Sea level rise is a pressing issue when it comes to climate change. Many coastal communities already face deadly and dangerous storms, flooding, and structural damage, and the number of those affected is likely to increase^[1]US EPA, O. Climate Change Indicators: Weather and Climate. https://www.epa.gov/climate-indicators/weather-climate (2016).. A study published in Nature used a technique called LiDAR (short for “light detection and ranging”, which can be used to measure elevation using planes or drones equipped with lasers) to gain a comprehensive look at how much land is close to (or in some cases, below) sea level, and how many people live there. The study found that around 267 million people live on land that is less than 2 meters above sea level, with most of the areas susceptible to sea level rise are concentrated in tropical parts of southeast Asia, with Indonesia having the largest amount of land at risk^[2]Hooijer, A. & Vernimmen, R. Global LiDAR land elevation data reveal greatest sea-level rise vulnerability in the tropics. Nat. Commun. 12, 3592 (2021).. With the current rate of melting ice in the Arctic (a major contributor to sea level rise) it’s likely that global sea levels could rise by around 1 meter within the century, drastically affecting cities and population centers^[3]Box, J. E. et al. Greenland ice sheet climate disequilibrium and committed sea-level rise. Nat. Clim. Change 12, 808–813 (2022).. 

What exactly would happen if the sea level rose that much? A collaborative study in New Jersey found that the 6 inches of sea level rise that have occurred in the state since 1980 more than doubled the number of houses at risk of flooding from a storm surge. They also reported that between 62,000 and 86,000 more homes were at risk of flooding at least once within a 30-year mortgage^[4]New Jersey’s Rising Coastal Risk. Rhodium Group https://rhg.com/research/new-jersey-flooding-hurricanes-costs-climatechange/.. What’s more, this threat of displacement, loss of life, and property damage is only projected to increase, with worse consequences if we are unable to meet global emissions goals. While we have some protections in place for coastal cities and communities, their short-term benefit comes at the expense of long-term protection.

Current Coastal Protections

The main coastal protections in use today take a few forms, this first of which is the use of seawalls, which are man-made (typically concrete) structures that are designed to bear the brunt force of large waves and reduce erosion. They are quite effective for this purpose — One study in Japan found that only seawalls 5 meters in height or more were effective in reducing mortality and structural damage against 4 tsunami events (in the years 1896, 1933, 1960, and 2011). However, while they may protect the land behind them, seawalls tend to divert seawater to nearby, unprotected areas, resulting in more erosion to their sides^[5]Seawalls and jetties — English. https://climate-adapt.eea.europa.eu/en/metadata/adaptation-options/seawalls-and-jetties.. Additionally, if the seawall is regularly hit by larger waves than it’s designed to withstand (a likelihood with sea level rise), the buildup of water behind the wall creates pressure from the opposite direction, which can weaken its foundation^[6]Seawalls and jetties — English. https://climate-adapt.eea.europa.eu/en/metadata/adaptation-options/seawalls-and-jetties.. In fact, the same tsunami study found that seawalls less than 5 meters actually exacerbated damages in vulnerable areas and increased the range of subsequent flooding^[7]Nateghi, R., Bricker, J. D., Guikema, S. D. & Bessho, A. Statistical Analysis of the Effectiveness of Seawalls and Coastal Forests in Mitigating Tsunami Impacts in Iwate and Miyagi Prefectures. … Continue reading. Somewhat ironically, the analysis posited that the temporary protection and feeling of safety seawalls provide also allows for more properties to be built along the shore, despite their inability to provide long-term security.

Another current strategy is the “hard shoreline,” in which the natural shore is reinforced with rock or other durable materials (called revetments) to break up waves and dissipate their energy before they hit the shoreline.Though this method of transforming the shore is used around the world and provides some protection from flooding and storm surges, its inflexibility, like the seawall, can be harmful in the long run: hardened shorelines replace and inhibit salt marsh ecosystems, which have their own advantages when it comes to flooding. Salt marshes are able to redistribute strong wave energy and promote soil retention, in turn reducing coastal erosion^[8]Fisheries, N. Understanding Living Shorelines | NOAA Fisheries. NOAA https://www.fisheries.noaa.gov/insight/understanding-living-shorelines (2022).. Additionally, as an ecosystem, these marshes also improve water quality, sequester and store carbon, and sustain a number of key species, including blue crabs (Callinectes sapidus), oysters, ducks, bald eagles (Haliaeetus leucocephalus), great blue herons (Ardea herodias), North American river otters (Lontra canadensis) and the occasional hungry bottlenose dolphin (Tursiops truncatus)^[9]6 Types of Wildlife and Plants That Thrive in Salt Marshes. https://pew.org/3tL6ftP.. When these ecosystems are replaced with hard shorelines, coasts not only lose habitats, but also may do the opposite of their intended effect. In North Carolina,  researchers studied the damages caused by Hurricane Matthew in 2016 on homes with hardened shorelines versus those with natural shores. It was found  that homes with hard shorelines experienced more damage than those without, suggesting that hardened shorelines are ineffective in protecting homes close to the water in storm events^[10]Smith, C. S. & Scyphers, S. Past hurricane damage and flood zone outweigh shoreline hardening for predicting residential-scale impacts of Hurricane Matthew. Environ. Sci. Policy 101, 46–53 … Continue reading. 

Figure 1: The varying forms of shoreline solutions^[11]Living Shorelines. NOAA Habitat Blueprint https://www.habitatblueprint.noaa.gov/living-shorelines/..

The Future is Living

The goal of coastal protections is to keep people and places safe against storms, and the current systems are falling short. Furthermore, they can often damage coastal ecosystems that provide habitat and necessary services for marine species. There is a need for better, more dynamic protections that can withstand the increasing power of storms but still safeguard against erosion and habitat loss.   

As with many environmental solutions, the path forward lies with embracing natural processes: salt marshes’ ability to absorb and reflect wave energy, as well as retain soil and vegetation despite intense flooding may provide a guideline for reclaiming eroded and lost shore habitat and enabling future sea level rise strategies. One such strategy is the “living shoreline”, which was initially pioneered in the Chesapeake Bay on the East coast of the U.S. NOAA defines living shorelines as “soft elements alone or in combination with some type of harder shoreline structure, like oyster reefs, rock sills, or anchored large wood for added stability^[12]Living Shorelines. NOAA Habitat Blueprint https://www.habitatblueprint.noaa.gov/living-shorelines/..” Put simply, a living shoreline is man-made, built by adding a soft fill material (often sand) to the shore to increase elevation and then recolonizing it with native grasses and vegetation. Additionally, this shoreline is sometimes accompanied by strategically placed oyster reefs or lines of rock to reduce some of the overall force of waves. 

The marsh sill has been widely used in the Chesapeake Bay for around 20 years and has shown promising results. In 2011, the category 3 Hurricane Irene hit the U.S. and created extensive flooding along the East Coast. A study conducted on various shoreline sites compared hardened shorelines, natural salt marshes, and these man-made marsh sill structures. After Irene, 76% of the bulkheads (fully concrete seawalls that armor the shore) were physically damaged, while salt marsh and marsh sill sites showed no damages or changes in elevation.

Figure 2: A sill marsh (foreground), with a natural marsh (background)^[13]Marsh Sill. Virginia Institute of Marine Science https://www.vims.edu/ccrm/outreach/living_shorelines/design/sills_breakwaters/sill/index.php..

Plant life briefly decreased with all the water, but recovered in under a year^[14]Administration. Two year study shows living seawalls promote regeneration in Sydney Harbour. The Lighthouse … Continue reading. The slight edge that sills had over natural areas was their increased elevation, which would prove vital with  higher sea levels. The results of this study suggest that these marsh ecosystems could be the better strategy when it comes to flood protection, and better yet, they boast much higher biodiversity! In fact, one study in Virginia comparing natural and living shorelines in habitat viability for several important species found that living shorelines were “functionally equivalent to natural marshes in nearly all measured aspects”^[15]Isdell, R. E. et al. Living shorelines achieve functional equivalence to natural fringe marshes across multiple ecological metrics. PeerJ 9, e11815 (2021).. Marsh ecosystems similarly outperform seawalls in terms of biodiversity: an analysis of 54 studies of shorelines on the Atlantic Ocean (mostly from North America) found that seawalls supported 45% fewer organisms and were 23% less biodiverse than natural shorelines^[16]Gittman, R. K., Scyphers, S. B., Smith, C. S., Neylan, I. P. & Grabowski, J. H. Ecological Consequences of Shoreline Hardening: A Meta-Analysis. BioScience 66, 763–773 (2016).. 

Finding a Middle Ground

Despite their flaws, seawalls may still be necessary for the foreseeable future to protect coastal communities as they adapt to the changing ocean. Though they may compromise the biodiversity and long-term health of coastal habitats, they can provide an immediate safety for houses and buildings. Seeing this conundrum, a lab in Australia decided to work on an experimental adaptation to seawalls, dubbing it the “living seawall,” a cost effective, low effort option to increase settlement of species on seawalls by simulating a rocky intertidal zone^[17]Living Seawalls. REEF DESIGN LAB https://www.reefdesignlab.com/living-seawalls.. Textured panels, designed to mimic natural features, can be secured onto the existing seawall, and at first glance they look a bit like a playground feature! 

Figure 3: Diagram of a living seawall from the Reef Lab^[18]Living Seawalls. REEF DESIGN LAB https://www.reefdesignlab.com/living-seawalls..

One of the first test sites of this new design was implemented 2 years ago in the Sydney Harbor, and monitoring efforts have demonstrated very promising results: the modified design hosted 115 species, which is 20% more than traditional seawalls^[19]Administration. Two year study shows living seawalls promote regeneration in Sydney Harbour. The Lighthouse … Continue reading. Additionally, some of the living seawalls had a cooling effect on the water, measuring 10°C less than normal seawalls. This temperature difference could be a lifeline for intertidal species, as warmer temperatures can limit the foraging ranges of key predators, decrease available dissolved oxygen (as warmer water holds less of it), and promote disease or bodily changes. A study on sea stars (Pisaster ochraseus) showed that the ambient temperature of the water and air affected how long they were able to forage in areas for food. When they reached a certain body temperature, they retreated into deeper water for longer periods of time to cool back off^[20]Szathmary, P., Helmuth, B. & Wethey, D. Climate change in the rocky intertidal zone: predicting and measuring the body temperature of a keystone predator. Mar. Ecol. Prog. Ser. 374, 43–56 … Continue reading. In the larger scheme of predators, this tendency of “overheating” could limit their ability to feed. Another intertidal species, the black abalone (Haliotis cracherodii) experienced higher rates of withering foot syndrome in warmer water, which decreased their populations significantly. This study took place in California, and showed that though the northern sites had no effect on the abalone, the southern sites (which were historically warmer than the northern) experienced an over 90% decline in individuals of all sizes^[21]Raimondi, P., Miner, C., Ambrose, R., Engle, J. & Minchinton, T. Continued declines of black abalone along the coast of California: Are mass mortalities related to El Niño events? Mar. … Continue reading. This suggests that southern areas of the world, in addition to rising sea levels, are at a greater risk of intertidal die offs after heat waves and the cooling effect of this experimental sea wall could thus have a life saving effect on the coasts in these areas.   

Implementation and International Use

How can we implement these natural solutions? For many cases, the responsibility lies with local governments, but it often requires cooperation from private landowners to install a living shoreline on their property. It requires a reasonable amount of effort to remove a hardened shore line, but fortunately creating the living shoreline itself is relatively straightforward. In fact, there are several online resources that can direct people on how to get started, making it accessible to those that enjoy a good DIY project. The Chesapeake Bay Foundation has a full guide (check it out here) including site evaluation, necessary materials, local funding opportunities, and construction tips. Another great resource for embarking on a living shoreline project is from NOAA and the Nature Conservancy, with information on building techniques and a breakdown of the costs associated with them. Overall, the cost to implement them is lower than installing a hard shoreline. The NOAA resource puts the cost of a hard shoreline between $80 to $1200 per linear foot versus the marsh sill at $75 to $150. When it comes to maintenance, the hard shorelines cost an average of $500 a year against $100 for natural alternatives^[22]How Much Does It Cost To Install A Living Shoreline. How Much Does It Cost To Install A Living Shoreline //www.clemson.edu/extension/living-shorelines/selecting-shoreline/cost.html., and though standard landscaping maintenance is necessary (for example, replanting after a particularly rough storm), it is minimal when the marsh has grown to the point of sustaining itself^[23]Natural and Structural Measures for Shoreline Stabilization. https://coast.noaa.gov/digitalcoast/training/living-shorelines.html.. The time it takes to design and create the shoreline is relatively short, with the process of acquiring a permit taking the most of the project period. In order for these marshes to form, they benefit from initial shelter from the waves, so areas with consistently high energy waves (like the west coast of the U.S., New Zealand, or parts of Japan) would likely make it difficult for grasses and plants to stay rooted, which are key in the design. 

When it comes to the issue of sea level rise, strategies like the living shoreline and seawall would be most needed in the places that are going to be most affected. As mentioned previously, southeast Asia has some of the most vulnerable areas, and some cities have begun to build up their defenses. The National University of Singapore has collaborated with the innovators of the living seawall to trial it in the Keppel Bay Marina, where it has been active for 2 years now. 30% of the land in Singapore, lies under 5 meters above sea level, making it a highly vulnerable area^[24]Climate change: How Singapore is saving its shores from rising sea levels | The Straits Times. The Straits Times.. The country’s prime minister announced that long term sea level rise protections could cost $100 billion or more, and a good portion of the rest of the country (up to 45% of it) has hardened its shorelines. Unfortunately, this situation is common across many coastal countries. Adapting to sea level rise is going to be costly, but it is going to affect all of us, which is exactly why we need to invest in solutions that are going to deliver significant results. As it stands, hard shorelines simply are not offering the best protection, and it is up to national leaders to make changes. Living shorelines are still only on the fringe of coastal conversations in Europe, Africa, and South America, and have only just begun in Asia. 

Continued monitoring of living shorelines projects is necessary to give a long term view into their effectiveness, as they have not been widely studied for that long. These strategies are comparatively new when compared to traditional shore armoring, but they have the potential to provide a range of services to coastal communities, both human and not. They could reclaim eroded shorelines, encourage biodiversity, and protect homes from flooding and storm damages. Altogether, “eco-engineering” techniques like this are a promising new frontier when it comes to the looming threat of sea level rise. Our coastlines are going to look different 50 years from now, and hopefully we will see less concrete, and more green.

Cate Hogan

Research and Writing Intern

Learn More About Cate