A New Model for Floating Wetlands

The National Aquarium has an ambitious mission to inspire conservation of the world’s aquatic treasures. 

With its prime location in downtown Baltimore on historic shipping piers, the Aquarium wants to localize this mission by restoring aquatic environments in its own backyard, the Chesapeake Bay. To that end, the Aquarium is planning to redevelop an inlet at the heart of its campus with a large-scale floating salt marsh. 

These recreated wetlands will serve multiple purposes. They will support greater biodiversity in the Inner Harbor and provide infrastructure for supplemental oxygenation of the water. They will also be an immersive experience for learning about the Chesapeake Bay watershed and its component ecosystems. 

Several major technical challenges stand in the way of realizing this vision. First, conventional floating wetlands are costly, and yet they typically last a mere five years. It would be prohibitively expensive for the Aquarium to replace such a large floating wetland structure (planned to be 16,000+ SF) twice per decade. 

Secondly, conventional floating wetland systems are topographically flat and not readily calibrated to create a range of microhabitats. They are incapable of supporting the ecological diversity that the Aquarium desires for this unique environment. 

Lastly, conventional floating wetlands are not stable enough to support maintenance personnel. For the Aquarium to be able to manage such a landscape, the structure needs to be designed with a high degree of stability. 

To realize the client’s vision, our designers (and our partners at Biohabitats, McLaren Engineers, and Kovacs, Whitney & Associates, continuing Studio Gang’s EcoSlip concept) had to create a durable and more topographically varied floating wetland. 

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A brief history of Baltimore’s Inner Harbor: in pre-Columbian times, there was tremendous biodiversity in this zone of the Chesapeake Bay. With the rise of the Industrial Revolution, the area became a major shipping port. Hard infrastructure development mirrored rising urban populations into the early 20th century, replacing natural shorelines. Humans reshaped the harbor to suit the needs of industry and shipping, which resulted in lost habitats and waning species diversity. 

The heavy industry eventually faded, and in the 1980s the Inner Harbor was one of the first post-industrial waterfronts transformed into a cultural amenity. Unfortunately, while the land surrounding the Inner Harbor’s water was revitalized, the water itself was largely neglected. 

Another significant development that affects the health of the Chesapeake Bay is sprawling urbanization throughout much of its watershed. Hard surfaces cover soil and prevent infiltration of rain water into the ground, so when rain falls on buildings and pavement, it carries lawn fertilizers, pet waste, and road salts into storm drains. Leaks in an aging network of sewer and stormwater pipes, running underneath the city, also added excess nitrogen and phosphorous to Inner Harbor waters. This polluted urban stormwater runoff joins suburban and rural runoff and ultimately flows downstream into waterways like the Inner Harbor. Excess nitrogen and phosphorous, transported in polluted stormwater runoff, is utilized by naturally occurring phytoplankton species and fuels an endless cycle of algae population explosions and crashes throughout the Inner Harbor. When the excess fertilizers that enabled the algal blooms to occur are consumed, a massive die-off of phytoplankton follows. The dead algae sinks to the bottom and provides food that fuels a major bacterial bloom. The rapidly growing bacteria population uses up all the available dissolved oxygen in the water and effectively smothers fish, crabs, and other aquatic life. 

Reversing years of environmental degradation and creating a renewed and thriving ecosystem requires a large-scale intervention capable of delivering a wide array of ecological services. Floating wetlands were a natural choice for the Aquarium’s project.  

However, as noted above, conventional floating wetlands have some significant drawbacks. They are typically made of polyethylene terephthalate (PET) injected with marine foam for buoyancy. Plants are placed in drilled holes to allow their roots to reach directly into the water. The PET layers are typically flat with upper layers extending out of the water – a form that does not mimic the varied topography and microhabitats of most wetlands or tidal shorelines. Thus only a limited number of aquatic species can thrive in them (falling well short of the Aquarium’s ambitions for this project). 

Additionally, with time, biomass accumulates from plants and bivalves that colonize the PET mesh, causing the entire wetland to sink under its own weight.  

Therefore, while current models of floating wetlands can serve decorative and educational purposes, they are ultimately more akin to a flower show exhibit than to a real-life habitat that is both durable and functional enough to achieve the Aquarium’s objectives. We had to develop a new floating wetland model and adapt an array of technologies from other disciplines to realize our goals. 

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In collaboration with the Aquarium and our multidisciplinary team of scientists and engineers, we designed a new kind of floating wetland. It improves upon the technologies of conventional floating wetlands while remedying their shortcomings in terms of habitat-creation capabilities and the lifespan of the final installation. These new technologies and variables have been prototyped and are currently being tested within the harbor on the Aquarium’s campus. 

First, we addressed the issue of topography. 

In lieu of a flat floating sheet of PET, our team created a layered topo-model with varied planting surfaces at different elevations, some submerged, relative to the water surface. In the middle of the prototype, a deeper channel provides habitats analogous to shallow salt marsh tidal channels. On the edges, the layers stack up to simulate the low and high marsh environments of the Chesapeake Bay. The prototype also features airlifts and air diffusers, which help to oxygenate and continuously circulate the water and prevent water stagnation in the channel and around the outer edges of the form. All together, these interventions create a variety of microhabitats, which will be utilized by a greater diversity of species and life stages of those species. 

Secondly, we addressed the issue of buoyancy. Conventional floating wetlands have what is called static buoyancy from integrated marine foam, which means they can generally restore equilibrium in response to pressure (ie, they don’t capsize or sink easily). Our design adds a rigid support structure underneath the PET layers with capabilities for adjustable buoyancy. This “skeleton” is made of high density polyethylene (HDPE) pipes and pontoon structures that provide the wetland with ballast. 

Adjustable buoyancy is essential to longevity. As the plants grow and become heavier, the PET bed can be raised or lowered by pumping water into or out of the pontoons as needed. This design feature also allows for easier maintenance and unique research opportunities. The pontoon structure acts similarly to a ship’s ballast system, whereby trim and list are controlled through adding and removing water. That way, the elevations of individual areas of wetland can be controlled, rather than solely raising and lowering the entire structure uniformly. 

The reserve buoyancy system within the PET layer is one of the most difficult and sensitive portions of the design. As buoyancy is directly related to the weight of water displaced, PET mesh itself has very little buoyancy in reserve to counteract the added weight of maintenance workers and waves. To address this issue, we filled hollow cavities in the PET layers above the waterline with marine foam, which is engineered to provide added buoyancy and stability to allow people to stand on the edge of the wetland without it swamping. The foam cavities are carefully spaced in linear strips to avoid interference with plantings. 

Additionally, we added a cementitious bonding coating to the PET to increase longevity with regard to ultraviolet degradation. 

The 200-SF prototype was shop-fabricated, transported in pieces, and then assembled in a shipyard on the Middle Branch River before being towed to its current position in the Inner Harbor in August 2017. Aquarium staff then planted it with over more than 1400 plugs of native plants. (The staffers were pleased to report that the wetland was stable and firm underfoot—a pleasure to work on compared with the small conventional floating wetlands that have been used on a small scale around the Inner Harbor.) Every square inch of this ecological powerhouse provides opportunities for a diverse range of organisms to grow, colonize, molt, spawn, or eat. 

Nine months into the experiment, preliminary results are promising. 

Almost immediately after implementation, Aquarium scientists observed a rapid colonization of the submerged woven PET material by biofilms, a type of beneficial bacteria that creates a sticky, living coating of the vast PET surfaces. Biofilms feed on excess nitrogen and other nutrients in the water and are the first step towards reaching broader biodiversity and recreating a more natural and multi-layered food web. 

By the third day, schools of killifish moved into the prototype’s central channel, and a blue crab was observed molting in the protected shallow water of the new habitat. More fish, anemones, and crustacean species soon followed, along with the arrival of larger species like wading birds and muskrat. The recreated wetland has brought several native species back to the Inner Harbor and into full view of people passing by. 

Going forward, the performance of the prototype will continue to be measured. Its impact on water quality will be monitored using data collection equipment installed nearby in the same inlet. This information will help us to calibrate and refine the design of the floating wetland system, so that it has maximum impact when it is fabricated at full scale. 

We’re excited to see what’s next for the Aquarium, the Harbor, and the Bay, and what role our newly designed wetlands can play in improving these vital and beautiful places.