Aquarium team creates first-of-its-kind floating wetland

Members of the project team for the National Aquarium’s new Harbor Wetland were asked to do something no construction team had ever done before: build a uniquely engineered, robust and beautiful floating wetland.

Staff at the National Aquarium had long dreamed of creating a wetland in Baltimore’s Inner Harbor to help reestablish a healthy ecosystem that had been lost to industrialization. Early experiments using off-the-shelf parts produced mixed success. They supported ample plant life and wildlife. However, just as a small, floating wetland achieved that success, it became so heavy that it sank or broke from its mooring and drifted away.

However, in 2017, the Aquarium developed a more highly engineered prototype wetland module with improved mooring, pontoons to keep it buoyant and other features tailored to conditions in Baltimore Harbor. The invention proved robust and ecologically successful, so Aquarium staff began exploring plans to build a much bigger version.

The Aquarium tapped The Whiting-Turner Contracting Company to lead the construction of a 104-module, 8,000-square-foot, adjustable-buoyancy, shallow-water wetland and 2,000-square-foot learning dock.

The start of construction in late 2023 “was scary,” said Charmaine Dahlenburg, Director of Field Conservation at the National Aquarium. “No one had ever done anything like this before, so there was no roadmap for us. We really didn’t know all the challenges we would face by scaling up the design until we faced them in the field, and then we had to make some hard decisions.”

A key challenge was creating a system to manage the buoyancy of the modules, which is controlled by the amount of air and water (ballast) in their pontoons. Different modules respond to ballast changes differently, depending on weights of vegetation and on whether they are completely surrounded by other modules or not. The project team had to revise designs for the pontoons, supply lines and control system to avoid any listing and to make system operations manageable for Aquarium staff.

“Logistically, this project was incredibly complex,” said Ben Kilduff, Project Manager at Whiting-Turner Contracting, who was featured in a video about the award-winning project.

In addition to dealing with a high-visibility site, limited working area and weather disruptions, the project team faced another unusual challenge.

“The other big thing that no one thinks about in construction is this thing [the wetland] is moving constantly,” Kilduff said in the video. “One of the most difficult parts of working in the harbor is the tides. They are raising and lowering constantly and that impacts what activities you can get done in a day.”

Electricians, mechanical subcontractors and other project partners had to figure out how to design and install systems, equipment and structures in or over a floating marine structure.
Excell Concrete Construction was tasked with creating a relatively small piece of the Harbor Wetland, namely a cantilevered, concrete slab at the edge of the pier that would serve as a tie-in for the floating dock. In total, it would amount to pouring about six cubic yards of concrete.

Determining how to pour that small concrete structure over water, however, was no small feat.
Excell President Ted Bowes spent weeks swapping drawings with a form company before finalizing a plan.

“We created a grid of steel beams, that are referred to as cribbing, on the pier and we cantilevered the edges of them out over the water,” Bowes said. “We placed high-strength rods down through the beams to support the formwork for the concrete extension. Then, to counteract the weight of the concrete that we would pour, we placed huge, concrete blocks on the opposite end of this steel grid.”

The counterweight blocks, he added, contained more concrete than the pier extension.
Members of the project team completed a huge amount of off-site fabrication, including the dock system, wetland modules, PET planting media, large mechanical equipment and all structural steel components. To minimize environmental impacts of construction, Whiting-Turner Contracting worked with subs to limit packaging of materials coming onsite and implemented logistics, such as mesh on site guardrails, to prevent debris from entering the water.

Project team members also made extraordinary efforts to avoid disturbing inhabitants of the Aquarium.

“We house marine mammals in our building, including the Atlantic bottlenose dolphins who are incredibly sensitive to sound and vibration,” Dahlenburg said.

When it came time to drive pilings for the Harbor Wetland, the project team used a bubble curtain mechanism to reduce the noise and vibration and followed a cautious approach to the operation.

“We had someone outside watching the construction and someone inside watching the dolphins,” she said. “Anytime they hit that piling, they would stop and wait to hear if there had been any change in the dolphins’ behavior and whether they could hit it again. It was a very long process and very careful.”

But it spared the dolphins from any distress.