Cities are sitting on a largely underused public resource: urban stormwater wetlands. If properly designed, these landscapes can reduce flooding, support urban wildlife, and serve as public space. A new report Design Guidelines for Urban Stormwater Wetlands — authored by an interdisciplinary group of researchers and students at the MIT Norman B. Leventhal Center for Advanced Urbanism led by Celina Balderas Guzmán, Heidi Nepf, and Alan Berger — advocates for the positive role wetlands can play in cities and outlines research that provides insights for landscape architects, engineers, and planners.
The authors make a case for the potential of urban wetlands, especially in a time of changing climate and deteriorating urban infrastructure. “Wetlands, the world’s most valuable terrestrial ecosystem, provide a multitude of ecosystem services: water treatment, flood protection, carbon storage, habitat, recreation, and aesthetic value,” they write.
And yet, in many cities, existing wetlands have been filled, paved, developed, or channelized, eliminating the benefits they provide. In this context, the authors see opportunity. “Just as urbanization has obliterated wetlands, urbanization can build them new,” they write. “While constructed wetlands are not in all aspects comparable to natural wetlands, they can partially restore some lost ecosystem services.”
However, urban wetlands present challenges for the prospective designer, not the least of which is understanding hydraulic dynamics well enough to create a design that is both beautiful and functional. This is where the team’s research steps in.
At MIT’s Nepf Environmental Fluid Mechanic Lab, the researchers tested dozens of different wetland landform configurations to better understand how “island size, shape, and placement affect hydraulic flow and provide ecological habitat.”
Researchers fabricated models of different topographies from high-density foam using a CNC milling machine. The models were then inserted into a flume (essentially a long, plexiglass tank that circulates water) for testing. The researchers used dye to track how different landform configurations impact the speed and direction of water flowing over the model.
In analyzing the results of these tests, the authors made some findings. First, topography matters. Topography describes the physical features of a landform. Results varied widely for the different landforms, meaning that certain design approaches are more or less appropriate depending on the goals of the design.
According to the authors, “wetland engineers and designers must make carefully considered design decisions based on hydraulic goals, balanced with ecological and urban goals as well.”
Second, in attempting to slow down water and filter pollutants, smaller interventions may be more effective. “Adding topography subtracts volume from a wetland’s potential water storage capacity,” they write, which means that “water will exit sooner simply because there is less water volume, leading to less pollution treatment.”
In their tests, the researchers found that models were most effective when the total volume of topography equaled approximately 10 percent of the total volume of the basin, although they caution this number may shift in real-world applications.
Of the thirty-four topographies tested, the team found two that provided the best balance of hydraulic performance and pollutant filtering capacity. They conclude the report by applying these topographies to two case study sites: Buffalo Bayou in Houston, Texas, and Taylor Yard, on the Los Angeles River in Los Angeles, California.
The case studies are intriguing, but may be frustrating to those hoping for a more detailed explanation of how to apply the team’s findings. However, the authors note that the studies are “urban design frameworks” and meant to be conceptual. Those seeking to transfer the team’s research to real world projects will likely find their topographic models to be helpful starting points, but will still need to develop unique design solutions that respond to site and program requirements.
Ultimately, Design Guidelines for Urban Stormwater Wetlands aims to “inform decision makers, planning agencies, consulting engineers, landscape architects, and urban designers about the efficacy of using ecologically-designed constructed wetlands and ponds to manage stormwater while creating new public realms.”
However, the authors do not present any hard and fast rules for designing urban wetlands. Instead, the report makes a compelling case for why constructed stormwater wetlands are an important and underused resource in urban areas, and provides information that may prove valuable to designers and public officials looking for ways to extract more public benefit from stormwater infrastructure.
“We hope this work gives practitioners and designers a new set of adaptable forms to work with and elaborate upon either in implementation or in future research,” says co-author Celina Balderas Guzmán, describing the study as “a crucial first step to explore forms and validate designs quickly and easily with scientifically rigorous metrics.”
In this respect, the report is a success, presenting imaginative possibilities for new urban spaces supported by hard research. As a resource for designers, Design Guidelines for Urban Stormwater Wetlands may not have all the answers, but it does have important ones.