Mangroves versus dykes? The mix may be the ‘best of both worlds’ to deal with rising South Florida seas

Along much of South Florida’s historic coastline, mangroves were nature’s way of protecting the coast from waves and hurricanes. As development moved closer to the water, levees became the method of choice for separating land and sea.

With two feet of sea level rise on the horizon by 2060, what is the right choice to extend the life of cities like Miami?

Science – at least for pure flood control – suggests that man’s response trumps nature’s solution, with some caveats.

Peter Sheng, a University of Florida researcher who has studied the protective value of mangroves in Florida, said they are an incredible tool when used correctly. In places like Cutler Bay, huge expanses of mangroves provide a barrier between hurricane waves over 10 feet and populated areas.

“If you have a narrow urban mangrove planter, it’s not enough,” he said. “Three meters of mangrove does nothing. It’s a pledge for the look. If you want to dissipate waves, you need at least 100 meters.

This calculation was evident in the federal government’s decision that the best method of protecting the Miami-Dade coast from future punishing hurricanes was a 20-foot wall stretching for miles along the coast, not the combination dense mangroves and short levees many residents. raised in public meetings.

In the end, the county and its residents rejected the wall, sending the US Army Corps of Engineers back to the drawing board for a new solution that the people of Miami-Dade will actually love, and that aims to protect them for decades. coming.

An aerial view of current conditions at Brittany Bay Park, where Miami Beach plans to spend $1.8 million to renovate the park and flesh out a living shoreline with mangroves.

How do mangroves stack?

It’s not that dykes are better than mangroves 100% of the time, said Rik Gijsman, a coastal engineer and researcher at the University of Twente in the Netherlands. In a 2021 paper, he and other authors analyzed a decade of research on the ability of mangroves to reduce flooding compared to a levee.

“It’s not a ‘either’ question. They are different systems and they can both be useful depending on local conditions,” he said. “I think we need both and it depends on the local conditions, how to integrate the two.”

Rod Braun, director of climate strategy for The Nature Conservancy, said mangroves have a long list of benefits, including controlling erosion, providing habitat for fish and other organisms that inhabit the bay. , improving water quality and storing carbon dioxide, a greenhouse gas that causes global warming. And unlike a concrete wall, they actually form a stronger, taller barrier over time.

“I think that makes it a nicer place to live,” he said.

And the dykes have problems. When the waves crash against the hard concrete, they dig into the sand below, making it a less friendly place for sea creatures or plants.

Aerial view of an island

Pedro Portal

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Miami Herald

On January 26, 2022, scientists from the Department of Environmental Resource Management (DERM) sail by Crescent Island B, which is part of the county’s former living shoreline projects on Biscayne Bay. Living shoreline projects use a variety of materials such as wetland plants, oyster shells, coconut fiber logs, sand, wood, and native rock.

The combination of the two, known as the hybrid approach, solves many of these problems. That’s what the City of Miami suggested for Morningside Park’s new shoreline, a 6-foot seawall in combination with mangroves and other landscaping. Then some residents complained that the mangroves would spoil their view and they were removed from the project.

But in the case of the corps plan, which was designed to protect the county from extremely high waves associated with hurricanes and not just occasional flooding, the data shows that even a hybrid approach might not be enough.

Gijsman said studies consistently show that to cut huge waves down to size, you need a lot of mangroves, an Everglades-sized amount of mangroves.

“Mangroves will never provide complete security for true storm surges,” he said. “To really attenuate the waves, it takes miles.”

Science is always out

Yuepeng Li, a Florida International University researcher who has published several papers attempting to quantify exactly how much protection mangroves provide, said there are many factors to consider in determining this. It doesn’t matter what species of mangrove you’re talking about, and even the root to branch ratio makes a difference.

Accounting for those variables, he said, it takes about half a mile of mangroves to cut through even a foot of storm surge.

“Even a mile of mangroves, I don’t think will significantly reduce thrust,” Li said.

One of the reasons builders stick with levees over mangroves, Gijsman said, is so you can build a concrete structure the exact same way every time and guarantee the same performance.

“We have realized for a long time already that the natural area contributes to flood protection, but it is not so easy to work because nature is unpredictable, and as an engineer you want to be on the safe side”, did he declare.

For example, engineers can calculate what it would take for a levee to crack and fail. As for the mangroves, they are not so safe. This makes it difficult to provide the “return on investment” numbers that organizations like the Corps rely on to make decisions.

And as the world gets hotter, wetter, and saltier, mangroves could perform less well.

The best of both worlds

To arm the coasts, dykes and mangroves are not mutually exclusive solutions. Increasingly, planners seek to combine the two strategies.

Landolf Rhode-Barbarigos, assistant professor at the University of Miami School of Engineering, said it was “all about the hybrid” of the two – using a levee to protect against flooding and plants like mangroves to provide environmental benefits.

    A rendering illustrating what the 10-foot-tall walls proposed by the Army Corps of Engineers, designed to protect downtown Miami from flooding caused by storm surges, could look like.

Curtis + Rogers Design Studio and the Miami DDA

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A rendering illustrating what the Army Corps of Engineers proposed 10-foot-tall walls designed to protect downtown Miami from flooding caused by storm surges could look on the Miami River alongside solutions based on nature as a living shore.

But calculating these eco-friendly additions is much more difficult than simply seeing how much less flood damage a city faces with or without a levee. Not everyone agrees on the value of having a few dozen fish around or a few pounds of carbon dioxide sucked out of the atmosphere.

Rhode-Barbarigos said that if planners changed the question they ask at the beginning from “what provides the best flood protection?” to ‘what makes this space the most good?’ then a hybrid solution is a clear winner.

“What you get with the hybrid, I want to believe, is the best of both worlds and not the other way around,” he said.

This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative created to cover the impacts of climate change in the state.

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