Feature

Cleaner Water Through Site-specific Floating Treatment Wetlands

By Michael Ross, SITES AP, ASLA, Assistant Professor, Department of Plant Sciences + School of Landscape Architecture, University of Tennessee and Mia Roark, graduate student, School of Landscape Architecture, University of Tennessee

I saw its tail move first. The Guadalupe Bass, Micropterus treculii, was hiding under the mass of willow roots rippling in the current. The undercut bank and riparian vegetation, particularly the exposed willow roots, provided a great ambush spot for Texas’s state fish, which is closely related to the smallmouth bass. The guad, as we lovingly refer to them, is a sucker for crayfish patterns. With my friend on the oars and a moderately accurate cast, scarcely had the fly hit the water and began to sink than the predator rushed out from cover and inhaled it. Experiences like this one on the San Marcos River in Central Texas shaped my view of urban creeks, streams, and rivers and particularly the biodiversity possible when water and habitat quality are high. These types of habitat conditions have been key to many great fishing experiences as well as being essential to my early development as a budding herpetologist as copious numbers of turtles, salamanders, and snakes can be found in root curtains and shady bank overhangs.

As I have moved around the country and traveled, I have seen the impact of urbanization on watersheds and waterways. Water quality issues and lack of good habitat have impacted streams, creeks, and rivers globally and it is particularly impactful in our state as Tennessee is known for the diversity of its aquatic organisms. But there is hope.

Floating Treatment Wetlands: An Opportunity for Cleaner Water

Maintaining effective interplay of aesthetics and function is one of the on-going balancing acts of contemporary landscape design, especially design based on ecological principles. Traditional approaches to landscape design often have embraced a Eurocentric approach to aesthetics that largely ignores ecological assemblages, species interactions, and habitat complexity. Given the environmental challenges facing our communities, the landscape industries are uniquely positioned to intercede.

One of the issues that arises with ever greater frequency is water quality. As our land development patterns continue to sprawl, impervious surfaces increase, as does the associated runoff pollution, volume, and velocity. As private land owners, municipalities, communities, and commercial entities begin to see their role in mitigating these by-products of growth, new tools that allow for tailored site-specific approaches gain in popularity. One such approach is the application of floating treatment wetlands (FTWs). These floating islands utilize regenerative systems inherent in plant growth to mitigate pollutants in the water, provide biodiversity and habitat, as well as aesthetic opportunities with seasonal flowering times and colors.

How FTWs Work

Floating treatment wetlands work by floating at or slightly above the surface of the water. Plants that have evolved to grow in saturated, moist, or emergent conditions are placed in planting pockets with a small amount of soil (Image 1). These planting pockets or moisture levels can be tailored to meet the design goals based on what conditions the individual plants prefer. In time the plants send roots out through the island and into the water. These roots represent the primary mechanism for phytoremediation. The roots pull in water, extract excess nutrients, and develop biofilm. These mechanisms are particularly useful in conditions where excess nutrients or suspended solids might be problematic. They also provide excellent habitat for fish, amphibians, and invertebrates that help fuel a larger food web. In effect, the roots act much like the bank overhang with the root curtain I described above. The shade and accompanying cooler temperatures, lower light intensities, cover, food, and cleaner, less turbid water provide meaningful benefits to highly impacted urban streams. These are all positives from the perspective of aquatic organisms as well as those critters that feed on them.

IMAGE 1: Installed commercially available floating wetland cell

Experiential Learning Part I: Teaching with the FTW Application

The added benefits of habitat creation and aesthetics are key components to well-designed green infrastructure and as part of the green infrastructure class I teach at the University of Tennessee. I have worked with students from Plant Sciences, Landscape Architecture, and Environmental Sciences to design and build FTWs utilizing cut shoots of manually harvested invasive bamboo, Phyllostachys aurea, as the structure and native wetland and emergent plants as our phytoremediating component (Image 2). Additionally, students are given the opportunity to plant and launch commercially-available units, much as they would spec as a landscape architect, designer, consultant or contractor.

IMAGE 2: Students harvest bamboo for the raft construction project.

Currently we are utilizing these islands as a way to address issues of water quality, habitat, and aesthetics in Knoxville’s Third Creek. Our project is part of a larger effort to improve the greenway and is based in the University’s shared concern for the waterway and its impact on the larger watershed, the Tennessee River, and the citizens of the state.

Experiential Learning Part II: A Student’s Point of View

The impact of this project extends well beyond the waterways and has directly impacted the students who are contributing to our team’s effort. Mia Roark was invited to share her perspectives and insights on the experiential learning that she gained. As she explains:

“With a bachelor’s degree in Ecology and Evolutionary Biology and an aspiring Master’s in Landscape Architecture, I have spent most of my academic career learning about environmental challenges facing our society. Personally, I have a specific interest in urban development and the implications it has on the natural environment and its processes. As population numbers rise and cities continue to expand, the rapid urbanization of areas has led to various anthropogenic complications, both ecologically and economically. However, as cities continue to develop, design also continues to evolve. Now professionals are generating more sustainable, innovative solutions to mitigate these anthropogenic complications that arise from development.

To further explore these solutions, I took Professor Mike Ross’s Green Infrastructure Theory course at the University of Tennessee in spring of 2021. Professor Ross has designed this course as an exploration of the history, theory, construction, and maintenance of sustainable design as a cost-effective, innovative solution to a myriad of ecological concerns. One of the major components of our course was the design, creation, and implementation of a floating treatment wetland (FTWs) for Third Creek.

Third Creek, which runs through the University of Tennessee’s campus, is heavily impacted by urban stormwater runoff from rooftops and pavement in the surrounding areas. Ultimately, this runoff has led to the pollution of the creek and degradation of the surrounding habitat. To help mitigate these concerns, Professor Ross tasked us with designing our own floating wetland treatment. FTWs act as engineered rafts that support native wetland vegetation, which provide habitat and attenuate pollutants from the water.

In small groups, we were instructed to design and build our own FTW from bamboo, cultivated from an invasive patch on campus, and any other biodegradable materials of our choosing. The design inspiration for my group’s floating wetland came from the traditional art of creating bamboo furniture. We adopted the strategy from Taiwanese artists to make small triangular cuts in a singular piece of bamboo to create a bend. This technique is not only visually interesting; but also creates a stronger corner in comparison to connecting two separate bamboo pieces. We then used deck screws to attach two bent pieces to the opposite sides of a rectangular box made from four pieces of bamboo drilled together to form a hexagon shape. This shape was chosen so that in the event we created more wetlands, we could form a honeycomb pattern by connecting them together. Once our shape was finalized, we assembled two additional identical hexagons and attached them on top of one another for buoyancy.

Prior to finalizing this model, we did a floatation test to insure our FTW would indeed float (Image 3). While our design was successful, we were apprehensive whether the structure could support additional weight. Therefore, to increase buoyancy we decided to add two side palettes of dry bamboo to the bottom of our wetland, similar to the structure of pontoon boats. Then, we latched each corner so that two pieces of bamboo meet with a waxed string for added support. Finally, we finished our FTW by creating a base for the soil and plants by adding split pieces of bamboo and coconut coir fiber to the top layer.

IMAGE 3: Students launch their bamboo raft constructed wetland cell

The process of designing, creating, and implementing a floating treatment wetland in an environment that we were familiar with was an invigorating, eye-opening experience (Image 4). We were able to observe the concerns surrounding Third Creek and integrate a solution of our own design from start to finish. Personally, I too often succumb to the notion that change must happen at a large scale. However, this opportunity taught me that it is possible to enact change in only two months. Though our design had its flaws, it was still successful in supporting native vegetation and providing habitat, which was the prevailing priority.

IMAGE 4: The bamboo wetland cell is transplanted and made ready for launch

After completing the Green Infrastructure Theory course in May of 2021, I began an internship with Professor Ross working on the enhancement of Third Creek and the greenway that runs alongside it. Largely, we are focused on the restoration of the riparian buffer surrounding the creek, the improvement of the quality of water, and the enhancement of the greenway users experience. The integration of more floating treatment wetlands, both commercial and student designed, will offer a unique, cost effective solution to the restoration of the degraded habit and water quality of Third Creek.”

The Long-Term View

Stream restoration is very important, but can often be a long and costly process. The benefits of green infrastructure can help offset costs and lessen the cumulative impacts of urbanization. FTWs are not a panacea for poor ecological design and management, nor will they make up for all of the lost riparian and aquatic habitat we have experienced in Tennessee. These structures can certainly help us gain lost ground, however. FTWs will play a role as an inexpensive solution available to private landowners, golf courses, lake side residential communities and urban parks and greenways. Through the efforts of University of Tennessee students and faculty, we are learning how to apply this agency across scales with multiple benefits in mind. In the long-term we are creating informed future professionals, clients, and constituencies that care about watersheds and swimmable, fishable urban waterways. Growers, consultants, contractors, and designers can all make a positive difference in the aesthetics and biodiversity of our urban waterways.

For more information about this project or to learn about other opportunities in Sustainable Landscape Design, or would like to know more about the University of Tennessee’s Masters in Landscape Architecture degree program, contact Michael Ross at mross28@utk.edu or (865) 9741606.

This article is reprinted with permission from Tennessee GreenTimes, Fall 2021.