Santa Cruz where his laboratory group is based.