Coastal Protection with Reefs:
Appeasing Nature with Nature

David Carvalho
David Carvalho Author
Fábio Cruz
Fábio Cruz Reviewer and Editor
Perspectives on the Sea
In Perspectives on the Sea, we look into how simulation can help us face societal challenges, particularly those related with the ocean.

We have already motivated here how the global oceans are fast evolving to become uncharted environments altogether, bringing foundational challenges to be solved.

These bleak forecasts force us to make intelligent and fast-tracked choices.
The power of simulation can be harnessed just for that.

In this post, we show that coral reefs are more than stunning ecosystems.

Just like other nature-inspired solutions such as wetlands, coral reefs can provide decisive coastal protection and we dive deeper as to how.

Coastal Protection with Reefs:
Appeasing Nature with Nature

Coral Reefs - the Rainforests of the Sea

When we think of reefs, coral reefs immediately spring to mind.

Fig. 1: A healthy and shallow coral reef. Credits Jim Maragos / U.S. Fish and Wildlife Service

The scaffolding structure that coral polyps build very slowly is the backbone of an extremely fascinating and diverse ecosystem with truly global impact.

How diverse?

Covering a combined area roughly half the size of France — a measly 0.1% of the entire ocean area – coral reefs host a whopping quarter of all marine species [1].

Apart from the ecological richness they bring, their value to the global economy is tremendous — with estimates varying from 30 billion to 10 trillion dollars per year [2] and benefitting about 500 million people [3].

Yet coral reefs hold an underappreciated power.

Protecting the coastline… with coral reefs

Besides the biodiversity they boast, coral reefs are also extremely efficient in protecting the shoreline from waves, storms, and floods [4].

About 71 thousand kilometers of coastline is protected by coral reefs worldwide, providing protection to nearly 200 million people, who live mostly 10 m below sea level and within 50 Km from a reef.

But how do they offer protection?

Just like other nature-inspired solutions such as wetlands, this protection is attained by providing a buffer between land and the shallow sea.

The intricate and harsh coral surface allows a huge deal of energy to be dissipated (as much as 97%) as incoming waves are dragged across them, leading to a wave height reduction of up to 84% [5].

Fig. 2: Depiction of a shoaling wave across a reef. Credits: Régis Lachaume

Reefs have usually a long slope which has the effect of creating fast currents moving toward the surface. As the mass from the undercurrents reaches the crest, the wave is now shallower, taller but much slower. This is the principle behind wave shoaling.

The consequence? Given an extended shallow region, these slow and tall waves are unstable and eventually break, arriving at the shoreline in moderate conditions.

This is a key survival mechanism for many islands.

Fig. 3: Degraded reefs eventually become smaller. Changes in their height have huge repercussion in their ability to shield the coastline. Credits: Coastal Resilience Lab

The offshore reef sections are absolutely critical to attenuate waves arriving at the slope. Consider, for instance, the topmost meter of reef. Without it, estimates are dire.

The expected damage from flooding would double, costs related to frequent storms would triple and extreme one-in-a-lifetime storms would increase by 91%, totalling 272 billion dollars in losses [6].

The benefits brought by reef-based solutions can in fact exceed those from erecting more conventional structures, such as breakwaters.

Take tropical environments, where projects to restore structural coral reef sections can be undertaken. There, erecting breakwater structures cost, on average 20K per meter. On the other hand, reef structures have a far cheaper median price tag of 1.3K per meter [7].

A call to arms

However, coral reefs are dying at an alarming rate.

Half of these reefs have been destroyed in the last 70 years [8].

Why exactly?

Seawater is now more acidified, warmer and contains an excess of nutrients. Unsustainable practices such as overfishing are becoming widespread and lead the corals to slowly bleach. Within weeks without proper care, they die.

Apart from the ecological loss, this degradation leads to substantial economic losses, estimated to be between 137K and 1.2M dollars over a period of 25 years for each kilometer of coral reef lost [9].

Regrowing these ecosystems is neither easy nor fast. Monitoring programs have established that some coral reef habits take up to 32 years to recover [10].

Fig. 4: Floating nurseries can be used to accelerate the growth of many different coral species. Credits: Coral Restoration Foundation

Very welcome initiatives to restore coral cultures around the world for ecological reasons are taking place.

In the context of Coastal Protection, artificial reef structures can recreate similar functions afforded by coral reefs.

This is a hopeful prospect.

Protecting the coast… with artificial reefs

Humans have long engineered the seabed to their own advantage.

Even though the underlying reasons were different — from blocking ship passage to boosting fishing yields — the principle behind protection remains the same.

By submerging large structures, the incoming waves can be somewhat dampened and attenuated as they move along.

Fig. 5: Reef blocks made of concrete on a beach in Phuket, Thailand. Credits: Go Eco Phuket

It is not uncommon to see indiscriminate objects, such as tyres, vessels, barges, shipwrecks, cars and even train carriages, to be used as reef components.

But from an Engineering standpoint, this is unsatisfactory.

Engineering artificial reefs

With more refined and tailored goals, the choice of materials used and structural geometry have become a critical feature when engineering artificial reefs.

Components are conventionally built with concrete. As we discussed before, this is not the most environmentally-friendly material.

Easily available in nature, rocks, stones, boulders and gravel, as well as stabilized ashwaste or harbor mud can offer similar integrity more sustainably and at a fraction of the cost.

For instance, a 5000 tonne reef costs about 60K euros if made with quarry rock but the double if made with ash blocks.

Low-cost and adaptable, geosynthetics are a range of high-drag materials which can also be used to cover obstacles or the seabed. Geotextile sandbags can behave similarly to reefs and offer added installation flexibility.

Fig. 6: Sandbags, containers or tubes can attain similar effects to naturally-occurring barrier reefs. These were built for the Boscombe reef in southern England. Credits: BNPS

Simulating Reefs

As we have been discussing, designing optimal solutions for Coastal Protection for a given scenario is a challenge.

However, simulation allows high-quality models to be executed and inspect how many proposals fare, offering a way to speed up the process by order of magnitude.

This problem-solving mindset paves the way for the New Blue Economy.

And we want to be a part of it.

In the next post, we will show how these reef structures can be simulated in useful protection scenarios.

🌊 Stay tuned! 🌊

References

[1] - Spalding, M. D., and A. M. Grenfell. “New estimates of global and regional coral reef areas.” Coral reefs 16.4 (1997): 225-230

[2] - Costanza, Robert, et al. “The value of the world’s ecosystem services and natural capital.” nature 387.6630 (1997): 253-260

[3] - Doney, Scott C., et al. “The impacts of ocean acidification on marine ecosystems and reliant human communities.” Annual Review of Environment and Resources 45.1 (2020)

[4] - Coastal Resilience Lab: Coral Reefs
An excellent pedagogical exposition of the benefits of coral reefs to protect the coastline.

[5] - Beck, Michael W., et al. “The global flood protection savings provided by coral reefs.” Nature communications 9.1 (2018): 1-9.

[6] - Ferrario, Filippo, et al. “The effectiveness of coral reefs for coastal hazard risk reduction and adaptation.” Nature communications 5.1 (2014): 1-9

[7] - Whitmarsh, Davide. “Artificial reef investment: an assessment of information needs in the analysis of project risk.” (2002)

[8] - Global coral cover has fallen by half since 1950s, analysis finds

[9] - World Wild Life: Coral Triangle

[10] - Turning the tables – how table corals are regenerating reefs decades faster than any other coral type

[11] - Baine, Mark. “Artificial reefs: a review of their design, application, management and performance.” Ocean & Coastal Management 44.3-4 (2001): 241-259

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