Salt marshes are enormously valuable for coastal protection

Salt marshes are nature’s coastal defenders, and are gaining recognition as an essential line of protection against rising seas and increasingly intense storms.

As images of coastal homes being swept away become a more familiar sight, the impacts of climate change are undeniable.

Salt marshes as a sustainable solution

Researchers at the Massachusetts Institute of Technology (MIT) investigated the role that salt marshes play in protecting coastlines.

The study, led by graduate student Ernie I. H. Lee and Professor Heidi Nepf, reveals that salt marshes are a cost-effective and sustainable solution for coastal defense.

“Each year, coastal storms threaten hundreds of millions of people, disrupt transportation networks, and produce billions of dollars in damage. In 2022, in the United States alone, coastal storms have resulted in economic losses over $165 billion,” noted the study authors.

“These costs are projected to increase with sea level rise and with more frequent and more severe storms promoted by climate change.”

Cost-effective coastal protection

The research indicates that implementing measures to protect and enhance salt marshes in front of protective seawalls can protect some coastlines, and the cost is surprisingly reasonable.

According to Professor Nepf, protecting coastal marshes is not just something that would be nice to do, but is actually economically justifiable.

The researchers discovered that the seawall behind the salt marsh can be significantly lower due to the marshes’ wave dampening effects. This saves on construction costs while offering the same protection from storms.

Small marshes, big impact

The study highlights that even a small marsh can make a difference. This is encouraging news for places where it was thought preserving smaller marshes wasn’t worth the cost.

“One of the other exciting things that the study really brings to light is that you don’t need a huge marsh to get a good effect. It could be a relatively short marsh, just tens of meters wide, that can give you benefit,” said Professor Nepf. “We show that it can make enough of a difference to be financially viable.”

Urban marshes to the rescue

While the benefits of natural marshes in mitigating storm damage are well-known, this study differs in its focus.

According to Lee, the study emphasizes the solution’s viability in urban settings, where space for marshland is constrained and seawalls are prevalent.

The MIT researchers developed a model that factored in the morphology of various salt marsh plants rather than relying on an empirical drag coefficient.

They assessed the plant species’ influence and changes in morphology across seasons without requiring field measurements for each condition.

The team based the cost-benefit analysis on a simple premise: how much of the seawall height could be reduced if a given amount of marsh accompanied it? This practical metric provided a concrete value to quantify the benefits of salt marshes.

A real-world example

Nepf and Lee validated their simulations using real-world examples. They studied local salt marshes in Salem, Massachusetts, where restoration projects are underway.

Their model showed a healthy salt marsh could replace an additional 1.7 meters of seawall height, maintaining the safety of pedestrians from wave overtopping.

Collating data for modeling a marsh is a labor-intensive process. To mitigate this, Lee developed a method using drone imaging and machine learning to enhance mapping.

Quantifying the value of ecosystem services

The White House Office of Information and Regulatory Affairs released guidelines for assessing the value of ecosystem services in federal project planning. Professor Nepf said they lack specific methods for quantifying value.

The study addressed this gap by providing practical tools to translate the wave attenuation capabilities of marshes into economic value.

The study also complements the Federal Emergency Management Agency’s toolkit for benefit-cost analysis of environmental services.

As Lee notes, this is one of the applications policymakers can consider to quantify the environmental service values of marshes.

To facilitate this process, the software that environmental engineers apply to specific sites is available online for free on GitHub. “It’s a one-dimensional model accessible by a standard consulting firm,” said Professor Nepf.

Salt marshes represent a win-win situation for coastal defense. They provide an environmental benefit and a cost-effective solution.

“This paper presents a practical tool for translating the wave attenuation capabilities of marshes into economic values, which could assist decision-makers in the adaptation of marshes for nature-based coastal defense, ” said Xiaxia Zhang, a professor at Shenzen University in China.

The study is published in the journal Communications Earth & Environment.

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