Part of the National Aquarium in Baltimore’s Master Plan is to transform the canal between Piers 3 and 4, in Baltimore’s Inner Harbor, into a 15,000 square foot floating wetland habitat. McLaren Engineering Group worked to transform their vision into a reality by engineering a living shoreline ecosystem that attracts native species while improving water quality, and providing visitors a unique perspective of the salt marsh habitat and the critical role it plays in the health of the Chesapeake Bay’s ecosystem.
The floating wetlands “S” curve design mimics the natural marsh shoreline and is made up of three separate islands, with the biggest being over 250 feet long and 80 feet across at its widest. A floating dock with ADA accessible gangways, extends out from Pier 4 bringing visitors closer to the green space that will create a home for numerous native species — crabs, mussels, wading birds waterfowl, eels, and other fish species.
The full-scale floating wetland will be layered with PET (polyethylene terephthalate / plastic) mesh, giving native salt marsh plants a place to grow. Airlift pipes will provide constant water flow to the topside’s shallow channels of the floating wetlands, creating running streams like the natural shallow channel microhabitats found in tidal salt marshes. An additional aeration system located under and around the perimeter of the floating wetland will constantly mix, destratify, and increase the dissolved oxygen levels in the upper six feet while improving the quality and overall health of the water column below.
To test the stability and resiliency of a large-scale project, McLaren’s Marine team engineered a 15-foot by 20-foot award-winning small-scale prototype that is currently exceeding expectations in the Aquarium’s waterfront campus in Baltimore’s Inner Harbor.
Small-scale floating wetlands have failed due to maintenance and short service lives — the more thriving the wetland became the heavier and more unbalanced the structure, causing them to tip or sink. McLaren and their project team took into consideration the lessons learned from these previous structures and designed a floating wetland with inert plastic materials, and an adjustable buoyancy system to counteract the accumulation of marine growth. This design solution blurs the boundaries between natural and structured urban environments, showing they can coexist and flourish together.
The 1-year prototyping phase started in the spring of 2017. A 15 foot by 20-foot prototype that anchors on two sets of existing guide pipe piles was developed to 60% design. The project team determined the best-detailing practices and operational limitations of the floating wetland before finalizing 100% construction drawings, saving the NAIB money, and while designing the most effective project.
The McLaren team found that the most prudent and cost-effective solution for creating stability in the low freeboard required by the plantings (highest marsh levels only extend 6 inches above water) was adding a controllable ballast system, counteracting the effects of added marine growth weight. By calculating the sinking rates from the prototype, the design team arrived at an estimated fouling load of 1.5 pounds per square foot per year, which will gradually taper off to near zero before year 10.
The controllable ballast system utilizes high-density polyethylene (HDPE) 30-inch diameter pontoons with adjustable water fill, referred to as the “dynamic buoyancy” system. The floating wetland is designed to easily accommodate additional pontoons being floated under the wetland, attached, and then pumped full of air to provide supplemental buoyancy. A portion of the HDPE pontoons will be filled with closed-cell marine foam, and will provide an unchanging buoyant force, referred to as the “static buoyancy system.” Both the foam fill and static buoyant force can be calibrated to match the weight of the wetland’s structural components, PET, and plantings.
With very little buoyancy in reserve to counteract the added weight of maintenance workers and waves, a reserve flotation system was engineered for added buoyancy and stability, allowing employees to stand on the edge of the wetland without it swamping. Cavities within the PET will be filled for the reserve system, with spray-applied closed cell marine foam, and carefully spaced in linear strips to not interfere with plantings. As the PET colonizes with biological material overtime and void space reduces, the wetland is predicted to become more stable and improve its ability to support persons and wave loads.
The top structural layer consists of 2-inch-thick fiberglass grating. The grating is environmentally inert, lightweight, and provides easy means of fastening the PET and fiber-reinforced polymer (FRP) threaded rods above the structure. The fiberglass grating also provides a sufficiently rigid base to support laminated PET layers with foot traffic and not cause damage. The wetland is designed to accommodate FEMA 100-year flood levels, resist winds, waves, and currents from a 100-year storm, locally support 40 pounds per square foot of personnel loading, have a service life of 30 years with minimal maintenance, and be able to withstand the catastrophic loss of a pontoon’s buoyancy without structural failure.