VICKSBURG, Miss. — Along the Florida coastline, forests of trees with a dense tangle of prop roots appear to be standing on stilts above the water. These trees, or mangroves, are not only magnificent to see, but are a key element in protecting coastlines and communities during coastal storms. Researchers at the U.S. Army Engineer Research and Development Center (ERDC) have partnered with the U.S. Army Corps of Engineers (USACE) Jacksonville District and the U.S. Naval Academy to explore the engineering value of Florida’s mangrove forests.
For more than a decade, the USACE Engineering With Nature (EWN) Program has pursued the intentional alignment of natural and engineering processes to efficiently and sustainably deliver economic, environmental and social benefits. One of these opportunities is the application of natural and nature-based features, which are landscapes, such as mangrove trees, that are used to provide engineering functions relevant to flood risk management.
“Natural and nature-based features have been identified as a pathway to increase coastal resiliency and reduce coastal storm impacts for quite a while,” said Dr. Duncan Bryant, a research hydraulic engineer with the ERDC’s Coastal and Hydraulics Laboratory. “Districts are really starting to ask how to implement them, how to put a value to them, and how to include them in future designs. Across the board, we’re seeing that need to understand how these features can serve effectively.”
“This research on mangroves is incredibly important,” said Dr. Jeff King, program manager for USACE’s EWN Program. “Duncan and this collaborative team are advancing practice through their ongoing investigations. The results of this effort will help inform our future approach for the use of mangrove forest to reduce flood and storm risk.”
With the substantial number of mangroves growing throughout their area of responsibility, the Jacksonville District reached out to the researchers at the ERDC to help them better understand how to successfully use the trees in their project planning processes as well as quantifying their added value.
“They kind of initiated all this,” said Bryant. “They were the ones asking the questions.”
To answer those questions, Bryant and his team began collaborating with professors at the Naval Academy to build a scale model of coastal mangroves. For their project, which began in early 2020, they focused on two specific components.
“First, we felt it necessary to narrow our consideration to the properties of mangroves in Florida,” he said. “Some other experiments have been conducted focusing on the same species in Asia, but they seem to have some different regional growth habits. Those different growth habits can influence the engineer performance, so we really wanted to concentrate on what they look like specific to Florida.”
With field reconnaissance provided by the Naval Academy team and using documents and dissertations from similar studies that gave an overview of the Florida area, the ERDC team was able to build a 1:2 scale model.
“That was the other aspect we felt was necessary,” said Bryant. “Previous studies have been done on a smaller scale, but when working with smaller scaled models, you could unintentionally skew your results. Because of the size, some physics, some property, some physical process is not being accurately represented. In this case, we’re talking about turbulence. We felt like going as large scale as we could, would help alleviate that concern.”
“Oregon State University actually built a 1:1 model,” he said. “Their focus is a little bit different than ours, but we have been able to share data and see how similar or how different our results are. We’ve learned a lot about the basic science and about our experiments. They’re doing some work, and we’re doing some work — so to get the most and learn the most, it really helps us to share and let each other know what we’re doing, how it’s going and what we’re learning.”
As phase one testing on the model began to wrap up in December 2020, Bryant and the team set their sights on the next phase.
“Now that we’ve completed testing on waves and transport, we’re actually going to switch gears a little bit. We’re planning experiments that will include sediment,” he said. “That’s a big deal for the Corps, especially considering dredging and where sediments are going and sediment retention. We want to do some experiments this summer to capture some of the physics and the processes that happen within the mangroves from a sediment retention standpoint. The Naval Academy will be sending four midshipmen to help us work on that.”
Looking toward the future, Bryant says, “I imagine a lot of this will move into design and a numerical modeling type of approach for planning,” he said. “Once you have a physical-based description of the processes and what happens inside of these forests, you can now move up to larger implementation. Once we understand that response, we can start using that to make designs — to make plans — to say, ‘Okay, how does this compare to that?’ and make educated decisions for the Corps.”
“ERDC is working to describe, measure and provide predictive capabilities for the use of natural and nature-based features in the districts for the districts,” he said. “We’re working toward that goal — that’s the big thing. We heard the districts, and we’re trying to execute that. And at the same time, I would say through the efforts of the EWN Program, it’s really moved the USACE into a leadership position across the world.”