Climate change effects, including rising sea level and increasingly powerful storms, have put many coastal communities at risk. To mitigate damage, governments have responded by building higher seawalls. But a new study finds there is a more cost-effective, green solution: Instead of blocking water with walls alone, absorb it with marshes first.
Previous studies suggest that areas behind marshes experience significantly less damage during coastal storms compared with those without a living buffer. While past research has assessed the protective benefits of individual marshes, the new study employed modeling techniques to compare how different shore profiles and plant species help reduce wave energy. Seagrass, for example, will simply bend and flow with a wave. Mangroves, on the other hand, are far more rigid and effective at absorbing ocean energy.
The researchers used their model to analyze how much shorter a protective seawall could be if it sits behind various types of marsh. Furthermore, they compared the cost of building a higher seawall with that of establishing or replanting a marsh.
To test the model in a real-world context, the researchers looked at a seawall elevation and marsh restoration project already underway in Salem, Massachusetts, U.S. The local municipality there has planted two species of native seagrass, Spartina alterniflora, locally known as cordgrass, and Spartina patens, or salt hay, to restore the marshes. Armed with information about species, density of plantings and coastal topography, the study’s modeling estimated the seawall could be up to 1.7 meters (5.5 feet) shorter if it is behind a healthy marsh.
In this way, the researchers say they can put a dollar value on a healthy marsh. “The value of the marsh is equal to the value of the reduction in [wall] construction costs, and that will vary depending on where you are in the world, because construction costs do vary,” Heidi Nepf, study co-author and a professor of civil and environmental engineering at the Massachusetts Institute of Technology, told Mongabay by phone.
But the value of a marsh doesn’t end at avoided construction costs. Marshes provide crucial habitat for marine animals and birds. They also sequester carbon and filter water on land before it enters the ocean, improving water quality. “But we didn’t include those other benefits because there are not currently good ways to quantify them,” Nepf said.
A seawall doesn’t provide any of those ecosystem services. Moreover, instead of absorbing the energy of a wave, a wall reflects it, potentially causing erosion and costly damage at neighboring sites.
Nepf said this research will give city planners the financial information they need to prioritize marshes as they look toward protecting communities from coastal flooding.
“Now you can put it in the budget sheet and actually do the analysis to see that this is economically justified rather than just relying on philosophical statements about how marshes and biodiversity are important,” Nepf said.
Banner image by Eric Koppel via Wikimedia Commons (CC BY_SA 3.0).
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