A bioswale is a vegetated, shallow, and channeled landscape element engineered to manage stormwater runoff. Unlike a simple ditch, a bioswale is a form of green infrastructure that collects water from impervious surfaces like streets, rooftops, and parking lots. These features work to slow and filter this runoff, reducing the volume and pollution that reaches conventional drainage systems and local waterways.
The Function of a Bioswale
A bioswale’s primary role begins when rainwater carrying pollutants from urban surfaces is directed into the channel. The gentle slope and vegetation work to slow the water’s velocity. This reduction in speed causes larger debris and suspended solids, like silt and sediment, to settle out of the water instead of being carried downstream.
As stormwater moves slowly through the swale, it percolates downward through the plant root systems and layers of soil. This is where the filtration process occurs. Plant roots and the soil matrix physically trap fine particles, while biological and chemical processes remove other contaminants. Microorganisms living in the soil break down organic pollutants, and the plants themselves absorb nutrients like nitrogen and phosphorus.
Much of the filtered water infiltrates deep into the ground, which helps to replenish local groundwater aquifers. In heavy rainfall, any excess water that doesn’t soak into the ground is collected by an underdrain system at the bottom of the swale. This system directs the treated water into the main storm sewer system. This entire sequence reduces flood risk and mitigates the environmental impact of urban runoff.
Key Design Elements
A bioswale’s effectiveness depends on its vegetation. These systems use hardy plants, often native species, chosen for their extensive root structures and tolerance to both wet and dry conditions. The dense root systems stabilize the soil, prevent erosion, and actively draw water and nutrients from the runoff. These plants can also be selected to enhance local biodiversity and provide habitat for pollinators.
Beneath the vegetation lies a carefully engineered soil media. This is a specific mixture of sand, compost, and soil designed to facilitate both filtration and proper drainage. The composition is calculated to retain water long enough for treatment to occur while preventing prolonged ponding. Below this soil layer, a gravel layer often provides additional storage and conveyance of filtered water to an underdrain.
The shape of the bioswale is also a design feature. They are linear with gently sloped sides, often with a slope of less than six percent. This geometry ensures incoming stormwater spreads out evenly, maximizing contact time with the vegetation and soil for treatment. Check dams, small barriers of stone or wood, may be installed to create pools, further slowing water and increasing infiltration.
Water enters a bioswale through an inlet, which could be a curb cut for street runoff or a pipe from a roof’s drainage system. While many are designed to allow all water to soak into the ground, some include an overflow structure. This feature acts as a failsafe during extreme storm events, allowing excess water to exit the swale and flow into the conventional storm drain system.
Common Applications
Bioswales are frequently integrated into parking lot designs, where they serve a dual purpose of landscaping and stormwater management. They are placed in medians or along the perimeter of the paved area to capture runoff laden with automotive pollutants. The “first flush” of rain, which carries the highest concentration of contaminants, is effectively captured and treated.
Another common application is along residential streets and within road medians. In these settings, bioswales replace traditional curbs and gutters, collecting runoff directly from the roadway. This approach helps to clean the water and can also contribute to traffic calming and neighborhood beautification, improving the community’s aesthetic appeal.
Corporate and university campuses, as well as public parks, utilize bioswales as part of a sustainable landscape design. In these larger spaces, they can be designed as aesthetically pleasing features that meander through the property, managing runoff from buildings, walkways, and other hard surfaces. This provides functional water management while enhancing the landscape.
Homeowners can incorporate smaller-scale bioswales into their properties to manage stormwater from a roof, driveway, or patio. A residential bioswale can be a practical way to prevent soil erosion and localized flooding. When strategically placed, it can turn a drainage problem into a functional garden feature.