The drama, beauty, and economic viability of coastlines make them highly coveted places to live. But it also exposes human coastal settlements to storm surges, inundation, and erosion. All of these threats are exacerbated by the way people have engineered the coast, altered the flow of rivers, and changed sediment supplies. Climate change also poses a serious threat, bringing rising sea levels, warming waters, and changing storm patterns.
The cost of natural hazards to coastal communities has increased substantially over recent decades. The proportion of the world’s GDP annually exposed to tropical cyclones has increased from 3.6% in the 1970s to 4.3% in the first decade of the 2000s. Billions of dollars are invested in reducing risks from coastal hazards and climate change, creating both threats and opportunities for natural systems. Most of those dollars are being spent on infrastructure like sea walls, jetties, and levee systems, yet natural coastlines can help buffer these threats, making them more important than ever to coastal communities.
Because of the role they play in mitigating the threat of storms, coastal wetlands have become part of the coastal defense strategy in some areas, which has led to active restoration of these habitats and even the creation of new marshes through hybrid engineering projects.
Three key processes
Coastal ecosystems play a remarkable role in shaping the physical structure of our coastlines and, in so doing provide critical services to people in reducing the physical impacts of erosion, storm-damage and flooding. These ecosystems support three key processes: wave attenuation, storm surge reduction and shoreline elevation.
- Wave attenuation: Wind and swell waves scour the coast and can drive erosion and the shifting of sediments. Larger waves can overtop beaches, dunes and artificial barriers such as seawalls causing flooding. Coastal ecosystems have a complex tangle of shoots, roots, shells or coral skeletons that cause friction, rapidly diminishing waves’ energy.
- Storm surge attenuation: Major storms and typhoons typically create a storm surge, a rise in the water level along tens or hundreds of kilometers of coastline and the end-result can be devastating floods. During storms, coastal ecosystems continue to reduce incoming overlying wind and swell waves, but where they are sufficiently extensive they also provide resistance to the landward flow of the storm surge itself. This way, even a partial reduction in surge heights can prevent large areas of flooding.
- Maintaining shoreline elevation: On average, sea levels are now rising by over 3 mm per year, with considerable local variation. Such rates are set to continue or increase over coming decades. While no human engineering can alter this
fact, many coastal ecosystems have a capacity to “grow” vertically, raising the elevation of the seabed or land on which they are growing. Coral reefs, oyster reefs, salt marshes, and mangroves have all been shown to be able to keep up with rising sea levels. Such processes are not guaranteed, as they can be countered by other natural processes of erosion or natural subsidence, but in at least some places they can may a remarkable difference. Reefs, mangroves, marshes and seagrass meadows can become dynamic self-maintaining barriers and coastal defenses.
Coral reefs protect coasts from erosion and flooding by reducing wave energy and supplying and trapping sediment found on adjacent beaches. Coral reefs reduce wave energy by up to 97 percent. Healthy reefs protect coasts even during cyclones with strong wave conditions. They also keep pace with sea level rise, and, unlike man-made coastal defenses, they require little or no direct maintenance costs.
Coral reefs worldwide are declining from multiple threats, however, ranging from direct destruction by coastal development, through to overfishing, pollution and climate change. While rising sea temperatures are having dramatic impacts in many areas, it seems likely that reefs can survive or quickly recover from climate change impacts if other threats are reduced or removed. In addition to dealing with climate change at the international level, which will take years, it is critical to deal with local reef stressors as a means of building near-term resilience so that the benefits from this first line of defense will continue.
Mapping Ocean Wealth: Coral Reefs’ Sea Defense Value.
In a first rapid assessment to show the relative value of reefs as sea defenses worldwide, we first mapped the areas threatened either by erosion and/or inundation. The resulting map clearly shows that many coral reefs—being far from people—are of zero value in coastal defense. By contrast the highest values are of course closest to areas of high human population densities. These do not always correlate with the healthiest coral reefs, or the most important reefs for other values such as tourism or fish production, however their importance in coastal protection is critical and should provide an important warning for decision-makers regarding the protection and improvement of water quality as a means to ensure the continuation of this ecosystem service.
Mapping Ocean Wealth: Flood Protection Benefits of Coral Reefs
To further assess the benefits of coral reef protection, TNC has worked with collaborators at the Institute of Environmental Hydraulics at the University of Cantabria (IH Cantabria) and UC Santa Cruz to combine ecological, engineering and economic approaches to value the benefits of reefs for flood reduction to people and property.
Using high resolution modelling of flood hazards and damages, we have estimated the expected benefit of coral reefs for storm flood reduction. Building on recommended approaches, we looked at the potential damage costs from four different storm return periods (one-in-10 year, one-in-25, -50, and -100-year storms). In each case, we compare flooding for scenarios with reefs at their present height with scenarios for a loss of one meter in reef height. We estimate the land, population and built capital ($) flooded across all coral reef coastlines to a 90 meter resolution (Figure 1). We examined flooding in cross-shore profiles every two kilometers for all coral reefs globally and summarized these in coastal units of 10 profiles, giving our summary coastal segments of approximately 20-kilometer stretches, across >71,000 kilometers of coastline with coral reefs. For each coastal segment, we then derive the expected benefit of coral reefs for flood damage reduction from local to global levels in social and economic terms.
What we found is that reefs provide significant flood protection savings for people and property with some of their most important flood protection benefits from the most frequent storms. Just small declines in the height
of the reef crest allow much more wave energy to pass through to flood coastlines. For one-in-10-year events, storm costs would more than triple if we lost just one meter in the height of reefs. Reefs provide significant benefits even for higher intensity, 100-year events where damages would increase to $219 billion if we do not manage reefs well.
es with the most to gain in annual benefits from reef conservation and restoration include Indonesia, Philippines, Malaysia and Mexico. For each of these countries, the annual expected benefits of reefs exceed $450 million per year. And this benefit is only from the topmost meter of reefs and for direct flood reduction to built capital. Reefs provide many other benefits and the effects of flood protection on people are widely felt across countries and economies. The countries that may see the greatest annual benefits relative to their GDP include many Small Island Developing States particularly across the Caribbean.
Mangrove forests form extensive, often continuous belts of vegetation, growing particularly vigorously in river mouths and deltas, places that are often of critical importance to human dwelling and commerce. Mangroves are important due to their dense root structures and low branches. This forms a critical barrier to waves and flooding.
During these extreme events mangroves play multiple roles. They may reduce the height of the storm surge, they reduce the wind waves that top the storm surge, they reduce wind speed across the water surface, which can prevent waves re-forming, and they can even provide some mechanisms for trapping debris, which is a major cause of death and injury during storms. Studies following an extreme cyclone in Orissa, India showed that villages that had maintained mangroves as a barrier between themselves and the sea had a lower death-toll from the storm.
Mapping Ocean Wealth: Mangrove Coastal Protection
The Mapping Ocean Wealth team undertook detailed reviews of all the existing research into the role of mangroves in coastal protection. The resulting publication describes how a 100-meter-wide belt of mangroves can reduce wave heights between 13 and 66 percent, and up to 100 percent where mangroves reach 500 meters or more in width. If mangrove forests are sufficiently large, they can reduce storm surge peak water levels between 4 and 48 centimeters per kilometer of mangrove. In low-lying areas, even such relatively small reductions in peak water levels can reduce flooding, and prevent property damage.
Other MOW-linked studies look at the costs of natural versus engineered sea defenses and show that restoring mangroves can be two- to five-times cheaper than building a concrete breakwater to provide the same degree of protection.
Densely packed beds and reefs of oysters form a natural breakwater, dramatically cutting wave heights. They can also occur naturally in intertidal areas where they provide a form of shoreline armoring, preventing erosion and protecting marshes.
In Mobile Bay, Alabama, large-scale efforts are now underway to construct living breakwaters along shorelines facing rapid erosion. Through the American Recovery and Reinvestment Act (ARRA), federal “stimulus funding” has been used to construct more than two kilometers of submerged breakwaters, which are designed to support the rapid settlement of oysters and so gradually morph into living reefs.
Design is a key element—the reefs must serve a function: protecting adjacent marshes and seagrass beds from their prior state of rapid erosion, and so also defending roads and valuable coastal property. One of the important criteria for this project was the creation of jobs—short-term employment during the design and construction phase, but more beside that. Economists have found other returns on the investment in the fisheries sector. Recent research shows that for every new hectare of oyster reef created in Mobile Bay, we can expect an additional 3,200 adult blue crabs every year.
While saltmarshes lack the high physical structures of mangroves, for smaller waves and surges their dense vegetation actually makes them more effective in wave attenuation. The tightly packed vegetation creates strong frictional resistance to waves, while both the aboveground plants and their dense root systems help to stabilize sediments and reduce erosion.
Several studies show attenuation of as much as 50 percent of smaller waves even by a barrier of just 10 meters—rates of attenuation that are typically more than double that of unvegetated mudflats. In fact there is considerable variation in how effective they are, influenced by the density and structure of the vegetation, but all studies point to the importance of saltmarshes, particularly in the first few meters of land where attenuation is greatest.
Top image: © Ethan Daniels. Image credits: