Thermal Evaluation of an Urbanized Watershed using SWMM and MINUHET: a Case Study of Stroubles Creek Watershed, Blacksburg, VA
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Urban development significantly increases water temperatures within watersheds, primarily from the construction of impervious surfaces for buildings and pavement. While thermally enriched runoff can be harmful to aquatic life, available research and guidance on predicting these effects is limited. The goal of this assessment is to provide guidance on how to achieve necessary temperature regimes that meet aquatic health criteria for sensitive species such as trout. To address this need, the Minnesota Urban Heat Export Tool (MINUHET) and U.S. Environmental Protection Agency's Storm Water Management Model (SWMM) models were utilized to simulate hourly streamflow, water temperature, and heat flux in an urban watershed in Blacksburg, VA for typical summer periods using continuous-based simulation. SWMM and MINUHET were combined in a unique, hybrid approach that emphasized each model's strengths, i.e., SWMM for runoff and streamflow, and MINUHET for water temperature. The watershed is 14.1 km², and is portion of Stroubles Creek located near downtown Blackburg, Virginia and the main campus of Virginia Tech. Streamflow, water temperature, and climate data were acquired from Virginia Tech StREAM Lab (Stream Research, Education, and Management) monitoring stations. SWMM and MINUHET were calibrated manually for the summers of 2016, and were validated for the summer of 2015. Model sensitivity analyses revealed that simulations were especially sensitive to imperviousness (SWMM predicted streamflow as outputs) and dew point temperature (MINUHET predicted water temperatures as outputs), both resulted in increased outputs of SWMM and MINUHET models, respectively. Model performance in simulating streamflow was evaluated using Nash-Sutcliffe Efficiency (NSE) and correlation (R²). NSE and R² values were 0.65 and 0.7 for the SWMM Model and 0.57 and 0.55 for the MINUHET model during the validation period, indicating that SWMM performed better than MINUHET in streamflow simulation. Streamflow temperatures were simulated using MINUHET with a NSE and R² statistical values of 0.58, and 0.83, respectively, demonstrating a satisfactory simulation of water temperature. Heat loads were simulated using the MINUHET and Hybrid models, demonstrating less Percent BIAS of the Hybrid approach in simulation of watershed total heat load than MINUHET alone. Furthermore, a number of practices were implemented to reduce thermal loading within a watershed. These included infiltration practices, methods for decreasing absorption of thermal energy primarily by reducing albedo, and increased vegetation canopies. An index titled Percentage of Time Temperature Exceeded 21°C Threshold (PTTET) for trout habitat was used to represent the effectiveness of thermal mitigation practices. Installing concrete pavement (thermal diffusivity: 15×10-7 m²/s, pavement thickness: 0.20 m, and heat capacity: 4.0×106 J/m³⋅°C) and Acrylic Coated Galvalume (ACG) roofs for all pavement and roofs, respectively, in the watershed reduced heat load by 45%, and the PTTET index declined 4.5%. Installing bioretention with 61 cm of media thickness, and soil-media infiltration rate of 25 mm/hr. for 53 selected parking lots in the watershed, resulted in 11.1% reduction in watershed heat load and 1.4% reduction in PTTET. Planting forest canopies for the entire pervious area of the watershed, sufficient to shade 90% of all lands, resulted in reduction in heat load by 24% and PTTET by 4.6%.
- Masters Theses