Trout are an important game species that provide a substantial economic impact in Virginia. Along with other cold-water fish species, trout are extremely susceptible to changes in stream temperatures. Urban development and the increase in impervious surfaces alter the hydrologic cycle in urban watersheds, limiting infiltration and increasing surface runoff. Impervious surfaces absorb and store solar radiation, resulting in higher surfaces temperatures, and then transfer this thermal energy to runoff during a rainfall event, resulting in higher runoff temperatures. Bioretention cells are a common stormwater control practice identified as a possible thermal mitigation practice in urban watersheds harboring cold-water fish species. However, design specifications vary by locality and few studies have explored how design characteristics impact the temperature reduction potential. The goal of this study was to investigate changes in stormwater thermal load due to bioretention cells.
In this study two bioretention cells with differing design approaches were monitored to quantify the thermal reduction impact that the bioretention cells have on stormwater from impervious surfaces. Both cells significantly reduced stormwater outflow volume, event mean temperatures and heat loads; however, outflow temperatures repeatedly exceeded the 21°C temperature threshold for cold-water fish species. This finding indicates this practice alone may not be sufficient to reduce runoff temperatures below biological stress thresholds. In addition, previous literature suggested that deeper cells may provide more cooling benefits as deeper soil layers are cooler and have more stable temperatures. In this study, the deeper cell was not as effective in reducing runoff temperatures, likely due to surface overflow and a shorter residence time in the bioretention cell. This finding indicates there is a limit to the effectiveness of cell depth in runoff thermal reduction and that other cell characteristics, such as subsurface drainage system length, may play an important role in runoff temperature reduction. / Master of Science / Cold-water fish species such as trout are a game species of large economic value that are very susceptible to changes in water temperature. Due to warmer runoff temperatures from urban watersheds stream temperatures are increasing, posing a potential impact on the cold-water fish found in these watersheds. Bioretention cells are a common method for treating and reducing pollutants from stormwater in urban areas. Recently, research has focused on the potential of bioretention cells to reduce runoff temperatures in urban watersheds. However, research is limited and does not fully address the bioretention design characteristics that may be beneficial for reducing runoff temperatures.
In this study two bioretention with differing design approaches were monitored during summer months to quantify and assess the potential for runoff temperature reduction. Both cells reduced runoff volume, temperature, and overall heat energy leaving the cell. However, outflow temperatures were typically above the stress temperature threshold for many cold-water fish species, indicating that this practice may reduce runoff temperatures to a level that will not stress these fish species. Previous research has suggested that deeper cells may provide more cooling benefits as deeper soil layers are experience cooler and more stable temperatures. In this study, the deeper cell was not as effective in reducing runoff temperatures as the shallow cell with a greater overall volume. This finding suggests that there is a limit to the effectiveness of deeper cells and that other cell characteristics, such as cell volume, play an important role in runoff temperature reduction.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/104073 |
| Date | 29 June 2021 |
| Creators | Paraszczuk, William Dale |
| Contributors | Biological Systems Engineering, Thompson, Theresa M., Hester, Erich T., Sample, David J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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