Bioswales

By Robin Lynch and Adam Sapin

With more stories of major flooding, drought, and eroding landsides in the news, people often ask, “What is the answer to solving these problems?” As a major component of a holistic Best Management Practices approach, bioswales are one of the many answers.

Both large- and small-scale projects can take advantage of updated bioswale techniques to improve city infrastructure—from the increase of biodiversity to water purification and the reduction of flooding. By understanding how these can best be utilized, designed, installed, and maintained, officials have the opportunity to change the way a new project is approached.

As communities look for ways to enhance their public spaces with aesthetically pleasing and environmentally stable elements, bioswales are evolving beyond their basic functionality to allow for more creative applications. No longer simply relegated to concrete drainage systems, bioswales not only can convey the natural beauty of their surroundings but take advantage of rain and other sources of runoff, where neighborhoods—predominantly surrounded by pavement—have experienced problems with pollution and flooding, and turn them into beautiful, adaptive solutions.

What Are Bioswales?
Bioswales consist of a swaled course, in the surface of the ground, with ≤1-percent drainage, and gently sloped sides ≤6 percent, and are comprised of natural materials, such as native plants, rocks, and soil.

Bioswales direct and absorb water draining from higher elevations, filter pollutants, and distribute the purified water to linked channels and aquifers at lower elevations, thereby hydrating species and wildlife in their path as the water continues its journey to local waterways and the ocean.

What Are Bioswales Used For?
According to the EPA, storm water runoff is one of the fastest-growing sources of pollution. When rain hits rooftops, parking lots, and roads instead of wetlands, forests, and grasslands, it tends to run into storm drains that are directly connected to waterways. Bioswales and other natural resources can hold excess water in place, filtering out sediment and pollutants before they reach waterways, while also helping to recharge groundwater and prevent flooding. Bioswales, as a critical component of green infrastructure, mimic natural habitats and absorb excess water. They help to conserve water, preserve water quality, reduce the amount of pollution in waterways, and improve natural habitats and urban centers.

The Exposition Light Rail Train & Bikeway project (LRT) and its adjacent Inaccessible Landscape Areas project (ILA), both of which run between Culver City and Santa Monica, Calif., were designed with bioswales running parallel to the trackway, in order to fulfill a critical infrastructural role by protecting the rail lines from flooding during seasonal rains. This important feature is partly why the bioswales cover a staggering 120 percent of the total linear feet of the ILA project (due to their implementation along both the north and south sides of the track in numerous areas) and 50 percent of the total linear feet of the 6.4-mile LRT project.

However, the sheer volume of bioswales on these projects not only protects the massive expanse of rail infrastructure and its passengers, but also contributes to the ecology of the entire surrounding region in the following ways:

1.      Bioswales facilitate water distribution. Bioswales move and distribute water across and within a greater level of landform surface area. The stretch of bioswales on the LRT project collects runoff from the surrounding areas, preventing pooling and flooding on hard surfaces that otherwise would have evaporated or advanced across impermeable surfaces via unabated sheet flow, directly into the storm drain system. Instead, the bioswales distribute filtered runoff into both the soil and connected waterways at an appropriate pace and across a greater amount of landscaped area.

2.      Bioswales recharge aquifers. Bioswales allow storm water to permeate its soil walls—infiltrating beneath the surface—via deep drainage or deep percolation to recharge the thirsty underground aquifers, hydrating the soil profile, from surface water to groundwater, deep in the vadose zone below plant roots. This process not only is important for sustainable groundwater management, but can help move excess salts that accumulate in the root zone to the deeper soil layers of the groundwater system.

3.      Bioswales increase biodiversity. An important component of the bioswale filtration process is the use of native plants. The bioswale gradually slopes downward at about 1 percent to a low collection or infiltration point. This slope, as well as the channel shape, creates microclimates where plants at the low point need to be able to withstand greater levels of shade and saturation, while plants at the top of the bioswale need to withstand greater sun exposure and dryness. The wide range within the bioswale microclimate requires a diverse selection of native plant species to successfully contribute to the functioning of the bioswale, which consequently increases the biodiversity of plant species, as well as other insect and animal species to which they are symbiotically connected.

4.      Bioswales facilitate water purification. As storm water runoff passes over impermeable urban surfaces, an extensive number of pollutants (such as animal waste, litter, salt, pesticides, fertilizers, oil, grease, and other potential pollutants) are swept up with it. The integrated system of a bioswale works in concert with the soil, the roots of native plants, and due to its slope and channeled shape, acts to infiltrate the storm water along with the pollutants it has collected; it then filters it via hyperaccumulating bioremediators before it runs into the waterways and underground aquifers.

5.      Bioswales reduce flooding. On the LRT project and the ILA project, with the conveyance of transportation infrastructure at stake, flooding was not an option, and bioswales were utilized to prevent it. Bioswales reduce flooding by their decreased elevation and connected channelization, which guide water towards them. Additionally, their increased surface area and the permeability of their soil and stone components both slow the force and drain the volume produced in a rain event. The plants flourishing within bioswales help absorb and store floodwaters, and their textured massing helps slow its velocity. Particular to the Expo projects, bioswales help keep the entire connected infrastructure of a transportation system (the tracks and guideway, the stations, and the adjacent bike path) from being submerged in water. On other projects, regardless of their location, bioswales can help reduce flooding by slowing and collecting the rapid stream of storm water, which helps prevent sidewalks, streets, open spaces, urban centers, and storm drain systems from becoming inundated with pools of standing polluted water.

Smart Installation
For the successful establishment of native species installed in and around bioswales, it is best to plant the species immediately prior to the rainy season. In this way, seedlings and delicate nursery stock can take root and thoroughly establish themselves using seasonal rains to bolster growth rather than relying solely on supplemental irrigation. This also avoids having to endure temperature stresses and fluctuations of hot and dry summer months.

Establishment And Maintenance
In order to ensure that bioswales are not overtaken by invasive weeds, they should be properly identified and removed via hand-pulling. It is best to remove these invasive weeds early before they begin seeding and become established. Chemical sprays are not recommended and should be strictly avoided as they may kill the desired native species that are often entwined with the invasive weeds. Additionally, as bioswales are connected to larger water bodies, the application of chemical sprays can infiltrate nearby aquifers and waterways that the bioswales are deliberately linked to, and which we depend on as sources of drinking water.

The vitality of our ecosystem is inherently connected, and bioswales are a holistic approach to its survival and restoration in their design, utility, function, and application. What we instill in them, they contribute back to us.

Robin Lynch, ASLA, is the owner of Regenerative Design Studios in San Pedro, Calif. Reach her at robin@rdsla.com.

Adam Sapin is a Landscape Design Associate at Regenerative Design Studios. Reach him at adam@rdsla.com.