By Xiangfei Li, Kerri Robinson, Danlin Su, Ross Bulat and Andrew Liu
Low impact development (LID) is a stormwater management and land development approach that mimics the natural hydrological cycle to manage stormwater at its source(s). Bioswale LID features have been increasingly applied in green field development and redevelopment to treat stormwater and enhance aesthetics of the communities. Bioswales are open channels with dense vegetation specifically designed to attenuate, treat and convey stormwater runoff.
Stormwater volume and flow rate are reduced as the water moves along bioswales. Contaminants are settled, filtered, absorbed and/or biodegraded as the water slows down and percolates through the vegetation layer into the ground. Bioswales can green a community, improve air quality, provide wildlife habitat, and contribute to flood mitigation.
Bioswales are being applied in Edmonton as part of the City’s LID implementation plan and environmental initiatives. The City promotes bioswale application and has developed design guidelines and provided training to LID practitioners. The use of bioswales is encouraged but not yet mandated. For them to become the norm, further understanding of how to address cold climate challenges is critical. The City keeps track of bioswale facilities, conducts demonstration projects, and monitors their performance to understand what would and would not work in Edmonton’s cold climate conditions.
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The City maintains an LID facility inventory map that includes seven types of LID features installed around Edmonton. See the map here: https://www.edmonton.ca/city_government/environmental_stewardship/low-impact-development-map.aspx
Bioswales in different land use settings were inspected, including parking lots, middle of roadways, and around buildings. Properly locating bioswales with consideration of traffic and snow removal/storage is important for the success of the bioswale application. Bioswales located along a bus route faced the most challenges related to snow plowing and storage. This is particularly important in a municipality like Edmonton, where winter conditions necessitate the extensive use of street sands along arterial roads and bus routes.
Mulch plays an important role in plant health and stormwater water quality treatment. It can prevent weeds, keep moisture in the soil, and strain pollutants in stormwater. On the other hand, mulch was observed to float in the channel of a few bioswales after storms. Mulch is a landscaping product that is not typically considered in drainage designs. Specifications rest with landscape architects by default. It is therefore important that landscaping architects understand the functions and limitations of mulch related to its role in LID facilities.
Mulch should be tested in water before being installed due to the lack of mulch specification for LID application and field experience.
Field inspections observed that flow bypassed curb-cut inlets due to improper grading and/or overgrowth of grass that can block flow coming into bioswales. This can be a design or construction issue.
Side slope instability
Steep side slope and lack of energy dissipation can cause side slope instability.
Parking lot bioswales accepting sheet flow are prone to vehicle damage. Curbs or chains around the bioswale should be considered to protect the bioswale facilities. In addition, snow should not be stored on a bioswale unless the facility is specifically designed to take it. Sand and gravel applied during winter maintenance can clog the soil mix of the bioswale during snowmelt and reduce its permeability. This can in turn cause ponding during storm events.
Salt in the snow can harm plants and vegetation in the bioswale. To mitigate snow impact, designers need to properly size the bioswale and locate it so that there are other spaces left for snow storage. Prioritizing street sweeping operations in areas upstream of bioswale facilities prevents highly concentrated sediments and sand from entering, avoiding potential soil clogging issues.
Sedimentation control during construction
Sedimentation control is critical to achieving the designed infiltration rate of bioswales. Field observations noted that bioswales on public lands were generally protected from sediments released as a result of construction activities. Bioswales located in the middle of the road in a greenfield development faced the most challenges because there are typically multiple contractors and home builders working in the stripped catchment area. It takes years for full build-out to occur and for the catchment areas to be completely stabilized with vegetation.
Sedimentation control post-construction
A common issue in established bioswales is the accumulation of sediments around bioswale inlets or even caking on the bottom of the facilities. Post-construction sediment control is an integral part of maintenance that affects the long term health and functionality of a facility.
Long term maintenance
Identification of the department(s) responsible for various aspects of inspection and maintenance should occur at the planning stage. Then, the relevant business areas are able to provide input into the facility design to ensure operational needs are considered. The City is in the process of implementing their Low Impact Development Construction, Inspection and Maintenance Guide to improve construction practices and long-term maintenance.
Based on monitoring results and field inspection information, future work to improve bioswale implementation should include the following:
- Start to clean up sediments accumulated around the inlets or in the bioswales.
- Enforce implementation of sedimentation control measures and reporting and include reports on sedimentation control into project acceptance inspection.
- Design with consideration of snow removal and sand/gravel impact on bioswales.
- Monitor water quality treatment efficiency.
Xiangfei Li, Kerri Robinson, Danlin Su, Ross Bulat, and Andrew Liu are with the City of Edmonton.