Bioinfiltration basins are vegetated, landscaped shallow depressions designed to capture stormwater runoff and rainwater which then infiltrates the soil recharging groundwater or restoring soil moisture. Some water that enters these basins is also dispersed by evapotranspiration. Similar structures include detention basins (hold and release), retention or infiltration basins (hold and infiltrate) and dry ponds. These systems are designed primarily to capture and retain stormwater, and allow it to infiltrate the soil or slow its release into a receiving water body.
Bioswales or vegetated swales are open, vegetated channels designed to move stormwater runoff from one point to another. They are an ecologically-functional alternative to underground storm drains. In some situations, they can be used in combination with storm drains.
Rain gardens are typically implemented on a smaller scale and are otherwise similar to bioinfiltration basins. They are a landscape feature that are typically planted with native perennials to slow the flow of stormwater runoff and allow infiltration into the soil. When installed on residential properties, rain gardens can help direct water away from homes.
Recommended Practices
Bioinfiltration sites (basins, swales, and rain gardens) are recommended to detain, treat and infiltrate stormwater runoff.
Replacing below-ground storm drains with above-ground bioswales can be a practical and cost-effective stormwater management solution under the right circumstances.
Increasing organic content of soils and protecting or creating wetlands in the built environment of urban and periurban areas will help restore the land’s capacity to absorb rainwater.
Project Considerations
Soil types. It is important to consider the infiltration capacity of underlying soils. In cases where infiltration/percolation rates are very slow, i.e., in Alberta’s clay soils, it is necessary to amend the soil to increase infiltration rates. This can be done by removing the clay to an adequate depth, filling the trench or basin with a coarse substrate (e.g., gravel) and using an organic-rich soil medium (e.g., sandy loam) for planting. Consult a soil specialist or a soil engineer for a site assessment and advice regarding adequate soil amendment.
Maintenance. These less conventional stormwater management systems often have unique maintenance requirements, particularly in cold climates where winter roads receive applications of sand and salt. One way to help manage sediment build-up is to incorporate concrete-lined “fore-bays” into the design, in which sediment can settle out and be mechanically collected on a periodic basis.
Backup system. Depending on soil types and climate conditions, a parallel backup or overflow stormwater system may be required (e.g., overflow basin, constructed wetland or smaller conventional storm drain system).
Containing traffic. Preventing vehicles from travelling on bioswales is important. Curbs can be incorporated into bioswales in such a way that keeps vehicles out of these structures while allowing water to enter the infiltration area.
The Business Side
Bioswales can present significant cost reductions for municipalities that choose not to implement the traditional curb-and-gutter system. It is estimated that a bioswale costs half to one-third the cost of a conventional engineered stormwater system.[1] Although there are still significant expenses for proper soil preparation and potential land acquisition, there is much less earthwork required to prepare a bioswale than to unearth the area required to lay down storm drains. Maintenance costs can also be reduced since the system is at the surface and therefore more accessible when repairs are needed. Furthermore, maintenance of a bioswale does not require prolonged road closures and traffic detours for upgrades or repairs since it is built along a right-of-way that is adjacent to the road.
Another economic benefit of these alternative stormwater management systems is the reduced demand on traditional stormwater conveyance systems. This can extend the life of traditional systems and reduce the need for upgrading capacity.
Other values provided by bioswales, rain gardens and bioinfiltration systems include flood attenuation and associated reduction of risk to infrastructure and property due to flooding, watering of urban vegetation through the swale/rain garden system, and protection of water quality in urban and periurban areas. Bioinfiltration, rain gardens and bioswales allow for stormwater to stay at the surface (as opposed to underground pipes), enabling ultraviolet radiation, microbial activity, sedimentation, and plant-based nutrient sequestration, thereby improving the water quality of the runoff.
The Nature Side
Bioinfiltration, rain gardens and bioswales act as filtration systems for runoff. Aquatic habitats of receiving water bodies are subsequently protected from pollutants that enter with stormwater (i.e., heavy metals, hydrocarbons, sediments, nutrients, pesticides, pet waste, etc.) and from the erosive forces of incoming high water volumes.
Vegetated bioswales also add biodiversity and habitat for wildlife and pollinators. When properly designed and landscaped, bioswales provide opportunities for natural area networks in urban and periurban areas, as well as the potential for wildlife movement corridors. Plants in these vegetated areas also provide carbon sequestration.
The Community Side
Bioinfiltration basins, bioswales and rain gardens can add a lot of visual interest and green space to urban and periurban landscapes that have large expanses of impervious surfaces, such as industrial areas, streets and parking lots. When linked to other green spaces, bioswales and infiltration basins can present opportunities for attractive recreational space.
Bioswales can be incorporated next to walking paths and linked to urban greenways to create green networks. The shade, shelter and added interest of a vegetated bioswale along a pedestrian pathway may encourage more pedestrian and cyclist activity. Consequently, this creates positive effects on local neighbourhoods and the municipality as a whole by reducing traffic and emissions and providing recreational opportunities. Within parks, these features can provide opportunities for public education through interpretive signage.
Adding green space and plants to the urban and periurban landscape has a positive impact on human physical and mental health and can increase real estate marketability in adjacent neighbourhoods.
Footnotes
[1] Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Ellicott City, Maryland. (June 2021).
Ahern, J. 2011. From fail-safe to safe-to-fail: Sustainability and resilience in the new urban world. Landscape and Urban Planning, 100(4), 341-343.
Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Ellicott City, Maryland. (June 2021).
City of Calgary. 2007. Stormwater sources control handbook.
Primeau, S., Bell, M., Riopel, M., Ewaschuk, E., & Doell, D. 2009. Green Communities Guide: Tools to Help Restore Ecological Processes in Alberta’s Built Environments. Land Stewardship Centre of Canada.