An excessive amount of nitrogen in Baltimore’s Inner Harbor is one of the major water quality issues undergoing improvement at the historic industrial waterfront. Surplus nitrogen levels fuel a harmful repeating cycle of algae blooms which block the sunlight from reaching underwater grasses, and during decomposition, create bacterial “dead-zones” that rob the water of oxygen. This hurts the native species of flora and fauna that reside in and rely on Chesapeake Bay’s native salt marsh environment.
As the city works to clean up and prevent polluted stormwater and sewage from depositing excess nitrogen into the Harbor, The National Aquarium in Baltimore’s conservation efforts are aimed at removing the nitrogen that is already there, as part of their overarching work to protect the areas waterways and wildlife. Part of the organization’s current Master Plan is to one day transform the canal between Piers 3 and 4 into a 15,000 square-foot floating wetland habitat.
Floating wetlands remove nitrogen from the water through the beneficial bacteria (biofilms) growing on its plastic base material and the new growth of native tidal marsh plants within that base. With roots taking up nutrients directly from the water, the intent is to use these plants to remove the excess nitrogen before it can fuel an algal or bacterial bloom.
To test the stability and resiliency of a future large-scale project, McLaren Engineering Group worked with architectural design firm Ayers Saint Gross to develop a 15-foot by 20-foot prototype living shoreline ecosystem that is currently exceeding expectations at the National Aquarium’s campus in the Inner Harbor. Experts from both the Aquarium and Biohabitats guided the development of the man-made structure by applying the sciences of marine biology, conservation planning, and ecological restoration into the design.
A major challenge with sustaining small-scale floating wetlands in the past is failure due to their long-term maintenance and short service lives. The more thriving the wetland, the heavier and more unbalanced the structure would become, causing it to tip or sink. McLaren and their project team took into consideration the lessons learned from these previous structures to design a floating wetland with inert plastic materials and an adjustable buoyancy system to counteract the accumulation of ongoing marine growth. This design solution blurs the boundaries between natural and structured urban environments, showing they can coexist and flourish together.
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 ballast utilizes a dynamic buoyancy system of high-density polyethylene (HDPE) 30-inch diameter pontoons with adjustable water fill. The entire structure is designed to easily accommodate additional pontoons to be floated under the wetland, attached, and then pumped full of air to provide supplemental buoyancy. A portion of the HDPE pontoons are with closed-cell marine foam, and 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 (polyester material), 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 were filled 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 becomes more stable and improves its ability to support live loads and wave loads.
The resilient wetland is designed to accommodate FEMA 100-year flood levels, resist winds, waves, and currents. Additionally, it can support 40 pounds-per-square-foot of live loads, has a service life of 30 years with minimal maintenance, and can withstand the catastrophic loss of a pontoon’s buoyancy without structural failure.
Four years after initial installation, the floating wetland prototype’s success has brought the National Aquarium’s vision of developing a large-scale version one step closer to reality. The man-made living shoreline ecosystem has attracted native species while improving water quality and providing visitors a unique perspective of the salt marsh habitat and its critical role in the health of the Chesapeake Bay’s ecosystem. It has successfully attracted native fish, reptiles, crustaceans, mollusks, and birds seeking food and shelter. The Aquarium continues to perform water quality monitoring, root system analysis, and core sampling to measure how much nitrogen is being removed, doing their part to sustainably improve the Inner Harbor.