A Mesocosm Study of Bioretention Functionality Under Frozen Conditions
Date
2023-06
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Abstract
Urbanization interrupts the natural hydrologic cycle through the development of large impervious areas that prevent infiltration and introduce contaminants to stormwater. Bioretention is a green stormwater management strategy designed to restore natural hydrologic conditions in urban areas by promoting infiltration and removing contaminants at the source. Bioretention technology has proven to be effective in warm climates, but questions remain regarding how the systems perform in cold climates. This study investigated how bioretention systems function under frozen conditions using 24 mesocosm cells in Okotoks, Alberta. The mesocosm cells were lined and had three types of media, three types of vegetation, and two different impervious to pervious ratios. Media types included two typical bioretention media as well as an experimental mixture of clay loam and woodchips. Simulated spring melt events were conducted at the end of March 2021 and March 2022, with an additional event included at the beginning of March 2022 to observe the influence of midwinter melts on bioretention performance. Datasets were analyzed to compare frozen performance among the different design parameters and simulated events, and were also compared to unfrozen conditions using existing datasets from a separate study conducted at the site. The results of this study showed an average 80% decrease in infiltration rate under frozen conditions relative to unfrozen, with significant difference in infiltration rate depending on media type. The findings show that hydrologic performance is largely based on media type, and more specifically, the ability of the media to support preferential flow under frozen conditions. Preferential flow also limited the effects of refreezing midwinter meltwater, as no significant difference in infiltration rate was seen following the midwinter melt event. Nutrient performance was comparable between frozen and unfrozen conditions, with most cells leaching relatively low concentrations of nutrients. Nutrient removal was largely dependent on media type, but plant nutrient uptake also appeared to play an important role. Overall, the results of this study showed that bioretention systems function well under frozen conditions, but the reduced infiltration rates and effects of preferential flow must be taken into consideration when designing the systems.
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Keywords
Hydrogeology, Bioretention, Low Impact Development, Stormwater, Frozen Soil
Citation
Elliot, S. (2023). A mesocosm study of bioretention functionality under frozen conditions (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.