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A Bioswale or biofiltration swale is a narrow strip of a vegetated area that redirects and filters stormwater. A typical bioswale is long, linear and shallow and is used to collect runoff water from non-porous surfaces such as roads, parking lots, rooftops etc. Besides treatment of stormwater, a bioswale helps in mitigating flooding potential and diverting stormwater away from critical infrastructure.
Bioswales are usually used as an alternative, or as an enhancement, to gutters, storm sewers, and traditional stormwater pipes. While gutters and stormwater pipes act merely as a conveyance system, bioswales act as a multifunctional conveyance system.
Bioswales are capable of holding the excess water in place filtering out dirt and pollutants before they reach waterways and at the same time also helping to recharge groundwater and prevent flooding.
Bioswales provide a way to conserve water, improve water quality, minimize the pollution in waterways and improve biodiversity in our burgeoning concrete jungles.
Design of a Bioswale
Bioswale systems are generally preferred for areas with permeable grounds and relatively low groundwater levels. The only construction activity it requires is the digging of a linear depression with slanted walls (Parabolic or trapezoidal shapes are recommended with side slopes no steeper than 3:1). A simple bioswale is divided into 4 different layers :
- The top layer of a bioswale is all about the closely-packed vegetation that provides a high amount of surface area for contact with stormwater. The thicker and heavier the grasses, the better the swale can filter out the pollutants. Specially chosen plants (native plants preferably) are planted that have high nutrient uptake ability.
- A layer of sand is laid beneath the vegetation that serves as an absorbent. It causes colloids in murky water to gather into larger masses and ease removal from the water.
- Below that layer is a layer of gravel, scoria or baked clay pellets enclosed in geotextile. These materials have large empty voids, allowing the rainwater to drain off. The layer is packed in geotextile to prevent the layer from becoming clogged by sludge or roots.
- An infiltration pipe/drain tube is situated below the second layer. In order to prevent a bioswale from overflowing its banks during heavy rainfall, overflows are added that are connected directly to the drain tube.
Once prepared, bioswales require very little maintenance as they need less water and no fertilizer.
Applications of Bioswale
- Parking lot Islands and medians
- Highway medians
- Landscape buff
- Curb cutouts
- Bioswale minimizes overflow, improves the quality of surface water and helps recharge the groundwater. In an event of groundwater flooding, bioswales can serve as drainage systems.
- Bioswale can help reduce heat stress in a locality.
- The vegetation and water together improve the air quality in an area.
- Bioswales with diverse vegetation increase the aesthetic and recreational value.
- Bioswales recharge aquifers.
- Bioswales increase the biodiversity of plant and insect species.
- Bioswales facilitate water purification and distribution.
- Bioswales reduce flooding.
- Bioswales protect sensitive areas.
- Bioswales increase community marketability